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BULLETIN 


OF THE 


_ MUSEUM OF COMPARATIVE ZOOLOGY 


AT 


HARVARD COLLEGE, IN CAMBRIDGE. 


VOL. XXX. 


CAMBRIDGE, MASS., U.S. A. 
1896 —1897. 


UnIvERSITY PRESS: 


p Sox, CAMBRIDGE, U.S. A- 


Joun W1LsoN AN 


613340 
v1, oe 


CONTENTS. 


No. 1.— The Early Development of Asplanchna Herrickii de Guerne ; a con- 
tribution to Developmental Mechanics. By H. S. Jennines. (10 Plates.) 
October, 1896 . Sika dooe pace 

No. 2.— Some Variations in the Genus Eucope. By A. Acassiz and W. M. 
WoopworrH. (9 Plates.) November, 1896 

No. 3.— Reports on the Results of Dredging in the “Blake.” XXXVII. 
Supplementary Notes on the Crustacea. By W. Faxon. (2 Plates.) No- 
vember, 1896 

No. 4.— On the Colors and Color-patterns of Moths and Butterflies. By A. 
G. Mayer. (10 Plates). February, 1897 

No. 5.— The Mesenteries and Siphonoglyphs in Metridium marginatum 
Milne Edwards. By G. H. Parker. (1 Plate.) March, 1897 . 

No. 6.— Photomechanical changes in the Retinal Pigment Cells of Palae- 
monetes and their relation to the central nervous system. By G. H. 
PaRKER. (1 Plate.) April, 1897 


PAGE 


273 


No. 1.— The Early Development of Asplanchna Herrickii de 


Guerne. A Contribution to Developmental Mechanics! By 
HERBERT S. JENNINGS. 
CONTENTS. 
PAGE PAGE 
introduction’. -s =<. . « 2. Maturation . . Sh Seyoeo. 86 wae 


8. Orientation of the Tevelpine Em- 
AVEA ach Opas eT Moe aM onmON EO. make! 
AM@leavarehvensecns Hel nett) settee LO 


“ney 
Part First. Developmental Mechanica: 4 
I. Statement of Problems. . ...- 4 
1. Cleavage ... Ce oer RAN RCS 

4 


A. Direction of ieee Sask Nomenclature .... . . 16 
(1) Berthold's principle of least sur- irst(Cleavagé (0s) os 2 =. 18 
PAGES IES wt « 4 Second “ Ao & ooo 2 
(2) Hertwig’s law o the Epudie in Thirds) 5 WRENS. ho) MN ye 
the longest axis of the eG Fourth ‘“ AG MA yO On 
plasmic mass 5 Fifth = Sela em phates, moo 

(3) Braem’s theory of ae in Sixth ne 


the direction of least resistance 5 

(4) Roux's theory of a compromise 
between the tendency imma- 
nent in the nucleus and the 
tendency due to the form of 
the protoplasmic mass 

(5) Heidenhain’s problem of a defi- 
nite angle of rotation . . 6 


a 
Seventh and Tater Gleawees . 45 
The Eetoderm. ... .. .+45 
The Entaderm)..%5 <3 7s 54 

III. Discussion of the Bearing of the 
Observations on the Problems . 58 
1. Cleavage ... i oer eetcet |. 
A. Direction of Cleavage eS bernie 58 
(1) Berthold’s theory of least srriees 58 


for) 


(6) Sachs’s law of the right- aneled 


(2) Hertwig's law of the spindle in 


arrangement of cell walls . if the long axis f 60 
(7) Rauber’s theory of inter-attrac- (3) Braem’s theory of least rete 69 
tion of asters . Aas (4) Roux’s theory 70 
(8) Braem’s principle of eainat re- (5) Heidenhain’s problem of a defi 
sistance at the two ends of the nite angle of rotation 72 
spindle . 3 7 (6) Sachs’s law of the Fone anced 
B. Equality or Inequality of Gieavazs i arrangement of cell walls. . 73 
C. Rate of Cleavage. . . iS (7) Rauber’s theory of the inter- 
D. Differentiation during Clearare As ae) attraction of asters . . . . 74 
2. Later Developmental Processes . 9 (8) Braem’s principle of equal re- 
Il. Descriptive Portion 70 sistance at the two ends of the 


1. Form and Structure of the eee - 10 SPINGle persia Nota Sot let fra a: 


1 Contributions from the Zodlogical Laboratory of the Museum of Comparative 
Zoology at Harvard College, under the direction of E. L. Mark, No. LXX. 

Contribution No. LXIX., under the direction of E. L. Mark, was published 
under the following title: “Descriptions of Three Species of Sand Fleas (Amphi- 
pods) collected at Newport, Rhode Island. By Sylvester D. Judd.” Proceed. 


U.S. Nat. Museum, Vol. XVIII. No. 1084, pp. 593-603, 11 Figures, August [7], 1896. 
VOL, xxx.— no. 1. 1 


2 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 

PAGE PAGE 
B. Equality or Inequality of Cleav- 2. Development. . . - © » - « 89 
Bre Progucte: Wereey mice Hee te A. Maturation . . =». « «2 » 389 
GC RateorOleavare’.) 3) wy... TE B. Cleavage .< + «© % (eaten 

D. Differentiations accompanying C. Summary on Maturation and 
Gleauaoaie se Perse poke ot nit Cleavage in the Rotifera . . 100 
. Gastrulation . . . . . . . 80|Part Third. Material and Methods. . 101 
3. General Cpantiefedons cha 83| General Summary. ..... - . 106 
Part Second. Discussion of Mattes A. Observations = . < (ce ouemee LUD 
bearing upon the Morphology of B. ‘Conelusions «9. 3) 3) ee od 
the Rotifera . . . . 87 | Literature\Cited ” 2.) "=: een 
1. Previous Knowledge of ae iene Explanation of Plates . .., . . - 117 


PIGITIGEIE © pro oy Nene! is blaine 1S 


INTRODUCTION. 


Tue following pages contain a study of the early development of an 
organism, with especial reference to recent theories in regard to the laws 
of cleavage and the relation of cleavage to morphogenesis. 

Many theories and so called laws have been set forth concerning the 
factors determining the manner and rate of cleavage. These have 

aken the form chiefly of theories in regard to the causes of the direc- 
tion of the spindle, of the equality or tnequality in size of the products 
of division, and of the relative rapidity with which the different cleav- 
age cells divide. Yet few attempts have been made to interpret con- 
sistently the cleavage of any given organism with relation to any or all 
of these theories. The sketch of Braem (94) with regard to the 
Echinoderm egg, and the recent studies of Ziegler (95) and zur Stras- 
sen (96) on the Nematode egg, are almost the only works that can be 
cited in which an attempt has been made to show the relation of any 
theory or theories to the series of normal cleavages in any animal. In 
other discussions the theories have been based upon experimental evi- 
dence or upon scattered observations. Yet it is, of course, the normal 
processes for which explanations are desired; scattered observations may 
be adduced for almost any view. It seems of the greatest importance, 
therefore, to show clearly the exact relation which the theories hitherto 
proposed have to the actual series of cell divisions in the development 
of particular organisms. 


1 In view of the close similarity of some of my conclusions with some of those 
in the more recent ('96) of two papers by zur Strassen, it may be proper to state 
that a copy of the present paper, exactly as here published, with the exception of 
some verbal alterations and the addition of a few references, was deposited with 
the Faculty of Arts and Sciences of Harvard University on April 80, 1896, while 
zur Strassen’s (96) paper was not received here till May 13. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 3 


Furthermore, there is much discussion of the question as to whether 
cleavage is a mere quantitative separation of a single mass into smaller 
masses similar in nature to each other and to the original egg, or 
whether it is accompanied by a differentiation of the separated blasto- 
meres, — as a result either of qualitative division or other changes. 

A third question of theoretical interest, somewhat related to the last, 
is whether the method of cleavage has a direct mechanical relation to 
future morphogenetic processes, or whether it is merely the passing of 
partitions through a mass of protoplasm, the order in which this occurs 
and the arrangement of the partitions being immaterial. For example, 
Is gastrulation a process independent of cleavage and merely requiring 
the latter as a prerequisite, —as the planting of seeds must be preceded 
by ploughing, — or is gastrulation in some way connected with or depend- 
ent upon the manner of cleavage? Stated in the most general terms, 
this is the question: Is cell division a direct morphogenetic factor, or 
are the real formative processes dependent upon the introduction of 
other factors after the cleavage is finished ? 

With these questions in mind, I have studied the development of an 
organism of the class Rotifera throughout those stages of development 
in which it is possible to make the cells the units of observation, — that 
is, through cleavage and gastrulation and somewhat later. 

Broadly stated, the object of the work may be expressed as the analysis 
of the early development of an organism into the simplest factors possible. 

The development of Asplanchna Herrickii has not been studied previ- 
ously, and in the course of this paper it will be necessary to discuss some 
matters which are of importance primarily to persons who are engaged 
particularly with the morphology of the Rotifera, and which are not of 
especial interest from a morphogenetic standpoint. In order to dis- 

_tinguish these two lines of discussion, I shall divide the work into two 
main portions. Part First will contain all matters bearing upon devel- 
opmental mechanics. Here will be found the minute description of the 
cleavage, gastrulation, and other processes, as well as a discussion of 
their bearing upon the problems of morphogenesis. Part Second will 
contain a brief review of previous knowledge of the organism studied, 
a comparison of the development, so far as traced, with the develop- 
ment of other Rotifera, and a discussion of some of the conditions de- 
scribed by other authors. These principal parts will be followed by a 
third, on material, methods, and other subordinate matters, and the 
whole will be closed by a summary of the more important conclusions 
arrived at. 


4 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


PART FIRST.—DEVELOPMENTAL MECHANICS, 


I. Statement of Problems. 


We shall deal in the following pages with (1) cleavage, (2) gastrula- 
tion, and (3) the relation of these to each other. 


1. CLEAVAGE. 


It will be necessary, in studying the cleavage and the factors deter- 
mining it, to enter into minute details as to the movements of asters, 
the form and dimensions of cells, and other similar matters; the effort 
of following this, in itself somewhat laborious, description will be much 
lightened by holding in mind the problems upon which it bears. I 
shall therefore give first a statement of the main theories which have 
been advanced as to the determining factors in cell division. 

Cell division presents three aspects, in each of which its nature is in 
some way determined. (A) As to the direction of cleavage: the posi- 
tion in which the new septum is to appear. Since this bears a definite 
relation, in general, to the position of the spindle leading to the cleav- 
age, we may speak of this aspect as the determination of the direction 
of the spindle. (B) As to the relative size of the two products; 
whether the division is equal or unequal. (C) As to the relative time 
of division, or the interval between successive cleavages. 

Besides these, we have (D) the question of the qualitative nature of 
cleavage. Are all the cells that are produced of similar structural and 
material character, or is cleavage accompanied by qualitative differentia- 
tion of the blastomeres, — either as a result of qualitative karyokinesis 
or otherwise ? 


A. Theories as to the Factors determining the Direction of the Spindle 
and the Position of the new Cell Wall. 


The theories as to the factors determining the direction of cleavage 
are numerous, and have been much discussed of late. General reviews 
of these theories will be found in Driesch (’92, p. 26), Braem (94, 
p. 340), Ziegler (’94, p. 136), and McMurrich (’95). I shall give here 
as brief and precise a statement of each theory as possible, first in my 
own words, then, so far as practicable, in a quotation from the author, 

(1) Berthold’s principle of least surfaces. — Berthold (’86) holds that 
the form and relative position of cells, and as a consequence their direc- 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 5 


tion of cleavage, is determined, partially at least, by the same factors 
which determine the form and relative position of soap bubbles in a mass. 
As a result of surface tension, the cells take such forms as to occupy 
the given space with the least possible surface areas. New septa will 
appear in such positions that their surfaces will be the least possible 
areas that could divide the cell into parts of the required size. “ Die 
Lamellensysteme ordnen sich so an, die einzelnen Lamellen kriimmen 
sich in der Weise, dass die Summe der Oberflichen aller unter den ge- 
geben Verhiltnissen ein Minimum wird.” (Berthold, ’86, pp. 219, 220.) 

(2) Hertwig’s law of the spindle in the longest axis of the protoplasmic 
mass. — According to Hertwig’s well known view, as a result of the inter- 
action of nucleus and protoplasm, the spindle during division comes to 
lie in such a position that its longitudinal axis coincides with the axis 
which passes through the greatest protoplasmic mass. “ Es lasst sich 
hier das zweite allgemeine Gesetz aufstellen, dass die beiden Pole der 
Theilungsfigur in die Richtung der gréssten Protoplasmamassen zu 
liegen kommen, etwa in derselben Weise, wie die Lage der Pole eines 
Magneten durch Eisentheile in seiner Umgebung beeinflusst wird.” 
(Hertwig, ’93, p. 175.) 

(3) Braem’s theory of separation in the direction of the greatest space 
for development. — This is a modification of the principle of least pres- 
sure, first enunciated by Pfliiger (’84). Since Pfliiger’s principle, con- 
sidered from a purely mechanical standpoint, seems irreconcilable with 
the nature of the material on which it was supposed to act, and since 
Braem’s view is based on an essentially different conception of the na- 
ture of the phenomena, I have not thought it necessary to take into 
direct consideration Pfliiger’s view. 

Braem holds that when an egg is subjected to unequal pressure, the 
spindle places itself in such a position that the resulting products shall 
have the freest opportunity for development; that is, in the direction of 
‘least resistance. The rule is not the expression of a purely mechanical 
force, but is to a certain extent teleological in character. ‘“ Die Spindel 
eines ungleichem Druck unterliegenden Eies stellt sich in derjenigen 
Richtung ein, in welcher der riumlichen Entfaltung der Zelle und ihrer 
Teilprodukte der freieste Spielraum geboten ist. Ich glaube, dass diese 
Fassung trotz oder vielmehr gerade wegen ihres teleologischen Gehaltes 
dem Wesen der Sache besser entspricht als die rein mechanische Deu- 
tung.” (Braem, 94, pp. 341, 342.) 

The result is held to be due to a sort of sensory power resident in 
the egg, ‘“‘eine Art Tastsinn, durch den es der Zelle m@glich wird, sich 


6 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


iiber ihre unmittelbare Umgebung zu orientieren und demgemass ein- 
zurichten.” (p. 342.) 

(4) Rouzx’s theory of a compromise between the tendency immanent in 
the nucleus and the tendency due to the form of the protoplasmic mass. 
— Roux holds that the spindle places itself in one of the positions of 
stable equilibrium in relation to the protoplasmic mass, — therefore, at 
least generally, in the longest axis of the protoplasmic mass, though 
sometimes at right angles to that axis, the factor that decides which of 
these positions shall be taken being an immanent tendency in the nu- 
cleus to divide in a certain direction. 

“Richtiger ist es zu sagen: Die Kernspindel der Furchungszellen 
stellt sich in die, resp. in eine Richtung festesten Gleichgewichtes der 
tractiven Einzelwirkungen der Protoplasmamasse. Diese Richtung 
entspricht tiberwiegend hiufig annihernd oder ganz der gréssten durch 
den Mittelpunkt der Protoplasmamasse gehenden Dimension. 

“Diese Richtung des Gleichgewichtes wird aber nicht vollkommen 
vom Protoplasma allein bestimmt, sondern sie kann, wie ich bereits 1884 
und 1885 auf Grund von Experimenten erschlossen habe, von der Lage der 
immanenten Teilungsrichtung des Kernes zu den Hauptrichtungen des 
Protoplasmakérpers abhangig sein; denn ich erhielt bei symmetrisch ge- 
stalteten ‘linsenformig’ deformirten, mit den gréssten Flache senkrecht 
stehenden Froscheiern zwe? Pradictionsrichtungen der Spindeleinstel- 
lung: die Richtung der gréssten und der kleinsten durch den Massenmit- 
telpunkt gehenden Dimension, erstere allerdings wieder die tiberwiegend 
hiufige.” (Roux, ’94, p. 152.) 

It is to be noted that this theory does not attempt to give any rule 
by which the position of the spindle is necessarily determined ; the ten- 
dency of the nucleus is simply “immanent,” and its factors unknown. 

In addition to these four well characterized theories, a number of 
less definite or partial views have been set forth, —some proposing fac- 
tors which may influence, though not alone determine, the position of 
the spindle. A number of the more important of these will be 
mentioned. 

(5) Heidenhain’s problem of a definite angle of rotation (‘“ Prob- 
lem der gesetzmissigen Drehungswinkel ”).— Heidenhain (’94, p. 719) 
thinks it probable, or at least possible, that careful investigation will 
show that in a given tissue the position which the spindle takes at the 
time of division is a result of its rotation through a definite angle, de- 
terminable for the given tissue, after the first formation of the spindle | 
by the separation of the asters. This separation of the asters is held 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. - 


to be at first in a line at right angles to the axis of the preceding spin- 
dle ; then, by a rotation through an angle characteristic for the tissue, 
the definitive position is reached. The position which the spindle is 
finally to take is therefore determined at the time the asters separate. 
“‘Soweit ich indessen die Lage ubersehen kann, ist die schliessliche 
Stellung der Spindel von dem Moment an fest gegeben, in welchem 
die Theilung des Muttermikrocentrums stattfand.” (Heidenhain, ’95, 
pp- 555, 556.) 

(6) Sachs’s view, that the walls separating the cells meet one another at 
right angles. — This (Sachs, ’78, p. 1070) can hardly be considered as 
more than a statement of a condition commonly found. Berthold (’86, 
p- 252) and Hertwig (’93, p. 177) have endeavored to show that the 
condition is explainable as a result of the theories proposed by them. 

(7) Rauber (83, p. 276) holds that there is evidence that the asters of 
the different blastomeres exercise an attraction fur each other in such a way 
that, in a given area composed of a number of cells, the spindles must 
take such positions as to bring about a condition of equilibrium among the 
asters. —“ Beurtheilt man die Verschiedenheiten der Furchennetzes von 
der Stellungen der karyokinetischen Achsen aus, so gewinnt es den 
Anschein, als ob die neu entstehenden Centren eines Blastomers auf 
diejenigen der angrenzenden Blastomeren einzuwirken verm6gen und die 
Richtung ihrer Achsen beeinflussen.”” (Rauber, ’83, p. 280.) 

(8) Braem’s principle of equal resistance at the two ends of the spindle. 
— Subordinate to his principle of least resistance, Braem holds that the 
spindle tends to take such a position that the pressure at the two ends is 
the same. ‘Es ist das Princip des gleichen Widerstandes, wodurch die 
horizontale Lage der Spindel bedingt wird. Wir miissen annehmen, dass 
der Kern von vornherein das Bestreben hat, sich gleichmiassig nach bei- 
den Seiten hin auszudehnen und somit auf eine dquale Zellteilung 
hinzuwirken.” (Braem, ’94, p. 345.) 

In the following description these theories will be kept in mind, and 
the bearing of the observations upon them pointed out. It will appear 
that, for certain of the theories, the conditions in the egg of Asplanchna 
present crucial tests. 


B. Equality or Inequality of Cleavage. 

The second aspect under which cleavage is determined is with regard 
to the relative size of the two products. What is it that determines 
whether the division shall be equal or unequal? 

Concerning the factors which determine the equality or inequality of 


8 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


cleavage, two theories have been proposed. According to the view 
which is perhaps that most generally known, the cause of unequal 
cleavage lies in the relative distribution of yolk material and formative 
protoplasm. The interaction between nucleus and cell contents, which 
determines the position of the dividing nucleus, exists only between the 
nucleus and the formative protoplasm, not between the nucleus and 
the yolk material. As a consequence of this interaction, the nucleus 
tends to take a position in the centre of the mass of formative proto- 
plasm. When one region of the cell is composed largely of yolk 
material, in a mere meshwork of protoplasm, while another region is 
made up eutirely of protoplasm, the dividing nucleus must separate 
equal masses of formative protoplasm, and thus may divide the entire 
mass into very unequal parts, —one containing a certain mass of proto- 
plasm only, the other an equal mass of protoplasm and a large additional 
mass of yolk material. The theory has recently been formulated by 
Hertwig as follows: “Die Folge dieser Wechselwirkung aber ist, dass der 
Kern stets die Mitte seiner Wirkungssphare einzunehmen sucht... . 
Wechselwirkungen finden zwischen dem Kern und dem Protoplasma, 
nicht aber zwischen ihm und dem Dottermaterial statt, welches bei allen 
Theilungsprocessen sich wie eine passive Masse verhilt. Ungleichmassig- 
keiten in den Protoplasmavertheiluug miissen sich daher auch auf Grund 
des obigen Satzes in der Lage des Kerns geltend machen, und zwar muss 
derselbe nach den Orten der grésseren Protoplasmaansammlung hin- 
riicken.” (Hertwig, ’93, pp. 172 and 174.) 

Braem’s principle of equal resistance at both ends of the spindle is in 
character related to this view of Hertwig. Besides the effect of it in 
determining the direction of the spindle, this supposed principle is like- 
wise of effect in determining the equality or inequality of cleavage, as 
appears from the quotation from Braem given on page 7. 


C. Determination of the Time of Division, or the Interval between 
Successive Cleavages. 


The same factor which is held to determine the relative size of the 
cells was also held by Balfour, with whom Hertwig agrees (Hertwig, ’93, 
p- 180), to determine the relative rapidity of cleavage. The greatest 
interval between successive cleavages is found in cells which contain the 
greatest amount of yolk relative to the amount of contained protoplasm. 
“The rapidity with which any part of an ovum segments varies ceteris 
paribus with the relative amount of protoplasm it contains; and the size 
of the segments formed varies inversely to the relative amount of the 
protoplasm.” (Balfour, ’80, p. 99.) 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 9 


D. Differentiation during Cleavage. 


Besides these questions in regard to the form and rate of cleavage, 
we have also the question of the qualitative nature of cleavage. Is 
cleavage merely a quantitative process, or is it accompanied by a 
differentiation of the separated cells? And if the latter is the case, by 
what means is this differentiation accomplished ? 

The view once maintained, that cleavage is entirely unaccompanied by 
differentiation of the separated cells, may be said to be nearly or entirely 
given up; the questions which remain relate to the means by which this 
differentiation is brought about. In regard to this several well defined 
views exist. 

1. Roux holds that the differentiation accompanying cleavage is a 
result of qualitative karyokinesis; i.e. at a given cell division the two 
products receive nuclear material of different nature. 

2. Driesch maintains that the differentiation which may accompany 
cleavage is due to the specific cytoplasmic structure of the egg, different 
parts of the egg being of different constitution, so that when this 
differentiated mass is separated into parts, these parts receive different 
sorts of cytoplasm. That is, the qualitative division is in the cytoplasm, 
not in the nuclear material. ‘‘Ich habe schon oben gesagt, dass ich ein 
Verschiedenwerden der Furchungszellen wihrend der Furchung gern 
zugebe, aber hierin nichts anderes als die Folge eines spezifischen 
Plasmabaus des Eies sehe.” (Driesch, ’94, p. 100.) 

3. According to Wilson and Hertwig the differentiation accompanying 
cleavage is due, largely at least, to the interaction of the blastomeres, 
after division has taken place. This does not exclude the possibility of 
the existence at the same time of a qualitative division of the cytoplasm, 
as stated above (2). 


2. LATER DEVELOPMENTAL PROCESSES. 


With regard to the later developmental processes, it will not be 
necessary to give here a review of the various factors and theories 
which have been set forth by different authors. Driesch (94) gives 
an extended analysis of the morphogenetic process and its factors, 
and Davenport (’95) presents a detailed list of the different processes 
concerned in development. It is sufficient here to propose a single 
question: What is the relation of the cleavage process to the secondary 
morphogenetic processes? Driesch’s well known experiments indicate 
that, in the case of the sea-urchin, the manner of cleavage is entirely 


10 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


unimportant for the later morphogenetic processes. Gastrulation, for 
example, occurs in the same manner, after the most varied and 
fundamental alterations of the cleavage. Is this a fact which is 
capable of generalization, — of application to different animals and 
different methods of gastrulation? Doubtless the only positive answer 
to this question must come from experimental studies; but a care- 
ful descriptive analysis of the process in Asplanchna gives results 
which, if the egg were a mechanism of the ordinary physical sort, would 
be definite and conclusive. 


II. Descriptive Portion. 


1. Form anD STRUCTURE OF THE Ecc. 


The development of the embryo in Asplanchna Herrickii takes place 
within the body of the mother, the egg lying enclosed in the enlarged 
oviduct, close to the ovary. The chief axis of the developing embryo 
bears no relation to the position of surrounding organs of the mother, 
the egg lying in the oviduct as it might within a protecting sac of 
any foreign material, its position determined by chance circumstances. 
In cases where two embryos are present, their axes may make any angle 
with each other. 

For study it is necessary to dissect out the eggs. A full account of 
the methods of work is given in Part Third; here it is important to note 
two facts: (1) All the work was done on preserved material ; (2) Each 
egg comes from a different individual, and is therefore in at least a 
slightly different stage from every other. A considerable number of 
eggs showing any given process, as, for example, the first cleavage, gives 
therefore a series of stages, 80 that a complete idea of the changes taking 
‘place may be gained. 

The unsegmented egg is approximately an ellipsoid of nearly equal 
axes, one end often slightly more pointed than the other. The form and 
proportions vary a little, as do also the absolute dimensions. In many 
eggs it is difficult to distinguish a more pointed end. The proportion of 
the longer to the shorter axis is about as 9 to 8, and the average dimen- 
sions of the egg are about 90 » through the longer axis by 80» through 
the shorter. Variations from a minimum of 84 by 70 to a maximum 
of 97 » by 83 » were observed. 

Whether an egg membrane is present or not is exceedingly difficult to 
decide ; and I have not succeeded in thoroughly satisfying myself upon 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. fi 


that point. The walls of the oviduct invest the egg closely, and gener- 
ally cannot be removed, so that a thin membrane, if present, could not 
be detected. In most cases where it was possible to remove the walls 
of the oviduct entirely, no membrane could be seen either in sections or 
whole preparations, the egg appearing to be naked. In a few cases, 
however, in which cleavage had recently taken place and the cleavage 
furrows were marked, it could be observed that the smooth outline of 
the egg was preserved even above the cleavage furrow, either by means 
of a membrane continuing across the furrow, or, what seemed from the 
appearance of the preparations more probable, owing to the presence in 
the furrows of a fluid mass, perhaps exuded from the egg. Such a case 
is shown in Plate 1, Fig. 4. On other grounds, however, it seems pos- 
sible that an extremely delicate membrane is present. Lameere (’90) 
states that in Asplanchna Sieboldii, which is likewise viviparous, it was 
possible to observe definitely a very delicate membrane surrounding the 
egg, especially at the time of the formation of the polar cell. 

This question of the presence or absence of an egg membrane is of 
importance from a mechanical standpoint, owing to its bearing upon the 
question as to what preserves the ellipsoidal form of the egg. The form 
is retained throughout all the early developmental processes ; cleaving 
cells do not project above the general surface of the egg, nor do the 
products of cleavage become spherical, touching at a few points only, as 
is common in the Mollusca and other groups. This retention of the 
ellipsoidal shape by the egg compels the cleaving cells to take various 
peculiar forms, which allow of a direct test of some of the theories of 
cell division above stated. It is also a most important factor in the 
process of gastrulation, so that it becomes of great interest to discover 
what it is that preserves this form. 

It is evident that surface tension would tend to produce a spherical 
rather than an ellipsoidal form. Roux (’95) has recently proved that 
blastomeres have a direct attraction for each other; but an equal 
attraction throughout the mass would produce a spherical form, and an 
unequal attraction, such as would produce a regularly ellipsoidal form, 
is very difficult to conceive of, especially as this attraction would have 
to vary regularly with the shifting of the coutents of the egg. A mem- 
brane of equal elasticity in all parts would likewise result in the produc- 
tion of a spherical form. The only direct mechanical factor that seems 
capable of explaining the continued ellipsoidal form is the presence of a 
non-elastic membrane of the exact size and shape of the egg. But 
during the later development the embryo enlarges and changes its form ; 


12 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the membrane cannot therefore be absolutely inelastic, but might be of 
such strength as to act as an inelastic membrane with regard to such 
slight forces as are exerted within the egg during its early development. 
The existence of a membrane of this peculiar character is, however, 
very improbable, and it loses all its explaining power when the egg of 
another rotifer, Melicerta ringens, is taken into consideration. In this 
species the egg is not a regular ellipsoid or oval, but is of an irregular 
shape, one side being curved in profile, the other straight. (See the 
figures of Zelinka, ’91.) This form is retained during development 
exactly as in Asplanchna, yet is not explainable on the assumption of 
a membrane. This question is discussed more fully later. 

The cytoplasm of the egg is closely filled with fine yolk granules. 
These are distributed uniformly throughout the egg (except that they 
are not present in the asters), so that there is no visible differentiation 
into regions containing greater and less amounts of yolk material. 

The development of Asplanchna priodonta Gosse was also examined 
for comparison with that of Asplanchna Herrickii. The egg of this 
species is similar throughout to that of Asplanchna Herrickii, save that 
it is smaller. The average dimensions are about 70pm by 60y. The 
egg of Asplanchna priodonta is shown in Figure 29, Plate 4, drawn to 
the same scale as the figures from Asplanchna Herrickii. 


2. MATURATION. 


The formation of the polar cell in Asplanchna Sieboldii has been 
described by Lameere (’90) from observations upon the living egg. 
The general features of the process are similar in Asplanchna Herrickii, 
though the finer nuclear phenomena differ from those described by 
Lameere. An account of the finer nuclear phenomena is, however, 
foreign to the purpose of this paper: it is necessary to describe merely 
the general features of the process, especially concerning the place where 
the process occurs, in relation to the later orientation of the embryo. 

As is now well known, but a single polar cell is commonly formed in 
the parthenogenetically developing eggs of the Rotifera. The subject 
has received full discussion, especially by Weismann und Ischikawa (’87) 
and Lameere (90). It may be noted that Zelinka (’91) observed that in 
a number of eggs of Callidina two polar cells were formed ; whether these 
arose by division of a single one, or whether the two were formed separ- 
ately from the egg, is not stated. In no case have I obtained any evidence 
indicating the formation of more than a single polar cell in Asplanchna. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 13 


In immature eggs the large germinative vesicle is commonly found in 
an eccentric position, with no very apparent relation to the axes of the 
egg. In Asplanchna Sieboldii, according to Lameere (’90), just before 
the movement of the germinative vesicle toward the spot where the 
polar cell is formed, it lies in the long axis of the egg nearer one 
pole, — in the position where later the first cleavage spindle is located. 
This is probably the case also in Asplanchna Herrickii; but before 
the germinative vesicle has begun to show the changes indicating 
the formation of the maturation spindle it is difficult in preserved 
material to get evidence as to the proper sequence of the stages 
observed. 

Just before the maturation spindle is formed, the nucleus moves 
toward the periphery of the egg, and begins to lose its spherical shape. 
It takes a position close to the surface, not at the equator, but nearer 
one of the poles of the egg, as shown in Figure 1. In cases where the 
differentiation into a more pointed pole and a blunter one is visible, 
the nucleus always lies nearer the more pointed pole. Here a spindle is 
formed, and the maturation division takes place. The polar cell thus 
formed does not lie upon the outer surface of the egg as a free body, 
but from the first is pressed into the substance of the yolk (Fig. 2), 
as if by a firm membrane, in the manner described by Lameere for 
Asplanchna Sieboldii. The nucleus begins to withdraw from the 
periphery, at the same time resuming the spherical form, leaving 
the polar cell a flattened, disk-like body, not projecting above the 
general surface of the egg. This condition of the egg is shown in 
Figure 2. 

From the first, therefore, the polar cell is imbedded in the substance 
of the egg, so that it cannot suffer displacement during the processes 
which follow. As will be shown in the course of this paper, the place 
where the polar cell is formed marks the point on the surface of the egg 
opposite to that at which gastrulation takes place. This is contrary to 
the statement made by Zelinka (91) for the egg of Callidina, and con- 
trary to his general statement for the Rotifera as to the relation of the 
place of polar cell formation to the later axes of the embryo. As this 
matter is not of especial interest from the standpoint of developmental 
mechanics, a full discussion of the difference between my account and 
that of Zelinka is reserved for Part Second. There it will be shown by 
evidence from Zelinka’s own work, as well as that presented here, that 
his general statement of the relation of the place where the polar cell is 
formed to the axes of the egg in the Rotifera cannot be considered true 


14 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


for other forms than Callidina russeola, whatever may be the conditions 
in that species. 


3. ORIENTATION OF THE DEVELOPING Emspryo. 


The first cleavage plane is transverse to the long axis of the egg, and 
divides it into two unequal parts (Figure 6). The plane passes through 
the place where the polar cell was formed ; the smaller cell includes, 
therefore, that end of the egg nearest to which the polar cell is located. 
As previously stated, this is also the more pointed end of the egg, when 
any ditference in the two ends is distinguishable. 

The second cleavage is approximately at right angles to the first, and 
nearly in the long axis of the egg. It also passes through the region 
where the polar cell was formed. 

As previously stated, the oval or ellipsoidal form of the egg is re- 
tained throughout the early development. This form is independent ot 
the precise arrangement of the material of which the egg is composed. It 
is as if the egg substance were enclosed in a rigid mould of oval or ellip- 
soidal form. Within this mould the (fluid?) contents may shift their 
position widely, without influencing the form of the mould. Thus, at the 
first cleavage, the material of the smaller blastomere occupies all of one 
end of the egg. In the ten-cell stage (Plate 3, Figs. 20-25) the same 
form is still preserved as if in a rigid cast, but the material which 
previously formed the smaller of the first two blastomeres has shifted 
from the end to one side of the egg. It therefore is necessary to have 
sores term by which to designate the two ends of this constant form, as 
distinguished from the shifting blastomeres themselves. I shall hence- 
forth speak of that end of the egg at which lies the smaller cell in the 
tr »-cell stage as the micromere end of the egg, while the opposite region, 
where the larger blastomere lies, will be called the macromere end. 
These terms refer to the form of the egg, without regard to the shifting 
contents. 

The orientation which I shall adopt for the egg itself is similar to 
that used by Wilson, Heymons, Conklin, Lillie, Kofoid, and other re- 
cent workers on cell lineage. The region where the polar cell is formed, 
and which afterward lies opposite the blastopore, will be called the ant- 
mal pole ; it marks the dorsal surface. The opposite point is the vege- 
tative pole, marking the ventral surface, — the position of the future blas- 
topore. Dorsad signifies always toward the animal pole, or place where 
the polar cell was formed; ventrad, in the opposite direction, toward 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 15 


the region where the blastopore is found at a late stage. The orien- 
tation is thus based upon the structure of the gastrula. The chief axis 
of the gastrula is the line connecting the animal and vegetative poles. 

The first cleavage plane, though coinciding with the chief axis of the 
gastrula, is, as shown by the later development, transverse to the long 
axis of the embryo. The smaller cell of the two-cell stage is anterior, 
since its products occupy the anterior margin of the blastopore ; the 
larger cell is posterior, its products forming the posterior lip of the 
blastopore. The second cleavage plane, though modified in the pos- 
terior part of the egg, is approximately longitudinal. In the four-cell 
stage (Plate 2, Fig. 8) the two cells A® and 4%, resulting from the 
division of the smaller cell, AB’, are respectively left anterior and right 
anterior, while C? and D*, produced by the division of the larger cell, 
CD’, are respectively right and left posterior. 

A section taken transversely to the chief axis of the gastrula will be 
spoken of as a transverse section. A section at right angles to this, 
passing from anterior to posterior and including the animal and vegeta- 
tive poles, is a sagittal section. A section at right angles to both of 
these, cutting both the animal and the vegetative pole and passing 
through the right and left sides, is a frontal section. 

As will be seen from the above, in the two-cell and four-cell stages, 
the micromere end coincides with the anterior, the macromere end with 
the posterior end.? 

The orientation given above is based upon the relation of the ege to 
the axes of the gastrula; the same is true of the orientation used in 
most of the recent works upon cell lineage. It differs fundamer ally 
from the orientation used by Zelinka (’91) for the developing erg of 
the rotifer Callidina russeola. In that species the egg is of the sime 
form as in Asplanchna. After extensive shifting during developmeat, 
the anterior end (in both Asplanchna and Callidina) comes to lie in the 
region of that end of the egg which I have called the macromere end. 
Zelinka calls this end of the egg, therefore, the anterior end, the oppo- 
site (my micromere end) the posterior end. Anterior and posterior in 
Zelinka’s orientation of course remain constant with regard to the form 
of the egg, but not with relation to the parts of the embryo. Thus, if 


1 It is of the greatest importance to observe that I do not use the terms “ micro- 
mere end” and “macromere end” in the same sense in which ‘“‘ micromere pole ” 
and “macromere pole” are sometimes used, as synonymous with “ animal pole ” 
and “vegetative pole.” The two terms are used only as a convenient way of 
indicating a peculiarity of the rotifer egg. 


16 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Zelinka’s orientation were adopted, the point where the polar cell is 
formed might be first ventral, then posterior, then dorsal, and later an- 
terior, and this would actually be the case in Asplanchna. In a special 
study of cleavage, with particular attention to direction, it is necessary 
that the orientation should give some constant basis for reference. It is 
therefore impossible for me to use Zelinka’s orientation in my work. 
The animal pole of the egg does retain, however, a constant relation to 
the position of the blastomeres and to the axes of cleavage, so that I 
have adopted this relation as a basis for orientation. 


4, CLEAVAGE. 


Nomenclature. 


For accurate comparative study of the direction and sequence of cleay- 
age in the different regions of the egg, such a system of nomenclature is 
needed as will indicate directly the relationships, and especially the com- 
parative age (measured in cell generations) of the blastomeres. The only 
system of nomenclature hitherto proposed which fulfils these demands is, 
I believe, that of Kofoid (94). I shall therefore use his system in the 
following account. 

The four blastomeres of the four-cell stage, and the cells derived 
from them, are designated respectively by the letters A, B, C, and D, 
beginning with the left anterior blastomere and passing around the egg 
to the right, i.e. in the same direction as the hands of a watch, —as- 
suming the egg to be viewed from the animal pole. The letters thus 
represent the same blastomeres as in Wilson’s work (92) on Nereis, 
Heymons’s (93) on Umbrella, Lillie’s (95) on Unio, and Kofoid’s (’95) 
on Limax. 

After the first equatorial cleavage, at which the four original blasto- 
meres are divided into smaller cells, the capital letters A, B, C, and D 
will be reserved to indicate respectively all the cells derived from the 
corresponding cell of the first four blastomeres ; and such a collection 
will be called a quadrant of the egg.! The separate cells will be desig- 
nated by the lower-case letters a, b, ec, and d, according to the quadrant 
to which they belong. Each letter will be followed by two exponents. 
The first exponent indicates the generation to which the cell belongs, 
the unsegmented egg being considered the first generation. Thus, in 


1 Each quadrant from the four-cell stage onward receives a specific color in the 
plates, so that the quadrants are instantly distinguishable by their colors. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. a LY 4 


the eight-cell stage (fourth generation) we shall have cells a’, 54, c*, 
and d*, Since, however, in this and in later generations, there are 
more than one cell of a given quadrant in a given generation, this first 
exponent must be followed by a second, serving to distinguish each cell 
from every other of the same quadrant. In “spiral” cleavage, this sec- 
ond exponent indicates the ‘‘quartet,” or layer of cells, in the embryo 
to which the blastomere belongs, the ventral cell being number 1, the 
next dorsal number 2, and so to the most dorsal quartet. In equatorial 
cleavages the same relation may be preserved in other types of cleavage 
than the spiral. Thus, in the eight-cell stage (fourth generation), the 
ventral blastomeres are a*!, 6*1, c*1, and d*, while the corresponding 
dorsal cells are a*?, 5*?, c#?, and d‘*. But in meridional cleavages, 
where there is no trace of the so called spiral, this criterion fails, and 
the second exponent can be used only for distinguishing the cells, not 
for indicating their relative positions. What is required is a rational 
system of applying the exponent such that no two cells of the same 
quadrant in the same generation shall have the same exponent. Follow- 
ing the suggestion of Kofoid (94), I have in meridional cleavages desig- 
nated the right derivative with the even exponent in even generations, 
and with the odd exponent in odd generations, — the left derivative of 
course receiving the reverse designation. This method of application 
was designed to preserve any possible homologies of the products of 
meridional with those of spiral cleavage, since in normal spiral cleavage 
the right derivative lies above the left in even generations, and so re- 
ceives the even exponent, while in odd generations the reverse is true. 
The results, however, have not shown any striking homologies with 
spiral cleavage, but the method of application has been retained, since 
no other seems to have any advantage over it. 

In meridional cleavages, the terms right and left will be used as de- 
fined by Kofoid (94, p. 180): “A miniature observer is imagined as 
placed in the principal (vertical) axis of the egg, with his head at the 
animal pole, facing the part or parts under consideration, and the terms 
right and left, upper and lower, are used as determined by this observer.”’ 

A full account of this system of nomenclature is given by Kofoid 
(94). In order to make clear the relation of the succeeding blasto- 
meres and their designations by this system, I give here a scheme of the 
nomenclature through the sixth generation, modified from that given by 
Kofoid. Only the products of quadrant A are carried out beyond 
the third generation, since the method is the same for the other quad- 
rants. Here a*1 represents the most ventral cell derived from the 

VOL. XXx.— NO. l. 2 


18 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


blastomere A, while a®® represents the most dorsal one, the others 
occupying intermediate positions. 


1 cell. 2 cells. | 4 cells. | 8 cells. | 16 cells. | 32 cells. 


First Cleavage. 


After the formation of the polar cell the nucleus (Plate 1, Fig. 2) 
returus to the position formerly occupied by the germinal vesicle in the 
longitudinal axis of the egg, lying nearer that end of the ovum in prox- 
imity to which the polar cell was formed (the micromere end). It takes 
such a position that a plane at right angles to the long axis of the egg 
and cutting the polar cell would also cut the nucleus. The distance 
from the centre of the nucleus to the nearer end of the egg is about 
two fifths of the length of the egg. Here two asters appear on oppo- 
site sides of the nucleus, the line joining them being oblique to the 
long axis of the egg. Though I have examined a large number of eggs 
at this stage, in no case have I been able to observe a stage in the 
process of forming the two asters of the first cleavage spindle (Fig. 3) 
from the single aster remaining after the formation of the polar cell 
(Fig. 2). 

Between the two asters a spindle is formed. This lies at first some- 
what oblique to the longitudinal axis of the egg, as shown in Figure 3, 
but before cleavage takes place the spindle swings into coincidence with 
the long axis (Fig. 4). The aster lying at the micromere end of the 
spindle is distinctly smaller than the opposite one (Fig. 3). The nucleus 
becomes divided into two small masses, which move toward opposite 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 19 


ends of the egg, but remain connected for a time by a distinct strand 
(Fig. 4). Meantime, before the first cleavage plane has appeared in the 
cytoplasm, the aster of the smaller blastomere has begun to divide, as 
shown in Figure 4. The two resulting asters separate at right angles 
to the axis of the first cleavage spindle. In the future larger cell the 
aster does not begin at once to divide. Both nuclei begin immediately 
to increase in size. The first cleavage plane passes through the point 
on the surface of the egg marked by the polar cell, transversely to the 
long axis of the ovum, and through about the middle of the strand con- 
necting the two nuclei. The strand is slightly thickened at the point 
where the first cleavage plane is to meet it (Fig. 4), indicating perhaps 
the formation of the “Zwischenkérper.” The cleavage plane is thus 
perpendicular to the axis of the spindle, and passes through its middle. 
I mention this fact on account of the difference between the first cleav- 
age of Asplanchna and that of Callidina. In the latter rotifer, ac- 
cording to Zelinka (91), the first cleavage plane is oblique to the 
spindle, and the spindle itself, even at the time of division, is oblique 
to the long axis of the egg. In another rotifer, Eosphora, the first 
cleavage plane is likewise oblique to the long axis of the egg (Tessin, 
’86), while in Melicerta ringens (Zelinka, 791) and Asplanchna Sieboldii 
(Lameere, 790) the first cleavage plane is transverse to the long axis, 
as in Asplanchna Herrickii. 

During and after the passage of the first cleavage plane through the 
_ eytoplasm, the egg retains its ellipsoidal form, and the resulting cells do 
not separate and become rounded, as occurs in the eggs of so many 
animals, but remain closely pressed together. In a large series of cases 
showing the first cleavage in various stages, the only indication of any 
change in the form of the egg or its blastomeres is a slight depression 
of the surface where the cleavage plane cuts the periphery of the egg, 
forming a shallow furrow. Here the edges of the two blastomeres are 
slightly rounded off, as shown in Figure 6, instead of fitting squarely 
against each other. The retention of its general form by the egg is 
characteristic of all cleavage stages. This surface of contact of the two 
blastomeres is curved, the smaller cell, AB’, projecting slightly into 
the larger. 

So far as the direction of the division is concerned, the first cleavage 
of Asplanchna evidently fits easily either the surface tension theory of 
Berthold, or Hertwig’s theory of the spindle in the long axis of the pro- 
toplasmic mass. Comparison with the first division of Callidina russe- 
ola as described by Zelinka (’91) develops an interesting fact. In 


20 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Callidina the first cleavage spindle is obligue to the long axis of the egg, 
therefore not in agreement with Hertwig’s law ; but immediately after 
division is finished, a movement of the egg contents takes place in 
such a way that the two cells occupy the same relative position as in 
Asplanchna, — such a position, therefore, as is demanded by Berthold’s 
theory of least surfaces. It thus appears that in Callidina the direc- 
tion of division itself is determined neither by the principle of Berthold 
nor that of Hertwig, but that the later arrangement of the cells might 
be held to be due to the action of Berthold’s principle. It is somewhat 
curious that the exact arrangement produced in Callidina by shifting 
should in Asplanchna result at once from the position of the spindle at 
the time of cleavage. 

No cause can be assigned, from the visible structure of the egg, for 
the inequality of the cleavage. The yolk granules are distributed uni- 
formly throughout the egg seeming no more abundant in the large 


oo? 


than in the small cell. 


Second Cleavage. 


As a result of the first cleavage, the egg is now composed of two un- 
equal blastomeres, an anterior, AB’, and a posterior, CD’ (Figs. 5 and 6). 

Tn the smaller blastomere, as previously stated, the aster has already 
divided and the two parts are separating at the time when the first 
cleavage plane passes through the cytoplasm (Fig. 4). The line along 
which they move apart is perpendicular to the axis of the first cleavage 
spindle, and also at right angles to a line connecting the polar cell with _ 
the centre of the egg. The forming spindle is thus parallel to the 
lateral axis of the embryo and consequently perpendicular to its dorso- 
ventral axis. The two asters take up their positions on opposite sides 
of the nucleus, and the axis of the resulting spindle has a direction 
parallel to the line joining the asters at their first separation (Figs. 5 
and 6). Meanwhile the nucleus has steadily increased in size, up to the 
time when it participates in the formation of the spindle. 

In the larger cell, CD’, the order of procedure is different. The 
nucleus begins to enlarge, as in the smaller blastomere, but the aster 
does not at once divide. The nucleus and aster together begin to 
migrate to the right. At the same time the aster comes to lie farther 
to the right than the nucleus, either because the two rotate on a com- 
mon axis, or because the aster, moving faster, creeps around the nucleus 
toward the right side of it. Thus, whatever the method, a condition is 
reached in which the large nucleus lies in the right anterior angle of the 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 21 


cell CD’, with the undivided aster at its right and slightly behind it 
(Fig. 5). 

By this time the two asters in the cell AB’ have completely separated 
and lie upon opposite sides of the enlarged nucleus. Thus the prepara- 
* tory stages for karyokinesis are much more advanced in the smaller 

cell, and it would be anticipated that this cell would cleave first. 

The single aster in CD* now begins to divide. The process seems to 
be accomplished very quickly, since in a series of nineteen specimens of 
the two-cell stage (each, of course, taken from a different individual) 
only one case was found exhibiting a transitional stage between that 
shown in Figure 5 and that shown in Figure 6. In this specimen the 
single aster had elongated slightly in the direction of the future 
spindle. When formed, the spindle takes an oblique position in the 
cell, extending from right anterior to left posterior. The aster at the 
left posterior end of the spindle is much the larger, in correlation appar- 
ently with the larger mass of cytoplasm surrounding it. The nucleus 
of CD has now overtaken in its metamorphosis that of AB’; the 
spindles are found in exactly corresponding stages, the chromatin being 
in both arranged in an equatorial plate (Fig. 6). 

Not only are the two spindles not parallel, as shown in Figure 6, but 
they do not lie in the same plane. If the two spindles are viewed 
exactly from the anterior or from the Epastenior end of the egg, the left 
aster in AB and the right aster in CD’ are seen to lie more dorsally 
than their mates. Viewed in this direction, the spindles cross each 
other at an angle of about twenty-five degrees. 

As a result of the dissimilarity in the direction of the two spindles, 
the two next cleavage planes, perpendicular to them, will not meet the 
first cleavage plane in a common line. The position and direction of 
the spindle in CD" are such that the cleavage plane cutting OD? 
would probably meet the first cleavage plane to the right of the line 
where the plane dividing AB’ would meet it. Since the right aster of 

- OD is farther dorsal than the left, the plane of cleavage of CD? 
would be inclined to the sagittal plane, — on the dorsal side toward the 
left, on the ventral side toward the right. 

The cleavage of the two cells now follows at almost or precisely the 
same time, the karyokinetic processes being found from this time on in 
the same stage. Ina series of thirty-one eggs from different individ- 
aals, each containing more than one and less than five cells, none con- 
tained exactly three cells. 

An examination of the four-cell stage after the completion of division 


22 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


(Plate 2, Fig. 8) shows that the cleavage planes have taken the posi- 
tions foreshadowed by the arrangement of the spindles. The plane 
separating C* from D® lies to the right of the plane separating A® from 
B*; the corresponding furrows on the surface are nearer together on 
the dorsal than on the ventral side. The blastomeres resulting from 
the division of AB” are equal, whereas CD" divides very unequally. 
The right derivative (C*) is much smaller than the left (D*), and is of 
approximately the same size as A? and B% The blastomeres B® and 
D® are in contact along the whole distance from the dorsal to the ven- 
tral surface of the egg, while A® and C® do not touch each other at all. 
The polar cell lies either at the junction of 6%, C®, and D*, as shown in 
Figure 8, or sometimes at the junction of A’®, B%, and D*. The egg is 
now markedly unsymmetrical. 

It is evident from the above description that this cleavage may be 
considered as belonging to the so called spiral type. Since the left end 
of the spindle is in each cell the higher, the cleavage is a left spiral, 
like the corresponding cleavage in Discocelis, Nereis, Limax, and indeed 
all forms with spiral cleavage except in the reversed cleavage of certain 
mollusks. This fact is striking, since the succeeding cleavages in 
Asplanchna do not belong to the spiral type. 

The relation between the axes of the embryo in later stages and the 
first two cleavage planes is as follows. The first furrow separates an 
anterior from a larger posterior portion, but the plane of separation of 
the parts bears no simple relation to the axes of the later embryo. 
(Compare Figure 8 with Figure 75, Plate 9, in which the parts derived 
from the first four cells are colored in the same manner as their parent 
cells in Figure 8, and note the great shifting.) The later sagittal plane 
of the embryo is coincident with a plane passing through the animal pole 
and the longest axis of the egg; that is, through the plane separating A® 
from B? (Fig. 8), and dividing the larger blastomere D? into two un- 
equal parts. The second cleavage plane therefore divides the right side 
from the left in the anterior part of the egg ; but in the posterior part it 
lies entirely in the right side. It is not until the seventh cleavage that 
the division into symmetrical right and left halves takes place on the 
posterior side (Plate 7, Fig. 58) ; indeed, certain cells containing material 
for both sides of the egg remain undivided till even a later stage. 


Third Cleavage. 


Immediately after the second cleavage, the aster in each of the four _ 
cells produced begins to extend dorso-ventrally, at right angles to the 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 23 


axis of the preceding spindle. An optical section of the egg along its 
chief axis, showing the asters in the cells b* and D*, is given in Plate 
2, Fig. 10. Spindles are formed in all four cells nearly or quite in 
the position indicated by the direction of separation of the asters. 

The tendency of the karyokinetic processes in the posterior half of 
the egg to gain upon those in the anterior half, shown during the last 
division, is continued and accelerated. Spindles appear in C* and D?, 
while the nuclei in A? and B? are still spherical and have distinct mem- 
branes. Figure 9 gives a view of this stage from the right side; the 
large spherical nucleus of B* is represented by a broken outline. The 
spindle in O% has a dorso-ventral direction, and its middle coincides with 
the middle of the length of the cell; the two asters are of equal size. 
In D? the spindle is nearer the dorsal side of the egg, and is inclined, 
passing from dorsal and anterior to ventral and posterior. The ventral 
aster is the larger. 

Cleavage takes place first in the larger cell D*, separating a large 
ventral blastomere, d*4, from a smaller dorsal one, d+. At the same 
time the two cells (considered as a whole) elongate dorso-ventrally. 
In so doing, the ventral blastomere, d*1, remains nearly stationary, 
while d*? moves in the direction of the animal pole of the egg. (Com- 
pare Figure 12, a sagittal section of a five-cell stage, with Figure 
10, the corresponding section of a four-cell stage, observing the posi- 
tion of the cells in relation to the general form of the egg.) Asa 
result of this, the dorsal end of the cell B*, and, to a less degree, the 
ends of A* and C*, are displaced in the same direction ; that is, the 
whole animal pole moves toward the micromere end of the egg. At 
the same time the cells A*, B?, and C® are slightly compressed dorso- 
ventrally. This is the beginning of that peculiar rotation of the blasto- 
meres in the eggs of Rotifera, described by Zelinka (’91) and others, 
which eventually results in the process of gastrulation. 

O* divides next, the cleavage being equal; the products are c*1 
and‘ ¢**. 

Before the cleavage is finished in D? and C*, spindles have been 
formed in B* and A®, division taking place in them in the order named. 
The cleavage is equal, as in C?, 

The order of cleavage, then, for the four cells, is as follows: D, C, B, A. 
This rhythm reappears in later cleavages. 

The third cleavage is therefore equatorial, dividing the egg into two 
layers of four cells each. The ventral cells are a*'—d*, the dorsal cells 
a*?_d*?, The egg is still slightly unsymmetrical. 


24 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Views of the eight-cell stage are shown in Figures 15 to 18. 

From a cyte-mechanical standpoint, the third cleavage may be char- 
acterized as follows. The first division of the asters is along a line at 
right angles to the axis of the previous spindles, and indicates the posi- 
tion of the spindles for the next cleavage. These lie in the long axes 
of the cells, and the cell walls are formed in the position demanded by 
the principle of least surfaces. 


Fourth Cleavage. 


Immediately after the division of D® (Plate 2, Fig. 12), the asters in 
d* and d*? begin to extend laterally, at right angles to the axis of the 
preceding spindles, and each becomes divided into the two asters for the 
following spindle. In d*’, Figure 11, the two asters for the succeed- 
ing cleavage are still connected by a striate band. Figure 11 shows a 
ventral view of the same egg as Figure 12, the five-cell stage. The 
corresponding dorsal view of a slightly later stage is shown in Figure 
14. The asters in d*? are moving apart in the same manner as in d*, 
save that the line of separation is slightly oblique, the left aster being 
higher. 

In the same way the asters in the cells a*1—c*1 and a*?—c*? become 
constricted, and divide at right angles to the axis of the preceding 
spindles, The dividing asters in c** are shown in Figures 14 (Plate 2) 
and 17 (Plate 3), and those of c** in the latter figure. Views of the 
other four cells would show similar conditions. 

From the manner in which the asters separate in all of the eight 
cells, one would be led to expect that the next cleavage would be merid- 
ional, at right angles to the third cleavage. This expectation is 
strengthened by the fact that the dateral dimensions of the cells in 
which the asters lie are considerably greater (in the quadrants A, B, and 
C, at least) than the opposite measurements (Fig. 17). 

But in a slightly later stage it is observed that the line joining the 
asters in d* has become oblique, like that joining those of d*? (as 
mentioned above). This oblique position of the asters in d** is shown 
in the ventral view (Fig. 15). The left aster (right side of the figure) 
has become ventral, the right one dorsal. The sagittal section (Plate I, 
Fig. 7) of a slightly later stage shows the completion of the rotation 
thus begun ; the line connecting the two asters and passing through the 
nucleus is now approximately dorso-ventral in direction. 

At the same time a similar rotation has taken place in the cell d*?, 
but the position taken by the two asters is not the same as in d*7. One 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 25 


of the two asters has become central, the other peripheral, as if the cell 
were about to divide into a deep and a superficial portion. This condi- 
tion also is shown in the sagittal section, Figure 7. The difference in 
the position of the asters in d** and d*-? is apparently due to simple 
mechanical conditions, —the form of the cell d*:? compelling the asters 
to take the position which they have. 

At the same time a slight differentiation in the cytoplasm of the cell 
d‘* becomes visible. As previously stated, the fine yolk granules are 
at first distributed uniformly throughout the egg. In this eight-cell 
stage, a slight concentration of the yolk granules in the ventral part of 
the cell d** may be noticed by careful observation. The condition at 
this time is shown in Figure 7; in the ventral part of d*" the yolk 
granules are a little larger and more numerous. As will be shown, this 
concentration of yolk becomes later much more marked, and its history 
is peculiar. 

A spindle is now formed in d*" in the position indicated by the asters 
of that cell in Figure 7, — that is, with a dorso-ventral axis, — thus pre- 
figuring another equatorial cleavage. The spindle is shown in Plate 2, 
Figure 16. 

Immediately thereafter the spindle is formed in d*?, and it appears 
that the position of the asters shown in Figure 7 (Plate 1) is not defint- 
tive. The asters shift, so that the spindle in d*? is dorso-ventral, like 
that in d*+, as is shown in Figure 16. Which of the two asters seen in 
Figure 7 becomes dorsal and which ventral, I have been unable to deter- 
mine. During the formation of the spindle in d*? the cell extends a 
little in the direction of the spindle, as is shown by a comparison of 
Figure 7 with Figure 16. 

Meanwhile, changes have been occurring in the quadrants A, B, and 
C. As the processes are the same in all three, the quadrant C will be 
selected as a type. 

At first, as described above, the asters separate tangentially, at right 
angles to the axis of the previous spindle (Plate 3, Fig. 17). This 
position is retained for some time, but in a later stage the line connect- 
ing the asters in c*? has become oblique, as shown in Figure 18, which 
exhibits a side view of the egg of which Figure 7 is a section. The 
asters in c** still retain their original position. 

Now follows the cleavage of the cell d*. This is accompanied by an 
increase of the dorso-ventral extent of the two products, as compared 
with that of the original cell. The division is unequal ; the ventral 
cell d** is much the larger, and retains the whole of the territory con- 


26 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


taining the larger yolk granules shown in Figure 7. The larger deriva- 
tive retains its position at the macromere end of the egg (Fig. 19). 
The smaller cell d°? is therefore pushed dorsad, and this, together with 
the extension of d*? at the time of the formation of its spindle, dis- 
places the animal pole, marked by the polar cell, still farther toward 
the micromere end of the egg. As a further result, the cells of the 
quadrants A, B, and C are still more compressed dorso-ventrally, so 
that, especially in a*1—c*, the lateral extent is much greater than the 
dorso-ventral (Fig. 19). 

Nevertheless, as Figure 19 shows, the rotation of the future spindle 
axis still continues. The line joining the asters becomes dorso-ventral 
first in the dorsal cells a*-?—c*?, while in a*-1—c* the asters are still 
oblique, as shown in c*?, Figure 19. In quadrant B of this same figure, 
the axis has become dorso-ventral in both cells. 

Now occurs the cleavage of d*?, with still further elongation, shifting 
of the animal pole toward the micromere end of the egg, and resulting 
greater compression of the cells of the quadrants A, B, and C (Figs. 
20-24). Without regard to this, the asters in the cells of these quad- 
rants continue their movements until the future spindle axes are in 
every case dorso-ventral. Spindles are now formed in all of the six 
cells, the spindle being in every case in the shortest axis of the cell 
(Figs. 20-24). 

The conditions at this stage are so significant from a cyto-mechanical 
standpoint, that I have thought it best to analyze and illustrate with 
especial fulness a typical egg at this stage. Figures 20-25 are views 
of asingle egg. Figure 20 shows the right side (quadrant CQ), Figure 
22 the left side (quadrant A), Figure 21 an anterior view (quadrant 5), 
and Figure 25 a posterior view (quadrant D). In Figures 23 and 24 are 
given respectively sagittal and frontal optical sections, for comparison 
with the surface views. 

In order to exclude possibility of error, the egg from which the above 
figures were taken was moved about so that the six cells belonging to 
the quadrants A, B, and C occupied successively the middle of the 
upper surface of the egg; careful camera figures of each cell were made 
in this position. Then optical sections were taken in the same way, 
both along the axis in which the spindles lie, and at right angles to 
these. Accurate measurements of the dimensions of the cells could 
thus be made; the results are as follows. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 27 


Dorso-ventral Lateral Ratio of Dorso-ventral to 
Cell. Measurement. Measurement. Lateral Measurement.° 
atl 25 (2Cu1) 52u 1to 2 (about) (2 to 5) 
at2 35m 50m 7 10 
pel 24u 42u 4“ 7 
pA2 35u 42u 5“ 6 
cnt 30u 49u 3 “ 5 (about) 
c#-2 334 52u 2“ 3 (about) 


After the spindles become completely formed, the cells begin to 
elongate in the direction of the spindles. A slightly later stage than 
that just described is shown in Plate 4, Figure 26. Comparing this 
with Plate 3, Figure 22, it is evident that the cell a*’ has stretched in 
the direction of the spindle to such an extent that the difference between 
the two axes of the cells is much diminished. Nevertheless, in both this 
cell and a*? the axes in which the spindles lie are distinctly the shorter. 
This is still true at the time of the division of the cells. Figure 27 
(Plate 4) shows the right side of the egg last considered ; in the quad- 
rant C’ the processes are much more advanced than in A. The nuclei 
have separated and the cytoplasm is dividing, yet exact measurements 
both of surface views and optical sections show that the greater diam- 
eter is still at right angles to the line joining the two nuclei. A frontal 
section, showing the greatest dorso-ventral extent of the cells of the 
quadrants A and (@ of this egg, is given in Figure 28. 

The two figures last mentioned show another fact of importance. The 
divisions do not separate the blastomeres into cells of equal size in the 
quadrants A, 6,and C. The completed cleavage is shown in Plate 4, 
Fig. 30 (anterior view). This, with the figures just cited, shows that the 
cells a*? — c*? divide very unequally, the dorsal derivatives, a**—c*-4, being 
very much larger than the ventral ones, a°*—c**. The inequality is less 
in the division of the ventral cells a*’—c*1. Although the ventral de- 
rivatives, a°**—c*1, occupy a larger area on the surface of the egg, there 
is little difference in actual volume, and such as occurs is in favor of the 
more dorsal cells a>-?— ¢5-?, 

The order of division is the same as in the last cleavage ; first, the 
quadrant D, then in order C, B, A. In quadrant D the larger cell 
d** divides first; in the other quadrants the cells are of equal size and 
divide at the same time. 

The important facts in this fourth cleavage, from a cyto-mechanical 
standpoint, may be summarized as follows. 

1 In every case the first measurement was taken through the two asters; in the 


case of a*! the real dorso-ventral extent of the cell, into which the spindle later 
moves, is but 204, — so that the ratio is as two to five. 


28 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


The asters in all of the eight cells after the third cleavage separate 
tangentially, at right angles to the direction of the preceding spindles. 
Having taken up positions on opposite sides of the nucleus, in every 
case the complex of nucleus and asters rotates in such a way as to bring 
the axis of the forming spindle into the same direction as that occupied 
by the spindle for the preceding cleavage. In six of the cells, this ro- 
tation is from a previous position in the longer axis of the cell to a later 
position in the shorter axis. In these six cells spindles are completed in 
the shortest axes of the cells, and division ensues in such a way that 
the newly formed septa are surfaces of greatest area, and the cells sepa- 
rate in the direction of greatest pressure. 

The cleavage in a*?-—c*? and in d** is markedly unequal. 

In two cells of equal ages but unequal size (d*" and d*?) the larger 
divides first. 

The exact changes in form during the divisions of the cells is a point 
worthy of careful attention. As the transformation of the nucleus 
giving rise to the spindle takes place, the cell elongates slightly in the 
direction of the spindle. (Compare c*1, Figure 24, with the earlier 
stage of the similar cell a* in the same figure.) As the spindle nar- 
rows and lengthens and the chromosomes begin to separate, the cell 
continues to elongate (Fig. 26, a*!, compared with Fig. 20, e*1, and Fig. 
22, at"). As the two new nuclei are formed and move apart, and the 
cytoplasm becomes constricted, there is a still further extension of the 
cells in the direction of the spindle. (Compare c*? and c*? with a*! and 
a*-?, in Plate 4, Fig. 28.) 

As Heidenhain (94%, p. 154) has recently urged, this elongation of 
the cell in the direction of the spindle is a point of great importance 
for a proper understanding of the conditions affecting the direction of 
cell division. In many later divisions in Asplanchna the spindle is first 
formed, as will be shown, in the short axis of the cell, and then this 
axis by stretching becomes the longer. It is possible that to this phe- 
nomenon is due the apparent general agreement of normal cleavage with 
the law of Hertwig, and that careful observation will in many cases, 
as in Asplanchna, show the so called law to be of little significance. 

A full discussion of the bearing of the facts above described is 
reserved until later cleavages have been examined. 

The foregoing description is based on a study of forty-two specimens 
from different individuals, showing the various phases of the fourth 
cleavage ; that is, each containing more than seven and less than six- 
teen cells. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 29 


This cleavage, as described above, differs in some respects from that 
of Callidina as described by Zelinka (’91). A discussion of the differ- 
ences will be found in Part Second. 

The cleavage of Asplanchna priodonta takes place to this stage in 
exactly the same manner as that of Asplanchna Herrickii. Figure 29 
(Plate 4) shows the egg of Asplancbna priodonta in the 10-cell stage. 


Fifth Cleavage. 


As a basis for an account of the following cleavage, it will be well to 
summarize the divisions which have already taken place, and to take a 
careful survey of the structure of the egg at the end of the fourth 
cleavage. 

The first and second cleavages pass through both animal and vegeta- 
tive poles and are therefore meridional. The third cleavage is at right 
angles to the dorso-ventral axis and is therefore equatorial. The fourth 
cleavage is parallel to the third, thus likewise equatorial. 

As a result of these cleavages, the egg now consists of sixteen cells, 
arranged in four dorso-ventral rows or guadrants, each quadrant con- 
sisting of four cells derived from one of the four blastomeres of the 
four-cell stage (Plates 4 and 5, Figs. 30-36). Passing from the ven- 
tral side dorsad, we may also distinguish four layers of cells, each layer 
containing one cell of each of the four quadrants. The layers may for 
convenience be numbered ; I will call the ventral the jirst layer, the 
others following in order to the fourth, which is at the animal pole. As 
a result of the shifting during cleavage, the animal pole has now come to 
be situated almost exactly at the micromere end of the egg; the oppo- 
site end is occupied by the large cell d*! (Figs. 30 and 33, anterior 
and posterior views respectively). The dorso-ventral axis therefore 
now coincides with the long axis of the egg. 

Of the four quadrants, three, A, B, and C, are alike in the size and 
arrangement of the cells of which they are composed. (See Fig. 30, 
anterior view.) The four cells composing any given one of these quad- 
rants differ in size. The cell of the fourth layer (next to the animal 
pole) is much the largest, while that of the third layer is much the 
smallest. The cells of the first and second layers are nearly equal in 
size; that of the first layer covers more of the surface of the egg (Fig. 
30), but that of the second layer is deepest (Fig. 32). The cells of the 
first, second, and third layers are much compressed dorso-ventrally, so 
that the lateral dimensions of the cells are at least twice as great as the 
dorso-ventral dimensions. In the third layer especially, the cells are 


30 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


deformed by the pressure to such an extent that the surface area ex- 
posed resembles the section of a biconvex lens. In the dorsal layer 
the compression is less; the cells are triangular in surface view, and the 
dorso-ventral extent is greatest. 

All the cells of quadrant D are much larger than the corresponding 
cells of the other quadrants (Figs. 31 and 33). The ventral blastomere, 
d*!, is much the largest cell of the egg, and occupies the entire ventral 
end at this period. Its position as shown by the section (Fig. 32) is 
worthy of careful attention. Its dorsal or inner surface, like the outer, 
is convex ; anteriorly the cell is partly covered by the ventral cells of 
the other quadrants, while the ventral end of cell d°? extends a slight 
distance ventrad of the middle dorsal portion of d*. 

The cell d*! is distinguished from all the others by a further peculi- 
arity. 1 have shown above (page 25 and Fig. 7) that in the eight-cell 
stage there is a slight concentration of yolk material in the ventral 
region of the cell d*?, where the yolk granules are a little larger and 
more numerous. At the division of d*?, this cloud of granules, as a 
natural result of its position, remains in the cell d*? (Plates 2 and 3, 
Figs. 16 and 19). At the same time it becomes more distinctly differ- 
entiated. The granules composing the cloud increase in size and range 
themselves about the periphery of the egg, next to its free surface 
(Figs. 23, 24, Plate 3, and Fig. 28, Plate 4). A narrow strip of the 
posterior margin of the free surface of the cell is without the granules 
(Figs. 20, 22, and 23). Ata time when the fourth cleavage is entirely 
completed, the granules have withdrawn still farther from the posterior 
margin of the cell, and show a tendency to concentrate at the free sur- 
face of the cell over its anterior half (Plate 4, Fig. 32). In the other 
cells, and in the remaining portions of d*1, the original finely granular 
cytoplasm is retained, so that I have not thought it necessary to repre- 
sent in the figures the yolk conditions in any region except where the 
cloud of granules is present. 

The cell of the second layer, d*-*, is next in size, then the dorsal cell, 
d>+, while the cell of the third layer is the smallest in quadrant D. 
The cells d>? and d** are very greatly compressed dorso-ventrally and 
elongated laterally, so as to form irregular flat plates, extending from the 
posterior surface of the egg two thirds of the distance to the anterior 
surface. (Compare Figure 31, left posterior surface, with Figure 32, sec- 
tion.) The dorsal cell d** is likewise compressed dorso-ventrally, so as 
to appear in a sagittal section (Fig. 32) as a low triangle. 

As a whole, the form and arrangement of cells are far from what is 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. ol 


demanded by the principle of least surfaces. Flat plates, such as we see 
in d*? and d***, and in all the cells of the third layer, retain their form 
in virtue of some force working strongly against surface tension. 

After the fourth cleavage, the asters in all the cells at first separate 
at right angles to the axes of the preceding spindles, as happened after 
the third cleavage. The later changes are essentially the same in all 
the quadrants, so far as the asters are concerned, so that quadrant D 
may be described as a type. 

Figure 31 (Plate 4) shows the conditions in the four cells of quadrant 
D, after the asters have divided. The two asters of each cell lie upon 
opposite sides of their nuclei in such a position that, if no change 
occurs, the ensuing division will be meridional. 

In the cells of the first three layers the asters retain their original 
positions. But in d** a rotation takes place, such as occurred in all the 
cells in preparation for the preceding (fourth) cleavage, so that the axis 
of the spindle in d** is at right angles to the axes of the spindles in the 
other three cells of the quadrant. This condition is shown in Figure 
33, and the completion of the division is shown in Plate 5, Fig. 37. 

The same processes take place in the other quadrants, so that all the 
cells of the first three layers have spindles extending laterally, while in 
the fourth or dorsal layer the spindles are directed dorso-ventrally 
(Fig. 40). 

We must now consider the cleavage in the several cells more in 
detail. 

As in previous cleavages, division takes place first in the ceils of 
quadrant D. The nucleus of the large ventral cell, d*7, is earliest to 
enter upon the karyokinetic process, followed immediately by d**, and 
a little later by d°* and d**. Figure 33 (Plate 4) gives a view of the 
posterior surface of the egg at this stage, showing the spindles in all the 
cells. As this figure shows, the spindles in the three cells d*1, d*-, 
and d°* do not lie in the middle of the cells, but nearer the right ends. 
(See definition of right and left, page 17.) In the dorsal cell, d**, the 
spindle is at right angles to those in the other cells. The plane of 
cleavage indicated in the three ventral cells is meridional ; in the dorsal 
cell it is equatorial, like that of the two preceding cleavages. The division 
of each of the four cells must be considered separately. 

To understand the cleavage of the large ventral cell, d5-!, it is neces- 
sary to observe accurately its position and relation to the other cells, 
A longitudinal section of about the same stage as that shown in Figure 
33 is given in Figure 34. Comparing this with the earlier correspond- 


32 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


ing section, Figure 32, it is seen that with the formation of the spindles 
in d®*-? and d>* these cells have yielded to the well known tendency to 
take a more rounded form at the time of karyokinesis; the inner parts 
of the cells have been withdrawn toward the surface and used in in- 
creasing the dorso-ventral dimensions of the cells. The animal pole 
has been thereby pushed still farther in the direction in which it has 
been steadily migrating, so that it is actually past the micromere end of 
the egg. The cells of the quadrants A, B, and C, being in a “ resting” 
condition, give way to the compression, and become much deeper and 
flatter than before. The cell d*-* retains its position at the macromere 
end of the egg, but lying in a concavity, partly surrounded by the 
other cells. The spindle lies in the deeper (more dorsal) parts of the 
cell, with its right end (Fig. 33) deepest, and close to the wall of 
the cell. A view from the ventral end of the egg (Plate 5, Fig. 35) 
shows that this “ right” end is really anterior, and that the spindle lies 
in an antero-posterior plane, coincident with the plane separating the 
quadrants 4 and &. The anterior (inner) end of the spindle lies close 
against the boundary between a**? and 6°. 

The division which now ensues is of an extraordinary character. The 
anterior end of the spindle is pressed against the periphery of the cell 
at the place above mentioned, and a minute vesicle is given off, which 
lies embedded between the cells a* and 6°. This, after the division 
is finished, is shown in Figure 38 (Plate 5), the vesicle being 
labelled d*?. 

During division, the granular cloud which was described as occupying 
the anterior half of the periphery of the cell moves still farther toward 
the anterior margin, and shows a tendency to concentrate into a more 
definite group; the individual granules become larger also (Fig. 38). 

In d* and d** the spindles are parallel to the spindle in d*, the 
right ends being nearer the boundary of the cells, and deeper within 
the egg. The latter fact is shown in the transverse section, Figure 36, 
passing through the cells of the third layer. The divisions are un- 
equal, as foreshadowed by the position of the spindles, but the inequality 
is much less than in the case of d*. The completed division is shown 
in Figure 37. The cleavage takes place first in d*?, 

At about the same time as the division of d** occurs that of the dor- 
sal cell, d***. Here the spindle is in the short axis of the cell; the 
cleavage is equatorial and unequal, the dorsal cell being much the 
smaller (Fig. 37). 

During the occurrence of the cleavage of these cells other changes, 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. ap 


which are of the greatest importance, have been taking place, partly as 
a consequence of these cleavages. As the constriction of the cyto- 
plasm in d5? and d** occurs, these cells show in a most pronounced way 
the tendency to become of a rounded form. The inner portions of the 
cell are withdrawn still more from the centre of the egg, until the an- 
tero-posterior measurements are no greater than their dorso-ventral 
dimensions. This is shown in the section, Figure 38. In this egg, a 
surface view of which is given in Figure 37, the cleavage of d°* is 
finished, and the products, d*? and d®*, have already passed into the 
“resting stage,” so that they take whatever form is impressed upon 
them by the surroundings. But d*? is just dividing into d°° and d™*, 
and the form shown in section by d®°* in Figure 38, as compared with 
the form of d°? in Figure 32, shows the change which I have been ; 
describing. 

At the same time the cells of the other quadrants, A, B, and C, are 
entering upon the stages preparatory to karyokinetic division. As a 
first step they also retract their deeper parts and bring their protoplasm 
into a more compact mass, as shown by a comparison of quadrant B 
in Figure 34 (Plate 4) with the similar quadrant, A, in Figure 38 
(Plate 5). 

As a consequence of this withdrawal of material from the inner parts 
of the egg, the large ventral cell d*’, which has now passed into the 
resting stage, moves inward to occupy the space which would otherwise 
be vacant, — being forced to do so, of course, by the greater dorso-ventral 
extension of all the other cells. The result is shown in Figure 38. 
This partial enclosure of d®** by the other cells is of course a stage in 
the process of gastrulation. 

Before the cleavage of all the cells of quadrant D is finished, the 
karyckinetic processes have begun in the other three quadrants. (See 
Plate 5, Figs. 39-42.) The first cells to show the characteristic nu- 
clear phenomena are those of the fourth or dorsal layer, a®**-c°*. As 
previously stated, the asters at first take up such a position in these 
cells as would lead, if unchanged, to a meridional cleavage. But in these 
three cells, as in d*-*, there is a revolution of the asters and nuclei, 
resulting in a dorso-ventral position of the spindles. At any given period 
the three cells are not in exactly the same phase of division, though very 
nearly so ; the order, beginning with the most advanced, is c**, b°-4, a>. 
The sequence is thus the same as in previous cleavages. The division is 
equatorial and the dorsal product is the smaller, as in the cleavage of 


the corresponding cell (d*-*) of the left posterior quadrant. In the three 
VOL. xxx. — No, l. 3 


34 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


cells of quadrants A, B, and C the spindles are not in the short axes, as 
in d*4, since these cells are not nearly so broad as that one, while the 
dorso-ventral dimension is about the same or greater (Plate 4, Figs. 30 
and 32). Figure 41 (Plate 5) shows the process of cleavage in these 
cells, while Figure 45 (Plate 6) shows the cleavage concluded. 

The division of the second layer follows upon that of the fourth, and 
again in the order c®*, 0-7, a°-?. The division is here meridional, as it is 
in the corresponding cells of the quadrant D, and equal, as it is not in the 
corresponding cell of the quadrant D. (See Figs. 39, 40, 43, and 44.) 

Next follow the divisions of the cells of the first layer, in the same 
order as in the previous cleavages, and, slightly later, the divisions in 
the third layer, also in the same sequence. The cleavages here also are 
meridional and equal. 

The nuclear conditions leading to these cleavages are shown in Figures 
39, 40, 42 (Plate 5), all from the same egg. A somewhat later stage 
is shown in Figure 43, which exhibits the conditions in the quadrants 
A, B, and C at a time when the divisions just described are completed 
in most of the cells. All the divisions are nearly finished except in the 
cells of the third layer (a*-8—c*), which still contain spindles. This 
view shows also another peculiar fact. During the cleavages the cells 
about the dorsal pole of the egg have shifted, and the cells a®* and c®? 
have pushed ventrad to such an extent that on the right side the cells 
c= and 6°4 have become completely separated, a part of the cell e&7 
lying between them. This condition is only transitory, however; the 
cell c®” is very soon pushed dorsad again, and the cells of the third layer 
again form a continuous row. (Compare Figure 47, Plate 6.) Figure 46 
shows the cells of the quadrant D at the close of this division, while 
Figure 45 is a view of the animal pole at the same stage. 

The features of the fifth cleavage may be summarized as follows. In 
the twelve cells of the three ventral layers, the asters separate after the 
fourth cleavage at right angles to the position of the preceding spindles 
and retain the position first taken ; the cleavage is therefore meridional. 
In all these cells the spindles are in the long axes of the cells. In the 
dorsal layer the asters at first assume the same position as in the other 
cells, but later a rotation takes place, and the spindles when formed 
have a dorso-ventral direction ; the resulting division is equatorial. The 
spindle is in the longer axis of the cells a°*, 6°*, and c**, in the shorter 
axis in d°*, 

The division is unequal in the four dorsal cells of all the quadrants, 
and in all the cells of the quadrant D. In the other cells it is equal. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 3D 


The sequence of cleavage is the same as in previous divisions, but 
with some modifications. In any given layer of cells, the order is D, 
C, B, A; a repetition of the sequence established at the third cleavage. 
In any given quadrant, the order of cleavage varies with the relative 
size of the cells. In the quadrant D the order is (beginning with the 
ventral cell) 1, 2, 4, 3, and this is also the order of relative size of the 
cells, beginning with the largest. In the other quadrants the order of 
cleavage is 4, 2, 1, 3, and this again is the order of comparative size 
beginning with the largest, except that 2 and 1 are so nearly of a size 
that it is difficult to say from observation that either is the larger. 

The fifth cleavage is accompanied, as a result of the changes in form 
of the cells during karyokinesis, by a partial enclosure of the ventral 
cell of quadrant D (d**) by the other cells. 

The above account of the fifth cleavage is based upon an examination 
of twenty-five eggs, taken from different individuals and showing differ- 
ent phases of the division; i.e. each egg contained more than fifteen 
and less than thirty-two cells. 


Sixth Cleavage. 


The first division belonging to the sixth cleavage, that of d*1, takes 
place coincidently with the last division of the fifth cleavage, that of a**. 
There is thus no resting period between the two cleavages. Neverthe- 
less, there is a sufficiently well characterized stage of thirty-one or 
thirty-two cells, just as the cleavage of d®? occurs, and it will be well 
to describe the egg in this condition as a basis for an account of the 
sixth cleavage. 

Figure 43 (Plate 5) shows the anterior surface just before this stage 
is attained ; Figure 47 (Plate 6) shows nearly the same surface after the 
fifth cleavage is finished. The posterior surface is shown in Figures 46 
(Plate 6), 53, and 54 (Plate 7); the animal pole, in Figure 45 (Plate 
G). Figure 48 shows a sagittal section, while a transverse section of a 
stage just later (looking toward the animal pole) is given in Figure 52. 

The principal axis of the egg still coincides with its long axis, the 
animal pole lying at or near the micromere end, the vegetative pole at 
the macromere end (Fig. 48). 

The egg now consists of (1) a single large cell, d®1, embedded within 
the other cells and appearing on the surface at the ventral end only 
(Plate 5, Fig. 38), and (2) of thirty-one smaller cells, partly surround- 
ing the larger cell d**. One of these smaller cells, d*?, is a minute 
vesicle embedded between the cells a*’, 6°", and d®*? (Figs. 38 and 42). 


6 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


ise) 


The other thirty cells show a regular arrangement. The four quadrants 
may be distinguished as at the beginning of the fifth cleavage, each 
quadrant now containing eight cells, showing a characteristic arrange- 
ment. From ventral to dorsal we may now distinguish five layers. The 
first three layers contain each eight cells, two from each quadrant. In 
the quadrants A, B, and C, the two cells of a given layer are equal 
(Plate 6, Fig. 47). In quadrant D there is great irregularity. The 
two cells of the ventral layer are extraordinarily unequal, constituting 
the large partly interior cell d*!, and the minute vesicle d*’, also 
enclosed within the other cells (Plate 5, Fig. 38). In the second 
and third layers the two cells are likewise unequal, though less 
markedly so. 

The fourth layer consists of a single large cell from each quadrant, 
that of quadrant D being the largest (Plate 6, Fig. 45). 

The fifth layer consists of four small cells at the dorsal pole of the 
egg (Fig. 45). The arrangement at the animal pole formed at the four- 
cell stage (Fig. 8) is still maintained. The quadrants Band D are in 
contact for a considerable distance, whereas A and C' do not touch. In 
one of the points where three cells of different quadrants meet (in this 
case 5°®, c®-§, and d®*) lies the polar cell. 

The first cells in which indications of cleavage are observed are again 
the large cells of the D quadrant, d*" and d**. Spindles are formed in 
these at about the same time (Plate 6, Fig. 48). The processes taking 
place in the two cells differ, and must be considered separately. 

In d*, after the giving off of the small vesicle d°*, the nucleus very 
quickly enlarges to its original size. The aster begins to elongate at 
right angles to the position of the previous spindle (Plate 5, Fig. 38). 
But at almost the same time a rotation takes place, and by the time 
the two asters are fully separated the line connecting them is seen to 
be nearly antero-posterior (Plate 5, Fig. 42). The movement continues 
until the axis of the complex becomes exactly antero-posterior, and a 
spindle is formed in precisely the same position as the spindle for the pre- 
ceding cleavage. This spindle is shown in the sagittal section, Figure 
48. Its anterior end lies just ventrad of the small vesicle formed at 
the previous cleavage. Division now takes place, and a second small 
vesicle is given off to the point in the median plane lying just ventrad 
of the vesicle formed at the fifth cleavage. Figure 49 shows the pro- 
cess of formation of this vesicle, and Figure 50 shows the condition of 
affairs after the division is finished. In later stages the two vesicles are 
visible, lying beneath the cells of quadrants A and B, in the place 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. at 


where they were given off. (Compare Figure 50, Plate 6, with Figures 
56, Plate 7, and 65, Plate 8.) 

Other changes occur at the same time in the large ventral cell. After 
the fifth cleavage the granular cloud in the cytoplasm gathered into the 
region of the anterior margin of the free surface of the cell (Plate 5, 
Fig. 38). As the spindle for the sixth cleavage is formed, the cloud 
becomes concentrated over a small area, at a slight distance from the 
anterior margin of the cell (Plate 6, Fig. 48). Then, as division takes 
place, the cloud moves up to the anterior margin, at the same time 
spreading out, and begins to pass beneath the cells of the quadrants A 
and B (Figs. 49 and 50). As the large nucleus moves away from the 
wall of the cell where the vesicle was formed, the granular cloud moves 
inward (dorsad) and spreads out between the nucleus of the large cell 
and the two vesicles (Figs. 51 and 52). The granules at this time have 
become very coarse and distinct. 

Meantime, cleavage is taking place in the cell d**. In this cell the 
changes occurring in the asters are peculiar. 

Immediately after the preceding cleavage, the cell, having passed 
into the resting stage, has been pressed into an irregular wedge-shaped 
form by the processes occurring in the surrounding cells (Plate 5, Fig. 
37, surface view, and Fig. 38, section, from the same egg). The cell 
has become very narrow at the level at which the nucleus lies, so that, 
apparently, there is not room for the asters to separate at right angles 
to the foregoing spindle. The nucleus is pressed closely against the 
ventral wall of the cell (Fig. 38), and the aster begins to extend 
obliquely along the dorsal side of it, between the nucleus and the dor- 
sal wall of the cell. When the aster has become completely divided 
and the products are on opposite sides of the nucleus, their common 
axis is already in the same direction as the axis of the spindle at the 
previous division. The same result is obtained as in the rotation at the 
fourth cleavage, though in a different manner. But the final position is 
not yet reached. 

As now situated, the asters lie in the Jong axis of the much elongated 
cell (Plate 6, Fig. 46). As the active condition preparatory to di- 
vision comes on, the cell withdraws its deeper parts (shown in Plate 5, 
Fig. 38), and its dorso-ventral dimension increases. Accompanying this 
change is a rotation of the nuclear complex, from a position with the 
axis in the greatest dimension of the cell, to a position with axis in the 
shortest dimension. This change is shown in progress in Plate 7, 
Fig. 53. A later stage is shown in Figure 54; here the spindle is 


38 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


completely formed. The dorso-ventral axis of the cell has greatly in- 
creased, but is still distinctly less than the width of the cell at right 
angles to the spindle. The cytoplasm has become grouped symmetri- 
cally about the spindle, with the latter in its short axis. A longitudinal 
section of the same egg is shown in Plate 6, Fig. 48; the completed 
cleavage (d7* and d**®) is shown in section in Figure 50, and from the 
surface in Plate 7, Fig.57. The cleavage is unequal, the ventral cell 
being much the smaller. 

Cleavage in the other cells of this generation takes place in a sequence 
that is complicated by various factors, so that the account will be 
clearer if the divisions of the cells are described in connection with their 
relative positions in the egg, reserving a discussion of the order of 
cleavage till the end. The divisions will be taken up according to the 
layers of cells, beginning with the ventral layer. 

First or Ventral Layer, consisting of the eight cells, a*1—d*" and 
q®?— d®-2, 

The cleavage of d*! has been described. The small vesicle d°? does 
not divide farther. The other cells of this layer divide equatorially 
into cells of equal size. Two of the spindles leading to this cleavage 
are shown in Figure 56 (Plate 7), and the completed cleavage in Figure 
61. The resulting fourteen cells are a’t-—d™', a’?-—d'?, ai?—cl8, 
and ate el, 

Second Layer, containing the cells a°?—d*? and a®-*—d*+, 

The cleavage of d** has been described ; it is equatorial and unequal. 
The remainder of the cells also divide by equatorial furrows, but the 
products are equal in size. One of the spindles is shown in e*, in Fig- 
ure 47 (Plate 6), and in Figure 55 (Plate 7) the nearly completed 
cleavage ; the nuclei in all but the products of ¢** are still connected 
by interzonal filaments. The same condition of the cell d** is shown 
in Figure 57 (the products being d™" and d%*). In all of these cells, 
except d°4, the spindles lie at first in the shorter axes of the cells, as 
indicated in Figure 47 (Plate 6); but as the karyokinetic processes 
progress, the cells elongate in the direction of the spindles until the axes 
in which the spindles lie are the longer. 

The products of this division are a7®—d7*, a7’®-—d™*, a™"-d"", and 
qs di, 

Third Layer, containing the eight cells a*-*—d*5 and a®®— d°*. 

In all these cells the division is meridional, not equatorial, as in the 
cells of the first and second layers. One of the spindles (6°*) is shown 
in Figure 55 (Plate 7). The cleavage in d** and d®*° is shown in Fig- 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 39 


ure 57. In this figure, d*° has already divided into d7* and d’”, the 
nuclei of which are still connected by interzonal filaments. In d*® the 
spindle is still present. The completed division in the anterior part of 
the egg is shown in Figure 61. The cleavage is equal in all the cells of 
this layer except d®** and c®°; in these it is unequal. The unequal 
cleavage of c®* is shown in Figure 58, and the same figure shows the 
unequal products (d’-4 and d7-*) resulting from the division of d°-*. 

By the division of the third layer a band of sixteen cells is produced, 
extending completely around the embryo. The cells composing the 
band are qi — di, gi-l0 = ft=10, gig and qi? — q7-2, 

Fourth Layer, containing the four cells a®7 -—d°*", 

In these cells the cleavage is meridional, as in the third layer, and in 
every case equal. The spindles in b*’ and c*" are shown in Figure 47 
(Plate 6). 

The eight cells resulting from this cleavage are a™%-d"- and 
qi-i4 a qdi34, 

Fifth Layer, containing the four cells a°*-d°**, situated at the animal 
pole of the egg. 

The four small cells at the animal pole of the egg divide equatorially. 
The spindle of d** is shown in Figure 59 (Plate 7) ; and of a®* in Fig- 
ure 60. The cleavage products are very unequal ; the dorsal cells so 
formed are very minute, so that the distinction between cell body and 
nucleus cannot be observed, and the cells cannot be distinguished from 
the polar cell lying in the same region. Figure 60 shows the cleavage 
at the animal pole completed except in the cell a*8. A group of three 
small vesicles, representing the dorsal cleavage products of the cells 
b°-°_ d®°, lie at the animal pole, surrounded by the four larger cells, — 
one of which is the undivided cell a**, while the others are the ventral 
cleavage products of 6%*— d*8, 

The eight cells thus produced are a™!5—d™-4 and a™16 — di-6, 

This, the sixth, cleavage may be tabulated as follows: — 


Direction of Nature of 
Layer. Cells. Cleavage. Cleavage. Product. 
First,or (a,,c,d)®land62 Equatorial Equal (a-d) 71 and 72, (a-c) 
Ventral (except d®1) (exc. d®1) 73 and 74 [and d®2]. 
Second (a,b,c, d) *? and ®* Equatorial Equal (a-—d) 75 and 7%; 
r (exe. d6-3) (a-d) 77 and 78, 
Third (a,b,c,d) ®5 and®® Meridional Equal (a-—d) 79 and 710; 
(exc. c86, d&6) (a—d) 7 and 722, 
Fourth (a, b,c, d) &7 Meridional Equal (a—d) 7:18 and 7-14, 


Fifth (a, b,c, d) 68 Equatorial Unequal (a-d) 7 and 716, 


/ 


40 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Since the minute cell d*? does not divide, we thus have produced 
sixty-three cells instead of the typical number, sixty-four. Such a stage 
does not, however, have an actual existence, since some of the divisions 
belonging to the seventh cleavage have taken place before all these 
cleavages are finished. 

Figure 61 (Plate '7) shows the anterior surface of the egg at the end 
of the sixth cleavage, Figure 58 the posterior surface, Figure 60 the 
dorsal pule, and Figure 63 (Plate 8) the ventral pole. 

Sequence of the Sixth Cleavage.— The order in which the cells divide 
is, as I have already stated, now complicated by several factors. ' 
(1) The divisions of the first four quadrants of the egg (at the third 
cleavage) were not synchronous, but followed in the order D, C, B, A. 
Other conditions remaining the same, —that is, with equal intervals 
between the ensuing cleavages, —the same order would obtain in the 

later stages. 

(2) As discussed on page 35, the sequence becomes modified during 
the fifth cleavage, so that the cells in any given quadrant divide in 
nearly or quite the order of size of the blastomeres, beginning with the 
largest. .In the three quadrants A, B, and C, the order is (the ven- 
tral cell in each case being considered number one) 4, 2, 1, 3, while in 
the quadrant D the order is 1, 2, 4, 3. This order would naturally 
reappear in the sixth cleavage, other conditions remaining the same. 

Both the above factors do influence the fifth cleavage, but with still 
further complications. The first factor appears in the fact, that in any 
given layer the general order of cleavage of the component cells is 
BOC, A. 

The second factor is shown by considering the cleavage of a single 
quadrant, as D. The order of cleavage for the large left hand cells of 
this quadrant is as follows, naming the layers from ventral to dorsal: 
1, 2, 13, —nearly or quite the same as at the last cleavage. 

But a third factor appears in comparing the large left hand cells d°* 
and d®* of the D quadrant with their small right hand sister cells d°* 
and d** (Plate 5, Fig. 37, Plate 6, Fig. 46, Plate 7, Figs. 54 and 57). 
The large cells of each pair divide first, though the age of the two, being 
sister cells, is exactly the same. 

Similar relations may be shown for the other quadrants, Two facts 
are worthy of pariicular notice. (1) The large cells a°’—e*" divide 
first of all the cells in the quadrants A, B, and C,—and long before 
the small cells a®*—c®8, which are of exactly the same age. (2) There 
are some variations which cannot be brought into relation with any of 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 4] 


the factors mentioned. Thus, Figure 56 (Plate 7) shows that the cell 
6°1 is dividing before the cells c** and 6°, though the cells are appar- 
ently of the same size, and from the sequence of preceding cleavages 
the cell c®t would be expected to divide first. However, such variations 
may be correlated with differences in the size of the cells, since it is 
impossible to calculate precisely the volume of cells which have such 
irregular forms, and are subjected to varying conditions with the chan- 
ging positions of the surrounding cells. 

Certain general facts appear from the preceding discussion of the 
sixth cleavage. (Compare the table of this cleavage, on page 39.) 

(1) Every cell of any quadrant cleaves with its spindle in the same 
direction as the corresponding cell of any other quadrant (except the 
large interior cell d*’). 

The cleavage of a single typical quadrant up to this time is shown in 
the annexed diagram (Diagram I.). 


DracramM I, 


Diagram of quadrant A, B, or C in 
the seventh generation. Only the second 
exponent designating the cells appears in 
the diagram, the first being in all cases 7. 
Thus, the cell labelled 5 represents (a, 8, 
or c)7-5, The arrows connect cells of 
common origin, and show the direction 
of the spindles at the preceding division. 

R signifies right; L, left; D, dorsal; 
V, ventral; according to the plan of 
orientation explained at page 14. 


(2) All the cells in any layer (series of cells occupying the same 
relative position between the dorsal and ventral poles of the egg) cleave 
with spindles in the same direction (except d*1). 

(3) All the cleavages are equal except in the dorsal (fifth) layer, and 
in d®+, c®*, and d°6, 

(4) There is a tendency for the largest cells to cleave fastest. The 
minute cell d*-? does not cleave at all. 

(5) The cell d*-" cleaves in such a manner as to form a marked ex 
ception to the method followed by the other cells. Its cleavage is very 


42 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


unequal, while all the other cells in the ventral layer cleave equally. 
Also, the spindle does not lie in the same direction as in the other cells 
of this layer. 

The exceptional nature of the division in d*1! evidently demands 
explanation. The regularity of the cleavage in the other cells of the 
egg is such that some special condition must be correlated with the 
markedly differing division of this cell. The cleavage of d*1 differs 
from that of the other cells of the ventral layer in the following points. 
(1) Its cleavage is very unequal, while the cleavage of the other cells 
is equal. (2) The spindle for the sixth cleavage in d*? lies in almost 
exactly the same position as did the spindle for the preceding cleavage, 
whereas in the other cells of the ventral layer the spindle for the sixth 
cleavage is at right angles to the position of the spindle for the fifth 
cleavage. 

As to the first point, — that of unequal division, — no special cor- 
relating factor for ¢his particular case seems necessary, since in many 
cases in this and preceding cleavages the cells varied as to the equality 
of the division, though alike in other respects. Thus, in the fifth 
cleavage, the three ventral cells of the D quadrant cleft unequally, 
though in all the other quadrants the division was equal ; and in this 
sixth cleavage the cells e** and d®**® divide unequally, though in all the 
other cells of the same layer the division is equal. 

But the second point is totally exceptional in the cleavage up to this 
time. The axial relations of the cells appear to be so distinct and con- 
stant, and there is such uniformity in the positions of spindles at a 
given cleavage among cells of similar origin and relative position, that 
one must look for some other marked difference in the cell d** that 
might occasion this change of axis. 

In what respects does the cell d*’ differ from the other cells of the 
egg? (1) In its greater size; (2) in its position. 

(1) The greater size evidently has nothing to do with the different 
direction of cleavage, since the same disparity in size was present in 
earlier cleavages, yet this cell divided in the same direction as did the 
other ventral cells. 

(2) The change of position is such as to bring about a fundamental 
change in the relations of the cell to the egg as a whole and to the 
other cells. Previously the cell d** formed the posterior cell of the 
ventral layer. At the time of the sixth cleavage the cell has moved 
toward the interior of the egg and its posterior surface is covered as far 
ventrad as its anterior surface. The cell is now central, and surrounded 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 43 


on all sides by other cells. It thus occupies a position in the egg which 
is fundamentally different from that occupied by any other cell. Cor- 
related with this fundamentally different position, the cell acquires a 
fundamentally different method of division. 

It is impossible to say whether any particular feature of the different 
position of the cell is the essential one in bringing about this altered 
method of cleavage. As the cell moves inward, it very probably 
accomplishes a partial rotation (see below); if the axes of the cell are 
definite, and determined within the cell alone, then this rotation would 
cause a change in the position of the axes of the cell d** in comparison 
with the axes of the other cells, and a different direction of the spindle 
would result. But any such explanation is hypothetical. 

During the later stages of the sixth cleavage, the process of gastrula- 
tion has made much progress. At the end of the fifth cleavage the 
large ventral cell d*-1 had already moved some distance toward the in- 
terior of the egg (Plate 5, Fig. 38). As the cells d®°* and d*° now 
withdraw successively their deeper parts and increase their surface 
extension during division, they push ventrad, displacing the posterior 
part of the ventral cell (now d™1). (Compare Plate 6, Figs. 48, 50, 
aud 51.) The large cell therefore pushes dorsad into the interior of 
the egg, occupying the space made vacant by the ventral extension 
of the other cells. Soon after, the anterior cells, belonging to quad- 
rants A, B, and C also enter upon the karyokinetic process, and in so 
doing likewise push ventrad (Plate 8, Fig. 64) at the same time vacat- 
ing of course a portion of the space before occupied by them near the 
animal pole. The cell d‘' therefore continues to move dorsad, so that 
at the end of this cleavage it is almost completely enclosed (Fig. 65). 

During this inward movement of the cell d™1, the cloud of granules 
previously described changes its position still further. We had traced 
it, after the sixth cleavage, until it occupied a position between the nu- 
cleus of d™' and the two vesicles formed at the fifth and sixth cleavages 
(Plate 6, Fig. 51). As gastrulation continues, the cloud of granules 
migrates still further dorsad, and later even crosses the dorso-ventral 
axis, so as to lie posterior to it, surrounding the dorsal aster at the next 
division of d’ (Plate 8, Fig 64). 

This movement of the cloud of granules possibly gives the key to the 
change of axis of division in the cell d74. When the posterior cells 
extend ventrad during division, as previously described (Plate 6, Figs. 
48 and 51), they push against the posterior side of the cell d*1, thus 
displacing the cell inward. But such an impulse from one side only 


44 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


would naturally, if the cell be a body not entirely fluid, give it a rotary 
motion. As a result of the partial rotation, the cloud of granules is 
now found underneath the anterior cells (Plate 6, Fig. 50). When the 
anterior cells extend ventrad, the form of d‘' is so changed by its 
change of position that the cells do not push against it, but glide over 
it, at the same time vacating a part of the dorsal region of the egg 
(Plate 8, Fig. 64). Therefore the cell d’", as it moves inward, con- 
tinues its rotation in the same direction as before, and the cloud of 
granules is brought to the dorsal end of the cell, and even farther. 

Such a rotation would explain clearly the peculiar position of the 
spindle in d*' at the sixth cleavage. To agree with the other cells of 
the ventral layer, the spindle would have to take a dorso-ventral posi- 
tion. (Compare Fig. 55, Plate 7.) But a rotation from posterior to 
anterior just before this cleavage would bring the spindle into an antero- 
posterior position, such as actually occurs. 

However, the explanation is not very satisfactory, for several reasons. 
(1) We have previously seen that the granules forming the granular 
cloud do move within the cytoplasm of the cell, so that this change of 
position of the granular mass may be due to simple migration through 
the protoplasm of the cell. (2) A study of the movements of the 
asters after the fifth and sixth cleavages does not give evidence of any 
such rotation. (See the account of the movements of the asters before 
the sixth cleavage, page 36, and before the seventh cleavage, page 54.) 
(3) Such a rotation explains the position of the spindle at the sixth 
cleavage only, — while all the later cleavages of the inner cells are like- 
wise out of relation to the divisions of the outer cells. (4) The ex- 
planation assumes that the position of the axes of cleavage is definite, 
and determined within the cell itself, so that, if the cell rotates, the axis 
rotates with it, — which is not proved. 

The only conclusion in which we are entirely justified is therefore 
merely this, that as the relation of the cell d°* to the other cells and to 
the egg as a whole becomes fundamentally changed, the method of di- 
vision likewise becomes fundamentally changed. 

At the close of the processes thus far described, the egg has evidently 
passed into the “gastrula stage” proper (Plate 8, Fig. 64). The blasto- 
pore is still large and lies at the macromere end of the egg. It is sur- 
rounded at first by eight cells, two belonging to each of the four original 
quadrants of the egg (Plate 7, Fig. 56). Later, as the sixth cleay- 
age is entirely finished, one of the cells (e™’) becomes displaced and is 
shut out from the margin of the blastopore (Plate 8, Fig. 63), which is 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 45 


itself diminished in size. The interior of the gastrula is occupied by 
the large cell d7! and the two minute vesicles d°? and d”? (Fig. 64). 
The interior cells are surrounded by a single layer of outer cells, except 
at the animal pole of the egg, where the small dorsal cells do not reach 
to the cells within the gastrula, but lie on the surface, making here a 
two-layered region. This region remains two-layered as long as it is 
possible to trace the history of the animal pole of the egg. 

The three inner cells d™, d*?7, and d’-*, with the products of the 
former, may henceforth be called the entuderm, the outer layer the 
ectoderm. 

Seventh and Later Cleavages. 

I have followed the cleavage through another generation, and, for 
parts of the egg, much farther. It becomes impracticable, however, to 
describe the cleavage according to the layers or series in which it takes 
place, as has been done up to this stage, owing to the complicated suc- 
cession of the divisions in the different cells, and to the great changes in 
position taking place while the cleavages are in progress. I shall there- 
fore now describe the processes in the general order in which they 
occur, and in so doing I shall consider separately (1) the ectoderm, and 
(2) the entoderm. 

THE EcTODERM. 

In discussing the changes taking place in the ectoderm, it will be well 
to distinguish for convenience of description two regions: (1) the (left) 
posterior part of the ectoderm, derived from the quadrant D; (2) the 
anterior and right lateral portions of the ectoderm, derived from the quad- 
rants A, B,and C. While the phenomena occurring in all of these regions 
are reducible to the same general scheme, so far as the method of 
cleavage is concerned, the irregularity in the size of the blastomeres 
forming quadrant D, their earlier cleavage as compared with the other 
quadrants, and the fact that some of the cells have passed inward to 
form the entoderm, give this region a peculiar and somewhat irregular 
character, which makes it convenient to discuss it separately. 

(1) The Quadrant D. —The entoderm cells belong genetically to this 
quadrant, but they will be considered later. 

In order to understand the conditions in quadrant D, and to see their 
relations with the arrangement in the other quadrants, it will be well to 
emphasize certain features of the last two cleavages. 

At the fifth cleavage, as previously described, the three ventral cells 
of quadrant D divided by meridional planes into unequal portions, the 
left derivative being in every case larger (Plate 4, Fig. 33, and Plate 5, 


46 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Fig. 37). The single dorsal cell divided equatorially into a small dorsal 
and a large ventral part. There is appended a diagram of the quad- 
rant D after this cleavage (Diagram II.). If the septa in the three 
ventral cells were moved to the middle of the cells, the diagram would 
represent the condition in any one of the other three quadrants. 

At the next division (sixth) the two ventral cells d*! and d*? have 
passed inward, becoming the entoderm, so that we may omit them from 
the present discussion. Of the other cells, the ventral pair (d°* and d**) 


DraGram II. Dracram III. 

Diagram of quadrant D in the sixth Quadrant D in the seventh genera- 
generation. Only the second exponent tion. Only the second exponent is 
designating the cells appears in the expressed, the first being in all 
diagram, the first being in al! cases 6. cases 7. 


The arrows connect cells of common origin, and show the direction of the spindles 
at the preceding division. R signifies right; L, left; D, dorsal; V, ventral. 


divide equatorially, d*-* unequally, d** equally (see Plate 7, Figs. 57 
and 58). The two next layers divide meridionally (Fig. 57), the cell 
d*-° unequally, the others equally. The dorsal cell divides equatorially 
and unequally. Diagram III. shows the ectodermal part of this 
quadrant at the end of the sixth cleavage. The actual condition in the 
egg at this period is shown in Figure 58, and at a slightly earlier stage 
in Figure 57. 

Comparison of Diagram III. with the type diagram for the other 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. AT 


three quadrants given on page 41, shows that the directions of the cell 
walls are the same in both, the inequality in the size of the cell in 
the quadrant D being the only difference. 

In the seventh cleavage, the spindle appears first in the cell d7-§, as 
shown in Figure 58, and the celi is divided meridionally into two equal 
cells, d*"! and d°% The finished division is shown in Plate 8, Figs. 
66, 67, and 68. ‘he plane separating these two cells is the median 
dorso-ventral plane of the embryo, as will be shown later. 

Shortly after this division is completed, spindles appear in d7*, d7%, 
d™, d7-8, and d™4, as shown in Figure 66. In d‘** the spindle is 
dorso-ventral, hence lying in the shorter axis of the cell; the cell 
extends and divides into two equal dorsal and ventral parts, d*-* and d*-1° 
(Figs. 67 and 68). The greater surface extension of d** in Figure 67 
is due to its being spread out in a thin layer over the surface of the 
entoderm cell. 

In d7™® and d™” (Fig. 66) the spindles are also dorso-ventral in 
position, and the cells divide equatorially into equal parts, d*", d*1°, 
d*-8 and d°-*° (Figs. 67 and 68). 

In d’-? and d™™ the spindles lie at right angles to those just described, 
and the cells divide meridionally and equally, forming d®-, d*-6, q8-2, 
EY a6 Ohta 

Figure 67 is a view of this region after these cleavages are finished. 
As shown in this figure, a certain amount of shifting has taken place 
during cleavage, by which the cell d’ has been excluded from its share 
in the boundary of the blastopore. As the cells divide, they withdraw 
their interior parts and extend in the direction of the spindle, as has 
been minutely described for other cleavages; in this way the dorso- 
ventral extent of quadrant D has been greatly increased. As a result 
the blastopore has been nearly closed (Fig. 63), and at the opposite 
end the animal pole has been pushed beyond the micromere end of the 
egg to its anterior side (Fig. 65). 

Thus far the six larger left hand cells have divided, leaving six smaller 
cells (at the right and at the dorsal pole) undivided (Fig. 67). 

Next, as shown in this figure, the cells d’*7 and d™® form spindles and 
divide. Each cleaves in the same manner as its larger companion cell 
has done, d™-’ equatorially, d’:* meridionally. The products, d838, d*14, 
d*-¥, and d*-°, are shown in Figure 68 (compare Diagram IV.). 

Next d’-” cleaves equatorially, like its companion cells d7-? and d™-, 
In d™-” the division is unequal, the ventral product, d*-, being much 
the smaller (Fig. 68). 


48 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


The cleavage of d7™ occurs much later, and is likewise equatorial. 
I have not thought it necessary to introduce a special figure to show the 
cleavage of this minute cell. 

In d7-® the cleavage also takes place late ; it is meridional and equal. 
The resulting cells, d°° and d*”, are shown in Figure 72 (Plate 9): 
The minute dorsal cell d'7* does not divide. 


Dracram IV. DiacraM VY. 

Quadrant D in the eighth genera- Diagram of quadrant A, B, or C, in 
tion, — except the dorsal cell, d*-18, the eighth generation. Only the second 
which does not divide farther. In exponent of the cells appears in the dia- 
the other cells only the second ex- gram, the first being 8. The small dor- 
ponent is expressed, the first being sal cell, (a. 5, or c)7*16, does not divide, 
in all cases 8. The arrows connect remaining thus in the seventh genera- 
cells of common origin, and show the tion. The arrows connect cells of com- 
direction of the spindles at the pre- mon origin, and show the direction of 
ceding cleavage. the spindle at the preceding division. 

R signifies right; L, left; D, dorsal; R signifies right; L, left; D, dorsal; 
V, ventral. V, ventral. 


There are thus 23 cells in the ectodermal part of the quadrant D at 
the end of the seventh cleavage. A diagram of this stage is annexed 
(Diagram IV.). Nearly this stage is represented in Figure 68. 

(2) The Quadrants A, B, and C.—Owing to the regularity in the 
size of the cells in these quadrants, and the fact that they are purely 
ectodermal, the conditions observed are fairly simple as compared with 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 49 


those in quadrant D. Figure 61 (Plate 7) shows the anterior surface of 
the egg at the end of the sixth cleavage, and a diagram of a single 
quadrant at this stage was given on page 41. Comparing either the 
figure or the diagram with the scheme (Diagram III.) given on page 46 
for the quadrant D at the same stage, the arrangement is seen to be the 
same except in two respects. (1) The cells belonging to the same lat- 
eral series are equal in A, B, and Q, unequalin D. (2) Four ventral 
ectodermal cells additional are present in each of the quadrants A, B, 
and ©; these are represented in D by the cells which have passed 
inward to form the entoderm. 

As shown by the spindles in Figure 61, the first cells to divide are 
714_ (7-14 The cleavage is meridional and equal. The 
resulting cells are a®®—c®*, a6 — 8 a7 — 81, and a**—c*. They 
are shown in Figures 72 and 75 (Plate 9). (Compare Diagram V.) 

Next follows the division of the six cells, a7-®-¢7* and a’8-c"*, which 
together form part of a transverse girdle, surrounding the egg. The 
cleavage is meridional and equal. The resulting 12 cells, a’ 2-8, a? — 
&P ab 815, and a®6—c*!6 forming as before a transverse girdle, are 
shown in Plate 8, Figs. 69 and 70, and Plate 9, Fig. 71. 

Next ensues the cleavage of the transverse row containing the six 
cells, a’°—c’* and a’"—c'", These cells, as shown in Figure 61 (Plate 7), 
are much flattened dorso-ventrally, and are of exactly the same form as 
the cells last discussed, which lie immediately dorsad of them. Moreover, 
each cell in this row corresponds in origin to a cell of the row last 
described, the two rows having been derived from the equatorial division 
. of a previously existing transverse row, as shown in Figure 55. If 
mechanical conditions are decisive in determining the direction of the 
cleavage, these two rows should cleave in the same manner, i. e. both 
meridionally. Nevertheless, as shown in Figure 69 (Plate 8), while the 
dorsal of the two rows divides meridionally, the cells of this ventral row 
all cleave equatorially. The axis of the cell in which the spindle lies is 
about half as long as the axis which is at right angles to it. The cells 
elongate in the direction of the spindles, and a very unequal division 
ensues. The ventral products are minute, while the dorsal ones are 
nearly equal in size to the mother cells. Figure 75 (Plate 9) shows 
the anterior surface of the egg after this cleavage. The twelve cells 
produced are a?9—¢*-, a810— ¢810, 3 _ 8. and a&4— 84, 

The division of the band of twelve small cells composed of a7? — c7-?, 
a0 — ¢7-20 gill _ 7-1 and a1? —¢7-2, and shown in Figure 61 (Plate 7), 
follows somewhat later. The cleavage is equatorial and the spindles lie 

VOL. XXx. — no. l. 4 


a?8_ ¢7-13 and a 


50 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


in the short axes of the cells, as shown in Figure 75 (Plate 9). Just 
before division the cells elongate until the axis in which the spindle lies 
is longest, as shown in the cell 6" of the figure last mentioned. 

The cells a™®—c™-, near the animal pole, cleave meridionally. The 
spindle in a7, and the cells 6° and c®.29, 58-30 and c®", are shown in 
Figure 72 (Plate 9). The minute cells a’’®—c", like d™, do not 
cleave further. The arrangement at the animal pole is now very 
irregular, and owing to this fact and the minuteness of the cells pro- 
duced at the last cleavages it is very difficult to be certain of the exact 
origin of any given cell, though the origin of the group cannot of 
course be doubtful. 

We have now accounted for all the cells of these quadrants except 
the four ventral cells of each quadrant, a7*?—e"1, a’? -c"?, a’? —¢"*, and 
a7-4— 7-4, These correspond in origin to the entoderm cells of quadrant 
D, and they do not cleave further until they are partly or entirely 
enclosed within the embryo, as will be shown later. 

A diagram of that part of any one of the quadrants A, B, and C 
corresponding to the ectodermal part of quadrant D, and showing the 
conditions at the end of the seventh cleavage, is given on page 48 (Dia- 
gram V.). A comparison of this diagram with that for the correspond- 
ing stage of the quadrant D (Diagram IV. on the same page) shows 
that the direction of the cleavage planes, and hence of the spindles, is the 
same throughout in all cells of corresponding position, though there are 
many differences as to the equality or inequality of the cleavage products. 

The general facts which may be deduced from the foregoing study of 
the seventh cleavage in the ectodermal parts of the egg are similar to 
those drawn from a study of the sixth cleavage, page 41. 

(1) Every cell of any quadrant divides with its spindle in the same 
direction as the corresponding cell of any other quadrant. 

(2) All the cells in any layer cleave with spindles in the same direc- 
tion (in spite of great differences in the form of the cells.) 

(3) No general law can be deduced as to the equality or inequality of 
the divisions. 

(4) There is a tendency for the largest cells to cleave fastest. Certain 
very small cells (at the dorsal pole) do not cleave at all. 

Other Changes during the Seventh Oleavage.—In almost every cleav- 
age which has taken place, whenever the division was equatorial, — the 
spindles taking a dorso-ventral position, —it will have been noticed that 
the axis in which the spindle was formed was the short axis of the cell. 
On the other hand, the cells in which meridional cleavages have taken 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 51 


place have commonly been already somewhat elongated in the direction 
of the spindle. Therefore, at the occurrence of division, the short cells, 
cleaving equatorially, have changed form greatly, becoming more elon- 
gated dorso-ventrally, while the cells which cleave meridionally have 
already been of sufficient length to permit of the extension of the spin- 
dle without much change of form. As a result, there has been a great 
extension of the ectoderm dorso-ventrally. This produces first a com- 
plete closure of the blastopore. (Compare Figure 65, Plate 8, with 
Figure 76, Plate 9.) A second result, due to the larger size of the 
cells of the quadrant D, and perhaps partly also to the fact that they 
cleave first, is the further displacement of the animal pole of the egg 
from the micromere end toward the anterior side (Plate 8, Fig. 65, 
Plate 9, Fig. 76). 

The closure of the blastopore is not sufficient to provide for the dorso- 
ventral extension brought about during the cleavage, so that, as a third 
consequence, the cells at the ventral pole of the egg, where the blasto- 
pore was previously situated, are pushed over or under one another, 
the ectoderm tending to become two-layered in this region. A compari- 
son of Figure 63 (Plate 8) with Figure 73 (Plate 9) shows the result- 
ing conditions. In the latter figure the entoderm (d*? in Fig. 63) is 
entirely enclosed, and the ventral cells a7?—e7*1, a™-?-—c™?, a’? —¢™3, and 
a4 —c™-* have become crushed together, and several of them, as a7? and 
b73, are almost hidden by surrounding cells. The beginning of the two- 
layer condition is shown in frontal section in Figure 80 (Plate 10), 
from the same egg as that shown in Figure 73 (Plate 9). 

Meanwhile other cleavages are taking place in quadrant D, leading to 
still further modifications of the structure of the egg. It is therefore 
necessary to return to a consideration of this quadrant. 

Quadrant D.— By reference to the diagram of the cells of this quad- 
rant at the end of the seventh cleavage (Diagram IV., page 48), it will 
be seen that there are now present, exclusive of the entoderm, twenty- 
three cells, arranged somewhat irregularly. Approximately the same 
stage, as it actually appears, is shown in Figure 68 (Plate 8). 

In this egg spindles have appeared in the large cells d*™ and d*??, and 
later they each become divided into two equal cells. It is worthy of 
notice that the two spindles are not parallel, but make a slight angle 
with each other. The two cells lie on opposite sides of the median 
dorso-ventral plane, so that the angle between the spindles indicates the 
beginning of a tendency to bilateral cleavage. The inclined position of 
the other spindles (in e7’ and c’-") is indicative of the same fact. The 


52 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


tendency toward bilaterality is apparently due to the crowding of the 
cells from all directions toward the blastoporic region, which lies at 
the ventral border of d*° (Fig. 68). The cells resulting from the 
division of d* and d*" are d*™, d°, d°:8, and d*™*; they are shown 
in Figure 74 (Plate 9). 

Next the four cells immediately dorsad of these, d*1", d*-1®, d*®, and 
d*-®, develop spindles in their short axes (Plate 8, Fig. 68), which as a 
result of extension become the long axes (Fig. 74, Plate 9),— the 
cleavage being in each case equatorial and equal. 

Now the four large cells d**”, d**°, d°-" and d*8, shown in the dorsal 
part of Figure 68 (Plate 8), cleave equatorially also. 

As a result of these many equatorial cleavages the quadrant D 
becomes greatly increased in dorso-ventral extent. The animal pole is 
forced farther upon the anterior side of the egg, toward the blastopore, 
so that the cells of the quadrant D come to occupy in the region of a 
sagittal section much more than half the circumference of the egg 
(Plate 9, Fig. 76). 

The two ventral cells d**® and d*’° meanwhile divide meridionally, 
completing the separation of quadrant D into two portions lying on each 
side of the median line (Fig. 74). However, the egg is not yet com- 
pletely separated by cleavage planes into right and left halves, for the 
entodermic cell d°* occupies the median plane at even a later stage than 
this (Plate 10, Fig. 83). 

This is the latest stage to which I have traced the cleavage in the 
ectodermal part of quadrant D. Diagram VI. shows the condition at 
this time. 

The dorso-ventral extension of the ectoderm, and consequent crowd- 
ing together of the cells in the region of the blastopore, are still further 
increased by the eighth cleavage of the large cells a*-*>— *-5; a®-25— ¢°-, 
a®-27_ ¢8-27 and a®-*-c®-*, belonging to the other quadrants, which like- 
wise divide equatorially and equally (Plate 9, Fig. 75). 

The blastoporic region has now become distinctly two-layered, as 
shown in Figure 77. The cells of quadrants A, B, and C are turned 
in and pushed dorsad, in the same manner as happened in early stages 
to the large ventral cell of quadrant D. The anterior lip of the blasto- 
pore thus becomes two layers thick, while the posterior lip is formed 
of a single layer of cells from the quadrant D, resting against the ento- 
derm cells. Between these ventral cells of quadrant D and the infold- 
ing cells of the other quadrants, a slight notch appears, marking the 
position of the blastopore. (At an earlier stage the blastopore was 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 53 


entirely closed, as shown in Figure 73.) The blastopore notch lies, not 
directly at the macromere end of the egg, but at some distance on the 
posterior side of it. 

As the cells of the anterior lip of the blastopore are turned inward, 
some of them begin to divide. Spindles in these cells are shown in 
Plate 9, Figs. 75 and 77. The form and position of the cells have 
changed so much at this time that it is impossible to determine with 
certainty whether the cleavage should be considered equatorial or merid- 
ional. In the cases figured the spindles are nearly or quite transverse, 
so that in some of the cells the division is meridional. 


Dracram VI. 


Diagram of quadrant D at a time when 
most of the cells have passed into the 
ninth generation. All cells in the ninth 
generation are bounded by continuous 
lines, and are designated by the second 
exponent belonging to the cell, the first 
exponent being in each case 9. In the 
other cells both exponents are given. 
The arrows connect cells of common 
origin, and show the direction of the 
spindle at the last division. 

R signifies right; L, left; D, dorsal; 
V, ventral. 


The cleavage of the ectoderm has now been traced to the eighth gen- 
eration in all parts of the egg, and in the greater part of the quadrant 
D to the ninth generation. 

With this ends the account of the cleavage of the ectodermal cells. 
The small size and the great displacements of the blastomeres, espe- 
cially in the regions of the blastopore and the animal pole, render it 
impossible to determine with certainty their identity in later stages, 
and the real direction of cleavage is masked by crowding and deforma- 
tion of the cells. It would perhaps be of little interest in connection 
with the laws of cleavage to carry the study further, as it is scarcely to 
be presumed that the later divisions would exhibit any phenomena dif- 


54 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


fering fundamentally in kind from those already shown in the earlier 


stages. 
We must now turn to a consideration of 


THe ENTODERM. 


The cells which I have called the entoderm — following in this Zelinka 
(91) and Tessin (’86) — are those derived from the single large cell of 
the quadrant D, which passes within the egg in the manner already de- 
scribed. I have already given an account of the first cleavages of this 
cell after it has become partially covered by the other blastomeres. As 
will be recalled, at the fifth and sixth cleavages the spindles occupied 
twice in succession the same position, one end lying in the anterior 
median line, between the ventral cells of quadrants A and B (Plate 5, 
Fig. 35, and Plate 6, Fig. 48). Here was given off at each of these 
cleavages a minute vesicle, the entire process. being comparable in ex- 
ternal features to the successive formation of two polar cells at a given 
spot on the surface of the egg. The two vesicles thus formed maintain 
their position for some time (Plate 7, Fig. 55), but as the surrounding 
cells become invaginated, I have found it impossible to follow their 
later fate. 

We will follow the cleavage of the large cell d7-’ (Plate 6, Fig. 50), 
which forms the greater bulk of the entoderm. 

After the sixth cleavage (Fig. 49), the asters in d’-? at once separate 
nearly in the dorso-ventral axis of the egg, as shown in Figure 50. The 
line joining them is at first a little oblique, the ventral aster being a 
little to the right. This obliquity soon corrects itself, and the asters 
come to lie in the sagittal plane. As the spindle is formed, its dorsal 
end moves to the posterior side, so that the spindle is no longer in the 
dorso-ventral axis of the egg. This stage is shown in Figure 64 (Plate 
8); as may be observed in this figure, the spindle is neither in the 
longer axis of the cell nor at right angles to it, but oblique. The cloud 
of granules, which soon after the last division occupied a region on the 
anterior side of the cell, underneath the two vesicles d*? and d™?, now 
surrounds the aster at the dorso-posterior end of the spindle. 

The cleavage is unequal, separating (Fig. 65) a smaller dorso-posterior 
cell, d°-, from a larger anterior one, d**. The cloud of granules 
remais in the smaller, dorsal cell, forming a band about its periphery, 
so as to leave a free space surrounding the nucleus. The position of 
the animal pole of the egg with reference to this cell should be carefully 
noted, as the relation remains constant, at least for a time, during the 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 5 


considerable shiftings which take place. As seen in Figure 65, the 
animal pole lies at the anterior margin of the cell d*-?. 

Before the eighth cleavage takes place, the blastopore has become 
closed and its anterior margin has begun to pass into the two-layer 
condition, as previously described. The larger cell, d*-, divides before 
its mate, by a spindle at right angles to the previous spindle. The 
cleavage is equal, forming the two large right and left cells d®* and 
d**, shown in Plate 10, Fig. 80. The plane separating these cells 
coincides with that separating the quadrants 4 and & on the anterior 
side of the egg, and also with that separating the two cells d*™ and 
d’-!2 on the posterior side (Plate 8, Fig. 68). As this plane also 
passes through the animal pole and the blastopore, it divides the egg 
into symmetrical halves, and is the median dorso-ventral or sagittal 
plane of the embryo. 

Later the small cell, d*-?, develops a spindle in the same direction as 
the spindle of the seventh cleavage, in the shorter axis of the cell, and 
divides into two very unequal cells. The anterior or ventral cell, d?-’, 
is a minute vesicle, whereas the dorsal or posterior blastomere, d*-4, is 
scarcely smaller than the mother cell. The process of budding off this 
small cell is shown in Plate 10, Figure 80. The vesicle lies between 
the two large cells d**t and d**, and, like the minute cells d®? and d7-, 
ean be traced but a short distance, soon becoming lost among the many 
cells by which it is surrounded. 

In the ninth cleavage, spindles are formed in the cells d*-1 and d*? in 
the position foreshadowed by the asters in Figure 80, — that is, antero- 
posterior, and at right angles to the preceding spindles, — and the cells 
divide equally, forming the four cells d®, d!°-?, d!°*. and d+. Figures 
76 and 81 show the entoderm at the close of these divisions — the 
nuclei of the cells in question being still connected in pairs by inter- 
zonal filaments. The blastopore is now present as a distinct notch ; its 
anterior or dorsal lip has become two-layered, owing to the folding 
inward of the cells of the anterior quadrants. The animal pole (pol. 
anm., Fig. 76) has moved a considerable distance on to the previously 
anterior surface, lying still at the anterior margin of the entoderm cell 
d°-4, As the side view (Fig. 76) shows, a frontal plane carried through 
the long axis of the egg at this stage would cut the nuclei of all the 
entoderm cells, as realized in the frontal section, Figure 81 (Plate 10). 

As soon as the cells, after the cleavage process is entirely finished, 
have lost their strong tendency to maintain a form as nearly spherical as 
possible, a sudden and considerable change of relative position takes 


56 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


place. The four entodermal cells, d!°1—d-* become much flattened 
dorso-ventrally, and the invagination of cells at the anterior lip of the 
blastopore increases in extent. ‘These cells press most strongly upon the 
anterior surface of the ventral entodermal cells, forcing them toward 
the posterior side. An early stage in the process is shown in Plate 9, 
Figure 77. Two of the cells of the invaginating ectoderm have flattened 
themselves against the entoderm cells in such a way as to form a direct 
continuation of the longitudinal series of interior cells. This longitudinal 
series is however tending to become curved by the displacement of the 
cell d®4 in a dorsal and anterior direction. This cell, together with the 
animal pole, has moved a very slight distance toward the macromere 
end of the egg. The animal pole now lies (Fig. 78) directly above the 
plane separating the cells d+ and d’? from d'? and d’-*, instead of 
above the posterior margin of the cells d? and d’°*, as previously. A 
frontal plane carried through the long axis of the egg would not now cut 
the cell d°* at all. Figure 82 shows a view from the animal pole, the 
outer layer of ectoderm cells being supposed to be removed from the 
dorsal half of the egg, while the entoderm cells remain in position. 

This process of rotation of the entodermic contents, as one might call 
the phenomenon, continues still farther. Figure 79 shows a stage in 
which the process is much more advanced. The ectodermal plug at the 
anterior lip of the blastopore has become very much thickened, and pro- 
jects farther posteriad; the four large entoderm cells d1—d™, of which 
of course but one pair is shown in the side view, are now so displaced 
that the plane separating the pair, which previously lay in the short axis 
of the egg (Figs. 76 and 78), now lies in the long axis (Fig. 79). The 
line connecting the centres of a given lateral pair is now at right angles 
to the line previously connecting them. 

At this time the five large cells constituting the entoderm — the 
minute cell d** not being traceable farther — begin to undergo karyo- 
kinetic changes preparatory to division. The spindles in various stages 
are seen in three of the cells in the side view, Figure 79; the same 
stage, slightly earlier, is shown from the animal pole in Figure 83 
(Plate 10), in which the covering ectoderm is supposed to have been 
removed from the dorsal side of the egg. As a comparison of Fignres 79 
and 83 shows, the spindles do not lie in parallel planes, so that no single 
view can give a complete representation of their positions. Nevertheless, 
Figure 83 shows that the arrangement is distinctly bilateral. The cell 
d** lies in the middle line, with its spindle in the sagittal plane of the 
egg; the spindles in the other cells all radiate outward from the region 


JENNINGS: DEVELOPMENT OF- ASPLANCHNA HERRICKII. 57 


occupied by d°*, the sagittal plane passing through this cell forming a 
plane of symmetry for all. The median plane thus indicated coincides 
with that already defined by the line separating the quadrants A and B 
anteriorly, and the boundary between the cells d** and d*- posteriorly ; 
it passes through the animal pole and the blastopore. 

This movement of the entodermal blastomeres is of course simply a 
continuation of the rotation inaugurated at the passage from the four- to 
the eight-cell stage. The ectodermic cells continually withdraw their 
deeper parts and increase their surface area at division ; in this way cells 
are continually forced in at the blastopore. These press upon the anterior 
and ventral aspects of the entoderm cells, forcing them backward, as 
already described. The pressure is greatest in the median region, so 
that the anterior or ventral ends of the cells on the two sides of the 
median plane are forced apart, the axes of the cells become oblique to 
the long axis of the embryo, and the oblique position of spindles shown 
in Figure 83 results. 

Beyond this point it is impossible to trace the development cell by 
cell. In Asplanchna there is a period, intervening between the stage to 
which it is possible to trace the cleavage step by step (about 120 cells) 
and the stage of recognizable differentiation of organs, during which the 
cells divide and become very minute. The cells probably reach the 
number of from 250 to 500, and the process of extension of cells and 
consequent “rotation ” and invagination of parts of the embryo continues. 
A sagittal section of the embryo at about the time of the beginning of 
differentiation of organs is shown in Figure 84. It thus becomes 
impossible to trace the fate of individual blastomeres, or even, except in 
a most general way, the fate of the different regions of the embryo during 
the later portion of the cleaving. From the small size of the adult roti- 
fer, | had hoped that it would prove to be a favorable object for an exact 
study of the cytogenetic history of organs ; in Asplanchna this turns out 
not to be the case. But, on the other hand, for a study of the factors in 
the early developmental processes it has shown itself well fitted. 

My study of the processes in the early development of Asplanchna there- 
fore closes with the stages shown in Figures 79 and 83 (Plates 9 and10). 

For critical comparison of my observations with those of Zelinka and 
other workers on the development of the Rotifera, the reader is referred 
to Part Second. 

I shall now proceed to a discussion of the bearing of the foregoing 
observations upon the problems already proposed, as well as upon other 
related subjects. 


58 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


III. Discussion of the Bearing of the Observations on the 
Problems. 


In the following discussion I shall adopt in general the order pursued 
in my “Statement of Problems,” taking up successively the various 
theories in regard to cleavage and gastrulation, and pointing out what 
bearing the observations above detailed have upon these theories. This 
will be followed by a résumé of the general conclusions which may be 
drawn from the work. 

We will therefore first take up a discussien of the cleavage, and of the 
theories bearing upon it. 


1. CLEAVAGE. 
A. The Direction of Cleavage. 


(1) Berthold’s theory of surfaces of least area. (See page 4.) — The 
two- and four-cell stages in Asplanchna agree well with the conditions 
demanded by the law of least surfaces. The peculiar arrangement of 
blastomeres in the four-cell stage, agreeing as it does with the four-cell 
stage of animals of the most various systematic positions, and with the 
four-cell stage of many plants, seems probably due to some very general 
law. In all these cases only three cells meet along one common line. 
As this is the arrangement demanded by the principle of least surfaces, 
the conclusion seems perhaps justifiable that this principle of least sur- 
faces is that common law. 

The eight-cell stage also fulfils the requirements of the principle of 
least surfaces. But from this stage onward, many of the conditions 
found are irreconcilable with the view that this principle is a determining 
factor. Six of the cells in the eight-cell stage divide in a manner that 
squarely defies the principle of least surfaces. Nor does the arrange- 
ment of cells in the resting periods agree better with the principle. As 
pointed out on page 30, the flat, almost disk-shaped form taken by the 
cells of quadrant D during the ten-cell stage (Figs. 23 and 25, Plate 3) 
and the sixteen-cell stage (Figs. 31 and 32, Plate 4) is widely at vari- 
ance with the demands of the principle of least surfaces. The form 
of the cells in quadrants A, B, and C during their resting period in the 
sixteen-cell stage (Figs. 30 and 34) is equally impossible of explanation 
on the least surfaces theory. Many other cases could be adduced in 
which this principle is contradicted, but a fuller discussion of these cases 
will be given under the next theory (Hertwig’s law). In general, any 
case which is not in agreement with Hertwig’s law is likewise inexplicable 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 59 


by Berthold’s principle, so that all the cases cited later as opposed to 
Hertwig’s law can be utilized equally well against the principle of least 
surfaces as a determining force. 

It should be noted that Berthold did not in any sense maintain that 
this principle is the deciszve factor in cell division, or the arrangement of 
cells in tissues. He recognized that the conditions in a living cellular 
body are widely different from those in a simple vesiculated fluid, and 
that the conditions actually found in plant tissues are often inexplicable 
by the principle of least surfaces, —in many cases, indeed, directly 
opposed to it. “ Aber nothwendig ist das in der Zelle nicht, wie bei den 
Fliissigkeitslamellen. Denn wir sahen schon frither, dass die Symmetrie- 
verhaltnisse der Zellen von der dusseren Form oft vollstaéndig unab- 
hingig werden, und auch unter Mitwirkung der dusseren Formverhiltnisse 
k6nnen bei dem ineinandergreifen der verschiedenen Factoren sich Thei- 
lungsrichtungen ergeben, die mit den Forderungen des Princips der 
kleinsten Flichen nicht in Uebereinstimmung stehen.” (Berthold, 86, 
p- 230.) Berthold (’86, p. 230, Taf. 4) describes and figures many cases 
in which the arrangement and division of cells is not in accordance with 
the principle of least surfaces. 

The fact that a single cell may at one time take such a form as that 
which d** shows in the surface view in Figure 37 (Plate 5), and in 
section in Figure 38, and at another time have the form exhibited by the 
same cell in Plate 7, Figure 54 (surface), and Plate 6, Figure 48 (section), 
while the shape of the egg remains unchanged, demonstrates that we are 
not here dealing with a problem of the statics of a vesiculated fluid ; 
a single simple principle can no more account for the forms taken than 
it can for the protean changes of shape of an Amceba, 

By this I of course do not mean to imply that it is not possible, and 
perhaps probable, that the laws of surface tension do, within certain 
limits, modify the form and arrangement of cells, as maintained and 
discussed at length by Berthold. Wherever the arrangement demanded 
by the principle of least surfaces is not in conflict with other purposes of 
the organism, or, to put it upon a less teleological basis, where it is not 
in conflict with stronger influences than the force of surface tension, the 
cells probably accommodate themselves to the demands of that principle. 
The point of importance, however, is that this is not a decisive factor, 
but may at once be overcome when other influences in the organism 
antagonize it. 

Zimmerman (793) holds that the general arrangement of cells in 
accordance with the principle of least surfaces in plant tissues is not 


60 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


due to surface tension at all, but to turgor. Turgor, however, can 
hardly be a factor in the cleavage of the egg, where no increase in size is 
taking place. 

(2) Hertwig’s law of the spindle in the longest axis of the protoplasmic 
mass. (Compare page 5.) — This is, so far as I know, the only principle 
for which the claim is made, that it is the decisive factor in determining 
the direction of the spindle. The statement quoted above (page 5) is 
from Hertwig’s general text-book on the subject, in which it is presum- 
able that care would be taken not to mislead the reader unacquainted 
with the literature into taking a special phenomenon for a law of general 
import. On the same page it is stated that ‘‘ Mit diesen Regeln stimmen 
die Erscheinungen, wie sie bei der Zelltheilung und insbesondere bei der 
Eifurchung beobachtet werden, fast ausnahmslos iiberein.” (Hertwig, 
793, p. 175.) ‘The “law” has been accepted by others in the same gen- 
eral bearing. Thus Ziegler (94, p. 154) questions the validity of cases 
apparently not in agreement with the rule, holding that they are due 
either to inexact study (cylindrical epithelium), or to the difficulties of 
determining in the presence of a mass of yolk (amphibian egg) which ts 
the longest dimension of the protoplasmic mass. This principle has 
become the most widely known and generally accepted of any of the 
principles which have been proposed in regard to the determining factors 
in cell division. I shall therefore discuss it at somewhat greater length 
than Berthold’s principle, analyzing in detail the evidence on the subject 
from my own work, and reviewing that advanced by others. 

A comparison of the very first cleavage of Asplanchna with that of 
Callidina (Zelinka, ’91) shows that in the Rotifera the form of the egg is 
not the factor determining the position of the first cleavage spindle. 
For in the two forms the first cleavage spindle bears the same relation to 
the animal pole, or place of polar-cell formation, but a different relation 
to the long axis of the egg. In Asplanchna the spindle at the time of 
division lies in the long axis of the egg (though a little earlier it is 
oblique), whereas in Callidina (Zelinka,’91, Taf. I. Fig. 5) the spindle at 
division is oblique to both the longer and shorter axes, — the place of 
polar-cell formation not being the same as in Asplanchna, but much 
nearer one end of the egg. The orientation of the spindle in these two 
rotifers, then, is constant with reference to the animal pole, but variable 
with reference to the form of the egg. 

The passage from the eight- to the sixteen-cell stage in Asplanchna is 
particularly instructive. The asters of the six cells of quadrants A, B, 
and C first separate in such a way that the line joining them lies in the 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 61 


long axis of the cell (Plate 3, Fig. 17), then a rotation takes place 
(Fig. 18) by which the line joining the asters, i. e. the axis of the future 
spindle, is brought into the shortest axis of the cell (Figs. 19-24). The 
six spindles are then formed in the shortest axes of the cells (Figs. 20- 
28), and the planes of cleavage accordingly coincide with the long axes 
of the cells. 

The simple fact that there are divisions in which the spindles lie in 
the shortest axis of the cells is of course a direct contradiction of Hert- 
wig’s law. The case becomes even more striking, however, when the 
movements of the asters are taken into consideration. They at first lie 
in the position demanded by the law, but move from this position to 
that which directly contradicts the law. (See pages 25, 26.) 

Hertwig (93, p. 175) has cited a similar phenomenon, described by 
Auerbach, as proof of his law. Auerbach (’74) observed in the eggs of 
Ascaris nigrovenosa, at the time of fertilization, that the two pronuclei 
often come together in such a way that the plane separating them lies 
in the short axis of the egg. Since the axis of the first cleavage spindle 
commonly coincides with the plane separating the pronuclei, the result 
in the eggs of this species of Ascaris would be that the spindle would 
occupy the short axis of the egg. But the two pronuclei after meet- 
ing undergo a rotation through an angle of 90 degrees, thus bringing 
the spindle into the long axis of the egg. Ziegler has recently observed 
with even greater clearness the same phenomenon in the eggs and 
cleavage cells of other nematodes (Ziegler, 95, Taf. XVIII. Figs. 40-42), 
and in the eggs of echinoderms (’94). He observed in nematodes in 
some cases that the line joining the two asters on opposite sides of the 
nucleus lies in the short axis of the egg, and that then follows a rota- 
tion of the whole complex, till the line joining the asters—the axis of 
the forming spindle — occupies the longest axis of the egg. Ziegler, 
like Hertwig, has interpreted this change of position as a confirmation 
of Hertwig’s law, and the interpretation is certainly the most natural 
and apparently well grounded that could be given. 

Nevertheless we have in Asplanchna an entirely similar phenomenon, 
but occurring under such circumstances as to give a direct contradic- 
tion, instead of a confirmation, of Hertwig’s law. 

It is instructive also to notice that in the eight-cell stage of As- 
planchna, notwithstanding the great variety in the form of the cells, 
the direction of the cleavage spindles is the same in all the cells. Thus 
d** (Plate 2, Fig. 15), though irregular in shape, is of such a form 
that it is possible to be confident that the spindle does lie in the greatest 


62 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


dimension of the cell. The blastomere d*? (Plate 2, Figs. 14 and 16) 
is so exceedingly irregular in form, that it is impossible to determine 
with certainty whick is the longest axis. The cells a*?—c*? are irregu- 
larly triangular, and the direction in which the spindle lies is the short- 
est line connecting any apex with the middle of the opposite side (Plate 
3, Figs. 18, 19). Finally, the cells a*’- e*! are approximately rectan- 
gular in form, with one of the axes much longer than the other; the 
spindles lie in the shorter axes (Figs. 20-22). In every case the spindle, 
whatever the form of the cell, lies in a meridian connecting the animal 
pole with the vegetative pole of the egg. The only rational conclusion 
from this fact is, that the position of the spindles is determined by 
some factor unconnected with the form of the cells. ; 

The theory that the direction of the spindles is due to their taking a 
position of equilibrium determined by the mutual attraction of spindle 
and protoplasm, so strongly insisted upon by Ziegler (’94, p. 140), is 
likewise inconsistent with the movements of the asters in the eight-cell 
stage of Asplanchna. Ziegler holds that, since the greater mass of pro- 
toplasm must exercise the greater attraction, the spindle in the short 
axis of the cell is in a position of unstable equilibrium ; if by any 
cause it is moved in the slightest degree to one side or the other, it 
must inevitably swing into the long axis of the cell, where alone it can 
be in a position of stable equilibrium. This he holds to be the explana- 
tion of the movements of asters and nuclei observed by Auerbach in 
nematodes, and by himself in nematodes and echinoderms, as mentioned 
above. An oblique position of asters and nuclei, such as is shown in 
the cell a*1, Figure 22 (Plate 3), is intelligible on this assumption if 
the movement taking place is from the shorter toward the longer axis. 
But in this cell, as in the other five of the quadrants A, B, and C at 
this stage, the movement is from the longer axis toward the shorter. 
The hypothesis that the movement is due to simple attraction between 
the protoplasm and the fundament of the spindle, varying with the mass 
of the protoplasm, is totally inconsistent with such a notion. 

The passage from eight to sixteen cells is not the only cleavage in 
Asplanchna which is irreconcilable with Hertwig’s principle. At the 
transition from the sixteen-cell to the thirty-two-cell stage, there is a 
similar regularity in the position of the spindles coincident with variety 
in the form of the cells. The four dorsal cells a**—- d** divide equa- 
torially, three of them with spindles in the longer axis; one, d**, with 
the spindle in the shorter axis (Plate 4, Fig. 33). 

Again, in the sixth cleavage the cell d°* snows the same phenom- 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 63 


enon of rotation of the spindle into the shorter axis of the cell (Plate 5, 
Fig. 37, Plate 6, Fig. 46, and Plate 7, Figs. 53 and 54). There is the 
same regularity in the direction of cleavage as noticed in the preceding 
cleavages, although there is variation in the form of the cells. Thus, 
d°*, the companion cell of d**, whose division with spindle in the short 
axis has just been cited, cleaves with the spindle parallel with that in 
d°*, but owing to the form of the cell the spindle in d** occupies the 
long axis (Plate 7, Fig. 57, cleavage finished). Tue ceils of the ventral 
layer (Plate 6, Fig. 47) divide with spindles in the same direction as 
those in the cells of the second layer, although in the one layer the 
spindles must thereby occupy the short axes of the cells, in the other 
the longer axes. 

In the seventh cleavage a still more striking case occurs. The mid- 
dle of the embryo is surrounded by two rows of eight cells each, of 
precisely the same form and size, the dorsal row composed of the blasto- 
meres a™°— d™ and a™-*~d".8, the ventral row of a> —d™* and a™7 -—d7"7, 
These two rows are shown in Figure 61 (Plate 7), for the quadrants A, 
B, and C, and in Figure 57 for the quadrant D. In both rows, every 
cell but one in each row (d™7’ and d’*, Fig. 57) is strongly flattened 
dorso-veutrally, so that the lateral extent of the cell is much greater 
than the dorso-ventral extent. If the form of the cell determines in 
any way the direction of the spindle, it is certainly to be anticipated 
that the direction of cleavage will be the same for the cells of both 
rows. On the contrary, all the cells of the dorsal row divide meridi- 
onally, while all the cells of the ventral row divide equatorially. In every 
cell of the dorsal row, except possibly d*’, the spindle lies in the long 
axis of the cell; in every cell of the ventral row, except d@‘"’, the spindle 
lies in the short axis of the cell. The exception of a single cell in each 
row gives a finishing touch to the proof that the form of the cells is not 
the factor determining the direction of cleavage. The fact that the cell 
d’* divides with its spindle in the same direction as the spindles of all 
the cells in the same row, but not, as in the other cells, in the long axis 
of the cell, while d%’ likewise divides with its spindle in the same 
direction as those of the other cells of z#s row, but not, like them, in the 
short axis, demonstrates that the dorsal row is not so constituted that 
all the spindles must take their positions in the long axis, nor the ven- 
tral row so that all must take their positions in the short axis. It 
demonstrates, in other words, that the relative dimensions of the differ- 
ent axes of the cells does not determine the direction of the spindles in 
either one way or the other. (The cleavage of these two series of cells 


64 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


is shown for the quadrants A, B, and Cin Plate 8, Figs. 69 and 70; 
in d‘* in Plate 7, Fig. 58 ; in d’-> in Plate 8, Fig. 66; in d‘" and d™*, 
in Fig. 67.) 

Still other divisions in which the spindle lies in the short axis have 
been followed out in the descriptive portion of this paper. 

We must conclude, therefore, that a very large number of cell di- 
visions in the cleavage of Asplanchna directly contradict Hertwig’s law 
that the spindle during division comes to lie in such a position that its 
axis coincides with the greatest axis of the protoplasmic mass. The 
characteristic feature of the cleavage is regularity in the direction of the 
spindles, coupled with great variation in the form of the cells, thus 
excluding any close relation between these phenomena. 

What is the evidence upon which this law has been based ? 

It is chiefly experimental, though there is likewise a certain amount 
of evidence based upon the observation of normal cleavage. 

Let us consider first the evidence derived from experiment. The ex- 
perimental studies of Pfliiger (84), Roux (’85), Driesch (92), Hertwig 
('93"), Born (95 and ’94), Ryder (793), and Ziegler (’94), on the effects 
of pressure upon the direction of the spindle, are so well known that it is 
not necessary to review them in detail. It is sufficient to state the general 
result. With rare exceptions it has been found that when the egg or 
the cleavage cell is so modified in form that one of the axes which may 
be passed through its protoplasmic mass is distinctly greater than the 
others, the spindle at cleavage comes to lie in this axis. I do not pro- 
pose to enter upon an analysis of these experiments, nor to attempt 
to explain in any different manner the results gained. A study of the 
works of the authors above cited, and a repetition of the pressure ex- 
periments upon the eggs of the toad (Bufo lentiginosus Shaw) during 
the spring of 1895, have convinced me that the explanation commonly 
given is the one most in agreement with the conditions, and, from the 
evidence, most probably correct, for these cases. But whatever we may 
hold as to the validity of the explanation for these cases, we know that 
the principle upon which it is based cannot be generalized, since in 
many other cases it is directly contradicted by the facts. Before sug- 
gesting how the experimental results may perhaps be reconciled with 
the apparently contradictory phenomena observed in other cases, it will 
be necessary to consider the evidence gained by other means, as well as 
such experimental evidence as is against the principle. 

First, then, we have the fact that the experimental evidence itself is 
not concordant upon this point. Roux (’85) found that under certain 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 65 


circumstances in deformed frogs’ eggs the first cleavage plane some- 
times, though rarely (Roux, ’94*, p. 274), passed through the greater 
axis of the cell, the spindle therefore lying in the shorter axis. Hycles- 
hymer (795) experimented as to the effects of pressure on the eggs of 
Amblystoma tigrinum, and found that when the eggs were compressed 
laterally to one half their normal equatorial diameter, there was little or 
no relation between the direction of cleavage planes and the greater or 
less dimensions of the egg. ‘The first vertical in the thirty-four eggs 
examined showed no constant relation to the compressed surfaces, in 
seven passing through the longest equatorial diameter ; in nine through 
the shortest ; in eighteen between the two.” (Kycleshymer, ’95, p. 353.) 
In experiments of a different nature, Morgan (’95) observed that the 
shaken eggs of Sphzerechinus often divide at the first cleavage into 
three equal blastomeres, and in such cases at the next cleavage the 
three spindles lie in the short axes of the cells. 

When we turn to the evidence from observation of normal cell 
division, there is the same disagreement that is met with in the experi- 
mental evidence. Until within a few years investigators in zodlogical 
lines have not paid attention to the exact relations of the spindle to the 
cell axes, so that little was to be found in the literature to emphasize 
the necessity of caution in accepting at once the generalizations from 
the first experimental results. In cases where cells containing spindles 
were figured, there was generally an apparent agreement with the 
principle that the spindles are in the long axis, but so long as it was not 
determined by observation whether this elongation of the cell was a 
consequence of the position of the spindle or a cause of it, the evidence 
was worthless, as pointed out by Heidenhain (94), and as clearly illus- 
trated in the preceding pages. 

In botanical literature the case was somewhat different, and seven 
years before Hertwig (’93) had stated that the phenomena observed in 
cell division agreed “fast ausnahmslos” with his law, Berthold (’86, 
p- 230), in his thorough and comprehensive work on the subject, had said : 
“Sehr oft theilen sich prismatische oder cylindrische Zellen der Liinge 
nach, wenn das Prinzip [of least surfaces] eine Querwand, der Quere 
nach, wenn es eine Lingswand verlangte. So theilen sich oft die Mark- 
zellen, die Zellen der Grundparenchyms sich entwickelnder Blatter 
nur quer, obwohl thre Hohe im Vergleich zur Breite nur gering ist” 
[Italics mine]. He had also given many examples of the conditions 
thus characterized. 


Recently the attention of zodlogists has been directed to a careful 
VOL. XXxx.— No. 1. 5 


66 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


observation of these relations, and a number of facts are now at hand 
which bear upon the subject. 

Ziegler (95) has studied the cleavage of certain nematodes (Diplo- 
gaster longicauda, Rhabdites teres, and Rhabditis nigrovenosa) with 
especial reference to the relations of cell form to the direction of cleay- 
age, and finds that the conditions in these cases confirm throughout the 
law of Hertwig. In the normal cleavage the spindle always places 
itself in the long axis of the cell, even though a rotation of the nucleus 
and asters from their first position is often necessary to accomplish this 
result. And in cases where the egg is deformed by some outer agent, 
as pressure by the walls of the oviduct or the like, the normal] position 
of the spindle is changed to agree with the changed form of the cells, 
the spindle lying in the long axis in every case. The agreement with 
Hertwig’s law is complete. 

On the other hand, zur Strassen (’95 and ’96) has studied the cleav- 
age of another nematode, Ascaris megalocephala, with the same ques- 
tions in mind, and has come to opposite results. In the two-cell stage 
one cell divides with the spindle in the long axis, the other with the 
spindle in the short axis, and in later stages a similar independence of the 
position of the spindle and relative dimensions of the cellisshown. Zur 
Strassen (’95, p. 86) concludes: “Ich halte vielmehr den Kern fiir be- 
fihigt, vermége ihm inhaerenter Eigenschaften eine gewollte Theilungs- 
richtung herbeizufiihren, selbst wenn mechanische Hindernisse von nicht 
unbedentender Héhe dem entgegenstehen.” 

Other observations bearing upon this question are much scattered. 
Cases are not uncommon in which authors have figured spindles in the 
shorter axes of the cells, but unless the observer’s attention has been 
especially directed to the question, such figures are of little value, since 
a slight change in the position from which the cell is viewed produces a 
foreshortening which gives very different apparent relative dimensions to 
the axes. Heidenhain (’95, p. 553) gives a number of such cases, from 
most of which the evidence is weakened by this consideration. How- 
ever, the case mentioned by Heidenhain of the germinal disk of the 
cephalopod egg as figured by Watase (91) cannot be explained away 
upon this ground. 

Some other instances may be mentioned. 

Tn the formation of the “germ bands” in the Polychet Amphitrite, 
according to Mead (’94, p. 467), “‘the axes of the spindles in these 
divisions lie in the shortest diameter of the cells, and apparently in the 
direction of greatest pressure.” 


- 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 67 


Wheeler (’95, p. 309) states in regard to the first cleavage spindle of 
Myzostoma: “In Myzostoma the spindle does not conform to O. Hert- 
wig’s law, but always lies at right angles to the long axis of the often 
very narrow protoplasmic pillar of the egg.” 

Castle (96, p. 250) states that in the division of the entoderm cells 
of Ciona the spindles in certain cases lie in the short axes of the cells, 
even when a shifting of the asters from a previous position in the long 
axis must have occurred to bring about this condition. Castle states 
that no mechanical explanation of this phenomenon offers itself, though 
he holds that “ other things being equal, it is true that the spindle arises 
in the longest axis of the cell” (p. 231, note). 

In the decapod crustacean Virbius, according to Gorham (’95), the 
ege is ellipsoidal in form, and the first cleavage spindle may occupy the 
long axis, or be more or less inclined to it, or may even be nearly at 
right angles to it. 

The cell divisions in the germ bands of Crustacea as described by 
Bergh (795), in which the spindles take the same direction for many 
cell generations, should be added here (see also MeMurrich, 95); though 
the evidence from these must be weakened in the eyes of the upholders 
of Hertwig’s law by the fact that before division the cells “ wachsen nur 
in der Weise, das ihre Langsdurchmesser dem Querdurchmesser ziemlich 
gleich wird und dann tritt die Theilung ein.” 

The positive evidence upon the question from observations of normal 
cell division is thus rather scanty, though doubtless some additions 
might be made by a further study of the literature. 

From both experimental evidence and observation of normal division 
the only conclusion possible is, that in some organisms the spindle does 
take a position in the greatest axis of the protoplasmic mass, apparently 
without regard to other factors, while in other cases the position of the 
spindle is determined by other factors, without regard to the form of 
the cell. 

The result is at first thought not very satisfactory, but this is not the 
only organic phenomenon with regard to which such a conclusion must 
be drawn. <A few examples will make this clear. 

Stahl (’85) found that the direction of the first cleavage in the spores 
of Equisetum is determined by the direction of the infalling rays of light. 
A general statement of the effect of the direction of the light rays on cell 
division would take a form similar to the unsatisfactory conclusion above 
given for the shape of the cells. 

If we leave cleavage and take up other growth phenomena, such as 


68 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


the direction of growth of plants and animals, such a conclusion would 
be reached with regard to almost every agent that ever exercises a 
determining power. “he effect of light is to cause certain parts to grow 
towards it, others to grow away from it; while in other organisms the 
direction of growth is not affected by it at all. The same is true of 
gravity, of heat, and of various chemical and physical agents. (See the 
extended list of such cases given by Herbst, 94.) In all these cases 
we are dealing with reactions to stimuli. It is only when we attempt to 
make one of these agents the only determining factor, and expect to see 
it act always in the same way, as a simple mechanical cause, that our 
result becomes unsatisfactory. 

It is evident that in this question of the relation of the form of cells 
to the direction of cleavage, we are dealing with a problem of a nature ~ 
similar to those which I have briefly stated above. Some organisms are 
so constituted as to react to rays of light by growing toward them, 
others are not. In the same way, some cells or nuclei are so constituted 
as to react to the influences determining form by bringing the spindle 
into the longest axis of the protoplasmic mass; others are not. In each 
case the result is due to a reaction to stimulus, or to an action of similar 
nature, and not to a simple mechanical action of the agent. 

In almost or quite all cases of reaction to stimuli, the result may be 
shown to be the accomplishment of a certain end that is of importance 
for the existence of the organism. The immediate explanation always 
takes a teleological form. It is not difficult to perceive a teleological 
aspect of this tendency of many cells to divide with the spindles in the 
longer axes. In cases where the purpose of cell division is merely to 
double the number of cells without alteration of form, division with the 
spindle in the longer axis is obviously the simplest method. A con- 
sideration of what would be the result if the opposite method prevailed 
shows this clearly. By continued division of a cubical cell and its 
products with the spindle in the shortest axis, a series of flat plates 
would be produced ; every cleavage would bring about a greater modifica- 
tion in the form of the resulting cells, and cells of such form would 
undoubtedly be very inconvenient for the purposes of the organism. 
Continued division with the spindles in the long axis would result, on the 
other hand, in the production of cells of the same form as the parent 
cell. It is not remarkable, therefore, if in many cases the cells are so 
constructed as to respond to a change of form by a corresponding change 
of position of the spindle, so that the resulting cells shall be as nearly as 
possible of the form of the parent. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 69 


But mere increase of the number of cells without change of form is 
not the only object which can be brought about by cell division, and 
when other purposes are to be accomplished the cells are so constituted 
as to react in a different manner. Thus, I have shown that in Asplanchna 
divisions of cells with spindles in the short axis is the method by which 
is brought about the continued extension of cells in one direction, with 
consequent gastrulation and a later invagination of ectodermal cells. 
In Amphitrite, where, according to Mead, the division in the germ 
bands is with spindles in the short axes, this method of division is per- 
haps necessary to bring about the elongation of the germ bands, and the 
same is doubtless true in the germ bands of Crustacea. It is of course 
not necessary, nor is it probable, that in these cases the position of the 
spindle in the short axis is a reaction to the form of the cell; more 
probably the position of the spindle is determined without reference to 
this, as I have endeavored to show for the cells of Asplanchna. 

The conclusion to which I have come is therefore similar to that 
maintained by Braem (’94), except that he seems to imply that the pur- 
pose of the cleaving cells is always the same, viz. to gain free space for 
the development of the products of division, whereas it appears to me 
that the facts indicate that the ends to be accomplished may be various, 
and the means by which they are brought about equally varied. This 
brings us to a nearer consideration of Braem’s view. 

(3) Braem’s theory of least resistance. (Compare page 5.)— As just 
stated, so far as Braem’s view is teleological, I must agree with him; but 
in so far as he seems to restrict his teleology to the accomplishment of a 
single purpose, — the attainment of the freest space for development, — 
it seems to me that the facts are against him. Certainly the principle 
of “least resistance” does not aid in understanding the cleavage of 
Asplanchna, where a large number of the divisions take place in what 
must be considered the direction of greatest pressure. Examination of 
the figures will show that, as a rule, the blastomeres in the resting 
Stages are much flattened dorso-ventrally and extended laterally, as if 
subjected to great pressure ; nevertheless, as shown in detail in the de- 
Scriptive portion and in the discussion of Hertwig’s law, when division 
takes place it is very frequently with the spindles in the dorso-ventral 
axes. The cleavage in this direction seems to have a purpose, but that 
purpose is not the gaining of the freest space for the development of the 
products of division, but the accomplishment of the process of gastrulation. 

' Since Braem’s principle is confessedly teleological, it was probably not 
intended to be rigidly applicable to all cases; indeed, the author states 


70 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


as much. But in view of the frequent departures from the principle of 
least resistance, it appears necessary that the object of any given method 
of cleavage should be Judged in each case for itself; so that Braem’s con- 
tention that his principle is fitted “den Verlauf der normalen Furchung 
in wesentlichen Punkten zu erklaéren ” must be considered unsuccessful. 
It may, by calling attention to one method in which cells react to a 
stimulus, “explain” the cleavage of some cells in the same sense that 
the growth of the stem and root of a plant may be said to be ‘ explained” 
by saying that the protoplasm of one is so constituted as to react to light 
by growing toward it, the other, on the contrary, so as to react by grow- 
ing away from it. But in other organisms the determining stimuli are 
of an entirely different nature, and the “explanation” must be sought 
anew for every organism. 

I am of course fully aware that the view here put forth, that the 
position of the spindle must be interpreted teleologically, and in many 
cases as a reaction to stimulus, is not an “explanation.” But I see no 
a priort ground for expecting a simple mechanical explanation for the 
direction of cell division, any more than we should of the direction of 
growth of a plant. Asa matter of fact, the phenomena show that such 
an explanation is not at present possible for either. 

(4) Roux’s theory of a compromise between the tendency immanent in 
the nucleus and the tendency due to the form of the protoplasmic mass. 
(Compare page 6.) — As remarked above, this theory is not definite, in 


o, inasmuch as one of its factors — 


the same sense as the three foregoing, 


the immanent tendency of the nucleus—is of an entirely unknown 
character. From the foregoing description and discussion it is evident 
that I must agree fully with this conception. The further question 
comes, In how far do the phenomena in Asplanchna lead to a recogni- 
tion of the second factor, —the tendency due to the form of the cells? 
It is evident that the form of the cell does not determine the main fea- 
tures of the direction of division,—the question as to whether the 
spindle shall be dorso-ventral or lateral in direction. But are there sub- 
ordinate features in which the form of the cell does affect the position 
of the spindle, as held by Roux? In other words, does the spindle 
always lie in either the long or the short axis of the cell, and never 
oblique to both ? 

An examination of the figures will show that in the large majority of 
cases the latter question is to be answered affirmatively. In the earlier 
stages, before the great changes in the positions of cells have occurred, the 
dorso-ventral axis of the egg commonly coincides with either the greater 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKILI. 71 


or the lesser axis of the cells, so that from these cases no evidence on 
our question can be gained. But in later stages the crowding together 
of the cells results in greater or less alteration of the cell axes in rela- 
tion to the axes of the egg. In Figure 68 (Plate 8), for example, the 
long axis of the cell d*-" is not parallel with that of d*; the two are 
wedged apart dorsally by the cell d*-", so that neither one points 
exactly to the animal pole of the egg. The two spindles are likewise at 
an angle with each other, and lie in the long axes of the cells, instead of 
exactly dorso-ventral. The same is true of the cells ¢”* and ce’? in the 
same figure. Again, in Figure 74 (Plate 9) the spindles of d*" and 
da, though in general direction dorso-ventral, form an angle with each 
other, the one in d*" being modified in position so as to lie in the 
longer axis of the cell, while the one in d**! lies in the shorter axis of 
the cell. In the entoderm the same thing is strikingly true of the 
spindles shown in Plate 9, Fig. 79, and Plate 10, Fig. 83, for there the 
spindles form various angles with one another, all lying in the longer 
axes of the cells. 

But these cases do not necessarily lead to the view that the form of 
the cell modifies even slightly the position of the spindle. It is 
possible that in each cell the axis of the spindle is determined other- 
wise, so that alteration of the position (not form) of the cell necessarily 
produces an alteration in the direction of the spindle. Thus in Figure 
68 (Plate 8), if the cells d* and d*” are ellipsoids of fixed form in 
which the two ends of the spindles have predetermined positions, in case 
the cells are forced apart, as in this instance by d*”, the ends of the 
spindles will be forced apart to the same degree. Though we know that 
the cells are not ellipsoids “of fixed form,” yet we also know that the 
form of the cell is greatly influenced by the direction of the contained 
spindle. It is possible that the cytoplasm of the cell tends to group 
itself symmetrically about the contained spindle, so that the direction 
of the spindle is the primary factor, the fact that it lies symmetrically 
in one of the axes of the cytoplasmic mass being a secondary result. 
This becomes very probable when we examine from this standpoint the 
change. of form of the very irregular cell d*%, shown in Plate 5, Fig. 
37 (surface view), and Fig. 38 (section), before the formation of the 
spindle, and in Plate 7, Fig. 54 (surface), and Plate 6, Fig. 48 (section), 
after the formation of the spindle. In this cell, before the forma- 
tion of the spindle, the shape is so irregular that it is not possible 
to distinguish a definite “short axis,” and no plane would divide the cell 
into symmetrical halves. But as the spindle is formed, the cytoplasm 


72 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


groups itself symmetrically about it, until just before division the 
spindle lies in the shorter axis of the cell, and a plane including the 
long axis of the spindle and the centre of the egg would divide the cell 
into symmetrical halves. 

If this be the true explanation, then the elongation of the cells in 
Plate 10, Fig. 83, in various directions, is due to the previous altera-— 
tion of the predetermined spindle axes. And certainly the elongation 
is not greater than is naturally the result of the position of the spindle, 
as has been shown for other cells. 

This view is strengthened by the fact that in certain cases, where 
apparently it is impossible for the cytoplasm to group itself symmetri- 
cally about the spindle, the latter takes a position which is oblique to the 
axes of the cell. Such a case is shown in the entodermic cell d™ of 
Figure 64 (Plate 8); the spindle lies in neither the longest nor the 
shortest axis of the cell, but oblique to both. 

It must be said, therefore, that the cleavage of Asplanchna gives no 
positive evidence of any effect of the form of the cell upon the position 
of the spindle ; on the contrary, the evidence on the whole is decidedly 
against it. Zhis, of course, cannot be generalized and applied to other 
organisms, as on the other hand the observed conditions in other organ- 
isms are not capable of giving generalizations which must hold for 
Asplanchna. Generalizations in the field of reaction to stimuli, which 
includes a very large proportion of organic activities, are exceedingly 
unsafe and are justifiable only after exhaustive examination of the phe- 
nomena; but in these matters scarcely more than a beginning of such 
an examination has been made. 

(5) Heidenhain’s problem of a definite angle of rotation. (Compare 
page 6.) — This certainly cannot be held to have any especial signifi- 
cance for the cleavage of Asplanchna; it is proposed by Heidenhain as 
applicable to systems of tissues in the later stages of organisms. I may 
point out some facts which bear upon the “ problem.” 

In the cleavage of Asplanchna, the angle which a given spindle makes 
with the preceding spindle may be either 0 or 90 degrees; but it is gen- 
erally either one or the other, not some angle intermediate between 
these. 

The time and method of rotation, when rotation occurs, cannot be 
said to be “ gesetzmissig.” Commonly the asters come to lie on oppo- 
site sides of the nucleus before the rotation begins, but in the cells d*? 
(Fig. 42, Plate 5) and d** (Fig. 37) the change of position begins at 
the same time with the division of the asters, so that by a separation of 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. te 


the asters accompanied by motions not in a straight line, the same 
result is attained that would otherwise be produced by a rotation after 
the asters were fully separated. ; 

In the larger cell at the two-cell stage, the change of position of the 
nucleus and spindle is not due to a rotation after the two asters are 
formed, but to a change of position of both nucleus and aster before the 
aster has begun to divide. 

In d*? (Plate 1, Fig. 7) the asters and nucleus at first rotate into a 
position which is not the final one, so that subsequently rotation in 
a different manner is necessary to bring the spindle into its definitive 
position (Plate 2, Fig. 16). 

In d** (Plate 5, Fig. 37, Plate 6, Fig. 46, Plate 7, Figs. 53 and 54) 
the definitive position taken by the spindle is at right angles to that of 
the preceding spindle, so that the simplest method of formation would 
be the natural separation at right angles to the axis of the preceding 
spindle, as commonly takes place in such cases. But, owing appar- 
ently to the peculiar form of the cell (Plate 5, Figs. 37 and 38, surface 
and section), the asters separate obliquely, taking up a position such 
that the line uniting them is parallel to the axis of the preceding 
spindle ; and the definitive position is reached only by a later rotation 
through an angle of 90 degrees (Plate 7, Figs. 53 and 54). 

There is thus no regularity about the method by which the asters come 
to occupy their definitive positions at the ends of the spindles. Appar- 
ently the early position of the asters 1s influenced or determined by the 
mechanical conditions within the cell, whereas the later position of the 
spindle is largely independent of such conditions. 

On the other hand, in the divisions of the ectoderm of Asplanchna, 
there zs a regularity in the final angle between the axes of two succes- 
sive spindles in any given “ layer” of cells, the second spindle being in 
all the cells of a given layer either parallel with or else at right angles 
to the preceding spindle. It is thus quite possible that there may be 
whole systems of tissues where there is such a regularity. But the fact 
that the definitive position may be reached by such various means 
renders the phenomenon of little significance for a mechanical theory 
such as that presented by Heidenhain. 

(6) Sachs’s view that the walls separating the cells meet one another at 
right angles.— An examination of the figures, especially the sections, 
shows that the condition above stated is not generally complied with in 
the cleaving cells of Asplanchna, so that it is not necessary to enter upon 
a discussion of this view. Neither is the regular alternation of spindles 


74 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


at right angles to one another —a theory often attributed to Sach — the 
rule in Asplanchna. 

(7) Rauber’s theory, that the asters of the different blastomeres exercise an 
attraction for one another, thus influencing the direction of the spindles. — 
There appears to be nothing in the cleavage of Asplanchna that would 
lead to such a conclusion as the above. Any attempt to explain the 
relative positions of asters in the different cells — in such a case, for ex- 
ample, as is shown in Figure 39 (Plate 5) — by the theory of attractions 
and repulsions among the asters, will be found to lead to purely arbitrary 
assumptions as to which asters exercise attractions on others and which 
do not; any general statement of a positive nature will be found to be 
inconsistent with the facts. Moreover, the irregularities in the migrations 
of the asters, to which attention was especially called in the discussion of 
Heidenhain’s “ Problem of a regular angle of rotation,” can with difficulty 
be harmonized with such a theory. 

(8) Braem’s principle of equal resistance at the two ends of the spindle. 
—This principle is discussed in connection with the question which 
immediately follows. 


B. What determines the Equality or Inequality of Cleavage Products? 
C. What determines the Rate of Cleavage ? 


Owing to the similarity of the factors supposed to determine the 
relative size of the cleavage products and the time of cleavage, it will be 
well to consider together the foregoing questions. 

In regard to the first question, we have (1) the theory of Hertwig, 
stated on page 8, that the spindle tends to take a position in the middle 
of the mass of protoplasm in which it is contained, and (2) the “ principle 
of like resistance at the two ends of the spindle,” set forth by Braem, 
and stated on page 7. Since these two theories lead to similar results, 
they may be considered together. 

It certainly does not aid in understanding the cleavage of Asplanchna 
to assume that “der Kern von vornherein das Bestreben hat, sich 
gleichmissig nach beiden Seiten hin auszudehnen und somit auf eine 
aquale Zelltheilung hinzuwirken ” (Braem, 94, p. 345), nor that “der 
Kern stets die Mitte seiner Wirkungssphire einzunehmen sucht” 
(Hertwig, 93, p. 172). No differentiation into more and less yolk 
bearing regions is visible. Yet the first cleavage is unequal, and the 
second equal in one cell and unequal in the other; the third likewise 
shows both methods, with a preponderance of equal divisions; and in 
the fourth all the divisions are again unequal. In no cleavage is there 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 75 


throughout an equality in the size of the two products in all the cells. 
The two assumptions above stated are strikingly contradicted, not only 
in the formation of the polar cell (Fig. 1), but also in the fifth (Fig. 
35, Plate 5) and sixth (Fig. 49, Plate 6) cleavages of the entodermal 
cell (d*! and d*1), and in the division of the entodermal cell d*? (Fig. 
80, Plate 10). In these cases the spindle moves against one side of th 
cell, and there a small vesicle is produced, the two products of cleavage 
being excessively unequal. The contradiction is emphasized by the fact 
that in the fifth and sixth cleavages of the entodermal cell (Figs. 35, 
48, and 49), although there is a concentration of yolk in one region of 
the cell, it bears no significant relation to the position of the spindle, and 
by the further remarkable fact that in the next or seventh cleavage 
(Figs. 64 and 65, Plate 8), this yolk cloud passes into the smaller 
blastomere. 

When one considers the method of formation of polar cell in all eggs, 
and the almost unlimited range in the comparative size of cells resulting 
from cleavage in a great number of organisms, the grounds for the 
generalization, that “der Kern stets die Mitte seiner Wirkungssphire 
einzunehmen sucht,” are difficult to comprehend, at least so far as they 
relate to the dividing nucleus. As a statement of fact for specific cases, 
with the word ‘“‘stets”’ omitted, I have no fault to find with it; but asa 
generalization it is, so far as it relates to the dividing nucleus, meaningless. 
The position of the spindle within the cell must be considered to be re- 
lated to the purpose of the ensuing division. It is of course probable 
that in many cases the primary purpose is to divide the formative proto- 
plasm equally between the two products, and this may determine the 
position of the spindle in, for example, the first, second, and third 
cleavages of the frog’s egg. But what determines the position of the 
spindle in the two divisions immediately preceding these, — in the forma- 
tion of polar cells? It seems to have been generally overlooked, even by 
those who have pointed out that polar-cell formation is cell division, that 
the formation of polar cells must be reckoned with in any general theory 
of cell division. 

An examination of the cleavage of many invertebrates shows that, as 
in Asplanchna, equal cleavage is no more the rule than unequal cleavage, 
even where there is no corresponding differentiation into yolk bearing 
and purely protoplasmic regions. The statement that the dividing 
nucleus tends to take a position in the middle of its sphere of action is 
true, if we consider the “‘ middle of its sphere of action” to be, as it 
actually is, the point where it divides. But with this interpretation the 


76 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


statement is of course utterly without significance. The real question 
is, What determines the point where it divides? Why in Asplanchna 
does the middle of the sphere of action in the fourth and seventh 
cleavages of the large ventral entoderm cell lie in such a position (Fig. 
16, Plate 2, and Fig. 65, Plate 8) as to divide the cell into parts which 
are only slightly unequal, while in the intervening fifth and sixth 
cleavages (Fig. 35, Plate 5, and Fig. 49, Plate 6) the middle of the 
sphere of action is at the periphery of the cell? Why at the first 
cleavage of the frog’s egg is the middle of the sphere of action in the 
centre of the mass of formative protoplasm, whereas at the divisions 
immediately preceding —the formation of the polar cells —it is at the 
periphery of the egg ? 

In regard to the question what determines the comparative rate of 
cleavage the case is similar. In view of the recent discussion of these 
two questions — the inequality of cleavage and the rate of cleavage — in 
the works of Kofoid (94, p. 196), MeMurrich (’95), Lillie (95, p. 45), 
zur Strassen (95), Ziegler (’95), and others, it seems scarcely worth 
while to insist upon the fact that the rate of cleavage and the equality 
or inequality of the products are related to the future morphogenetic 
processes, and show in many cases no relation whatever to accumulations 
of yolk. Yet the so called laws, according to which these matters are 
determined by the distribution of yolk, are repeated in O. Hertwig’s 
text-book on the cell (Hertwig, 93, pp. 174 and 180), and reaffirmed in 
the latest edition of his treatise on embryology (Hertwig, ’96, pp. 67 
and 84). 

In Asplanchna the main facts in regard to the sequence of cléavage 
are as follows. 

(1) The order of cleavage is, within very narrow limits, constant. If 
a number of eggs are taken showing successive stages of the division of 
any given cell, the series will show corresponding successive stages in 
the nuclear history of the other cells. 

(2) There is a typical order for similar cells of different quadrants of 
the same egg. This order is D, C, B, A. 

(3) There is a marked general tendency for larger cells to divide 
first. In every case where two cells of the same origin are of different 
size, the larger divides before the smaller. : 

This is a very general phenomenon, as has been repeatedly pointed 
out of late, even in cases where the larger cell contains the greater 
amount of yolk. The “law” recently formulated by Hertwig (96, 
p. 67), that “die Schnelligkeit der Furchung proportional ist der Con- 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKITI. 77 


centration des im Theilungsstiick befindlichen Protoplasmas,” must be 
considered —like the corresponding one in regard to the equality or 
inequality of cleavage products—as an example of the immature 
generalizations to which embryology has fallen heir through the acci- 
dental circumstance that the amphibian egg was for a long time the chief 
object for the study of cell division. It seems to be exceedingly diffi- 
cult to grasp the fact, every day becoming more evident, that because a 
statement is true for the eggs of some of the lower vertebrates it does 
not follow that it must be true for the cells of all organisms. 

(4) The rhythm of cleavage has an important relation to the other 
processes of morphogenesis. If cleavage took place coincidently in all 
the cells of the egg, the latter still retaining its form, there could 
apparently be no “ rotation” of the cells upon one another, and conse- 
quently no gastrulation. The tension in all directions would be the 
same; none of the cells would be moulded to fit the extension of 
neighboring cells, and all would retain approximately the positions held 
at the beginning. 

It would perhaps be possible to carry this into detail, and show that 
the earlier cleavage of the large ventral cell, leaving it in a resting 
condition, and therefore plastve (as indicated by a comparative study of 
the forms taken by resting and by dividing cells) when the other cells 
divide and extend, is directly favorable to gastrulation. 

The question as to the factors determining the time and the equality 
or inequality of cleavage, it will be seen, does not at present admit of 
any direct simple answer. Cleavage is a part of the process of morpho- 
genesis, and its rhythm and other features are related to the nature of 
the form to be produced. 


D. As to Differentiation accompanying Cleavage. 


The facts bearing upon this question to be derived from the observa- 
tion of the early development of Asplanchna are few in number. The 
principal phenomenon to which attention must be directed is that of the 
segregation and migration of the cloud of granules, described on pages 
25, 30, 37, and 54. As will be remembered, a concentration of granules 
begins in the ventral region of one of the cells at the eight-cell stage 
(Fig. 7) and becomes more and more marked as successive cleavages 
take place, till a well defined cloud of very large and distinct spherules 
occupies the anterior and ventral margin of the cell d*1 in the stages 
immediately succeeding the sixteen-cell condition (Fig. 32, Plate 4, and 
Fig. 48, Plate 6). Then occur the remarkable migrations shown in 


78 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Figures 49, 50, 51, and 64 (Plates 6 and 8), until the cloud of granules 
is enclosed at the seventh cleavage in the smaller of the two eutoderm 
cells (Fig. 65). This whole process shows clearly that other changes 
of astriking chacacter are taking place at the same time as the division 
of the egg into smaller portions; evidently cleavage is not a mere 
separation of the egg into smaller masses, each similar to the other and 
to the original egg. The final destiny of this granular mass is not 
known, but such a peculiar and well characterized phenomenon as it 
exhibits cannot be considered meaningless. 

The differentiation in this instance is of the kind admitted by Driesch 
(see page 9), in that it is cytoplasmic in nature. It is not, however, 
a direct consequence of the original distribution of materials within the 
egg; the migrations of the granules show that processes are taking 
place in the cytoplasm that are only indirectly connected with cell 
division. 

We have in this case a distinctly visible differentiation accompanying 
cleavage. Certain other phenomena give evidence that there are like- 
wise invisible differentiations accompanying the process. 

At the division of the second “layer” of ectodermal cells in the 
sixth cleavage, shown in Figure 55 (Plate 7), the two rows of cells a’7>- 
cS g™-6_ ¢7-8 gi-7_—¢l-7, and a™®—c™® are produced. The cells of these 
two rows, as shown in Figure 61, are of the same size and the same 
form, having similar relations to the surrounding cells and to the axis of 
the embryo. Yet, as has been repeatedly stated, all the cells of one row 
divide meridionally and equally with spindles in the long axes, the cells 
of the other row equatorially and unequally with spindles in the short 
axes. What causes this difference ? 

The difference must, of course, be due either to a different stimu- 
lus from the outside, or to a different structure of the cells. The 
problem may be expressed clearly in this way: If one of the cells of the 
more dorsal row, as a7* (Fig. 61), could be removed and placed in 
the position now occupied by a™’, in the more ventral row, would it 
change its method of division? That is, would it cleave equatorially 
and unequally, with its spindle in the short axis, like the other cells of 
the ventral row, instead of meridionally and equally, with its spindle in 
the long axis, as it actually does? 

There is, of course, no way of answering this question directly. It 
scarcely appears probable, however, that there is such a difference in the 
influences affecting the two cells as to cause so fundamental a difference 
in the cleavage. And if there is not, the only alternative is, that there 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 79 


was a qualitative division, nuclear or cytoplasmic, at the preceding 
cleavage. 

This conclusion is, of course, speculative, but the history of the 
cloud of granules directly proves that in Asplanchna the cleavage is 
accompanied by differentiation. 

The recent experimental evidence, showing that in certain organisms 
the cleaving cells in early stages all possess the inherent capacity to 
produce an entire animal, has led to a rather widespread impression 
that cleavage has been shown to be a process of little or no significance, 
being merely a quantitative division of a mass into smaller masses of a 
similar nature. This view apparently receives confirmation from gen- 
eralized statements of the results of such experiments; for example, 
the following from Driesch (’94, p. 69): “Es liegt also nach allem 
gesagten in der That kein Grund vor, in der Furchung etwas anderes als 
reine Zellteilung zu sehen ; ja die Gleichheit der Furchungskerne ist 
direkt durch Versuche bewiesen.” A summary of the evidence which 
has been adduced in regard to this matter shows that such a statement 
as the above conveys only a small part of the truth and must lead to 
error unless carefully interpreted. The evidence that cleavage is accom- 
panied by differentiation may be summarized as follows. 

(1) It is directly proved by observation that in certain cases the nu- 
eleus differs in structure in different blastomeres in early cleavage stages, 
and that this differentiation is correlated with a different fate of the differ- 
ing cells. This Boveri has shown for Ascaris megalocephala (Boveri, ’94), 
and Meyer (795) for certain other nematodes. 

(2) It is directly proved by observation that in certain cases the cyto- 
plasm of the different cleavage cells in early stages is of a different 
structure, reacting differently to chemical reagents, and this difference 
ts correlated with a different fate of the different cells. Thus, in the 
sixteen-cell stage of Nereis, “the somatoblast can always be recognized 
at a glance” from its different color (Wilson, ’92, p. 390). A similar fact 
has been shown above in regard to the cytoplasmic differentiation in 
Asplanchna, but here I have not been able to determine the fate of the 
cell which receives the differentiated granules. 

(3) It has been shown that in many cases the different blastomeres of 
early cleavage stages give rise to definite structures in the adult. This 
fact in itself of course admits of two interpretations, but taken in con- 
nection with the facts stated under (1) and (2) it becomes of great 
significance. ; 

(4) It has been shown experimentally, that in some organisms sepa- 


80 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


rated blastomeres give origin to parts of the embryo only. The most 
complete and satisfactory cases are those of ctenophores as described by 
Chun (’92) and confirmed by Driesch und Morgan (95), and of the 
gasteropod Ilyanassa, by Crampton (96). 

It is difficult to conceive how a more complete demonstration could be 
possible, that cleavage is accompanied in many cases by differentiations 
which are not ‘expressed by the phrase “reine Zellteilung,” and that 
these differentiations are of the utmost significance for the future 
development of the organism. Any amount of evidence that in other 
cases there is no differentiation cannot in the least shake confidence in 
this demonstration. 


2. GASTRULATION. 


In addition to the problems bearing directly upon cleavage, the plan 
of the present work included a study of some of the later morphogenetic 
processes, affecting masses of cells and leading to the differentiation of 
organs, in order to determine the relation of cleavage to these. Of these 
processes, gastrulation and the ensuing invagination of ectodermic cells 
to form the pharynx (Zelinka, ’91) were studied. These are in reality 
parts of a single process, so that they may be treated of together under 
the title of Gastrulation. 

In regard to the relation of cleavage to gastrulation, the result is 
evident from the account given in the descriptive portion of this paper. 
No separation of the two processes is possible; gastrulation is an 
accompaniment and a consequence of cleavage. At the passage from the 
four-cell stage to the eight-cell stage begins a displacement of the 
blastomeres ; this displacement, or “‘rotation,” continues in later cleay- 
ages in the same direction, and is still in operation at the latest stage 
examined, when it is no longer possible to follow the development cell 
by cell. As one of the phases of this displacement during cleavage, 
the large ventral cell of quadrant D gradually moves inward, followed 
later by a similar transference of the ventral cells of the other quadrants 
to the inside. The entire process has been followed step by step in the 
descriptive portion of the paper, so that it is not necessary to go into 
details here. In its general features the process is as follows. As the 
ectodermal cells begin to pass into the karyokinetic condition, they 
withdraw their more internal parts and increase in surface extent. The 
egg as a whole retains its form and size, so that the withdrawal of the 
internal parts of one cell necessitates an inward movement on the part 
of others; the result is a gradual inward migration of the ventral cells. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 81 


The inward movement of the large cell of quadrant D, rather than that 
of the other cells, seems due to several causes: — (1) The inequality of 
the cells. The large ventral cell having a much greater radius of 
curvature has less surface tension, and therefore may more easily change 
its form. (?) It thus yields to pressure, and fits itself to the changing 
form of the smaller cells. These are thus able to creep over it, as it 
were, and surround it. The greater quantity of cytoplasm in the 
entoderm cell as compared with the size of the spindle seems also to 
result in less change of form at the time of karyokinesis. The large cell, 
in virtue of its mere size, conducts itself on the whole passively with 
relation to the more active smaller cells. (2) The sequence of cleavage 
is possibly connected with this. At a given cleavage the large cell 
divides first, so that, when the karyokinetic stretching of the other cells 
takes place, the entodermic cell is in a resting condition, and therefore 
passive. (3) The direction of the spindles, which is prevailingly dorso- 
ventral, results in a continued dorso-ventral extension of the cells, so 
that invagination would naturally take place at one of the two ends. 
The developing egg may be likened to a fountain in which there is an 
upward movement within, an outflow above, at the animal pole, and 
a downflow about the periphery. 

The enclosure of the large ventral cell of quadrant D is what has been 
considered gastrulation proper by Zelinka and Tessin, only the products 
of this cell being spoken of as entoderm. But after this enclosure is 
complete the process continues, unchanged in character, the ventral cells 
of the other quadrants following that of quadrant D to the inside of the 
embryo, as shown in Figures 76-79 (Plate 9). 

A necessary condition for all this displacement is of course the 
retention by the egg as a whole of its form and outline. If the blasto- 
meres should separate and project above the general level, in the manner 
that is common for the cells of mollusks (see the figures of Unio by Lillie, 
’95) at the time of karyokinesis, no compensating inward movement of 
the other cells would be necessary, and apparently therefore gastrulation 
would not take place. The retention of the regular form appears thus 
to be of the highest importance for the development, and the question 
arises as to how this form is preserved. As previously stated, no mem- 
brane is visible; and any uniform force, such as surface tension or a 
centripetal attraction, would produce a spherical instead of an ellipsoidal 
form. The development of the egg proceeds as if it were enclosed in a 
rigid mould of oval or ellipsoidal form, so that the contents of the mould 


are rotated, while the form is retained. The retention of this shape 
VOL. xxx.— No. 1. 6 


82 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


seems to me inexplicable on any simple mechanical ground ; the form at 
this stage apparently must be judged from the same standpoint as that 
of the adult, which no one would attempt to refer to a simple mechanical 
principle. Of course the assumption of the presence of a non-elastic 
non-extensible membrane of ellipsoidal form, which later becomes elastic 
and extensible, would explain the retention of the shape, and is open to 
any one who chooses to make it. But several facts speak against this 
view, aside from the general improbability of the existence of a membrane 
of such peculiar and changing qualities: —(1) The negative evidence 
that no such membrane can be demonstrated in preserved material. 
(2) In the sea-urchin and in Amphioxus,‘as shown by Driesch (93) and 
Wilson (’93), development takes place as well when the membrane is 
removed as when present. This of course does not show that the same 
is true for Asplanchna, but it does show that the importance of the egg 
membrane has been overestimated for some cases. (3) In the rotifer 
Callidina, investigated by Zelinka (91), the egg is of the same form as 
in Asplanchna, yet the cells sometimes put forth short amceboid pro- 
cesses, which of course would be impossible with a close membrane. 
(4) In another rotifer, Melicerta ringens, the egg is not a regular oval or 
ellipsoid of rotation, but one side is flattened while the other is curved, 
and this irregular form is retained during the shifting of the blastomeres, 
as is the case in Asplanchna (see the figures of Zelinka, 91, and of Joliet, 
’83). Such a form would not be preserved even by such a non-elastic 
membrane. 

The facts given under (3) and (4) seem to me to render entirely in- 
admissible the explanation that the form of the egg in Asplanchna is 
due to the presence of a membrane, since this would leave the exactly 
similar phenomenon in the related forms Callidina and Melicerta without 
explanation. 

The factors concerned in gastrulation may be summarized as follows : — 

1. The form of the egg. 
. The change in the form of the cells at cleavage. 
. The direction of cleavage. 
. The inequality of the cleavage. 
. The sequence of cleavage. (?) 

The process of which gastrulation is a part begins with the third 
cleavage, and continues through all the period in which it is possible to, 
trace the development cell by cell, and apparently much later. 

The process of gastrulation as above described for Asplanchna is 
similar to the method briefly set forth by Ziegler (95, p. 402, note) for 


Or em OO bo 


‘ 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 83 


Rhabditis nigrovenosa: “Ich stelle mir die Mechanik des Gastrulations- 
vorganges so vor, dass die Ektodermzellen nach ihrer Theilung sich 
abflachen und in Folge dessen ausbreiten; dabei schieben sie die 
Mesodermzellen iiber die Entodermzellen heriiber. Es kann dies um so 
eher geschenen, da die Mesodermzellen zum Zweck der Theilung sich 
kugelig zusammengezogen und dabei an die Oberfliche des einschichtigen 
Epithels emporgehoben haben.” 

On the other hand, zur Strassen (’95 and ’96), in his careful studies 
on the early development of another nematode, Ascaris megalocephala, 
came to an eutirely different conception of the factors at work in the 
displacement and extension of the ectoderm cells. Zur Strassen holds 
that four cells of common origin constitute an “elementary mechanism,” 
the two pairs of which attract each other in such a way as to bring about 
the movements which actually occur. The interaction is thought of as — 
something having the nature of the “cytotropism” of Roux (95). It 
is not necessary to discuss the matter further here, since there is no 
occasion to call in any such action in the gastrulation of Asplanchna. 
Doubtless the inter-attraction of cells exists in the rotifer, as elsewhere, 
but it seems to have no determining significance for the movements 
which take place. 


3. GENERAL CONSIDERATIONS. 


I shall next take up certain general considerations upon the mechan- 
ics of cleavage and development, which do not naturally fall under the 
discussion of any of the theories above considered, together with a 
general review of the results gained. 

The egg of Asplanchna at the four-cell stage might be compared to 
the egg of an echinoderm, with the exception that one of the four 
blastomeres is much enlarged, and of a different form from the others. 
What variation in the cleavage will be induced by the differences in 
form ? 

As we have seen, the form of the cell in Asplanchna does: not affect 
the direction of the spindle at cleavage. Indeed, the most character- 
istic feature of cleavage in Asplanchna is difference in form and size of 
the blastomeres, coupled with identity in the direction of spindles, in cells 
having the same general relations to the axes of the embryo. 

In a closer consideration of the factors determining the position of 
spindles, it is evident from the phenomena described that the question 
is not a simple one, but must be resolved into (1) what determines the 
direction of separation of the asters; (2) what determines the position 


84 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


of the asters before the spindle is formed; and (3) what determines 
the movements of the asters into the definitive position occupied by 
the spindle. 

(1) and (2). In regard to the first two questions, two general facts 
are worthy of notice. 

First, there is a tendency, other things being equal, for the newly 
formed asters to separate at right angles to the axis of the preceding 
spindle, and in such a way that the asters are not to be distinguished as 
deep and superficial, but either as right and left, or dorsal and ventral. 
No explanation for this fact is apparent, and it is not in every case true. 
Thus in the two-cell stage the nucleus and aster in the large cell migrate 
to the right before the aster divides, and the separation of the two 
newly formed asters is not in a plane at right angles to the axis of the 
preceding spindle. 

Secondly, the position of the asters before the formation of the spindle 
may apparently be modified by the simple mechanical conditions sur- 
rounding them. Thus, in the cell d*? (Fig. 14, Plate 2) the asters are 
modified in position almost immediately after they begin to separate, so 
that very soon we actually have in this cell the condition which may be 
considered least typical, — an inner and an outer aster (Fig. 7, Plate 1). 
Tn the thin cell d** (Figs. 37 and 38, Plate 5, and 46, Plate 6) the form 
of the cell apparently operates to cause the two asters to separate in 
such a way that almost from the first the line joining them has the 
same direction as the axis of the preceding spindle. Such facts give 
the impression that before the formation of the spindle the position of 
the asters is undetermined, and indifferent for the general structure of 
the cell, except that the two asters always lie on opposite sides of the 
nucleus, 

(3) As the karyokinetic changes take place, the asters migrate into 
definite positions, apparently by a rotation of the whole complex com- 
posed of the nucleus and its two accompanying asters. This rotation 
is into a definite position, without regard to either the form of the con- 
taining cell, or the previous position of the asters; that is, the end to 
be gained is constant, while the means of gaining it vary. Thus, at the 
divisions to form the sixteen-cell stage, the line passing through the 
asters and nucleus rotates in d*1 from a lateral to a dorso-ventral posi- 
tion, and into the greatest axis of its cell (Figs. 11, 15, and 16, Plate 2). 
In d+? it rotates from a position in which one aster is central, the other 
peripheral (Plate 1, Fig. 7), likewise into a dorso-ventral position (Fig. 
16). In a*!-c*? and g*?-—c*? all the axes rotate from a position of 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 85 


lateral extension to a dorso-ventral position, and thereby from the greater 
into the lesser axes of their respective cells (Figs. 17-22, Plate 3). In 
d®°* the asters do not separate at right angles to the previous spindle, as 
commonly occurs, but the line joining them is parallel to the preceding 
spindle, i. e. lateral (Figs. 37 and 38, Plate 5, and 46, Plate 6) ; later, 
by a rotation into the short axis of the cell, the dorso-ventral position is 
attained (Plate 6, Fig. 48, Plate 7, Figs. 53 and 54). It is not possible 
to refer these and the other changes described in the general account 
of the development to any simple factors. We can refer the changes in 
position of the asters, and consequent manner of cleavage, only to the 
structure of the protoplasm and the (molecular?) processes occurring 
within it. 

The fact that the spindles take definite positions with relation to the 
axes of the developing embryo, but without regard to the form of the 
cells, seems to indicate that there is some influence governing the egg 
as a whole, which is related to its form, and that the position of the 
spindles is regulated by this. The determining factors in the position 
of the spindles would therefore lie, not within any given cell itself, but 
outside of it. But there are certain facts which seem to render this 
very doubtful. As discussed on pages 70, 71, in later stages the cells 
become displaced by the changes taking place during gastrulation, and 
there is a corresponding change in the position of the spindles ; they 
are no longer either parallel with or at right angles to the dorso-ventral 
axis of the egg. This is shown especially in Figures 68 (Plate 8) and 
83 (Plate 10). If a changed position of the cell with regard to the 
axis of the embryo results in a correspondingly changed position of the 
cleavage spindle, it seems to follow that the position of the spindle is 
determined within the cell. 

I do not, however, consider this conclusion as well established. It 
still seems possible that the spindles are all placed with reference to some 
influence resulting from the axial relations of the egg as a whole, — 
though not necessarily in all cases either in the dorso-ventral axis or at 
right angles to it. 

Comparison of the conditions in Asplanchna with those reported by 
other observers for other organisms shows that there are cases in which 
the form of the cell does determine the position of the spindle. In the 
same way we know that there are cases in which the direction of the in- 
falling rays of light determines the position of the spindle (Stahl, ’85). 
But the result is not universal for either agent, so that we must hold 
that the effect in both cases is of the nature of a reaction to stimulus. 


86 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


The form of the cell and the direction of pressure cannot therefore 
be used in explaining in any general way the direction of cleavage, as 
proposed by Hertwig and Braem. The method of reaction and the 
purpose of the reaction must be determined for each class of cases in 
itself. 

A consideration of the process of gastrulation leads, though from the 
opposite direction, to conclusions of a similar nature. The form and 
direction of cleavage are related to the later morphogenetic processes. 
Gastrulation is a result of the method of cleavage, —and the method of 
cleavage must be looked upon as adapted to the purpose of accomplish- 
ing gastrulation, 

The relation of the form of cleavage to the later morphogenetic pro- 
cesses is shown in a different way in such forms as Nereis (Wilson, ’92) 
and Unio (Lillie, 795), where it has been possible to show the exact 
relation of later organs to individual blastomeres. Cleavage in many 
cases is itself a direct morphogenetic process, the exact method of 
which can be referred to no more simple mechanical factors than can 
. the characteristic form of the adult. 

I do not of course wish to generalize this statement; it is evidently 
true for many forms, but may not be true for all. The evidence upon 
which a contrary view is sometimes maintained for certain forms, as 
the echinoderms, seems however inconclusive. The formation of the 
micromeéres has been shown to be preceded by a differentiation in the 
cytoplasm (Morgan, ’94), which would naturally lead to the conclusion 
that the micromere formation is a process having a definite signification 
for morphogenesis. But Driesch (’93) showed that the micromeres 
might be removed and a normal larva still produced. From this, how- 
ever, it does not follow that the formation of micromeres is without 
definite morphogenetic significance, any more than it follows that the 
fundaments of limbs are of no morphogenetic significance because a 
normal larva results after the embryonic limbs have been removed from 
a young amphibian. The formation of micromeres apparently segre- 
gates a certain amount of substance which needs to be localized in a 
definite region. If this segregated material is removed, there is no 
evidence proving that similar material is not again segregated, at per- 
haps a later stage. As Roux insists, it is important to distinguish 
the normal method of development from induced development due to 
injury. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 87 


PART SHCOND.—DISCUSSION OF MATTERS BEARING 
UPON THE MORPHOLOGY OF THE ROTIFERA. 


Our knowledge of the development of the Rotifera is due chiefly to 
the work of Zelinka (’91). This author has given a full and careful 
description of the development of Callidina russeola Zel. from the egg to 
the adult form, with a briefer, but still extended, comparative account of 
the development of Melicerta ringens. Earlier works on the embryol- 
ogy of the Rotifera are due to Salensky (’72), Joliet (83), Zacharias 
(84), and Tessin (’86); but all of these works are incomplete and in 
many respects inaccurate, so that they have been almost completely 
superseded by the work of Zelinka. In discussing the development of 
Asplanchna I shall therefore restrict myself chiefly to a comparison with 
the results of Zelinka, drawing upon the accounts of other authors only 
where there is special occasion. 

Since my work has been done primarily from the standpoint of cyto- 
mechanics, and not with regard to the morphology of the Rotifera, it has 
an entirely different aim from that of Zelinka. It thus results naturally 
that, in giving an account of the bearing of my studies on questions 
relating primarily to rotifer morphology, emphasis must be laid chiefly 
upon the points in which my results differ from those set forth in 
Zelinka’s paper. The plan of my work required a more minnte-study of 
the cleavage than was demanded for Zelinka’s purposes, and as a natural 
result I shall be compelled to criticise his account of the cleavage in 
regard to certain details. Furthermore, it will be necessary to show 
that Zelinka has been inconsistent in his account with regard to the 
place where the polar cell is formed, and hence is mistaken in his state- 
ment of the axial relations of the egg and embryo in the Rotifera. But 
all these are matters of detail, not affecting in any important way 
Zelinka’s general conclusions, and I wish to say at the beginning that I 
fully appreciate the thoroughness and excellence of Zelinka’s researches 
upon this difficult group, and make the criticisms and corrections con- 
tained in the following pages in no spirit of disparagement. 

Asplanchna Herrickii de Guerne, the form upon which my studies have 
been made, is not closely related to any of the species of Rotifera whose 
development has been previously described. Callidina and Melicerta, 
investigated by Zelinka (91), belong respectively to the groups Bdelloidea 
and Rhizota of Hudson and Gosse (’86). Rotifer and Philodina, studied 
by Zacharias (’84), belong also to the Bdelloidea. Brachionus, investi- 


88 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


gated by Salensky (’72), is one of the Loricata. Only Eosphora, upon 
which the work of Tessin (’86) was done, belongs, together with 
Asplanchna, to the swimming Illoricata. But even these two are widely 
separated, Eosphora possessing a foot and anus, like the majority of the 
Rotifera, whereas Asplanchna possesses neither. 

The only work which has been done upon the early development of 
any species of Asplanchna is that by Leydig (54) and Lameere (’90). 
The latter observed in the living egg of Asplancha Sieboldii the formation 
of the polar cell and the first and second cleavages, but did not carry his 
work further. Leydig (’54) observed the cleavage of the egg, and 
figured the four-cell stage in the same species, but does not give any 
detailed description. I know of no other work dealing with the early 
development of any species of Asplanchna. 

I have examined the early stages of both Asplanchna Herrickii de 
Guerne and Asplanchna priodonta Gosse, of which the material collected 
in the Great Lakes contained about equal numbers. Asplanchna Herrickii 
was chosen for special investigation on account of the greater size of the 
embryos and adults. The development of Asplanchna priodonta was 
examined for comparison, and notes upon the embryology of this species 
have been given in connection with the fuller discussion of Asplanchna 
Herrickii. 


1. Previous Knowledge of Asplanchna Herrickii. 


It will be well to give here a brief review of previous references in the 
literature to the little known species Asplanchna Herrickii. It was 
first figured by Herrick (’84, Plate V.), and described in the explanation 
of plates as “ flask-shaped Rotifer, hermaphrodite, with eggs and sperm,” 
but no further description was given and no name was proposed. 
Herrick in a second paper (’85, p. 61) again mentions this form, but 
adds nothing to the description. 

In 1888, Jules de Guerne (’88) reproduced Herrick’s figure of the 
jaws, and on the basis of this held that the species was new, and pro- 
posed for it the name Asplanchna Herrickii. 

Hudson and Gosse (’89) included Asplanchna Herrickii in their list 
of “ doubtful species.” 

Daday (’92) admits Asplanchna Herrickii as a valid species. 

Up to this time no observer since Herrick bad reported finding th.s 
species. But in this same year, 1892, Wierzejski (92) gave an account 
of its presence in Galicia, with a description, and figures of the jaws 
and the peculiar glandular organ which Herrick had mistaken for 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 89 


a testis. Again, in his Rotatoria (Wrotki) Galicyi, Wierzejski (’92*) 
gives a figure of the entire animal, with special figures of the character- 
istic glandular organ, the trochal field, the jaws, and the excretory 
system, together with a brief description in the Polish language. 

Asplanchna Herrickii was afterward reported by the author (Jennings, 
94) as occurring in Lake St. Clair, and by Levander (’95) as occurring 
in the neighborhood of Helsingfors. 


2. Development. 


The unsegmented egg of Asplanchna Herrickii is similar in form to 
- that of Callidina russeola, investigated by Zelinka, but slightly smaller, 
the maximum dimensions in Callidina being 130 u by 90 p, while the maxi- 
mum dimensions observed in Asplanchna Herrickii were 97 wp by 83 py. 

No thick-shell “winter eggs” were ever observed by me in the 
specimens taken; possibly later in the fall these would have been 
found. 

In regard to the details of the developmental process, reference must 
be made to Part First; the purpose here is merely to point out such 
observations as are of importance from the standpoint of rotifer 
morphology, noting especially any differences between my account and 
those of other writers. 


A. MATURATION. 


As stated on page 13, the place of polar-cell formation in Asplanchna 
has a different relation to the axes of the egg from that ascribed to it by 
Zelinka in Callidina. In the ensuing discussion I shall, for convenience 
of comparison, use the orientation adopted by Zelinka; that is, what I 
have called the ‘‘macromere end” is anterior, the “‘micromere end” 
posterior. That side of the egg upon which later the blastopore is 
found, occupied in early stages chiefly by the quadrant D, is ventral, the 
opposite side dorsal. These terms have no constant relation to the 
terms of orientation employed in Part First. 

As previously described and figured (Plate 1, Figs. 1 and 2), the polar 
cell is formed near one of the ends of the ellipsoidal egg, and the place of 
formation is cut by the first and second cleavage furrows (Figs. 6 and 8). 
The same is true for Asplanchna priodonta. 

In these two species of Asplanchna, therefore, the place of polar-cell 
formation is the same, with reference to the form of the egg, as that 
described by Lameere (’90) for Asplanchna Sieboldii, and by Zelinka 
(91) for Callidina Leitgebii (p. 53) and Melicerta ringens (p. 117). In 


90 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


Callidina russeola and Discopus synapte, according to Zelinka (’91, p- 53), 
the polar cell is formed almost exactly at one of the ends of the ellipsoidal 
egg, though a very little to one side. This difference is a point of little 
or no significance ; an examination of Zelinka’s figures (Taf. I. Figg. 1-5) 
shows that the polar cell in Callidina russeola occupies the same position 
with regard to the axis of the first cleavage spindle as it does in the three 
species of Asplanchna. Possibly the egg of Callidina russeola is forced 
by the shell to take such a form that the axes of the egg, as indicated by 
the first cleavage spindle, do not coincide with the apparent axes indi- 
cated by the shape. The place of formation of the polar cell, as might 
be expected, is correlated with the axis indicated by the cleavage spindle. 
After the first cleavage a rotation occurs in Callidina russeola, bringing 
the apparent axis into agreement with the real axis. 

But with regard to the place of polar-cell formation in its relation to 
the orieutation of the egg as shown by later development, a remarkable 
disagreement exists between the condition in Asplanchna Herrickii and 
Asplanchna priodonta on the one hand, and the description given by 
Zelinka of Callidina russeola on the other. The following is Zelinka’s 
statement of the orientation of the egg of Callidina with relation to the 
place of polar-cell formation :— 

“Es verdient hervorgehoben zu werden, dass von dem Augenblicke 
an, als das Richtungskérperchen gebildet wird, simmtliche Richtungen 
im Riiderthier-Eie orientirt sind. An dem Pol, in dessen Niihe das 
K6rperchen austritt, finden wir spaiter das Vorderende, am gegentiber- 
liegenden das Hinterende, waihrend die Flache, in der es erscheint, 
zur Riickenflache wird.” (Zelinka, ’91, p. 54.) 

Accepting the later orientation of Zelinka, the above statement be- 
comes accurate for Asplanchna Herrickii and Asplanchna priodonta if 
“ Vorderende” and “Hinterende” are interchanged, and “ Bauchfliche ” 
is substituted for “Ritckenfliche”; in other words, the ortentation 
of Asplanchna with reference to the polar cell is precisely the opposite 
of that of Callidina. The statement for Asplanchna would read: “ At 
the pole in the neighborhood of which the polar cell appears, we find 
later the posterior end, at the opposite pole the anterior end, while the 
surface on which it appears becomes the ventral surface.” This state- 
ment, while correct if we relate the orientation of the animal simply to 
the form of the egg, as is done by Zelinka, contains one false implication. 
While that surface of the egg on which the polar cell is formed does later 
become the ventral surface of the animal, —the same form being re- 
tained to a late stage, —yet during the processes of development that 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 91 


part of the material of the egg immediately surrounding the region of 
polar-cell formation is moved to the posterior end of the egg, and, later 
across this and even upon the dorsal side. The same is doubtless true 
for the corresponding region (opposite the place of polar-cell formation, 
according to Zelinka) in Callidina. It marks the animal or upper pole 
in Asplanchna, lying at the opposite end of the egg from the blastopore 
during gastrulation, and is the common point of meeting for the blasto- 
meres derived from the four quadrants of the egg. In Callidina russe- 
ola, according to Zelinka, the region where the polar cell is formed lies, 
not at the opposite end of the gastrula from the blastopore, but at the 
dorsal margin of the blastopore, and the cells of this region are later 
invaginated to form the fundament of the pharynx; the real animal 
pole of the egg lying at a distance from the point of polar-cell forma- 
tion. The whole of Zelinka’s general discussion of the early develop. 
ment of the rotifer egg is based upon this peculiar position of the polar 
cell. (See Zelinka, 91, pp. 132-135.) His general statement of the 
place of polar-cell formation is as follows: “ Das Richtungsk6érperchen 
kommt an der dorsalen Seite des kiinftigen Embryo hervor, bei Melicerta 
dem spiteren hinteren Pole naher, bei Callidina fast am spiiteren vor- 
deren Pole des in beiden Fiillen langlichen Eies.”’ 

The difference between our accounts is seen by comparing Figure 6 
(Plate 1) and Figure 8 (Plate 2) with Zelinka’s Figures 8, 9, and 10 
(Tafel I.). In the two-cell stage in Asplanchna (Fig. 6), when the 
smaller cell, 4B’, is turned away from the observer, who looks down 
upon the polar cell, the spindle in the larger cell is seen to occupy such 
a position that the smaller product of the division of CD will lie to 
the right, and in Figure 8 this condition is shown to be realized when the 
division has taken place, the cell O? lying to the right of the polar cell. 
In Zelinka’s figures, however, the small cell II (= (*), derived from 
the division of the larger blastomere of the two-cell stage, lies to the 
left of the polar cell, when the same orientation is adopted; i. e. with 
the smaller blastomere (A = AB’) of the two-cell stage away from the 
observer. It therefore follows that, if the position of the polar cell is 
dorsal in Callidina, it must be ventral in Asplanchna, using Zelinka’s 
orientation. Later stages show the same contrast. Thus Figures 13 
and 14 (Plate 2), representations of the eight-cell stage of Asplanchna, 
show the polar cell in the position already described, (the dorsal pole of 
the egg being directed toward the observer,) whereas Zelinka’s Figures 
15, 16, and 18 of Callidina show it at the opposite pole. Figures 19, 
20 (Plate 3), 38, 41 (Plate 5), and 59 (Plate 7) show the polar cell at 


92 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


later stages in Asplanchna, and demonstrate clearly that it lies at the 
animal pole of the egg, opposite the blastopore. 

Moreover, a careful examination of Zelinka’s own work shows that 
tle general statement above cited cannot be considered correct for all 
Rotifera. His general conclusions are based throughout primarily upon 
his more complete study of Callidina, and in his ‘ Theoretischer Theil ” 
he seems to have overlooked the fact that in Melicerta the place of 
polar-cell formation observed by himself was, not only not near the 
auterior end, as noted in the above general statement, but also not upon 
the dorsal side. 

The egg of Melicerta is described (Zelinka, ’91, p. 115) as an elon- 
gated ovoid with a sharper and a thicker or blunter end ; one side is 
cylindrical in form, so as to appear straight in profile, while the other is 
swollen, presenting in profile a convex outline. Zelinka says in regard 
to the orientation of the egg given by Joliet: “Da ganz richtigerweise 
das dickere End als das Kopfende, das diinnere als das Hinterende 
bezeichnet wird, sowie ferner dieser Autor dessgleichen richtig die vorge- 
bauchte Flaiche als die ventrale, die cylindrische als die dorsale ansieht, 
so ware die Orientirung durch diese Form des Eies eigentlich erleich- 
tert,’ etc. Now, in Zelinka’s figures of the maturation stages of 
Melicerta (Taf. V. Figg. 74-76) the polar cell is clearly shown to be 
formed at this convex, and therefore ventral side. Furthermore, his 
description of the process of maturation is as follows: “ Das Keimblas- 
chen zeigt zuerst noch seine wohlbegrenzte sphirische Gestalt (Fig. 73) 
und liegt etwa in der Mitte des Eies, sodann wandert es, indem es leb- 
haft seine Gestalt verandert, gegen den hinteren Pol, wird zu einem 
halbmondférmigen Fleck mit gekerbten Randern (Fig. 74), dessen Kon- 
vexitiit der Bauchfldche zugekehrt, welcher es sich rasch ndhert. Knapp 
unter der Oberfliiche zerlegen die Kerben den Kern in drei eng an 
einander liegende ungleiche rundliche Stiicke (Fig. 75), deren der Ober- 
flache zunichst liegendes aus dem Ei gepresst wird.”? (pp. 116, 117.) 
The figures show clearly the convexity of the curved nucleus directed 
toward the convex ventral surface of the egg, the gradual approximation 
of the nucleus to that surface, and the formation there of the polar cell, 
as described in the above passage. In the “ Erklirung der Abbildungen,” 
these figures (73-77) are said to be “Rechte Seitenansichten,” which 
would make the convex surface and the place of polar-cell formation 
dorsal, as required by Zelinka’s general statement. But this contradicts 
the description just quoted, and there is other proof that the orientation 


1 The Italics are mine. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 93 


of this first row of figures (73-80) of Melicerta is incorrectly given in 
the “ Erklirung der Abbildungen.” Thus, Figure 78 is said to be a 
“ Dorsalansicht,” which would bring the blastomere II (= C#) upon 
the left side. But this blastomere is, in all other rotifers whose develop- 
ment is known, formed on the right side, and Zelinka states concerning 
this very figure, “ Zuerst folgt der Kerntheilung die Zelltheilung in dem 
erossen Blastomer, die Theilungsebene steht senkrecht zur Kernspindel 
und schneidet ein Stiick an der rechten Seite schief heraus (Fig. 78, 
I]).”? (p. 117.) Again, Figure 79 is said to be a ventral view, whereas 
the same considerations as in the last case show it to be a dorsal view. 
Figure 80 is said to be a ventral view of the stage shown in Figure 78. 
Yet it is evidently a later stage than Figure 78; it really represents a 
ventral view of a stage similar to Figure 79, though the latter itself is 
said in the “Erklarung” to be a ventral view. In the text, Zelinka 
states correctly that Figure 79 is a dorsal, Figure 80 a ventral view 
(p. 118) of the same stage (p. 117), contrary to the statements in 
the “ Erklarung der Abbildungen.”’ 

In Melicerta, therefore, the polar cell is formed in the same position 
as in Asplanchna Herrickii and Asplanchna priodonta, and marks the 
animal pole of the egg. 

It seems very improbable that between these three forms and Calli- 
dina there should actually be such a difference in regard to the place of 
polar-cell formation as is brought out by the above comparisons. Ze- 
linka’s account of Callidina is full and clear upon this point ; in both 
his description and his figures the polar cell is traced to a late stage, 
when a mistake in the orientation is impossible. There seems, however, 
to be one opportunity for error. The polar cell in Callidina is not em- 
bedded between the blastomeres, as in Asplanchna, but lies free upon the 
surface of the egg. This is shown by Zelinka’s figures as well as by his 
descriptions. On page 62 he says, “ Das Richtungskérperchen hat sei- 
nen Platz, den es frither eingenommen, verlassen und liegt nun ganz auf 
den kleinen Zellen.” The egg of Callidina is enclosed by a rigid shell or 
membrane, which is separated from the egg itself by a space, except at 
the sides. The shell retains constantly the ellipsoidal form, while the 
egg itself may change its form and rotate within the shell. The original 
place of polar-cell formation, with respect to this shell, is near one end, — 
the end next to. which les later the anterior extremity of the embryo. 
But immediately after the first cleavage the egg rotates within the shell 
through an are of about 90 degrees, and the region of polar-cell forma- 


1 The Italics are mine. 


94 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


tion is carried to the equator of the egg. The polar cell itself is shifted 
at the same time, and it seems possible that during this rapid, rotation 
it may be transferred to a region of the egg different from its original 
position. Such shiftings of the polar cell, though to a less extent, 
are mentioned by Zelinka (pp. 55, 58). During the “sehr schnell ver- 
laufendes Phiinomen” of the rotation of the egg within its shell, the 
relatively different shifting of the polar cell might have been over- 
looked. This is, however, only the suggestion of a possibility, for which 
there is no direct evidence in Zelinka’s work. 

It is to be noted that in the above discussion I have employed 
throughout the orientation of the egg used by Zelinka, and not that 
adopted in my own account of the development. 


B. CLEAVAGE. 

The first cleavage in the two species of Asplanchna differs from that 
of Callidina russeola in being exactly transverse to the long axis of the 
egg. In the latter form, according to Zelinka, the first cleavage plane 
is oblique to the spindle, and the spindle itself is oblique to the long 
axis of the egg. By a change of their relative positions immediately 
after division, the two cells are later brought into the same position 
relative to each other as in Asplanchna, and even at first the cleavage 
plane in the two forms occupies the same position relative to the place of 
polar-cell formation. The difference thus is of little importance, except 
that, from a cyto-mechanical standpoint, it shows that the form of the 
egg does not determine the position of the first cleavage spindle. In 
Eosphora also (Tessin, ’86) the first cleavage plane is oblique to the 
long axis of the egg, whereas in Melicerta ringens (Zelinka, ’91) and 
Asplanchna Sieboldii (Lameere, ’90) the first cleavage plane is transverse 
to the long axis, as in Asplanchna Herrickii and Asplanchna priodonta. 

The second and third cleavages in the two species of Asplanchna 
(Plate 1, Fig. 6, Plate 2, Figs. 8-16) are essentially similar to the 
corresponding cleavages of Callidina and of other rotifers in which 
the development has* been described. For convenience in compar- 
ing the later stages, I give here a table showing the correspond- 
ence between the cells of Asplanchna in the eight-cell stage and those 
of Callidina. 


Asplanchna. Callidina (Zelinka, ’91). Asplanchna. Callidina (Zelinka, *91). 
TES NO ear oc Les et eonee. eet Chloe. 2) Lees 
Oe a ia a aa « i las, goes Hi AF oo) ee eee 
(RE BS reg en del |) el 


b42 . . . . . . . . by di2 . . . . . . . . Ill 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKIL. 95 


Lameere (’90) and Leydig (’54), have given figures of the four-cell 
stage of Asplanchna Sieboldii. Beyond this stage there are no other 
published figures of the cleavage of any species of Asplanchna. 

In regard to the fourth cleavage (Figs. 19-30, Plates 3 and 4), a 
remarkable difference is to be observed between the cleavage of As- 
planchna and that of Callidina as described by Zelinka. In Asplanchna 
the cleavage takes place up to (and beyond) this point with the greatest 
regularity both as to direction of spindles and as to sequence. The first 
and second cleavages are meridional, the third equatorial, and the fourth 
again equatorial. The sequence of cleavage is in every case D, C, B, A 
(see nomenclature of cleavage, page 16). In Callidina, according to 
Zelinka, the rhythm and regularity of the process is destroyed after the 
third cleavage by the remarkable circumstance that the cell d*? (1, Ze- 
linka) divides twice in succession before the fourth cleavage of any of 
the other cells. After these two divisions of d*}, the six cells of the 
other three quadrants are said to divide in the same succession that 
occurs in Asplanchna, while it is not until all these cleavages are finished 
that the cell d*? (III, Zelinka) is separated into two blastomeres. Be- 
fore this division of d*? takes place, the egg consists in Callidina, as in 
Asplanchna, of four rows of four cells each, but in Callidina the method 
of origin of the four cells of quadrant D is stated to be different from 
that in the other quadrants. In this quadrant the three dorsal cells 
(posterior, Zelinka) are said to arise by successive cleavages of the 
large ventral (anterior, Zelinka) cell, while in each of the other three 
quadrants the four cells arise by the halving of the two cells previously 
present. 

In Melicerta, according to Zelinka, the cleavage up to this point is as 
in Asplanchna; the cell I (d**) divides first, then III (d*), then the 
cells of the other three quadrants. Later the cleavage of Melicerta 
differs from that of Asplanchna, but up to the end of the sixteen-cell 
stage the processes are the same in the two, 

In Eosphora, as described by Tessin (’86), the cleavage is like 
that of Asplanchna, except in the unessential particular that his cell 
a’ (=d*") divides before the cleavage of a (=d*). The sixteen-cell 
stage is reached by the cleavage of the same cells as in the two species 
of Asplanchna and in Melicerta. 

In view of the regularity of the cleavage in these four forms, one 
might be led to suppose that the irregularity described in Callidina by 
- Zelinka was due to defective observation. Zelinka has noted this point 
with particular attention, and states that he is certain of the difference 


96 BULLEIIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


between Callidina and Eosphora, as described by Tessin. Nevertheless 
it is possible that the slight ¢/me variation in the cleavage of Eosphora 
may have misled Zelinka as to the point which needed especial care. 
In Asplanchna Herrickii and Asplanchna priodonta, in Melicerta and in 
Callidina, the large cell d*:* divides first, followed immediately (in all 
except Callidina, at least,) by the division of d*?. In Eosphora, by a 
slight relative delay of the cleavage in d* (a, Tessin), the cell d*? (a”, 
Tessin) divides first. Zelinka states that he has observed with especial 
care that the first cell (IV, Zelinka) given off in this quadrant takes 
origin from d*? (I, Zelinka), This is doubtless true; the important 
point, however, is the origin of the nezt cell formed. Though this also 
is stated to arise from the ventral cell of the series (I, Zelinka), it 
seems possible that Zelinka was thrown off his guard by the supposed 
greater care necessary for determining the exact method of the preced- 
ing cleavage, and that the statement with regard to this one is really a 
mistake. The cell IV in Figure 23 (Taf. II.) of Zelinka’s work might 
be the same cell as V in his Figure 24, while the cells called III and IV 
in Figure 24 might have arisen by the division of the previously exist- 
ing cell III (=d*?). This would bring the conditions in Callidina 
into agreement with those in Eosphora, Melicerta, and the two species 
of Asplanchna. This is, of course, a mere suggestion, which indeed is 
rendered rather improbable by the nuclear conditions in the cells under 
discussion shown in Zelinka’s Figure 24. There can, of course, be no 
question about the manner of division in Asplanchna. Figure 16 (Plate 
2) shows the spindles in d‘? and d*?, and the accomplished division of 
d*1 into d>1 and d®*? is shown in Figure 19 (Plate 3), while d*” still 
contains a spindle. I have observed similar conditions in many other 
specimens. 

In view of the essential similarity of the process in Eosphora, as 
described by Tessin, to that in Melicerta, as described by Zelinka, and 
in the two species of Asplanchna, as observed by me, and in view of the 
fact that the cleavage of the quadrant in question (Y) in these four 
forms may be said to agree completely with the general plan of cleavage 
as exhibited in the other three quadrants, — while in Callidina the con- 
ditions in this quadrant are anomalous,—the following remark of 
Zelinka (91, p. 61) seems hardly justifiable: “Da, wie spiter gezeigt 
wird, auch Melicerta in der Entwicklung unserer vorliegenden Form 
folgt, so muss der Vorgang bei Eosphora als eine bemerkenswerthe 
Verschiedenheit aufgefasst werden.” As above shown, the difference 
between Eosphora and the other forms consists merely in a slight 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. oF 


variation as to the relative time of cleavage of two blastomeres (d*! and 
d*),—a phenomenon which is exceedingly common both in different 
individuals of the same species and in closely related species, and to 
which little or no significance can be attached. It is Callidina which 
shows a “bemerkenswerthe Verschiedenheit,” since here the rhythm 
and regularity of the cleavage are completely destroyed, if the division 
is correctly described by Zelinka.* 

Beyond the fourth cleavage it becomes very difficult to compare the 
processes in Asplanchna with those described by other observers for 
other rotifers. As above described, even in the fourth cleavage one of 
the cells of the D quadrant (d**) was formed in a different manner in 
Callidina, according to Zelinka, from the method in Asplanchna. This 
in itself makes an exact comparison of the fifth cleavage in the two 
species impossible. But, considering for convenience the cells corre- 
sponding in position at the sixteen-cell stage as equivalent in the two 
forms, we find the following to be the process in Callidina as described 
by Zelinka. 

The first blastomere of the sixteen-cell stage to divide in Callidina is 
said to be the dorsal cell of the D quadrant, d*4 (III, Zelinka), whereas in 
Asplanchna the very unequal division of the ventral cell d°+ takes place 
first (Fig. 33, Plate 4, and Figs. 35 and 38, Plate 5). The cleavage of 
d** is followed in Callidina by the division of d*3 and d*? (IV and V., 
Zelinka). The division is meridional, as in Asplanchna (Fig. 37), but the 
products are equal, whereas in Asplanchna they are unequal. 

Now, according to Zelinka, the products (at the fifth cleavage) of the 
division of d°* (d°-" and d**, Zelinka’s III, and III,) are themselves 
divided by meridional furrows. Thus the sixth cleavage takes place in 
these cells before the fifth has been accomplished in any of the other 
quadrants. 

Following this, the large ventral cell of the D quadrant, d*! (1, 
Zelinka), divides equatorially, giving off on its dorsal side a small cell, 
VI, which lies between the products of the division of d>”. 

Thus the cleavage of the quadrant D is much less regnlar than in 
Asplanchna, where the ventral cells all cleave meridionally and unequally, 
the dorsal cell equatorially and unequally, the direction of cleavage 


1 Zelinka’s statement, quoted above, that Melicerta follows the same method of 
division as Callidina, depends merely upon his interpretation of a real variation; 
the actual divisions to form the sixteen-cell stage are the same in Melicerta as in 
Asplanchna and Eosphora, and different from those of Callidina, as may be seen 
by consulting Zelinka’s (’91, p. 121) description. 

VOL. Xxx.— No. l, 7 


98 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 


being the same as in the other three quadrants (see Figs. 33 and 37). 
In Asplanchna, moreover, the fifth cleavage in the other quadrants is 
well advanced before any of the divisions for the sixth cleavage in 
quadrant D have taken place. 

Zelinka does not follow the cleavage in the other three quadrants cell 
by cell. He states that the dorsal cells of these quadrants (all?) are 
divided by planes parallel to the long axis of the egg, as is the case for 
all but the fourth or most dorsal layer in Asplanchna, and that the 
ventral cells a*", 6°, and ec*? (a,, b,, and II, Zelinka) are the last to 
divide. In Asplanchna, as shown in Figures 39-44 (Plates 5 and 6), 
all the cells of these quadrants divide meridionally except those of the 
dorsal layer, which divide equatorially. 

It is obviously impossible to compare in detail the cleavage in the two 
forms at this time, or to reduce the condition described for Callidina to 
the regular scheme of cleavage exhibited in Asplanchna. Certain facts 
are perhaps worthy of notice, as showing the possibility that the cleavage 
in Callidina is not so different from that of Asplanchna as would be 
inferred from what is indicated above. The relation of the cells in 
quadrant D are somewhat similar in Figures 30 and 34 of Zelinka’s 
work to a later condition in Asplanchna,—a condition reached, how- 
ever, in a very different way, and shown in Figures 58 (Plate 7) and 66 
(Plate 8). Further, the unequal cleavages of this quadrant are very 
confusing, and easily overlooked. Zelinka’s work was apparently done 
almost entirely on living material, which does not lend itself as well as 
does preserved material to precise orientation of the object, and to its 
rotation in such a way as to permit views from all directions. It was 
only by bringing together a complete series, in which the karyokinetic 
process in every cell was represented in various stages by several 
specimens, that I was able to determine absolutely the course of events 
here. The remarkable unequal division of the cell d*’ is especially 
liable to be overlooked ; I did not observe it till the break in the rhythm 
of cleavage at this point set me at work upon a minute study of a series 
of eggs separated in cleavage conditions by very short intervals only. 
It is worth noting that in Callidina, shortly after the time for this 
cleavage to occur, Zelinka figures (Taf. II. Fig. 31) a small vesicle lying 
almost exactly in the place occupied by the minute vesicle given off at 
this cleavage in Asplanchna, viz. between the ventral cells of quadrants 
A and B (a, and b,, Zelinka). Zelinka considers this to be the polar 
cell, although at a previous stage he had observed that the polar cell 
had become displaced and now lay farther dorsad, on the outer surface 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 99 


of the smaller cells of the egg. The condition shown in his Figure 31 
he (791, p. 62) considers to be an exception. Later, he states that the 
polar cell becomes surrounded by small spherules, indicating that it is 
degenerating and falling to pieces. As shown in my Figures 51 and 
52 (Plate 6), the vesicle d°’ produced by the division of d*1 in 
Asplanchna also becomes surrounded by large granules or spherules ; 
but these are not of the nature assumed by Zelinka; they are the result 
of a concentration of granules in the ventral cell d°’, traceable from 
the eight-cell stage onward. In view of these facts, it seems possible 
that a similar division actually takes place in Callidina, and that the 
small cell lying between a, and }, in Zelinka’s Figure 31 is the small 
product of this cleavage, —the equivalent of d°-? in Asplanchna. 

Tessin, in his study of Eosphora, also failed to follow the cleavage 
in detail to the 32-cell stage. He (86, p. 282) speaks of “ fortgesetzte 
Aequatorialtheilungen” of the cells in the three smaller quadrants ; his 
figures show the three quadrants composed each of a single row of six 
cells (Figs. 22 and 23). This corresponds to the condition in Asplanchna 
at a time when the dorsal cells (a°*-c**) have divided equatorially, but 
when the remainder of these three quadrants are as yet undivided. 
Next, all the cells, except the large ventral cell d*1, are said to divide 
meridionally. It is probable, therefore, that the formation of the minute 
cell by the division of d°* was overlooked, and that the cleavage is 
essentially as in Asplanchna. The cells of quadrant D are said to 
divide unequally at this cleavage, as is also the case in Asplanchna. 

The sixth and later cleavages of the ectoderm have not been studied 
in detail by other observers, so that a comparison of my results with 
observations on other forms is impossible. Diagrams of the sixth, 
seventh, and eighth cleavages for Asplanchna are given on pages 41, 46, 
48, and 53, that for the eighth cleavage being incomplete. 

The divisions of the entoderm cells have been followed somewhat further 
by Tessin and Zelinka, so that for these a comparison may be made. 

Nothing comparable to the unequal fifth and sixth cleavages of the 
entoderm cell (forming the small vesicles d*? and d™?), shown in Fig- 
ures 38 (Plate 5) and 49 (Plate 6), have been reported by other 
observers. 

Later than these the cleavage of the entoderm in Eosphora (Tessin, 
*86) takes place as in Asplanchna (Figs. 64, 65, and 76-83, Plates 8-10) 
up to a stage comparable with that shown in Figures 77 and 78 (Plate 9), 
except that no cleavage of the smaller dorsal cell d*-? corresponding 
to the unequal division which I have shown in Figure 80 (Plate 10) was 


100 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


observed by Tessin. He did not follow the divisions of the entoderm 
further. 

In Callidina a division takes place in the same manner as the seventh 
in Asplanchna (Figs. 64 and 65, Plate 8), separating d** and d*-2 The 
cleavage of d*" into d*? and d*” also follows, as described above for 
Asplanchna. 

An unequal cleavage of d*?, as shown in Fig. 80 (Plate 10), was not 
observed in Callidina. The cleavages which next ensue are described by 
Zelinka as variable. The two cells corresponding to my d*? and d*? 
divide in the same direction as the corresponding cleavages of Asplanchna 
(Fig. 76, Plate 9, and Fig. 81, Plate 10), but the dorsal cells d1°-? and 
d’°-4 are smaller than the others. The cell d®* (e, Zelinka) divides by 
two successive divisions, at right angles to each other, into four cells; 
one of these divisions corresponds to that indicated for this cell in Fig. 
83 (Plate 10), while the other is at right angles to this. The order in 
which these cleavages occur in Callidina is, however, variable. 

According to Zelinka, each of the four cells corresponding to my d*?, 
d?-?, d°?, and d** now divides into three parts, but the details of these 
cleavages are not given. 

In Melicerta the cleavage of the entoderm is traced by Zelinka to a 
four-cell condition, but the process is entirely different from that in 
Asplanchna, Eosphora, and Callidina, so that it would not be of interest 
to review the facts here. 

The process of gastrulation takes place in Callidina and Eosphora, and 
probably in all other Rotifera, in a manner essentially similar to that in 
Asplanchna; the large ventral cell of the left posterior quadrant is 
enveloped by the other cells during the process of cleavage, and becomes 
the entoderm. 


C. Summary oN MATURATION AND CLEAVAGE IN THE ROTIFERA. 


In general, the following facts are shown for the early development 
of Asplanchna, as compared with previous accounts of the development 
of Rotifera. 

1. The polar cell is formed at the animal pole of the egg, at a point 
opposite that where the blastopore is later found, and not at the dorsal 
or anterior margin of the blastoporic region, as stated by Zelinka for 
Callidina and Melicerta. 

2. A much greater regularity, and in a certain sense symmetry, are 
shown in the direction and rate of cleavage than has been shown for 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKI. 101 


other Rotifera. Cell lineage is traced to a much later stage than has 
been done for other rotifers. 

3. In other respects the development of Asplanchna, so far as 
observed, agrees in general with that of Callidina as described by 
Zelinka (’91). The development of organs was not traced in Asplanchna, 
the purpose of the work lying chiefly in the domain of cyto-mechanics. 


PART THIRD.—MATERIAL AND METHODS. 


The material for the studies here presented was collected by means of 
towings from Lake Michigan and certain small lakes connected with it, 
in August and September of 1894. Such towings were killed and pre- 
served by a variety of methods. For killing, the following reagents were 
tried : (1) Flemming’s stronger chrom-osmo-acetic mixture ; (2) Klein- 
enberg’s picro-sulphuric mixture, weaker solution; (3) Henneguy’s 
fluid, consisting of Kleinenberg’s weaker fluid plus 10% glacial acetic 
acid; (4) picro-nitric acid ; (5) alcoholic corrosive sublimate; and (6) a 
mixture of corrosive sublimate and formalin. The best results were 
gained by the use of Flemming’s mixture. The eggs were considerably 
darkened, but this defect was easily corrected by bleaching with chlorine 
generated from chlorate of potash and HCl. Henneguy’s fluid and 
picro-nitric acid also gave good results. By alcoholic corrosive subli- 
mate the eggs were commonly shrunk, and with Kleinenberg’s fuid the 
shrinking was excessive. The towings after killing were preserved partly 
in 80% alcohol, and partly in a mixture of equal parts of glycerine, 
alcohol, and water. Both these preservatives gave satisfactory results. 

As is well known, the development of the embryo takes place in 
Asplanchna within the body of the adult. The developing egg lies in 
the posterior part of the body of the mother, enclosed in the greatly 
distended oviduct or uterus, and with the ovary of the adult closely 
applied to it. It was necessary to pick out the Asplanchnas one by one 
from the quantities of Crustacea and other plankton with which they 
were mingled. This was done by using capillary tubes. It was necessary, 
moreover, to assort them with respect to the state of development of the 
contained embryo, if an embryo were present. This is a process involv- 
ing great labor, as, in order to determine even approximately the stage 
of development of the embryo, it is necessary to examine the animal 
with the compound microscope. The majority of the specimens contain 
an embryo; not rarely two are present, in different stages of development, 
and in a single case I observed three. 


102 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


In order to study the eggs, it is of course necessary to dissect them 
from the mother. This, again, is a tedious and delicate process, and it 
is rarely possible to free the egg entirely from the closely applied ovi- 
duct and ovary: This fact causes excessive trouble later, since the 
fragments of oviduct and ovary attached to the egg prevent one’s 
placing it in any desired position or rolling it about at will, under a 
cover-glass. 

For surface study of the early stages the eggs were then mounted in 
glycerine under a cover-glass supported by bits of capillary tubing thick 
enough to allow free motion to be given to the object. It was found 
impossible to use to advantage any stain for this study, because all the 
stains tried colored the cytoplasm more than the nucleus, and made the 
egg so opaque that cell boundaries and nuclei could not be distinguished. 
For the stages from one to about sixteen cells, eggs fixed in Flem- 
ming’s solution are the best, as the slight darkening produced by this 
reagent is of advantage in stages where the egg is cleft into but few 
cells. For later stages this darkening isa disadvantage ; eggs killed by 
other methods, or bleached after fixing with Flemming’s fluid, must be 
used. 

The eggs were moved about by rolling the cover-glass on its rollers, 
and drawings were made of different views thus obtained. It is here 
that the ovary and bits of oviduct attached to the egg cause infinite 
delay and vexation, in preventing the eggs from rolling easily or rest- 
ing in any except certain positions. The time required for the work is 
certainly doubled, perhaps more than doubled, by this. 

With early stages, camera drawings can be made at once ftom the egg 
after a favorable position is gained; but after the egg has reached a 
stage of about thirty cells, it is necessary first to roll the egg and make 
many tentative free-hand drawings of the different surfaces, until 
together they show the whole surface of the egg and the relation of 
every cell to all surrounding cells. The egg is then oriented and a 
camera figure made which shows the exact form of the cells in the 
middle region of the upper surface, and the position of all the nuclei of 
that surface. The cell boundaries about the periphery corresponding 
to these nuclei are then supplied from the free-hand sketches. This 
method, while not giving mechanical accuracy for the form of the. cells 
about the periphery of a late stage of cleavage, does permit of complete 
accuracy so far as the relations of cells to one another are concerned ; 
any other method with eggs in which the cell boundaries are so 
faintly marked on the surface is impracticable. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII 103 


Sections were made of numbers of eggs, but optical sections are much 
more instructive, permitting exact orientation and revealing the struc- 
ture fully as well as actual sections, so that most of the figures in which 
sections are represented were made from optical sections. 

In the descriptive portion of this paper, details have been given of 
the movements of asters, nuclei, and other cell contents, as well as 
of the cells as units. As the entire account was gained from a study 
of preserved material, the question is a justifiable one, — Is there suffi- 
cient evidence that the movements actually occur as above described, or 
are the stages figured and described merely chosen at will from a mass 
of material and arranged arbitrarily in series ? 

The number of eggs used in determining the course of events in a 
given cleavage has been stated in several cases in the text. Thus, 31 
eggs were studied coutaining more than one and less than five cells; 
42 containing more than seven and less than sixteen, etc. In all, more 
than 250 eggs from Asplanchna Herrickii and 50 from Asplanchna prio- 
donta, between the single cell stage (Fig. 1) and the stage containing 
five entoderm cells (Fig. 83), were mounted in glycerine and studied. 
Each egg, of course, came necessarily from a different individual, — 
since, where two embryos were present in the same adult, one at least 
had passed to a stage in the formation of organs. Of many of these 
eggs examinations were made which may be called exhaustive; i. e. 
every cell with its nuclear conditions was carefully figured. Thus, 
from the egg of which Figure 68 (Plate 8) gives one view, at least 
twenty drawings were made, thongh but one is shown in the plates. 
The figures given, therefore, represent by no means even a considerable 
part of the evidence upon which the description is based. After de- 
termination of the exact order of events, drawings of typical cases were 
selected for illustrating the paper. 

The determination of the sequence of the stages observed is greatly 
lightened by the almost entire constancy in the relative order of events 
in the different cells. Very slight variations occur in regard to certain 
processes, as in the case of the migration of the cloud of granules, as 
mentioned in the Explanation of Plates, under Figure 51. But, in gen- 
eral, a number of eggs representing a series of events in a given cell 
show corresponding series of events in the other cells. It is not neces- 
sary, therefore, to rely upon the conditions within the cell under ex- 
amination for determination of the sequence of stages in this cell. 
Even this- would probably be possible, however, from the fact that the 
nucleus in any cell increases in size steadily from the time the cell is 


104 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


formed to the beginning of the transformation of the nucleus and asters 
into the spindle figure —so that the relative size of the nucleus in the 
same cell from different eggs gives a measure of its relative age. 

An illustration will make clear the method used. Perhaps the most 
difficult problem was presented by the movements of the asters and 
nuclei in the two large ventral cells of the quadrant D, at the fifth and 
sixth cleavages. These cells are d*" and d** before the fifth cleavage ; 
d®+ and d°* after the fifth cleavage. Figure 31 (Plate 4) shows the 
position of the asters in all the cells of this quadrant in the resting 
sixteen-cell stage. Figure 32 shows the nuclei in quadrant B in the 
same egg, as seen in a longitudinal section. Inthe egg shown in Fig- 
ure 33 the spindles are already completed in quadrant D, indicating 
that this egg is older than that shown in Figures 31 and 32. Figure 
35 (Plate 5), a ventral view of this stage, shows that the nuclei in a®.? 
and 6° are much larger than the nuclei in the corresponding cells of 
Figures 30 and 31, — which likewise indicates that this is an older stage 
than is represented in the latter figures. Figure 35 shows the exact 
antero-posterior position of the spindle in d*?, while in the other quad- 
rants the nuclei are shown to be still in a resting condition. 

Figures 37 and 38 together show all the cells of this quadrant during 
or just after division, the four dorsal cells being still connected by inter- 
zonal filaments.’ It is therefore a stage /ater than that shown in Fig- . 
ures 33 and 35. This is in agreement with the larger size of the nuclei 
in quadrant A, Figure 37, as compared with those of quadrant A in 
Figure 33, and also with the presence of a spindle in a*-*. In this egg 
the nucleus in d*! (Fig. 38) has resumed its nearly spherical form, and 
at the side of the cell where the anterior end of the spindle was located 
(Fig. 35) is a small vesicle, d*? (Fig. 38). This represents one product 
of the division. 

In d** at this stage (Figs. 37 and 38). the single aster is slightly 
extended in a direction which is oblique to both the longer and shorter 
axes of the cell. 

The spindles in the cells of quadrants A, B,and Cin Figures 39-42 
(all from the same egg) prove that this is a later stage than that last 
considered. In this egg the line joining the two asters in d®? (Fig. 42) 
is oblique to the antero-posterior plane of the embryo, and nearly dorso- 
ventral. 

In the egg shown in Figures 43-46, division is completed in some of 
the cells of quadrants A, B, and C, proving this to be a later stage than 
those shown in Figures 37-42. In this egg the two asters in d** (Fig. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 105 


46) are at opposite sides of the nucleus, the line joining them being 
parallel to the lateral axis of the embryo. 

In the egg seen in Figure 53 (Plate 7), the fifth division is finished 
in all the cells of the egg, (as shown by the study of the other quad- 
rants, which are not figured,) so that this is a later stage than that 
shown in Figures 43-46. The asters in d°? have taken an oblique 
position. 

In the egg of which Figures 48 (Plate 6) and 54 (Plate 7) are repre- 
sentations, the nuclei in the recently formed cells of quadrants A, B, 
and C have enlarged, and the spindle is completely formed in d*’ (being 
dorso-ventral in position). Both of these facts show that the egg is older 
than the one shown in Figure 53. In this egg (Fig. 48) we find that a 
spindle is present in the entoderm cell d*’, occupying nearly the position 
foreshadowed by the position of the asters in Figure 42, and almost 
exactly the same position as the spindle at the foregoing division (Fig. 35). 

The egg seen in Figure 49 is still older, as shown by the presence 
of spindles in d** and d*" (seen endwise), and the advanced condition of 
cleavage in d**. Here d®! is just dividing, forming d™* and the second 
small cell, d™**. 

Figure 50 is older than Figure 49, since d**, d*’, and d®** have divided. 
In this egg we find that d®*? has been separated into two cells, d7° and 
d™®, and there is a second small vesicle, d‘-*, in the position where it 
was seen in the process of formation in Figure 49. 

The above is sufficient to illustrate the method of work; the rest of 
the account might be analyzed in the same way. It is important to 
remember, however, that the description is not based merely upon the 
cases figured. “Thus, for the processes just analyzed, more than thirty 
egos, showing various phases of the changes occurring, were studied, 
while only eleven different eggs are represented in the figures of these 
stages. 

The foregoing work was done in the winters of 1894-95 and 1895-96, 
in the Zodlogical Laboratory of the Museum of Comparative Zodlogy at 
Harvard University. It gives me pleasure to acknowledge my great 
indebtedness to the Director of the Laboratory, Professor E. L. Mark, 
for advice and assistance which have been of the greatest value to me 
throughout my work. 


106 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


GENERAL SUMMARY. 


A. Observations. 


1. Many divisions take place during the cleavage of Asplanchna in 
which the spindle lies in the shortest axis of the cell, in the direction of 
greatest pressure, and the ensuing division results in the production of 
contact surfaces of greatest area. 

2. In the cleavage of the ectoderm of Asplanchna any cell of any one 
quadrant cleaves in the same direction as the corresponding cell of the 
other quadrants, though the forms of the corresponding cells may vary 
excessively. Conversely, cells of the same form and with similar relations 
to surrounding cells, but belonging to different layers or series, may 
divide with spindles in exactly opposite directions. 

3. The entodermal cell follows the same rhythm and direction of 
cleavage as the other cells, so long as it remains on the exterior and thus 
corresponds in position with other cells of the egg. When it becomes 
enveloped by the other cells, so as to come into different relations with 
the axis of the embryo, its plan of cleavage changes, showing uo definite 
relation to that of the ectoderm. 

4. All the cleavages in the ectoderm are to a late period either 
equatorial or meridional, so that the position of any given spindle is - 
either parallel or perpendicular to that of the preceding spindle. 

5. There is no regular alternation in the direction of spindles. Equa- 
torial cleavages may follow successively for three or more generations, 
and the same is true of meridional cleavages. 

6. The position occupied by the two asters after they have passed to 
opposite sides of the nucleus does not indicate ,the direction of the 
ensuing spindle. This may occupy the position indicated by the asters, 
or the definitive position may be gained by a rotation of the asters and 
nucleus at the passage into the karyokinetic condition. 

7. There is no “regular angle of rotation” (Heidenhain) in a mechani- 
cal sense, since (a) in cells of different layers, in one case the angle may 
be zero, in the other case 90 degrees; and (b) even in cells where the 
direction of the previous spindle and the direction of the following spindle 
are the same, the asters may move in an entirely different manner. In 
one cell the rotation may be directly through an angle of 90 degrees, 
and in a single plane, while in another there may be complex movements 
and rotation successively in different planes. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 107 


8. The position and movements of the asters in the resting stage seem 
partly determined by the form of the cell. 

9. The rotation of the nucleus and asters into the definitive position 
at the time of karyokinesis often takes place from the longer into the 
shorter axis of the cell, and apparently from the direction of least pressure 
into the direction of greatest pressure. 

10. The form of the cells in many cases does not conform to the law 
of minimal surfaces, being (a) changeable, and (b) even in the resting 
stage widely at variance with the conditions required by the law. 

11. Many of the cleavages are unequal, sometimes extremely so, but 
the inequality shows no significant relation to accumulations of yolk 
material. (See 16.) 

12. The sequence of cleavage is (within very narrow limits of varia- 
tion) constant, and shows no relation to accumulations of yolk. There 
is a general tendency for larger cells to divide faster, but not all the 
facts regarding the succession of cleavages show relation to the com- 
parative size of the cells. 

13. In the resting stage the cells seem to be passive, taking what- 
ever form is impressed upon them by the surrounding cells. As the 
cell passes into the karyokinetic condition it becomes more rounded, 
the cytoplasm tends to group itself symmetrically about the spindle, and 
the cell elongates in the direction of the spindle. 

14. The spindle generally (not always) lies in either the longest or 
the shortest axis of the cell, as maintained by Roux. But apparently 
this is due in Asplanchna to the fact that the cytoplasm tends to group 
itself symmetrically about the spindle. 

15. A change of the relation of a cell to the axes of the egg, as by a 
displacement due to the other cells, results in a change of the position of 
the spindle with reference to the axes of the egg. 

16. During cleavage a cloud of granules is segregated in a portion of 
the cell which is to form the entoderm ; this mass passes from the an- 
terior and ventral side of the entoderm cell to its posterior and dorsal 
side, and is there separated off at the seventh cleavage into the smaller 
entodermal cell. 

17. The egg retains its ellipsoidal form throughout all the processes 
of development, up to a late stage, though as cleavage progresses the 
blastomeres shift extensively their positions with relation to this form. 
This retention of the ellipsoidal form by the egg cannot be referred to 
any simple mechanical factor. (See pages 81, 82.) 

18. Gastrulation accompanies cleavage, and advances step by step 


108 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


with the withdrawal of the deep parts of the peripheral cells and their 
dorso-ventral extension, consequent upon frequent equatorial divisions. 


19. As to facts bearing upon the special morphology of the Rotifera, — 

(a) The polar cell is formed at the animal pole of the egg, at the 
point opposite that where the blastopore is later found, and not at 
the dorsal (or anterior) margin of the future blastoporic region, as 
stated by Zelinka (’91) for Callidina. 

(b) The cleavage of Asplanchna was traced to a later stage than has 
been done for other rotifers. A much greater regularity, and in a cer- 
tain sense symmetry, are shown in the direction and rate of cleavage 
than has been shown for other species. 


B. Conclusions. 


20. It results from 1, 2, 3, 5, 7, and 9 that the direction of cleavage 
is not determined by any simple mechanical factors or relations of form. 
Specifically, the course of cleavage in Asplanchna is inconsistent with 
any general validity of (1) Hertwig’s law of the spindles in the longest 
axis of the protoplasmic mass, (2) Berthold’s law of least areas, and | 
(3) Braem’s and Pfliiger’s principle of least resistance. 

21. It results from 11 that no simple factor can account for the 
equality or inequality of the cleavage. Specifically, the conditions 
in Asplanchna are inconsistent (a) with Hertwig’s view that the 
dividing nucleus takes a position “in the middle of its sphere of 
action,” so far as that expression has any definite significance, and 
(b) with Braem’s principle of “like resistance” at the two ends of the 
spindle. 

22. It results fron 12, as well as from a comparison with the cleavage 
of many other invertebrates, that no simple factor, such as greater or 
less quantity of yolk, will account for the sequence of cleavage. 

23. It is a natural conclusion from 15 and the latter part of 14, that 
the direction of the spindle is not due to an influence in the egg asa 
whole, connected with its axial relations, but is determined within each 
cell itself. However, I do not consider this conclusion at all well 
established. 

24. It results from 5, 6, 7, 8, and 9 that the problem as to what de- 
termines the position of the spindle is resolvable into several: (a) What 
determines the direction of separation of the newly formed asters? 
(b) What determines the position of the asters during the resting stage 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKIT. 109 


of the cytoplasm? (c) What determines the rotation of the asters and 
nucleus as the cell passes into the karyokinetic condition ? 

25. It may be concluded from 20, 21, 22, and 24 that the final posi- 
tion of the spindle and manner of cleavage are causally determined 
by processes —of an unknown character —taking place within the 
protoplasm. 

26. The definite relation of the position of the spindle to external 
conditions observed in some cases — such as to the form of the cell, the 
direction of pressure (!), and the direction of the incoming rays of light — 
is to be interpreted as a reaction to stimulus, dependent in every case 
upon the specific structure of the protoplasm, and variable with that 
structure. 

27. The manner of division is related to the purpose to be attained 
by the given division, and to the general morphogenetic changes in the 
organism. In Asplanchna the method of cleavage is adapted to bring- 
ing about gastrulation. 

28. It follows from 16 that cleavage is not merely a quantitative 
division into similar units; it is accompanied by other developmental 
processes, some of which are distinctly traceable. 

29. Gastrulation in Asplanchna is not a process distinct from cleay- 
age, but is an accompaniment and a result of cleavage. The process 
of which it forms a part begins at the third cleavage and is not finished 
until much later than what is commonly spoken of as gastrulation 
proper.’ 

30. Gastrulation in Asplanchna may be analyzed into several factors. 

(a) The form of the egg, or the influences determining it. 
(b) The direction of cleavage. 

(c) The inequality of cleavage. 

(d) The sequence of cleavage (1). 

(e) The changes in form taking place as the cells divide. 


1 It may be well to state expressly that Ido not consider the above as in any 
sense a general explanation of the process of gastrulation. My aim has been to 
give as nearly as possible a correct account, from the standpoint of developmental 
mechanics alone, of the facts in regard to the early development of a single form. 
The origin of the process of gastrulation in phylogeny is not touched by this 
account. It is a common phenomenon in the organic world, that the same end is 
accomplished by different means in different cases; doubtless in many forms gas- 
trulation is brought about in a way that bears no resemblance to the process in 
Asplanchna. In general, the whole question of the origin of processes to which 
an end or purpose can be assigned lies entirely without the field of the present 


paper. 


110 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


All of these must, according to 17, 25, and 26, be considered as 
determined by the unknown (molecular?) structure and activities of 
the protoplasm. 

31. It follows trom 30 that the early development of Asplanchna, to 
a stage somewhat beyond gastrulation, may be analyzed into two fac- 
tors: (1) the influences determining and preserving the form of the 
egg as a whole, and (2) processes occurring in consequence of the spe- 
cific (molecular ?) structure and activities of the protoplasm. 

Both of these factors, which perhaps should be considered as different 
manifestations of one, are from a causal-mechanical standpoint, entirely 
unknown. “ Damit werden die causalen Bedingungen der Entwicklung 
vorzugsweise in das Moleculargeschehen verlegt und entziehen sich 
vorderhand grossentheils unserer weiteren Erforschung.” (Roux, ’85%, 
p- 427.) 


ss CC 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 111 


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sions of Space. Contrib. Zod]. Lab. Univ. Penn., Vol. I. No. 1, pp. 37- 
50, Plate II. 

Sachs, J. 

'78. Ueber die Anordnung der Zellen in jiingsten Pflanzentheilen. Arb. d. 
bot. Inst. Wirzburg, Bd. II. Heft 1. A/so, Gesammelte Abhandl. iiber 
Pflanzenphysiol., Bd. II. pp. 1068-1125, Taf. IX., X. (Citations from 
the latter.) 


Salensky, W. 
'72. Beitrage zur Entwicklungsgeschichte des Brachionus urceolaris. Zeit- 
sehr. f. wiss. Zool., Bd. XXII. pp. 455-466, Taf. XXXVIII. 
Stahl, E. 
’85. Hinfluss des Beleuchtungsrichtung auf die Theilung der Equisetum- 
sporen. Ber. deutsch. bot. Gesellsch., Bd. III. pp. 334-340. 


Strassen, O. zur. 
95. Entwicklungsmechanische Beobachtungen an Ascaris. Verhandl. deutsch. 
zool. Gesellsch., Juni, 1895, pp. 83-95, 6 Figg. 
Strassen, O. zur. 
96. Embryonalentwickelung der Ascaris megalocephala. Arch. f. Entw.- 
mech., Bd. III. pp. 27-105, Taf. V.-IX. , 
Tessin, G. 
’86. Ueber Eibildung und Entwicklung der Rotatorien. Zeitschr. f. wiss. 
Zool., Bd. XLIV. pp. 273-302, Taf. XIX., XX. 
Watase, S. 
"91. Studies on Cephalopods. I. Cleavage of the Ovum. Journ. Morph., 
Vol. IV. pp. 247-302, Plates IX.-XII. 


116 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


Weismann, A., und Ischikawa, C. 
’'87. Ueber die Bildung der Richtungskorper bei thierischen Hiern. Ber. d. 
Naturf. Gesellsch. Freiburg i. Br., Bd. III, Heft 1, pp. 1-44, Taf. I-IV. 


Wheeler, W. M. 
'95. The Behavior of the Centrosomes in the fertilized Egg of Myzostoma 
glabrum, Leuckart. Journ. Morph., Vol. X. pp. 805-311, 10 Figs. 


Wierzejski, A. 
‘92. Zur Kenntniss der Asplanchna-Arten. Zool. Anzeiger, Jahrg. XV. 
pp: 345-349, 2 Figg. 
Wierzejski, A. 
92". Rotatoria (Wrotki) Galicyi. Akad. d. wiss. in Krakau, pp. 1-106, 
Tab. IV.-VI. 
Wilson, E. B. 
92. ‘The Cell-Lineage of Nereis. A Contribution to the Cytogeny of the 
Annelid Body. Journ. Morph., Vol. VI. pp. 361-480, Plates XIII.-XX. 


Wilson, E. B. 
93. Amphioxus and the Mosaic Theory of Development. Journ. Morph., 
Vol. VIII. pp. 579-630, Plates XXIX—XXXVIII. 


Zacharias, O. 
’84. Ueber Fortpflanzung und Entwicklung von Rotifer vulgaris. Zeitschr. 
f. wiss. Zool., Bd. XLI. pp. 226-251, Taf. XVI. 


Zelinka, C. 
91. Studien iiber Raderthiere. III. Zur Entwicklungsgeschichte der Rader- 
thiere nebst Bemerkungen iiber ilire Anatomie und Physiologie. Zeitschr. 
f. wiss. Zool., Bd. LIII. pp. 1-159, Taf. I-VI. Also, Arb. a. d. zool. 
Inst. Graz, Bd. IV. pp. 323-481, Taf. I-VI. (Citations from the former.) 


Ziegler, H. E. 
94. Ueber Furchung unter Pressung.  Verhandl. Anat. Gesellsch., Ste 
Versamml., pp. 132-146, 13 Abbild. — * Discussion,” pp. 151-156. 


Ziegler, H. E. 
’95. Untersuchungen iiber die ersten Entwicklungsvorgange der Nematoden. 
Zugleich ein Beitrag zur Zellenlehre. Zeitschr. f. wiss. Zool., Bd. LX. 
pp. 351-410, Taf. XVII.-XIX. 


Zimmermann, A. 
’93. Ueber die mechanischen Erklarungsversuche der Gestalt und Anordnung 
der Zellmembranen. Beitrage zur Morphologie und Physiologie der Pflan- 
zeuzelle, Bd. I., Heft II. 8, pp. 159-181. Tubingen. 


JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 117 


EXPLANATION OF PLATES. 


All the figures represent preparations of the eggs of Asplanchna Herrickii de 
Guerne, except Figure 29, Plate 4, which represents the egg of Asplanchna prio- 
donta Gosse. All were drawn, with the aid of the Abbe camera lucida, to a mag- 
nification of 525 diameters. 

The four blastomeres of the four-cell stage are distinguished by different colors, 
and the same color is retained throughout for all the cells (constituting a “ quad- 
rant”) derived from each of the four blastomeres thus distinguished. The quad- 
rant A is blue; B, orange; C, yellow; and VD, red. 

The prominent granulations in the ventral portion of quadrant D are figured ; 
but all other granulations are omitted, except in the case of Figure 7, where the 
general granulation is also represented, though somewhat diagrammatically. 

For an explanation of the system of nomenclature used in lettering the cells see 
page 16. In somecases for want of room the letters have been omitted, the expo- 
nents only being expressed. In such cases the color indicates to which quadrant 
the cell belongs. 

In all the figures where it is possible, the animal pole of the egg is above, the 
ventral pole below. In views of the dorsal or ventral poles of the egg, the anterior 
end is above. Unless otherwise stated, figures represent surface views of more or 
less transparent eggs; these are shaded, whereas sections — optical or actual — are 
not shaded. 


ABBREVIATIONS. 


bl’po. Blastopore. cl. pol. Polar cell. pol.anm. Animal pole. 


Fig. 
Fig. 


Fig. 


Fig. 


Fig. 


Fig. 
Fig. 


JENNINGS. — Asplanchna. 


me 


PLATE l. 


Egg showing the maturation spindle. 

Egg slightly older than that shown in Figure 1; the polar cell is formed 
and is embedded in the egg; the cleavage nucleus, preceded by the 
deep aster of the maturation spindle, is moving from the place of 
polar-cell formation toward the interior of the egg. 

First cleavage spindle, early stage. 

Longitudinal section (actual). The two nuclei formed at the first cleay- 
age are separating; the notch and the granules at the periphery 
indicate the beginning of the formation of the first cleavage plane. 
The aster in the anterior (upper) end of the egg has divided, while 
the opposite aster is still entire. 

Two-cell stage seen from the dorsal side; the anterior end above. 
There are two asters in the cell AB’, while the aster in CD” is 
still undivided. 

Two-cell stage, from the dorsal side; spindles in both blastomeres. 

View of an eight-cell stage; optical section through the quadrants B and 
D, showing the distribution of the yolk granules. Observe the con- 
centration of granules in the ventral part of d#1, and the position of 
the asters in d!! and d*2. 


= ASPLANGHNA. 


ENNINGS 


J 


Metsel lith Bester. 


B 


Buit.Mus. Comp ZOOL. VOL. XXX. 


{] 


ig. 11. 


Fig. 


Fig. 


JENNINGS. — Asplanchna. 


. 10. 


, 18. 


. 14. 


15. 


16. 


PLATE 2. 


Four-cell stage, viewed as a transparent object from the animal pole. 
The anterior end is above. 

Four-cell stage, later than Figure 8, seen from the right side. Spindles 
in C3 and D?; spherical nucleus in 5%. 

Four-cell stage, optical section through the cells 6? and D3, showing the 
division of the asters in preparation for the third cleavage. 

Five-cell stage, from the ventral side. The anterior end is above. The 
aster in d#1 has divided laterally, the two parts being still connected. 
Spindles in A#, 53, and CU? seen endwise. 

Optical section, approximately sagittal, through the quadrants B and D, 
from the egg shown in Figure 11. 

Oblique view of the eight-cell stage, shortly after the third cleavage. 
The animal pole is marked by the polar cell; the anterior median 
line by the boundary between quadrants A and B. 

Dorsal view of the egg represented in Figure 13, showing the oblique 
position of the asters in d*?, and the lateral extension of the aster 
in c#2, 

Ventral view of the egg seen in Figures 13 and 14, showing the oblique 
position of the asters in d#1, and the lateral extension of the aster 
in c#1, 

Optical, nearly sagittal, section of the eight-cell stage, through the quad- 
rants B and D, showing the dorso-ventral direction of the spindles in 
d*1 and d#?, Notice also the change of form of the quadrant B, as 
compared with the same quadrant in Figures 10 and 12. 


JENNINGS 


a. 


PLATE. 


B. Meisel lith, Bosten. 


Vou. XXX. 


P ZOOL 


Com 


Buiu.Mus 


ASPLANCHNA 


, ve t iat Lh 
a), BAe. fiad Py) eat 
Oe 
aie Cee See 
ae ones =) oe 

l ote 


Fig. 17. 


Fig 


Fig. 19. 


Jennies. — Asplanchna. 


. 18. 


PLATE 3. 


Right side of the eight-cell stage, same egg as that seen in Figures 13, 14, 
and 15, showing the lateral extension and beginning of division of the 
asters in c#1 and c*?. 

Right side of a slightly older stage than Figure 17, showing the com- 
pletion of the division of the asters. In c*1 the line joining the asters 
is lateral; in c*? it has already become oblique. 

Right side of a stage later than that shown in Figure 18, containing nine 
cells. The line joining the asters in c? has become dorso-ventral, 
while that joining those of c*1 has become oblique. The cleavage of 
d+ into d®1 and d®? has occurred (compare Fig. 16), while d*? is 
still undivided. 5 


Figs. 20-25. Different views of an egg in the ten-cell stage, to show the position of 


the spindles in relation to the exact form and dimensions of the cells. 


Fig. 20. Right side of a stage later than Figure 19, but containing still only ten 


ig. 21. 


ig. 22. 


Fig. 


Fig. 


. 23. 


24. 


25. 


cells. The line joining the asters in c*1 has become dorso-ventral and 
the spindleis formed betweenthem. Likewise in c**; d*2has divided 
into d*3 and d*4. 

Anterior surface of the egg represented in Figure 20; the spindles occupy 
the shorter axes of 41 and b*?. 

Left side of same egg showing the nuclear conditions in quadrant A. 
The spindles are not yet formed. 

Right face of optical, nearly sagittal, section, through quadrants B and 
D,from the egg shown in the three preceding figures, to exhibit the 
exact dorso-ventral extent of the cells 5*1 and 6*?, as compared with 
the lateral extent of the same cells in Figure 21. 

Dorso-ventral, approximately frontal, optical section of the egg shown 
in Figures 20-23, showing the greatest dorso-ventral extent of the 
cells of quadrants A and C, for comparison with the lateral dimen- 
sions of the same cells, shown in Figures 20 and 22. : 

Posterior surface (quadrant D) of the egg shown in Figures 20-24. 


JENNINGS- ASPLANCHNA. 


B. Meisel lith, Boston. 


HS.J.del. 


Vou. XXX. 


risCte) oe 


But..Mus. Come 


JENNINGS. — Asplanchna. 


PLATE 4. 


Figs. 26-29. Ten-cell stage. 


Fig. 26. 


Fig. 27. 


Fig. 28. 


Fig. 29. 


Left side of an egg in a ten-cell stage, slightly older than that shown in 
Figures 20-25. Note the dorso-ventral elongation of the cells a*1 
and a*?, as compared with the same cells in Figure 22. 

Right side of the egg shown in Figure 26. The cytoplasm is beginning 
to become constricted in the cells of quadrant C. 

Dorso-ventral, approximately frontal, optical section of the egg shown in 
the two preceding figures, viewed from the anterior side, to show the 
greatest dorso-ventral extent of the quadrants A and C at this stage. 

Right side of the egg of Asplanchna priodonta Gosse, in the ten-cell 
stage, showing the spindles in the shorter axes of the cells. 


Figs. 30-36. Sixteen-cell stage. 


Fig. 30. 
Fig. 31. 


Fig. 32. 


Fig. 33. 
Fig. 34. 


Resting condition, seen from the anterior side. 

Left posterior view of the egg seen in Figure 30, showing the position of 
the asters in all the cells of quadrant D. 

Dorso-ventral, nearly sagittal, optical section through the quadrants B 
and D, from the egg shown in the two preceding figures. 

Posterior view of an egg slightly older than that shown in Figures 30-82. 

Nearly sagittal optical section, through the quadrants B and D, in the 
same stage as that shown in Figure 33. 


JENNINGS; ASPLANCHNA . | PLATE. & 


‘ B. Meisel lith, Boston. 


Buti.Mus. Come 700L. VOL. eo ar 


nce’ tell Oi siye is 
re 


Jennines. — Asplanchna, 


PLATE 5. 


Fig. 35. Ventral view of a stage similar to that seen in Figures 88 and 34, Plate 4, 
showing the antero-posterior position of the spindle in d®1. 

Fig. 36. Transverse optical section of the sixteen-cell stage, through the cells of 
the third layer, exhibiting the position of the spindle in d*8. The 
section is viewed from the dorsal side. (Compare Plate 2, Fig. 8.) 

Figs, 87-42. Twenty-cell stage. 

Fig. 87. Completion of the fifth cleavage in quadrant D. Note the oblique posi- 
tion of the elongated aster in d®. 

Fig. 38. Sagittal optical section of the egg shown in Figure 37. The section 
passes, on the anterior side, between the cells of quadrants A and 5, 
showing the cells of quadrant A. 

Figs. 89-42. Different views of one and the same egg. 

Fig. 89. Left side, showing the asters and spindles for the fifth cleavage in quad- 
rants A and B. : 

Fig. 40. Right side of the egg shown in Figure 39. Observe the more advanced 
karyokinetic stages in quadrant C, as compared with quadrant A, 
Figure 59. 

Fig. 41. View looking down upon the animal pole. 

Fig. 42. View of the same egg from the ventral pole, showing the oblique posi- 
tion of the asters in d®1, 

Fig. 48. An older stage than that given in Figures 39-42; the fifth cleavage in 
progress; the egg contains twenty-seven cells. The cells c*? and 
b*3 are forced apart by the cell c®". 


PLATE 5. 


JENNINGS- ASPLANCHNA. 


B Meisel hte Beste. 


BuLi.Mus, Comp. Zobu. VOL. XXX. 


a —_— 
> 
* 
> 
o 
~- 
cy 
4 ~ 
head , 
7 
* 
: L _ 
58 =: 7 ~ 
: - ~ 
7 
a 4 
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Fig. 


ig. 48. 


ig. 49. 


ig. 50. 


Fig. 


Fig, 52. 


JENNINGS. — Asplanchna. 


. 47. 


44, 
. 45. 
- 46. 


51. 


PLATE 6. 


Right side of the egg shown in Figure 43, Plate 5, 27-cell stage. 

Dorsal pole of the egg shown in Figures 45 and 44. 

Posterior view of the egg shown in the three preceding figures. Note 
the lateral position of the asters in d®3, 

Right anterior surface of an egg, showing the quadrants B and C in the 
sixth generation. ‘ 

Sagittal optical section of the 32-cell stage, viewed from the right side, 
and showing the spindles in d*! and d*3. 

Sagittal optical section of an egg slightly older than that seen in Figure 
49, showing the process by which the cell d*? is formed at the sixth 
cleavage of the entoderm. 

Sagittal optical section of an egg a little older than that seen in Figure 
49, showing the recently divided and separating asters in d‘1, and 
the beginning of migration of the cloud of granules which lies at the 
anterior ventral margin of the cell d™1. 

Sagittal optical section of about the same stage as that shown in Figure 
50. The nucleus of d71 has moved away from the periphery of the 
cell, and the cloud of granules is distributed between it and the small 
cells dé? and d™?2, 

Apparently there is some slight variation in regard to the changes 
in the entoderm cell as compared with the other cells. From the 
condition of the remaining cells of the D quadrant, one would infer 
that Figure 51 is younger than Figure 50, though the migration of 
the cloud of granules is more advanced. 

Transverse optical section of the egg shown in Figure 51, through the 
region marked in Figure 51 by the cell d*%. The section is viewed 
from the ventral side. 


B Meisel lith Basten. 


Buu.Mus. Comp Zo6.. VOL. XXX. 


8 


ty sagt dana cla Dom 
4 eH hs ‘ . 


A“ #* ¥ ML LS a | 
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JENNINGS. — Asplanchna. 


. o4. 


g9 


PLATE 7. 


Quadrant Dat a stage a little older than that shown in Plate 6, Figure 46. 
The rotation of the asters in the cell d®3 is in progress. Thirty-two 
cells. 

Later stage than Figure 53, showing the final dorso-ventral direction of 
the spindle in d*. A section of this egg is represented in Plate 6, 
Figure 48. Thirty-two cells. 

Anterior surface of 49-cell stage, showing the sixth cleavage in progress. 

Ventral end of the egg seen in Figure 58, showin the large cell d@1 
nearly enclosed by the other cells. 

The sixth cleavage in quadrant D. 38-cell stage. 

Completion of the sixth and beginning of the seventh cleavage in quad- 
rant D. Same egg as that shown in Figures 55 and 56. 

Dorsal view of the egg seen in Figure 57. 

Dorsal view of a stage later than the preceding, representing the same 
egg as that shown in Figure 58. Sixth cleavage nearly completed. 
Anterior view, showing the completion of the sixth cleavage in the quad- 

rants A and B in all the cells except a®°. Sixty-nine cells. 


—- 


JENNINGS- ASPLANCHNA. 


Buit.Mus. Comp ZOOL. VOL. XXX. 


a 
‘ At 


N 
er li ai 


i. ; Di Thon 


JENNINGS. — Asplanchna. 


PLATE 8. 


Dorsal view of an egg in approximately the same stage as that of Plate 
7, Figure 61. The foursmall unlabelied cells at the point of meeting 
of the four quadrants are a™1®-d‘16. About 69-cell stage. 

Ventral view of the egg shown in Figure 61, exhibiting the nearly com- 
plete enclosure of the entoderm (to which the cell d*1 belongs) by 
the ectoderm. 

Sagittal optical section of the egg represented in Plate 7, Figures 55, 56, 
and 59. The cloud of granules in the entoderm cell d‘-! now sur- 
rounds the dorsal aster of the spindle of the seventh cleavage. 

Sagittal optical section of the egg represented in Figures 61 and 63, 
showing the completed seventh cleavage in the entoderm; the cloud 
of granules forms a ring around the nucleus in the smaller entoderm 
cell d8?,. The ring being cut twice, appears in the form of two groups 
of granules, one anterior, the other posterior to the nucleus. 

Posterior view, showing the seventh cleavage in quadrant D. 

Left posterior view; the seventh cleavage nearly finished in quadrant D; 
the sixth not yet finished in quadrant A. From the egg represented 
in Figures 61, 63, and 65. Sixty-nine cells. 

Posterior view of a later stage than that represented in Figure 67. The 
line separating d* and d*!2 is the posterior median line; the spin- 
dles are seen to be arranged symmetrically with respect to this line, 
so that the cleavage is becoming bilateral. The entoderm is entirely 
covered by the ectoderm; the region where a7! and d®*9 are in con- 
tact shows the place where the entoderm cells formerly occupied the 
surface. 82-cell stage. 


Figs. 69-74 give different views of the same egg, a 94-cell stage. 
Fig. 69. Anterior surface, showing the finished meridional cleavage forming the 


Fig. 70. 


cells a1 — cS, g8-12 — ¢& 12, q3-15—¢8-15, and a’:16—c®16, and the spindles 
for the equatorial cleavage of a™5-c™> and a™‘7—-c7"7, 
Right side of the egg shown in Figure 69. 


JENNINGS- ASPLANCHNA . PLATE. 8. 


B. Meisel lith Bostos. 


Buut.Mus. Comp. ZOOL.VOL. XXX. 


% 


va 


fh 


“ 


tt 


JENNINGS. — Asplanchna. 


Figs. 


Fig. 7 


Fig. 
Fig. 


Fig. 


. 76. 


PLATE 9. 


Left side of the egg shown in Figures 69-74. Ninety-four cells. 

Dorsal view of the egg represented in Figures 69-74. The small 
cells in the centre, at the point of meeting of the four quadrants, are 
qi-16 — qi-16. 

Ventral end of the same egg, showing the crowding together of the 
cells of the quadrants A, 5, and C in this region. 

Posterior view of the same egg, showing spindles for the ninth cleavage 
in some of the cells of quadrant D, and the ninth cleavage completed 
in other cells of that quadrant. 

Anterior surface of a later stage, containing about 120 cells. At the ven- 
tral end (lower part of the figure) the cells are much crowded and 
many of them are very small. The vesicles immediately below the 
cells 08-15, a8, and u®4 are the small ventral products of the cleav- 
age of a75—c™> and a™7-c77, the spindles for which are shown in 
Figures 69 and 70, Plate 8. 


76-79. Successive stages in which the ectoderm is conceived to have been 


79. 


removed from the right side, to show the entoderm cells. 
Egg at the stage shown in Figure 75. A frontal section of this egg is 
given in Plate 10, Figure 8]. ; 
Slightly older stage than Figure 76, viewed in the same way. 
Slightly older stage than Figure 77, showing the change in the position 
of the cells of the entoderm and of those at the animal pole. A view 
of this egg from the animal pole is shown in Plate 10, Figure 82. 
Later stage than Figure 78. The entoderm cells have changed position 
still further, and are approaching cleavage. 


P 


JENNINGS= ASPLANCHNA . PLATE. 9. 


pol.anme. 


‘BL no 


Butt. Mus. ComMP ZOOL. VOL. XXX. 


JENNINGS. — Asplanchna. 


Fig. 80. 


Fig. 81. 


Fig. 83. 


Fig. 84. 


PLATE 10. 


Optical section (approximately frontal) of an egg in the stage seen in 
Figures 69-74, showing the very unequal division of d%-2, 

Optical section (approximately frontal) of a stage later than Figure 80, 
showing the five large entoderm cells. A side view of the same egg 
is seen in Plate 9, Figure 76. 

Stage slightly later than that of Figure 81. The ectoderm is supposed 
to have been removed from the dorso-anterior part of the egg, dis- 
closing the position of the entoderm cells. A side view of the same 
egg is shown in Plate 9, Figure 78. 

Later stage than the preceding, viewed in the same manner. A spindle 
has appeared in each of the five large entoderm cells. 

Optical sagittal section of an embryo at about the time of the beginning 
of the formation of organs. 


JENNINGS= ASPLANCHNA 


Bunt. Mus. COMP ZOOL. VOL. XKX. 


No. 2.— Studies from the Newport Marine Laboratory. 
Communicated by ALEXANDER AGASSIZ. 


XL. 


Some Variations in the Genus EHucope. By ALEXANDER AGASSIZ AND 
W. McM. Woopwortg. 


WE examined for various points nearly four thousand specimens of 
Eucope (3,917). 

Among these we found nine specimens with only three radial canals, 
twenty with five, and three with six radial canals. 

There were fourteen specimens in which one of the radiating canals 
forked, the forking distal or proximal to the genitals being nearly equally 
divided. 

No less than thirty-nine specimens showed distinct traces of serrations 
or spurs from one or more of the radial canals. 

In eight specimens the radial serrations or spurs were not well defined, 
and the position and number of the radial canals were indistinct. 

In eight specimens marginal tentacles were observed, which had be- 
come united at the circular canal, sometimes with the tentacle next to 
the tentacle with the otolith. 

In six specimens there were marked spurs projecting from the base 
of some of the marginal tentacles. 

In eight specimens there were two otoliths in each sense capsule. In 
four there were three. 

In the other specimens the principal variations extended only to the 
degree of development of the cycles of the marginal tentacles and of 
the genital organs. The latter showed in some cases peculiar leaf-like 
expansions extending laterally from the radial canals. 

The radial canals were four in number in an overwhelming majority 
of the specimens examined. 

The study of some of the variations in the genus Eucope was under- 
taken with a view of calling attention to the changes undergone in a 
species of jellyfish, of which great numbers are always easily obtained 

VOL. XXX. — NO, 2. 1 


j hep BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


during the summer months. The size of the full grown Eucope is so 
small that with an ordinary hand lens striking variations can at once be 
detected, and it is possible with a low power to pass in review with 
comparative ease a large number of specimens. 

We hope also to call the attention of zodlogists to the advantages 
of photography, not only in an investigation of this kind, but also to 
.its application for ordinary purposes of delineation. (See Plate VI. 
Figs. 3-6.) 

Dr. Woodworth photographed the specimens reproduced here on 
Plates I.—-VI., and he has written a short account of the methods he 
followed. 

In reviewing the variations we have observed in one species of Eucope 
(E. diaphana), we may call attention to the similarity of these variations 
which occur in this simple Medusa to structures found sometimes in 
closely allied genera or families, and even in some cases to characters 
of groups considered as only distantly related to the genus we have 
examined. 

The great number of marginal tentacles in Eucope they have in com- 
mon with the A.quoride. 

Eucope shares with the Oceanide the limited number of marginal 
tentacles connected with sense orgaus. Pendant leaf-like expansions of 
the genital organs recall those of Melicertidz. 

The presence in Eucope of spurs at the base of the marginal tenta- 
cles recalls similar structures in Zygodactyla, Halopsis, and the like. 

The forking or branching of the radial canals below the genitals is 
found also in Willia and in the Berenicide, a family allied to the Hquo- 
ride ; the forking is symmetrical in the latter, and asymmetrical in the 
former genus. 

The increase in number of the radial canals from the pouch at the 
base of the manubrium is a structural feature which is characteristic 
of the Aquoride. 

The serration of the radial canals is a generic character of Saphenia 
and allied genera. 

The branching or sending off spurs from the radial canals of Eucope 
is a structural feature found in Gonionemus, Ptychogena, Polyorchis, 
and allied forms. The lateral offshoots in Polyorchis being, however, 
arranged in regular succession on each side of the radial canal, much 
like the rounds of a ladder. 

The anastomosing of the radial canals is a feature now characteristic 
of the Discophores. 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 123 


In the increase and special arrangement of the otoliths in the sense 
organ of Eucope we find the first trace of the specialization of the sense 
organs of such genera as Oceania, Tiaropsis, and the like. 

There were no variations noted in the shape of the digestive cavity, 
or in the number of actinal lobes of the manubrium,’ even in specimens 
with five or six radial canals in place of the normal number (four) o 
radial canals. The actinal folds were always found to be four in number. 
In one case only have we found the radiating canal originating from the 
circular canal. (Plate VIII. Fig. 19.) 

The origin of the peculiar club-shaped intertentacular appendages 
characteristic of Halopsis and Laodicea, as well as the spur at the base 
of the tentacles in many Alquoridee, may be referred to the spur-like 
appendages of the marginal tentacles of Eucope figured in Plate VIII. 
Figs. 4-13. 

And it may not be far out of the way to look upon the coalescence 
of adjoining marginal tentacles with sense organs as the first indication 
of such structural features as the radial marginal tentacles of Eucheilota, 
or even of Boungainvillia, Margelis, or Nemopsis. 

It is interesting to note that in Echinoderms there are five radial canals, 
and four or six or more are considered monstrosities, while in Acalephs 
four or its multiples are the normal number of radial canals, and five or 
less are variations. 

The specimens of Eucope showing numerical or structural variations 
were as a rule fully developed males or females, the eggs and sperma- 
tozoa being apparently in a healthy condition. 

It would be an interesting study in heredity were it possible to breed 
the variations in Eucope here enumerated, and ascertain how far the 
structural characters acquired in the variations we have observed can be 
transmitted, and lead perhaps finally to the formation of types which 
we have been accustomed to look upon as having no structural relation 
with the genus. 

But it is also possible that in a comparatively simple genus like 
Eucope these variations are not necessarily to be considered as heredi- 
tary ; they may indicate possibilities in mechanical combinations which 


1 Variations in the manubrium have been observed in Tubularian Hydroids, such 
as Lizzia, Dysmorphosa, Hybocodon, Dipurena, and Sarsia; but as they are usually 
connected with phenomena of reproduction and of budding they have only a dis- 
tant connection with the ltne of the present investigation. See an interesting 
paper by Hartlaub on the reproduction of the manubrium of Sarsia, in Verhandl. 
d. Deutschen Zool. Gesell., 1896. 


124 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


have become characteristic of jellyfishes, and result from their mode of 
life and the simplicity of their structure. Their repetition in closely 
allied genera may denote structural affinity, while in distantly allied 
groups it may be the result of mechanical combinations, and in no way 
indicate any affinity. 

The four primary segments of Eucope are of uniform size in the ma- 
jority of the specimens examined. Whenever there is suppression of a 
radial canal, as in the case of specimens with three radial canals, the 
segments are sometimes uniform (Plate III. Figs. 3, 6), or one of the 
segments, as in Plate III. Fig. 1, is nearly 180°, indicating the total sup- 
pression of the fourth radial canal at its normal point of development. 
See also Plate III. Fig. 4, in which one of the segments is smaller than 
the two from which the fourth section has been cut. 

The inequalities which exist in the segments of some of the specimens 
can best be expressed by a table : — 


2 Meee 6, 2.5. 
1; 1; 13-73 1; 1;—1; it” 1 ae 1; nh aon Te 
5 5 3 3 4 4 PARE SI 3 
ile is AMS 1;—1; 1 AX: Sian Ee 1? 1; 73-15 1? 1; poh 1’ Mee 
2 OO Sree 
i WG ale i; g==ile 1’ sa i poids a an Va 
~a—_— ——S_ Ss 
1, 15, 25,25 25 1.25 85 35 35 | 35 4 4 3.6. 
PEST Pate iy yes wh Dee ey ae 1? 1? ore ae ae 
WL: toh ee Ae ee Ake 
1; 1553 1 pa pists 1’ 
The —— indicates a fork of the radial canal. 
paw ia a2 Ok ate) yore b= 0° a 5 
isto Re LN Ua ac ice st 
1. 12 Taye TEA alae les: 


In this table 1 expresses the smallest segment; the approximate 
dimensions of the others are represented by multiples of it thus: : indi- 
cates that a segment is twice as wide at the periphery as the smallest ; 
*, that it is three and a half times as wide. Of course, the value of 1 


is a different value in each case. 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 125 


1; 1; 1; is the formula for the segments of a Eucope in which they 
are of equal size, the radial canals forming an angle of 120° at the 
centre. 

1; 1; 1; 1; and 1; 1; 1; 1; 1; would each denote the formula 
for segments of equal dimensions in a Eucope with four and one with 
five segments. 

The formation of additional radial tubes may be due to the growth 
of independent tubes from the pouch at the base of the digestive cav- 
ity, or from the forking of tubes, the new canals eventually reaching 
the marginal canal. Im one case we observed a radial canal which 
had its origin at the periphery and did not extend to the base of the 
manubrium (Plate VIII. Fig. 19). Such a formation of a new radial 
canal from the circular canal suggests a similar structure in the short 
canals, in which are found clusters of lasso cells extending at right 
angles from the periphery between the. primary radial canals of Willia, 
and perhaps other Medusz, in which we have clusters of lasso cells 
extending a short distance on the outer surface of the umbrella from the 
marginal canal. 

In the great majority of the specimens of Eucope observed, the 
radial canals are tubes with walls nearly parallel all the way from the 
base of the digestive cavity to the marginal canal (Plates I.-VI.). But 
in a great many instances this parallellism does not exist, and we find on 
the edge of some of the radial canals slight serrations, as in Plate VII. 
Figs. 1-4 and 6. These serrations vary greatly in size, and in some 
cases become short spurs (Plate II. Fig. 4, Plate III. Fig. 3, Plate VII. 
Figs. 2, 3, 5, 7), or even spurs of considerable length (Plate VII. 
Figs. 6, 9, 10); the longer spurs becoming often the forks of the 
primary radial canals (Plate III. Figs. 1, 2-5, Plate VI. Figs. 1, 2, 
Plate VII. Fig. 5), either above or below the genital pouches. Or the 
spurs may form connecting canals between the radial tubes (Plate VII. 
Fig. 4, Plate VIII. Fig. 20), or a rudimentary circular canal round the 
base of the manubrium (Plate VII. Fig. 8). 

Starting with the normal state, in which the genitals are equally 
developed, we find five or six variations, which cover by far the greater 
number of the specimens examined. 

The greatest number of the specimens (622 out of 1146) examined 
for variations in the genital organs were normal, the four genitals being 
equally developed ; the females were more numerous than the males; 
of the latter there were 175, and of the former 447. This stage is 
represented in the table by 1; 1; 1; 1; in which 1 means that the 


126 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


genitals are fully developed, and equally so on each radial canal, of 
which there are four. 

The next stage represented by the formula, 1; 1; 1; 0; the genitals 
were atrophied on one of the radial canals and equally developed on 
the others. Seventy-eight specimens of this stage, 49 females and 29 
males. 

The next most frequent stage is that in which two adjoining genitals 
are fully developed ; the others are of the same size, but less well de- 
veloped ; that stage is represented by the formula 1; 1; 2; 2; Out of 
the 1,146 specimens examined there were only 74 specimens of this 
stage, of which 45 were females and 29 males. 

Next comes the stage in which only one of the genitals is fully de- 
veloped ; the others are less so, corresponding to the formula 1 ; 2; 2; 2; 
39 females and 27 males. 

In the order of frequency of occurrence comes : 

Eucope with the formula 1; 1; 1; 2;— 28 females and 17 males. 

Then comes the stage in which the genitals were unequally developed : 

20 females and 21 males. 


Next, 
Eucope with the formula 1; 0; 0; 0;— 24 females and 13 males. 
“ “cc “c ts 2 0; 0; —Tp “c “ 3 “ 


ce ee “ée 


2g 210) Oy | ae ‘Ot ae 

In specimens with three radial canals we observed only one specimen 
in which the genitals were uniformly developed. On Plate III. are seen 
(Fig. 1) a specimen in which one of the canals forks at the extremities of 
the heart-shaped genitals, forming three primary segments of nearly 
equal size extending to the centre of the disk, with a small sector cut 
from the outer edge of two adjoining segments. 

Figure 4 of the same plate shows a specimen with three radial canals 
and four genital pouches, but the canal which forks subdivides above 
the genitals so near the centre of the disk as to subdivide the disk into 
four nearly equal segments. 

A variation similar to that of Plate III. Fig. 1, for a three-rayed 
Eucope, has been observed in a four-rayed Eucope (Plate III. Fig. 2), in 
which the fifth sector is a comparatively small triangle cut out from the 
periphery of two of the adjoining sections, extending to the centre of 
the disk. 

In Figure 5 of Plate III. the forking of the four-rayed Eucope, taking 
place nearer the centre of the disk, subdivides the disk into segments of 
more uniform size, and it closely resembles a five-rayed Eucope. 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. a IPATi 


In five-rayed Eucope only one specimen was observed with equally 
developed genitals (Plate V. Fig. 1). Those figured on Plate VI., as 
well as the others on Plates IV. and V., all show a very great range 
of variation in their development. In two cases (Plate IV. Figs. 3, 6) 
two of the genitals are wanting. 

In Plate VI., Figures 1 and 2 are those of two specimens with five 
radial canals, both of which fork so near the base of the manubrium 
as to divide the disk into six nearly equal segments. 

The accompanying table will show the other variations which have 
been observed, and their frequency. 


1146 SPECIMENS EXAMINED FOR VARIATIONS IN THE DEVELOPMENT OF THE 
GENITAL ORGANS ON EACH OF THE Four RapbiaL CANALS. 


With Three Radial Canals, or Three with Fork. 


1; 2; 3; 3 females, and 1 male. 


TR Wea US 2) Da a eas LEM 
Tdi 5/1 females 
Os Dis ote 


With Four Radial Canals. 


1; 1; 1; 1;* 447 females and 175 males. 
1 Nhe BO 1 a £8) f anit) ies 
eeths Der X45 “s Fae a7) HPS 
DD Ds. 39 s TVA (0 eae 
Sie Z 1 28 ey by 
1; 2:3; 4; 20 se Coa NE ke 
0 0 05 9 24. “s Je BS 
LS TEG AORN oe SB < ae Qu OS 
iD -careO.. (S19 cs «¢ yates 
Rees Os VAG < + poll At oes 
2 UO 10 s fe Sr 
fo 1 De); 6 s = Suns 
TO Ne Gis te 5 « id Ay) 
IS IR Asa ie 5 se fi yg 
gs CE a3 4 cs ef Bi ee 
Her 0s (O42 3 cs <a ee 
Pe Os 82s) oi 3 cf “ 7 
Mee tO: 2s 4 males. 


*1, fully developed genitals; 2, genital smaller than 1; 3, genital smaller 
than 2; 4, genital smaller than 3; 5, genital smaller than 4; 0, genital organ 
absent. The semicolon (;) indicates the position of the radial canal. The —~ 
indicates the forking of the radial canal. 


128 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


LAO se 20s 4 females. 
Th Ae Ass rate 3 cs 
OF 010) 305 5 specimens. 
D2 Orel 2 females. 
ae VAG hey 1 <e 
Ls 2:3 Soaks i < 
Diwali ier As 1 ss 
TORO ene 1 male. 
Us aisle Be 1 female. 
Ome Os 1 cs 
be 4G 4s 1 e 
es oeuZe Os 1 “ 
ILS Wag (Ve eee 1 male. 
LOS oe 1 female. 
Fees os as 1 ss 
Bs Osos 45 1 < 


With Five Radial Canals, or Four and Five with Fork. 


1+:2- 2-2-1025 a female: 

1; 2; 38; 4; 5; 2 females and 1 male. 
alee (2749 Ms il oe 

Ls 22 2s d204= i amales 

1; 3; 3; 4; 4; 1 female. 

eo eos et < 
LiPo) em ol 

ie ahaha is ibe. ib rrrel bey 

BRE satis pes iia: ST ec 

1; 1; 3; 4; 4; 1 female. 
Ha HORS yee ac tay, a | ts 

Aros O=alomale: 

1; 3; 38; 4; 4; 4 females and 1 male. 
1; 1; 4; 4; 2; 3 females and 1 male. 


The variations in the shape of the genitals from a circular to an ellip- 
tical outline to lobate, foliate, or elongate shape (Plate VIII. Fig. 21) is 
usually connected with the growing together of the genitals of adjoin- 
ing radial canals (Plate VIII. Fig. 18) or with their separation into two 
distinct bodies when the radial canals fork (Plate III. Figs. 1, 5). 

It will be seen that in Eucope, as has been observed in Aurelia, the 
tendency in the numerical variation of the genitals is greatest in the 
direction of an increase rather than a diminution in their number. A 
similar tendency is also noticed in the numerical variation of the radial 
canals of the otoliths. 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 129 


The youngest specimens of Eucope observed (eleven in number) had 
twelve tentacles in all, one at the base of each of the four radial canals 
and two in each quadrant with a sense organ, and without any tentacles 
intermediate between the radial and the sense organ tentacle. It is 
possible that these specimens may belong to a different species, as the 
youngest specimens of Eucope diaphana I have raised from the hydroid 
already had 24 tentacles. But of course it is possible that the above 
stage may have dropped from the reproductive calycle at an earlier stage 
of development. 

Were the marginal tentacles to develop regularly we should have as 
the third stage a Eucope with 48 tentacles, a fourth stage with 96, 
and a fifth stage with 192. 

But as far as our experience shows, we find normal Eucope only in the 
first and second stages, and even then there are numerous variations. 

The formule for the normal stages are, calling T any marginal 
tentacle, 

T,, the primary tentacle at the base of the radial canals ; 

To, the primary tentacle with a sense organ at its base; 

t, the first set of marginal tentacles appearing between T, and T, in 
the stage with 24 marginal tentacles ; 

t, the second set of marginal tentacles on each side of ¢,, in the stage 
with 48 marginal tentacles ; 

t;, the third set of marginal tentacles on each side of ¢, in the stage 
with 96 marginal tentacles ; 

t,, the fourth set of marginal tentacles on each side of fs, in the stage 
with 192 marginal tentacles ; 

ts, the fifth set of marginal tentacles on each side of ¢,, in the next 
Stage with as many (theoretical number 31 tentacles) as 16¢;, 8¢,, 4¢5, 
2t,, 1t; tentacles between the (T, and T,) radial canal and sense organ 
tentacle, or 95 tentacles in each quadrant between the radial canals, or a 
total theoretical number of 384 marginal tentacles. 

The formula of the youngest stage, or first stage, would be 

a(t) fb 4) 2 15 = 127, 
or two tentacles with a sense organ in each quadrant. 
That of the second stage, 


= (4) T. + (4) 2 To + (4) 3¢, = 24 T, 
or two tentacles with a sense organ in each quadrant with a tentacle of 
the ¢, order between each and the radial canal and sense organ. 


1 Proc. Boston Soc. Nat. Hist., June 4, 1862, p. 92. 


130 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


That of the third stage, 
= (4) T. + (4) 2 To + (4) 34, + (4) 64 = 48 T, 
the same as the second stage, with an additional tentacle of the ¢, 
order intercalated on each side of the ¢, tentacles, making 11 tentacles 
in each quadrant, or three between each of the three primary divisions 
ef the quarter. 
The fourth stage, 


= (4) T. + (4) 2 To + (4) 34, + (4) 6te + (4) 124, = 96 T, 
or a tentacle of the third order, ¢t,, intercalated on each side of ¢,, thus 


making 7 tentacles in each of the primary divisions of the quarter. 

The fifth stage, with 192 tentacles, would have for its formula, 
= (4) T.+ (4) 2 To + (4) 34. + (4) 6. + (4) 12, + (4) 24t,= 192 T, 
or a tentacle of the fourth order, ¢,, intercalated on each side of ¢g, 
thus making 15 tentacles in each of the primary divisions of the quarter. 

The theoretical formula for the sixth stage, with 384 tentacles, 
would be, 
= (4) T, + (4) 2 To + (4) 34, + (4) 6t, + (4) 124, + (4) 24t, 4+ (4) 484, = 

384 T, 
a tentacle of the fifth order, ¢;, having been intercalated on each side 
of the tentacles of the ¢; order, thus making 41 tentacles in each of 
the primary divisions of the quarter. 

It is not necessary to carry these stages any further, as the largest 
number of tentacles observed in any primary division of each quadrant 
is not greater than thirteen, which would limit it to a modified stage of 
192 marginal tentacles. 

The accompanying tables have been arranged so as to indicate the 
association, whatever their number may be, with the highest number of 
marginal tentacles occurring in any primary quadrant. This number 
may be a normal number between any T, and To, or between any Ty) and 
To, and may be due to the absence of one of the T, tentacles, or of both, 
in the quadrant. 

A glance at the tables shows how few of the specimens examined 
possessed the normal number of tentacles. We should have as the 
normal stages of each quadrant for the four stages up to 192 tentacles. 

T., To, To, T., first stage, with 12 marginal tentacles. 

T., ti, To, 4, To, 4, T., second stage, with 24 marginal tentacles ; or, 
as in the tables, 1, 1,1; 1, 1,1; 1, 1,1; 1, 1, 1; of which eleven speci- 
mens were seen from the number tabulated for numerical variations. 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 13] 


The formula for the third stage, with 48 tentacles, is, 
Te, tay t1, toy To, tay tay te, To, to, th, be, Tey 
or, as in the table, 3, 3, 3; 3, 3, 3; 3, 3, 3; 3, 3, 3; of which fourteen 
specimens were found from the number tabulated for variations. 
In the fourth stage the formula is, 


be ts, te, ts, hh, ts, to, ts, iN ts, to, ts, th, ts, te, tg, To, bss te, ts, ty, ts, to, ts, i 


or 96 marginal tentacles. 

Of this normal stage only two specimens were observed. 

Of the fifth stage, with 192 marginal tentacles, of which the follow- 
ing is the formula, not a single normal stage was observed : 


he ts, ts, ts, to, ts, ts, ts, hy ts, tz, ta, to, ts, ts, ts, di ts, ts, t4, to, t4, ts, tg, hy 
ly ts, tg, boy ty, bs, ty, To, ts; ts, ty to, ts; ts, ts, to, tg, tz, t4, his ts, tz, ts, toy 
ts, ts, ty, Te 


But a few specimens were seen with the normal number of 15 tentacles 
in some of the primary divisions of one quadrant. 

Of the sixth stage, with 41 tentacies in each of the primary divisions 
of the quadrant, not a single specimen was collected. 

It is interesting to note that in the specimens which may be said to 
belong to the second stage, some of the primary divisions of the quad- 
rants remain in the first stage with only one tentacle, and others with 
two, the third tentacle not having developed. 

Between the second and third stage, in a number of specimens, the 
greatest number of tentacles in a primary division of the quadrant is 
four, and the smallest one. In a number of cases two or even three of 
the quadrants remain in the second stage, and only in one quadrant do 
we find four tentacles in a primary quadrantic division. Similarly, we 
find a number of specimens with five tentacles as the largest number in 
any primary quadrantic division, and some of the quadrants in the second 
stage, but none in the first. Even when we come to six tentacles as the 
largest number of tentacles in any primary quadrantic division, we still find 
some of the primary quadrantic divisions in the second stage, and occa- 
sionally a whole quadrant or a quadrantic division above the first stage, 
as is the case in the second stage and stages intermediate with the third. 

Only four specimens typical of the normal third stage were observed, 
and the great majority of the other specimens in which seven tentacles 
were found in a quadrantic division belonged to stages approximating 
the third more nearly than the second. Only a small proportion of the 
specimens were observed in which the quadrantic divisions belonged to 


MUSEUM OF COMPARATIVE ZOOLOGY. 


BULLETIN : 


134 


With not more than six tentacles in any subdivision : 


Be em Ace Oa al Cet ORT REA Hee 
1D CO H 10 1 HID HID © H 19 6 19 19 19 1D 
WAP TOW POW is Aid Toa oW o OW 
WoO Sic ows Ao Aid id odo AS isc 
DO HD 19 19 19 1D Hc 19 19 0 19 1 HH 19 © 
Wid id ad is Sid id SOAP id SC Sid is 6 oF Ors 
Oris OO iwad as Gis ff Wad ef id Hf id iid o 
LD Hid. IM HID 19 1 HOO 69 O19 19 H 19 © 
Wis iPad Grad Dis Sid Gad Ors rs 9 od oo id 
TSP Ot Had Pa Pid Tid id To did S 
HOLD HH 19 10 HD 19 19 1 tH CO OH 1D H H 0D 19 
WD Hg Sad adad ff dod ot od ad oS od oid 
WiDs isis os ed Hod od AP od ad od df Aid oF id oS 
* 


" 


1D CD19 eH HID AOD OO © 19 1H 19 19 19 19 19 
COA POTS Ow dA TST iT So TSS Aid 


” 


MDotpoterotodaedotooow 
Wid Haig a Pa ai Sid id SOW TT Sis oS 
Sars Haid ad Paid id Did Pad fad Wad oF id ad 
COHNHKOOGC AMC OO OM Hint ow 
Dt At odid ff Aodad Pad a Pada ad dT SDT oa 
WW OOD Had ef tf od od i Wf tf od aad od dad od 


© 19 19 10 6D 19 19 10 H 19 19 10 NI OH oH SH 1D CO 09 10 
WOO OD POS OO wid id Pad Oris iT O19 wh 
OOD rad ch Pas gf Pais SF APaid aD id ad Pid Sid oS 


Oris ois Sid os od Has Pad od ad WP iT Prd S 


Third Stage. 


With not more than seven tentacles in any subdivision : 


mMwmoNior~ HM MODOOrRHAHOMNOnMErEr OOS 
roorwe errors rer rt oonso crow 
Misr Cio GV ASV TS Sid ws Tw a ido ow 
OMOorMMoONMMO® ONnMorNonrattHn oo 
wrigKr Te YL Te CY Tr SAK SSO CK CS 
Misi his o& was 1s ID Cis Gris wii isas oO Ors 
MD OMIDE~ WIDOW WL DOO MOO qi 1919 © O H 
KSC SSK SSO SSSONM SHO BDSOOSOOOS 
Gis Pi is Sisidid Owsowst ws Ggodsds cd wi 
OM OOMMDOHHHOnPMONrErOOrODDO OM 
Sottero er wo iw asd MAS. TA TS re iH 
WS OD QT TAD AD Pa wad ad asd od Hod ad OF oF OF ad rad od 
* 


— 


N 


el 

MOD DO OOM MN OO Mm M19 19 OD Hm 10 HOO HO 
Le PML as OOM ow qe) ene Sf eh mS 1) 0) 
Kod oP SSK STN OP wo Aigo w row 6 


an on ep 


RP HODMROSrEFOrP NM NHNMHONMEr OH OE 
Kor TKT oor re~rSrecos omor~~ om 
KOSS SOMO SMD DCP OK Cr Ts ooo 6 


” nen a 


ee Ue ae Ke Aa heen, been, AGU Atay Were Ne ae SIMA 
KewKrcanNTrwWocKr Tow sidte rr Kor SS 
Mase isid sis COTS TS icKr wr wor ss 
CER OSSCM RM OOSOPoOoSMMD EW Oo oD 
Keir sd wer wer cr yTowrcorcc dH 
Waid Sis HiT CIS Oaidid ad oF Was wad ds wf oF 


135 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 


Ne) 
on on on -” mn Fn -" na" wn on eR RN Veil ns - “_~— en 
ODOM MOOrKrMOW 19 ~~ © 
rrrYrrrrd Kr ort Sor YY 
rece TW DSO r KD MK K Ss 


Seosororrwow YolC ls 
Kcr sKr cos ssn aowrKV YL 
Yr Kr cK GSO WOKr TO OO YS 
SSeS tenis ceo Kens eek iis Woube k ce 
orordsccs cod coor sr rrr Y 
sooo cot SOD oO WO DK or Ks 
SOM OKR HN OSOHOHDOSUNKS 
rose coo ddA dood Kr corr Y 
Wis iDaid es Hid iis oid eo Dow Kos 
* * * * 


i 


ae 
O~-MOMKronsoMDOMrONHS Oo 
~eKr so Wd cK ic oKr TY Co K a LY 
SEK Git KC SWAPS STS Wed isos cwor 
1D re Hid rm 0 10 H 19 19 OO 19 OD ~~ 1 19 
SHO Drom ideoHKY Kr co iG rr sos 


woe SMe cid Kr ero Ww ts So OSK OS . 


Oe OFoowK DOOD HOSOSOOSS 

woreorKYr KY sow rod Or OOS Y 

Hor O seo odo OOO MO OC Or Kor 

SroetsoowooSHOOHWOUDN SL 

oroerYreod do sea o Oa Cor Kr or 

Ordad od Pad Pad Wed Dad Tid id od aad a 
* 


With not more than eight tentacles in any primary subdivision : 


on 


— 


on 


al 


CoH OM~M OM O10 re O 


arr rd cor rrTn 
ES Oid Or Os So 


is) 


So 


on 


N 


CGoeoorMmoonoo 
wyoororcoerrn 


ars “aS cae opteonie 9 @f 


iA) 


on 


ss 


MOOrnonowonwmn wo 
cororreocowns 


Norrorwooorm 
or SSW Wis iid ood 


* 


an 


re © © 19 10 


on 


* 


fo @) wm 1 © 


= i eS ~ a" ~ co Yel Neti Yo 
DHNOOrANDOroOM 


mr 19 =H CO & 19 OC H 19 19 10 
Lr oig Kr Pad Sadie Lis 
LOS Do toed odo ois 


mS me O10 HH CO H © br- 


on 


wD 


rorwrdoawarred? 
Orr iDOY wo od wo 


~oOomtomnroar iow 
Oris Sid WO wo ois is 
Oris Sos Hod od did 


* 


* 


With not more than nine tentacles in any subdivision : 


Ss 
12 00 I 


With not more than ten tentacles: 


1D oO 
sents 
See 
wt © 
19 Ba <= 


*5,4,5; 6,5,5; 6,7 
*3,4,4; 10,1; 3,5, 
*5, 6,7; 9,5,2; 3,1 


VOL. Xxx.—NO. 2. 


136 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


With not more than eleven tentacles : — 


* 11,5; ae ha 3, 11, 105 6, 4,85, 9; 8; 105°6;),.354- 
2 le Scuo.10.. 45 1b, 10,135) Osh es *4, 6,5; 2, 8, 6,7; 1, 11,2; 

So; sno 1 10, 10;, 11,:7, 10; *3;'5, 55°45, 65 4041 ii 

*3; 11,6; 5, 4, 2; 8, 8,4; 7, 7,8; 7, 8,115, 10;-7,. 75 11) Gage. 


With not more than twelve tentacles : — 

#5, 652) 5256, 65 1; 616s 1227): *3,6,2; 3,83 10, 979s 10 ines 
cite eM Guu itt pal he 7.7, (Gee be 8, 7, 12; (2 forks). 

¥5 7 Ae 4a, Os 0; 1,45 12,05 


With not more than thirteen tentacles : — 
6° 6.4.63; 6.1; 42 5 i515 *4,4,5; 6,4,4; 13,5; 5, 6,4; 
*8,3,13; 5; 11,8, 11; 12,7, 10; *9,7,8; 10,7, 10; 13, 9,7; 
With not more than fourteen tentacles : — 
Re Ted et OAl, G2 Dy kg Os Laie £50,156; 1,65 1g o;Gto es 
a PTO tlw leb De iets 
Fourth Stage. 
With not more than fifteen tentacles :— 
*2, 5,5; 2,15, 6,7; 9, 10; 9,2; *6, 7,10; 6,7, 15; 12, 14,5; 
¥6°6) 7-7) 64155 6,25 1,/8:/6 *7, 8,6; 6, 153 3, 1557 Suis 
DUG Mp 1 Os On ie ais ds Odie 
With not more than seventeen tentacles : — 


*17; 15; 15; 16; no otoliths. 


With not more than eighteen tentacles : — 
*6,6,6; 7,7; 9; 18,7; *9,7,9; 18,7; 11,17; 11, 18; 
With not more than twenty tentacles : — 


*7, 6,7; 7, 7,8; 8; 1,8; 20,8; 


With not more than twenty-seven tentacles : — 


#916; S027 500, ils 


137 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 


Table showing Variations in the number of Radial Canals, of Quad- 


rants, of Primary Divisions of Quadrants, or of the Otoliths. 


umber of 
Radial 


Canals. 


Number of N 
Otoliths. 


Irregular. 


State at Ht tt tt a 


MO mM ee & mH 


oD st Om OE Ss 

en CD ese em wa a oik2) 
ute © 1D me OO SH on 
DD AOI Or SS ag 
Taek iS See oye 

10 eg epee eens nee an 
Se be ee CMI eOC EOE rks an 
© 16 aco tH r= OO Mm c— oe ee) 


on 
hen 


SAE alex, es 


sH 


11, 8, 11; 12, 7, 10; 


5, 6 


on 


on 


oO 


sH 


9,10; 9,2; 


7 
» 45 


re 
wv 


on 


KL SOHO OSHMMAPOSHMOGONDOD NOK OK AHH ODN aor onror moo 


> 


lO HHH HO HH OHH HH OOD OH tH HH HD OD OH OH OH tH 


Gee hs 1G) ES Co S565 OS be RO Ste a he Cr 
N . on * seh 
oS me © me 
i 3 ~ s eee 
~ Co tt et as as as 
wo < (c'o} ae res ° . BR Dn 
4 ars Hach fon PS Pe op no ee ~ 
ee eon dex: sete ies gseea) Send Be ea Bo Rencs 

eee ed fe ak aie ae ee 

me eee | " Co = a ae) " Be hc es 
oo BN cee A oes ReneS o(r eed Woolen ee » Pg 
ver HAN H Pate 6) . A) SN St) Sr is * Wireinugs ACES 
Den = a en on H ~~ oO ~ 
Te tT oom Oat © Soe mee AN Sep see ca eee 
oO x 5 ” nn a a a” e al "ox = I~ * ” ” a” 
Oosowe Go mo Me SOs) wat 05) 0) Go) Ma = 
GS ma Pal a inur - oe - .~& Creer ee 
Gente in na oe Re Sener pen coh es Sa eH meget eet bw esol inca ee ener ho) See 
Be ine CGANGH Sto pai Ome CR CiRe Wnt nae Gener been Sys ao ee Fe 
moO yn OO en cH OH St 1 OH Wt) se sem Airis wo beet 
Tete ace easel) ie ae eS ER IUCN Ee aM CHS Tot an Ne Sats 
BGR HOO eh ACTS) es Se ee aS. Ne MS ee 


- - 


ns en SS: on en 


n” 


ee a Le RN en rata i a QE ONT ee nN NT, Se One ORIN ORT LON aN FRAN eR ome 
~ MT CATS SSS SSS PST TASC S oH ow is eo O Wr mm OO 


at 


SAWS SF SOAS MGT TOO A ST ah od 9 69 oS asd SF od oF IO gh Pt od oF Hid as SS Sid ot His 


138 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Number of 
Irregular “Otoliths, Radial 
RA Getis tly Osos tOsn20s Ol 6 5, 1 forking. 
OIG. Ss fi viene: heme 3 3 
6.5; 6s 7,11; Os nose nosy OO; O05 8 5 
1, 6,53. 1, 8 10 aia 5 4 
2.3: 35 a0 U3 4m aes 2, 0, 45 9 4 
6.6. 72)1G) Lar 7G.2 18.165 6 4 


On Plate IX. will be found a number of diagrammatic figures giving 
an idea of the irregular growth of the marginal tentacles. The lengths 
of the tentacles are drawn as fully expanded, the position of the radial 
and circular canals is indicated, and the otolith tentacles are marked by 
across. The structure and length of the marginal tentacles of Eucope 
are such that the comparative length of adjoining tentacles is readily 
observed, owing to the slight degree of contraction and expansion they 
possess. 

In Figures 1 to 6, 10, and 14 (Plate IX.), we have the normal num- 
ber of tentacles (seven) in each of the primary quadrantic subdivisions. 
It is noticeable that ¢, can only in the case of Figures 4 and 10 be dis- 
tinguished from the two f2, while the four ¢, are of nearly uniform size 
in all the figures except Figure 6. 

In Figures 7, 12, 13, 16, 17 (one primary division), 18, 19, and 22- 
26, there are only five marginal tentacles in each primary quadrantic 
division. In the greater number of these figures it is possible to dis- 
tinguish ¢, or ¢, and fj, while the irregularly developed tentacles are 
part of the tf; cycle. In Figures 20-22 (one sector), and 27-29 (two 
sectors), the cycles ¢, and ¢, can be distinguished, and the irregular- 
ity of development occurs in the ¢, cycle, which may appear at different 


points of the circular canal, as is seen by comparing Figures 20-22, 27, 
and 28. 


Figures 8-10, 17, 22, 29, and 30 show the irregularity in time of the 
development of the marginal tentacles in the different sectors of the same 
quadrant, as well as the irregularity in the growth of the three cycles 
t-t; in adjoining sectors. In Figures 8, 29, and 30, the marginal 
tentacles between the otoliths are in the same stage of growth, but 
the tentacles of the right or left sectors are in very different stages of 
growth. In one case (Fig. 30) only one ¢, is developed in each sector ; 
in Figure 29 the cycles of ¢, and ¢, are normal, while they are most 
irregular in the sectors of Figure 8. 

When six marginal tentacles occur in one sector the irregularities in 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 139 


. the # and ¢; cycles are very marked (see Figs. 8, 9, 11, 15, and 17); 
¢, can only be distinguished from the ¢, pair by its position. 

The coalescence at the base of adjoining marginal tentacles to form 
a double tentacle with two spurs and two lashes is not uncommon. 
During the summer there were fourteen specimens met with having 
double tentacles ; in all except two cases they were connected with the 
tentacle riding a sense organ. 

The sensory tentacles usually have only one otolith; we however 
observed thirteen cases in which each sense organ contained two (Plate 
VIII. Figs. 15, 17), and five in which there were three otoliths (Plate 
VILL. Figs. 14, 16), and one in which there were no otoliths in any of 
the quadrants. 

An examination of the table on page 137, in which the more interesting 
of the variations observed have been collected, will show how large a 
number of specimens show great variation in the number of the sense- 
bearing tentacles. Among specimens of Eucope with the normal number 
of quadrants we find the otoliths bearing tentacles vary from eight, the 
normal number, two in each quadrant, to three on one side and ten on 
the other. As will be noted, there are only five cases in which the sensory 
tentacles are greater in number than in the norm, while the number 
of cases in which they are suppressed is quite large. Their increase 
does not always accompany an increase in the number of radial canals. 
Two out of four specimens with five radial canals possessed nine sensory 
tentacles, another only seven, and one eight. 

There seems to be no correlation between the number of marginal 
tentacles in any sector and the number of sensory tentacles. 

The primary sector with the largest number of tentacles has often 
only one sensory tentacle, while that with a smaller number has two. 

In a specimen with quadrants of unequal size, and with an unequal 
number of marginal tentacles, in each of which the formula is 

Gre A od =e, fy Os 
there are six otoliths, one on each of the tentacles at the base of the 
radial canals between the first and second and the second and third 
quadrants, and the others as marked by the comma in the third and 
fourth quadrants. 

In another specimen with very unequal quadrants there are nine oto- 
liths, there being three in the largest quadrant, as shown by its formula. 
the first quadrant being the smallest, the last the largest : 


1, 3, 2; 2,8, 4; 2,3, 3; 3, 1, 3, 4; 


140 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


In a specimen in which two small quadrants are adjacent, there _ 
are only six otoliths. Taking the two smaller quadrants first, the 
formula is 

62d, Se 6 6 Gy 755-6, bis 
The last quadrant is the largest. 
The formula of another specimen, with unequal quadrants, is, 


BAG tGe iy, dso leadoaudie 
This shows only four otoliths, one quadrant without any, and two with 


only one. 
In a similar specimen with a formula of 


1.8.11 19.07) 10 Boor 


there are only six otoliths, with three normal quadrants, one not having 
any sense organs. 

In a specimen with two adjoining radial canals forking below the 
genitals, making four quadrants with two small sectors cut out of two 
of them, there are ten otoliths, the formula being 

eens =“——_—— 
10,9; 95: LOT Gy 2, ©, 2,105. Ug Liaus Oy wy ks 

In a second specimen, forking similarly at the extremity of one radial 

canal, the formula was 
—\ —,, 
GSTs Gilg Goes te os 20h, 

In a specimen with an eccentric digestive cavity and quadrants un- 

equally developed, the formula is 

TA, OD 21,0 sds, 5 Os 
or only five otoliths, neither the first nor the third quadrant having 
otoliths, while the fourth has two side by side. 

In a specimen with equally developed quadrants, but with a long 


spur above the genitals at right angles to the radial canal, barely reach- 
ing the marginal canal, the formula is 


a 
go Pll. 6: 0,4, BEN, Oo, Gs) i545 Os 
there being three tentacles between the fork (spur) and the nearest 


canal. 
In a very irregularly developed specimen, with the formula 


6 Greig ie Die 18, 1s 


there were only three otoliths. 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 141 


In a specimen with only three radial canals, one of which forks above 
the genitals, the formula is 


= 
2,15, 6,7; 3, 10,1; 9,2; 2,5, 5; 


the otoliths being normal in number, but irregular in their distribution. 
In another Eucope with three radial canals, we find seven otoliths 
represented in the formula 


[eit 24 G5 2 8. 6) 75 


the last sector being somewhat larger than the others. 
In a specimen with three equally developed sectors with the formula of 


121A, G6 7,105 6,.7,,.15.; 


there are six otoliths, the normal number in each sector, but the other 
marginal tentacles are most irregularly developed. 

In still another Eucope, similar to the preceding one, with three 
sectors of equal size and well developed genitals with six otoliths, two 
in each sector, the formula was 


Mer W040. 9. 7-9 78: 


Finally, a Eucope with three quadrants of the same size had only 
three otoliths ; its formula is 


9, 6,9; 27; 8,9; 


The formation of spurs (Plate VIII. Figs. 4-13) takes place usually 
at the base of the marginal tentacle, at its connection with the circular 
canal, but cases have been observed in which the spur shoots off from 
the lash of the tentacle (Plate VIII. Fig. 5). The formation of spurs 
is often accompanied by the atrophy of the inframarginal knob of the 
marginal tentacles (compare Plate VIII. Figs. 9-13, with Figs. 4-8). 

It will be noticed that in the numerical variation of the segments the 
tendency is not to doubling the number of normal (4) segments, but 
either to add one or two, or to reduce the segments to three. 

In Sarsia, Agassiz observed a specimen with six radial canals and 
Romanes one with five; he also observed a Sarsia with six radial canals, 
six ocelli, and six tentacles, like that seen by Agassiz, the only speci- 
men in thousands examined. 

According to Bateson, the numerical variations in Aurelia tend in two 
directions, i. e. to forms with six and twelve segments instead of the 
normal eight. 

The same tendency in Aurelia to vary in the direction of six and 


142 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


twelve segments was pointed out by Brown, and was also noted in the 
older papers of Ehrenberg and Romanes. 

According to Romanes, monstrous forms of Aurelia aurita are of fre- 
quent occurrence. Abnormality consisted in multiplicity and abortion 
of parts. All cases of asymmetrical multiplication applied to litho- 
cysts, and always occurred in the same manner. When there were nine 
instead of eight lithocysts, the extra one was always fully developed 
and in close proximity to one of the normal lithocysts. 

In symmetrical abnormalities all parts of the organism were equally 
affected. Thus all examples of multiplication extended proportionally 
to ovaries, nutritive canals, lithocysts, and tentacles, the effect being to 
increase the number while adhering to the type of the natural segments. 
In all cases the degree of abnormality was the same; e. g. 6 ovaries, 
24 unbranched radial tubes, 12 lithocysts, and a six-lobed manubrium. 
All parts and segments thus increased one third their normal number. 
Romanes calls attention to the fact that this is the same proportional 
increase as in Sarsia, with six canals, and explains it as accidental. 
Supernumerary lithocysts always occur at the ends of the faintly 
colored radial tubes, never at the ends of the darker ones. 

Segments and lobes of the manubrium may be multiplied without the 
ovaries increasing in number. Again, segments may multiply and manu- 
brium and ovaries remain normal. Processes of multiplication may not 
extend to all quadrants of the umbrella. Multiplication of parts may 
be confined to one side of the umbrella, thus doubling or tripling organs 
on one side only. 

Abnormalities usually are symmetrical. When they are not, the 
manubrium and ovaries are not affected, the segments only being multi- 
plied. Abortion of parts takes place in the same symmetrical way 
as multiplication: there may be one ovary and six segments, and three 
ovaries instead of eight and four. Segments and ovaries may also be 
reduced to one half the normal number. In these two cases the 
manubrium is not affected. Abortion of parts was observed in the 
ovaries only. Partial suppression of ovaries was of frequent occurrence. 
The most prevalent case was where one ovary was smaller than the other 
three. Reduction also occurs in two alternating ovaries (i. e. opposite ?). 
Sometimes three adjacent ovaries were reduced in size. 

Total suppression of one ovary was more rare. Only in twelve cases 
in thousands was total suppression of two ovaries observed: some- 
times it was two adjacent ones, and more frequently the two opposite 
ones that were absent. In one case three ovaries were absent, the 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 143 


specimen being otherwise fully developed. In no case was it observed 
that deficiency or absence of ovaries entailed a corresponding deficiency 
or absence of other organs. Reduction or suppression does not occur 
in any other organ than ovaries in A. aurita. 

H. C. Sorby found, among A. aurita collected in Suffolk and Essex, a 
“few per thousand” abnormal specimens exhibiting sixfold, fivefold, 
threefold, and partial twofold symmetry. References to variations in 
Aurelia, Clavatella, Sarsia, and Stomobrachium may be found also 
in Bateson’s “ Materials for the Study of Variation,” pp. 421-429. 
Edward T. Browne examined 383 specimens of A. aurita. He found 
that eight specimens (2.08%) exhibited numerical variations in the geni- 
tal sacs, buccal arms, and tentaculocysts. The number of the genital sacs 
and of the buccal arms varied from three to six. He concludes that 
there appeared to be a correlation between genital sacs and buccal arms, 
but that the tentaculocysts vary independently of these. Eighty-seven 
eases (22.8%) showed variation in the number of tentaculocysts. 
Twenty of these had less, and the remainder more, than the normal 
number. The range of variation in tentaculocysts was 6 to 15. 

The preceding observations on the variations of Aurelia show some 
striking differences from those we have made on Eucope. While in 
Aurelia there is a general correlation between the number of segments 
of genital sacs, of buccal lobes, and of tentaculocysts, there is no such 
correlation in the variations of Eucope. The sense organs in Eucope 
vary, both in number and in position, irrespectively of the number of 
radial canals and of segments. Neither multiplication nor abortion of 
parts in Eucope is symmetrical. The suppression of genital sacs is quite 
common in Eucope, while it is rare in Aurelia. In Eucope suppression 
is not limited to genital sacs; as in Aurelia, it extends to the otolith- 
bearing tentacles. As far as we have observed, the number of terminal 
folds of the manubrium does not vary in Eucope, and is not correlated 
to the number of segments. 

The apparatus used in making the photographs was the large photo- 
micrographic apparatus of Zeiss, with some modifications, direct sun- 
light being employed by means of a heliostat of the automatic kind, 
and all exposures were instantaneous. The camera was always used in 
the horizontal position, so that with an objective of low power the full 
length of the bellows could be employed to obtain sufficient magnifica- 
tion with the least loss of light. The objectives employed in photo- 
graphing Eucope were those of 35 and 70 mm. focus, the lower power 
being employed with the larger specimens. The exposures were made 


142 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


twelve segments was pointed out by Brown, and was also noted in the 
older papers of Ehrenberg and Romanes. 

According to Romanes, monstrous forms of Aurelia aurita are of fre- 
quent occurrence. Abnormality consisted in multiplicity and abortion 
of parts. All cases of asymmetrical multiplication applied to litho- 
cysts, and always occurred in the same inanner. When there were nine 
instead of eight lithocysts, the extra one was always fully developed 
and in close proximity to one of the normal lithocysts. 

In symmetrical abnormalities all parts of the organism were equally 
affected. Thus all examples of multiplication extended proportionally 
to ovaries, nutritive canals, lithocysts, and tentacles, the effect being to 
increase the number while adhering to the type of the natural segments. 
In all cases the degree of abnormality was the same; e. g. 6 ovaries, 
24 unbranched radial tubes, 12 lithocysts, and a six-lobed manubrium. 
All parts and segments thus increased one third their normal number. 
Romanes calls attention to the fact that this is the same proportional 
increase as in Sarsia, with six canals, and explains it as accidental. 
Supernumerary lithocysts always occur at the ends of the faintly 
colored radial tubes, never at the ends of the darker ones. 

Segments and lobes of the manubrium may be multiplied without the 
ovaries increasing in number, Again, segments may multiply and manu- 
brium and ovaries remain normal. Processes of multiplication may not 
extend to all quadrants of the umbrella. Multiplication of parts may 
be confined to one side of the umbrella, thus doubling or tripling organs 
on one side only. 

Abnormalities usually are symmetrical. When they are not, the 
manubrium and ovaries are not affected, the segments only being multi- 
plied. Abortion of parts takes place in the same symmetrical way 
as multiplication: there may be one ovary and six segments, and three 
ovaries instead of eight and four. Segments and ovaries may also be 
reduced to one half the normal number. In these two cases the 
manubrium is not affected. Abortion of parts was observed in the 
ovaries only. Partial suppression of ovaries was of frequent occurrence. 
The most prevalent case was where one ovary was smaller than the other 
three. Reduction also occurs in two alternating ovaries (i. e. opposite ?). 
Sometimes three adjacent ovaries were reduced in size. 

Total suppression of one ovary was more rare. Only in twelve cases 
in thousands was total suppression of two ovaries observed: some- 
times it was two adjacent ones, and more frequently the two opposite 
ones that were absent. In one case three ovaries were absent, the 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 143 


specimen being otherwise fully developed. In no case was it observed 
that deficiency or absence of ovaries entailed a corresponding deficiency 
or absence of other organs. Reduction or suppression does not occur 
in any other organ than ovaries in A. aurita. 

H. C. Sorby found, among A. aurita collected in Suffolk and Essex, a 
“few per thousand” abnormal specimens exhibiting sixfold, fivefold, 
threefold, and partial twofold symmetry. References to variations in 
Aurelia, Clavatella, Sarsia, and Stomobrachium may be found also 
in Bateson’s “ Materials for the Study of Variation,” pp. 421-429. 
Edward T. Browne examined 383 specimens of A. aurita. He found 
that eight specimens (2.08%) exhibited numerical variations in the geni- 
tal sacs, buccal arms, and tentaculocysts. The number of the genital sacs 
and of the buccal arms varied from three to six. He concludes that 
there appeared to be a correlation between genital sacs and buccal arms, 
but that the tentaculocysts vary independently of these. Eighty-seven 
eases (22.8%) showed variation in the number of tentaculocysts. 
Twenty of these had less, and the remainder more, than the normal 
number. The range of variation in tentaculocysts was 6 to 15. 

The preceding observations on the variations of Aurelia show some 
striking differences from those we have made on Encope. While in 
Aurelia there is a general correlation between the number of segments 
of genital sacs, of buccal lobes, and of tentaculocysts, there is no such 
correlation in the variations of Eucope. The sense organs in Eucope 
vary, both in number and in position, irrespectively of the number of 
radial canals and of segments. Neither multiplication nor abortion of 
parts in Eucope is symmetrical. The suppression of genital sacs is quite 
common in Eucope, while it is rare in Aurelia. In Eucope suppression 
is not limited to genital sacs; as in Aurelia, it extends to the otolith- 
bearing tentacles. As far as we have observed, the number of terminal 
folds of the manubrium does not vary in Eucope, and is not correlated 
to the number of segments. 

The apparatus used in making the photographs was the large photo- 
micrographic apparatus of Zeiss, with some modifications, direct sun- 
light being employed by means of a heliostat of the automatic kind, 
and all exposures were instantaneous. The camera was always used in 
the horizontal position, so that with an objective of low power the full 
length of the bellows could be employed to obtain sufficient magnifica- 
tion with the least loss of light. The objectives employed in photo- 
graphing Eucope were those of 35 and 70 mm. focus, the lower power 
being employed with the larger specimens. The exposures were made 


144 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


with a ‘‘ Low” pneumatic shutter, this model being chosen on account 
of the small amount of space it occupies, allowing it to be introduced 
between the camera and the microscope. This was accomplished by 
clamping the shutter to the collar on the front-board of the camera, 
another similar collar being screwed to the front of the shutter for the 
light tight connection with the microscope. With the microscope in the 
horizontal position the light was taken directly from the mirror of 
the heliostat and diffused by means of a disk of blue ground-glass 
placed in the substage immediately behind the iris diaphragm, and then 
passes through a simple condensing lens to the object on the stage of 
the microscope. The immediate source of the light, the ground-glass, 
is thus brought near to the object to be photographed, giving a brilliant 
illumination and permitting the use of a small diaphragm. 

The most difficult task was to confine the animals to be photographed, 
more particularly with the microscope in the horizontal position. The 
device which proved most serviceable for flat or discoidal objects was a 
parallel compressor of the model of Hermann Fol. Rings were cut 
from pure rubber tissue of different thicknesses, the ring to be em- 
ployed for any particular object being a little thicker than the object 
itself. The rubber ring was then placed on the lower plate of the com- 
pressor and pressed into contact with it by means of the finger. The 
object is then brought into the centre of the ring, and water added with 
a pipette until the inside of the ring is completely filled, and the upper 
part of the compresser carefully screwed down until it comes in contact 
with the rubber, the superfluous water being at the same time squeezed 
out. If this be done with care, the inside of the ring will be com- 
pletely filled with water and contain no air bubbles. There should be 
just enough pressure to allow the upper glass of the compressor to come 
in contact with the object. This can be determined by holding the com- 
pressor vertically, and screwing down the upper plate until the object 
ceases to sink. The compressor can now be clamped to the stage of the 
microscope in any position. By employing rubber rings of sufficient thick- 
ness, aquaria can be contrived in this way one eighth of an inch in depth. 
In photographing the rounder and plumper forms, any pressure upon 
the animal would produce a sensible change in shape. Such forms, 
therefore, were placed in small deep watch glasses and confined by 
glass rings, the microscope being placed in a vertical position, the camera 
however remaining horizontal. The connection between the microscope 
and camera was effected by means of the prism end of an Oberhauser’s 
camera lucida, to which the light tight collar had been adjusted‘ by 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 145 


means of an adapter. The light in this case was centred upon the 
substage mirror of the microscope, and thus upward. Jellyfish that 
have been for some time in a small quantity of sea water become par- 
tially stupefied by the consumption of the air in the water, and are then 
more quiet and their tentacles are better extended. 

For larger objects, such as Medusze and Ctenophora, a Zeiss series 
II. 1:8 photographic lens with an iris shutter was made use of. A 
reversing prism fastened to the front of the lens allows the use of a 
horizontal camera in photographing animals in open dishes. Work of 
this description is done out of doors, illumination being obtained by a 
series of mirrors, the arrangement of which varies with the nature of 
the object and the view desired. The work is still in an experimental 
stage, and it is hoped to give in a subsequent paper a more detailed 
account of methods and results. 


The following authors have noted variations in Acalephs :— 


Agassiz, L. 

Contributions to the Natural History of the Acalephs of North America. 
Mem. Amer. Acad., Vol. IV. p. 248, Pl. IV. Fig. 4, Pl. V. Fig. 5. 1849. 
Sarsia. 

Bateson, W. 
Materials for the Study of Variation, pp. 424-429. London, 1894. Aurelia. 
Brown, E. T. 

Aurelia aurita: Numerical Variation. Nature, Vol. L. No. 1300, p. 524. 

1894. 
Brown, E. T. 
On the Variation of the Tentaculocysts of Aurelia aurita. Quar. Jour. Micr. 
Sci., Vol. XXXVII. Pt. 3, pp. 245-251, Pl. XXV. 1895. 
Brown, E. T. 
On Variation of Haliclystus octoradiatus. Quar. Jour. Micr. Sci., Vol. 
| XXXVIII. Pt. 3, pp. 1-8, Pl. I. 1895. 

Claparéde, Ed. 

Beobachtungen iiber Anatomie und Entwicklungsgeschichte Wirbelloser Thiere, 
p.5. Leipzig, 1863. Eleutheria. 

Ehrenberg, C. G. 

Ueber die Akalephen des rothen Meeres und den Organismus der Medusen 
der Ostsee. Abhandl. Akad. Wiss. Berlin, 1835, pp. 199-202, Taf. I., II. 
Berlin, 1837. Aurelia, Stomobrachium. 


146 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 


Filippi, F. 
Sopra due Idrozoi del Mediterraneo. Mem. R. Acad. Sci. Torino, 2d ser., 
Vol. XXIII. pp. 375-383. 1866. Eleutheria. 


Forbes, Edw. 
Monograph of the British Naked-eyed Meduse, p.65. London, 1848. Sto- 
mobrachium. 


Herdman, W. A. 
Pentamerous Aurelia. Nature, Vol. L. No. 1296, p. 426. 1894. 


Hincks, T. 
British Hydroid Zodphytes, p. 71. 1868. Clavatella. 


Hornell, J. 
Abnormalities in Haliclystus (Lucernaria) octoradiatus. Preliminary Note. 
Natural Science, Vol. III. p. 53. 18938. 


Hornell, J. 
The Lucernarians as Degenerate Scyphomeduse. A Note upon the Phylogeny 
of the Order. Natural Science, Vol. III. p. 204. 1893. Haliclystus. 


Krohn, A. 
Beobachtungen tber die Fortpflanzung der Eleutheria, Quatref. Arch. fir 
Naturg., Jahrg. 27, Bd. I. pp. 157-172. 1861. 


Romanes, G. J. 
An Account of some New Species, Varieties, and Monstrous Forms of Meduse. 
Jour. Linn. Soc., Vol. XII. p. 528. IL. Vol. XIII. p. 190, Pls. XV. and 
XVI. 1874-76. Aurelia, Sarsia, Stomobrachium, Tiaropsis. 
Sorby, H. C. 
Symmetry of Aurelia aurita. Nature, Vol. L. No. 1298, p. 476. 1894. 


Unthank, H. W. 
Pentamerous Aurelia. Nature, Vol. L. No. 1295, p. 418. 1894. 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 147 


EXPLANATION OF PLATES. 


Plates I.-VI., from photographs taken by W. McM. Woodworth. 


PLATE I. 


To illustrate the variation in the development of the genital organs. The diame- 
ter of the disk of the specimens of Eucope figured varied from 3.5 to 4.5 mm. 


Fig. 
Fig. 


Fig. 
Fig. 


Fig. 
Fig. 


Fig. 


Fig. 


Fig. 
Fig. 


Fig. 


Fig. 


ite 
2. 


3. 


4. 


Adult male, with equally developed genitals, their formula being 1, 1, 1,1, 

Adult male, with only three fully developed genitals, their formula being 
pi her Ie 3 

Male, with the genital formula 1, 2, 3, 4, all in different stages of 
development. 

Female with two atrophied genitals, formula being 1, 1, 0, 0, 

Male of the formula 1, 1, 3, 4, 

Male with only one large genital pouch, with the formula 1, 4, 4, 4, 


PLATE IU. 


Male, with two atrophied genital organs, and with unequally developed 
radial canals, three of which have spurs running at a slight angle 
from them. 

Male, with unequally developed genitals, and with one radial canal forking 
both above and below the genital pouch. 

Young Eucope with undeveloped genital organs. 

Male Eucope with spurs on two of the radial canals, with unequally de- 
veloped genitals. 

Female with only two fully developed genital pouches, and one atrophied 
radial canal. 

Male with genital formula 1, 1, 1, 0, 


148 


BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


PLATE IIL. 


Figures of Eucope having three or more radial canals, and forking either above 


or below the genital organs. 


Fig. 1. Eucope with three radial canals, one of which forks at the extremities of 


Fig. 


Fig. 


Fig. 


Fig. 


Fig. 


Fig. 


2. 


3. 


co 


the heart-shaped genital pouch. 

Male Eucope with four radial canals, also forking at the genital pouch. 
Nearly equally developed genitals. 

Female Eucope with three radial canals, one of which sends off a prom- 
inent spur above therudimentary genital pouch. Genitals very un- 
equally developed. 

Male with three radial canals, one of which forks well above the genital 
organs of the branches. Genital organs nearly equally developed. 

Male with four radial canals. The genital organs corresponding to the 
forks of one of the canals are barely united. Genital pouch atrophied 
in one quadrant. 

Male Eucope with three unequally developed genital organs. 


PLATE IV. 


With figures of Eucope having five or more radial canals. 


Female Eucope with five radial canals and the same number of un- 
equally developed genital pouches, with the formula 1, 2, 3, 4, 5, 
Young male Eucope with five radial canals, unequally developed genitals, 
and a sixth rudimentary radial canal. 

Female Eucope with five radial canals, two fully developed genital 
pouches, one less so, and two radial canals without genital organs. 

Female Eucope with five radial canals, and only one fully developed gen- 
ital pouch. 

Female Eucope with five radial canals, one of which is atrophied below 
the genital pouch. Genital organs unequally developed. 

Female Eucope with five radial canals and unequally developed genitals, 
one of the canals having no genitals, and forking. 


PLATE V. 


Male Eucope with five radial canals and five nearly equally developed 
genital pouches. 

Male Eucope with five radial canals and traces of a sixth, and very un- 
equally developed genitals. 

Female Eucope with five radial canals subdividing the disk into very 
unequal segments, only two of the genital organs fully developed. 
Female Eucope with unequally developed genitals, but with five very 

symmetrical segments. 


AGASSIZ AND WOODWORTH: VARIATIONS IN EUCOPE. 149 


Fig. 5. Female with five radial canals, one of which forks at the periphery, and 
five very unequally developed genitals. 

Fig. 6. Male with five radial canals, unequally developed genitals, and segments 
of unequal size. 


PLATE VI. 


Fig. 1. Male Eucope with five radial canals, one of which forks above the genitals 
close to the base of the digestive cavity, dividing the disk into six 
different-sized sectors. 

Fig. 2. A male Eucope similar to Figure 1, with more fully developed genitals. 

Fig. 3. Young Mnemiopsis Leidyi A. Ag., magnified 4. 

Fig. 4. Doliolum sp. in profile, magnified #2. 

5. Annellid larva (Aricidea?), magnified 19°. 
6. Ectopleura ochracea A. Ag., seen in profile, magnified %2. 


PLATE VIL. 


To illustrate the formation of spurs, branches, and anastomosing canals from the 
sides of the radial canals. 


Fig. 1. Slightly projecting spurs on side of radial canal. 

Figs. 2, 3, 6, 7, and 10 show quite prominent hook-like lateral prolongations from 
the sides of the radial canals. 

Figs. 4, 5, and 9 show forks of the radial canals in Figure 4 above the genitals, in 
Figure 5 below. 

Fig. 4 shows a transverse canal connecting adjoining radial canals. 

Fig. 8 shows a circular canal connecting the radial canals below the base of the 
mapubrium. 


PLATE VIII. 


Fig. 1. Marginal sense-bearing tentacle with three otoliths. 

Figs. 2 and 3 show the coalescence of adjoining marginal tentacles, Figure 2 with 
one otolith, Figure 3 with two. 

Figs. 4-13 show the mode of formation of an abnormal basal spur frequently 
seen jutting out from the marginal tentacles, either with or without 
otoliths. 

Figs. 14-17 show the numerical variation of the otoliths in the sense organ of the 
marginal tentacle. 

Fig. 18 shows the coalescence of male genital organs of adjoining radial canals. 

Fig. 19 shows the formation of a radial canal shooting up from the circular canal. 

Fig. 20. An abnormal Eucope with confluent radial canals and ovaries developed 
on the main and the lateral canal. 

Fig. 21 shows lateral leaf-like expansions of male genital organs which have not as 
yet run together, as in Figure 18. 


150 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


PLATE IX. 


To illustrate the irregular development of the marginal tentacles belonging to 
the t,, ta, and tg cycles. 


Figs. 1-6, 10, and 14 are normal sectors belonging to the third stage of development. 

Figs. 7-9, 12, 13, 16-19, and 22-26 show sectors belonging to the same stage of de- 
velopment, but there has been great irregularity in the succession and 
growth of the third cycle of tentacles. 

Figs. 20-22, 27, and 28 show sectors with only one tentacle of the ts; cycle in each 
primary division. 

Figs, 29 and 380 belong to the second stage of development . 


PLATE 


ARTOTYPE, E. SIERSTAOT, N. Ye 


f, DIERBTAOT, He Ye 


PLATE 


z 
5 
5 


woop 
OODWORTH PHOTO. 


EUCOPE 


nn. Y 


ARTOTYPE, E. SIERSTAOT. 


woob 


EUCOPE 
PLATE ll. 


ARTOYYPE, E. BIERSTADT, Me ¥ 


WOODWORTH PHOTO. 


Le 


—. BIERSTADT, 


ARTOTYPE, 


| 


PLATE 


EUCOPE 


AATOTYPE, E- BIENBTADT, MY 


WoopworrH 


ie 


EUCOPE | 


Sb eT, 


N.Y. 


BIERSTADT, 


ARTOTYPE e 


EUCOPE 


WOODWORTH PHOTO. 


PEATE -V: 


ARTOTYPE, 


EUCOPE 
PLATE V. 


ARTOTYPE, E MIERSTAOT, Me ¥ 


Woo: 
VOODWORTH PHOTO 


NT nce tt een 


ARTOTYPE 


€. 


PLATE 


BIERSTADOT, 


N. 


Vi. 


Y. 


PLATE VI. 


WOODWORTH PHOTO 


_ B-Metsel lith Basten. 


Hi 


i 
| 
a 
aN 


4“ 


6 


10 


4 


i 


16 


0 


18 


29 


10 


No. 3. — Reports on the Results of Dredging, winder the Super- 
vision of ALEXANDER AGASSIZ, in the Gulf of Mexico and the 
Caribbean Sea, and on the East Coast of the United States, 
1877 to 1880, by the U. S. Coast Survey Steamer “ Blake,” 
Lieut.-Commander C. D. SicsBEE, U. S. N., and Commander 
J. R. Bartuett, U.S. N., Commanding. 


[Published by Permission of Cartite P. Parrerson and W. W. DUFFIELD, 
Superintendents U. S. Coast and Geodetic Survey. ] 


XXXVII. 


Supplementary Notes on the Crustacea. By Water Faxon. 


THE following notes were made while identifying some of the “ Blake” 
Crustacea that were retained as “duplicates” when the bulk of the 
collection was sent to A. Milne Edwards in Paris, and some (Macrura) 
that were returned by Milne Edwards undetermined. The notes chiefly 
consist of hitherto unpublished locality records, which add something 
to our knowledge of the distribution of many species. They also in- 
clude descriptions of six new species (five Macrura and one Schizopod). 
Detailed lists of the dredging stations occupied by the “ Blake” will be 
found in the Bulletin of the Museum of Comparative Zodlogy, Vol. VI. 
No. 1, and Vol. VII. No. 4. 


DECAPODA. 


Anamathia hystrix (Stimps.). 


Station 300. 82 fathoms. 1 ¢. 


Anomalothir furcillatus (Srimps.). 


Station 159. 196 fathoms. 1 9. 
Off Port Royal, Jamaica. 100 fathoms. 19. 
VOL, XXX. — NO. 38. 


154 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Pericera cornuta celata (A. M. Epw.). 


Station XX. 50 fathoms. 2 specimens. 


Picroceroides tubularis Miers. 


Station XXI. 33 fathoms. 1. 
The rostral horns and preocular spines are longer than in the male speci- 
men figured by Miers. 


Lambrous pourtalesii Stimps. 


Station XXX. 51 fathoms. 
We SR aby & 


2 &. 
ee 
Neptunus (Hellenus) spinicarpus (Stimps.). 


Station 149. 60 to 150 fathoms. 1 9. 


Achelous spinimanus (Lartr.). 


Station 144. 21 fathoms. 29. 


Calappa flammea (Herpsz). 


Station 144. 21 fathoms. 124,19. 


Acanthocarpus alexandri Stipes. 

Station 148. 208 fathoms. 1 ¢,19. 
© 149; 60'to 150) fathoms.” 1 ¢* 

Myropsis quinquespinosa Srimps. 


Off Port Royal, Jamaica. 100 fathoms. 1 @. 


Tliacantha subglobosa Srimps. 


Station X. 103 fathoms. 1 9. 


Cyclodorippe antennaria A. M. Epw. 


Station 238. 127 fathoms. 1 9. 
“ 946. 154 « Oey: 
« 974. 209 “ ee ae 


FAXON: NOTES ON* THE CRUSTACEA. 155 


Iconaxius caribbzus, sp. nov. 


Plate I. Figs, 1-4. 


Similar to [econazius acutifrons Bate, but different in the form of the ros- 
trum, which is much broader than in J. acutifrons, less triangular in its out- 
line, and broadly rounded at the anterior end; the upper border of the 
propodite of the larger cheliped, moreover, is entire, not denticulate as in 
I. acutifrons. The eyes are larger, and more heavily pigmented. 

The margins of the rostrum are minutely denticulate, as in J. acutifrons, 
the median keel entire. 

Length, 17 mm. 


Station 166. 150 fathoms. 1 specimen. 
weecos. 88 1 ee 
= 2Al. 163 : 3 


Reese: 2a, 1 (type). 

Lives as a commensal in Sponges of the genus Farrea. 

The genus Jconaxius, of which four species have been previously described, 
has a wide distribution in the warm and temperate seas. It has been recorded 
from such remote localities as the Arabian Gulf, Banda Sea, Japan, Kerma- 
dec Islands, and the Gulf of Panama. It is now for the first time recorded 
from the Atlantic. 


Polycheles crucifer (W.-S.). 


Station 29. 955 fathoms. 3 specimens. 


5 135. 4D0te 1 sé 
Peek Boar 1 (exuvie). 
Pena G20. 1 specimen. 
cs Ieee LIST be ee 1 oo 
188: Oia ese 1 ss 
eul9O: yg A 1 se 


Polycheles agassizii (A. M. Epw.). 


Station 129. 314 fathoms. 3 specimens. 


ar melee. 303) | teas 
phen DesBie b Dye =v abs ae 
re eae) Hays 
ORR 297 le ae 


Polycheles sculptus Smiru. 


Station 211. 357 fathoms. 
a DOT. 573 cs 
ae 230. 464 =. 


specimens. 


hm i! 
” 
wn 


156 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Station 245. 1,058 fathoms. 1 specimen.! 
ae 257 553 a 2 4: 
265 576 =e 2 
‘ 268 955 se 1 ‘ 
VII 610 £ 1 es 
co XeViLS 600 é 2 s 


Nephropsis agassizii A. M. Epw. 


Nephropsis agassizii A. M. Edw., Ann. Sci. Nat., Zool., 6 sér., Vol. IX. No. 2, 
1880. 
Station 195. 5024 fathoms. 1 9. 
SF OVO 2G ee Mar be 
DD ie Oils £ Le 


Nephropsis aculeata Smiru. 


Nephropsis aculeata Smith, Proc. U. S. Nat. Mus., Vol. II. p. 431, 1881. 

Nephropsis agassizii Smith, Bull. Mus. Comp. Zoil., Vol. XV. p. 44, Fig. 240, 
1888 (nec A. M. Edw.). 

Nephropsis rosea (W.-Suhm MS.) Bate, Rep. “ Challenger” Macrura, p- 178, Fig. 39, 
Pl. XXIII. Figs. 1, 2, Pl. XXIV. Fig. 1, 1888. 


Station 185. 333 fathoms. 3 specimens. 


co 188. 372) 1 “s 
cS) 222. 422 7 ** 3 a 
Mn 996. ADA Hgts 
c 230. 464 “* 1 2 
“<6 ? ? sé 1 “ 


There are two species of Nephropsis in the West Indian region, N. agassizii 
A. M. Edw., with two pairs of lateral spines on the rostrum, and N. aculeata 
Smith, with only one pair of rostral spines. N. agassizii was very inade- 
quately described by A. Milne Edwards, and the type specimen, from the 
Strait of Florida, 1,500 metres, has never been returned to Cambridge. Soon 
after, the other species, N. aculeata, was described by Smith from specimens 
obtained off the south coast of New England, in 100 to 126 fathoms. Subse- 
quently Smith and other authors supposed that N. aculeata was identical with 
N. agassizii. The chief differences between the two species are the following. 
In N. agassizii the rostrum is armed with two or two and a half pairs? of 
lateral teeth ; in N. aculeata there is only one pair of lateral rostral spines ; 


1 Identified as P. agassizii by A. Milne Edwards, and so recorded by him in 
Bull. Mus. Comp. Zo6l., Vol. VIII. p. 66, 1880. 

2 The third lateral spine may occur on either the right or the left side of the 
rostrum. 


FAXON: NOTES ON THE CRUSTACEA. 157 


the shell is less coarsely granulated, but more spiny in the former species than 
in the latter ; the two lines on the proximal half of the rostrum in both species, 
_ widely diverging as they pass backward over the gastric area, are marked by 
small tubercles in N. aculeata, by distinct acute spines in JN. agassiziw ; the 
top of the small median twbercle on the gastric area is truncated in N. acu- 
leata, while in N. agassizii it is bluntly triangular, passing into a slight 
median longitudinal carina both in front and behind ; the abdominal pleure 
are produced into longer spines in JN. agassizit than in N. aculeata, and the 
spines moreover trend more distinctly backward, forming a stronger angle with 
the vertical axis of the pleura; the outer surfaces of these pleure are quite 
smooth in N. agassizii, while in N. aculeata they are conspicuously granulated 
both on their margins and on the distinctly raised central field ; the lateral 
borders of the abdominal terga, which form a festoon on each side of the ab- 
domen, are more strongly convex in the former species; another distinction is 
apparent in the sixth abdominal somite, viz. in N. aculeata the antero-lateral 
margin of the pleura is shorter than the postero-lateral border, whereas in the 
other species the antero-lateral border is longer than the postero-lateral; the 
tergum of this somite in NV. aculeata sends off a granulated ridge from near its 
posterior lateral angles, —a ridge which runs forward into the upper, depressed 
portion of the pleura ; this ridge is not found in N. agassiziv. 

Nephropsis rosea Bate is without much doubt a young individual of N. acu- 
leata. N. atlantica Norman? is very similar to N. agassizii, but has a sharp 
spine on the anterior margin of the second abdominal pleura. 


Stenopus hispidus (Otiv.). 


Station 11. 37 fathoms. 1 specimen. 
ONES ae id 

. 1 “ 

2 


ie NBA LB: =A 


Pontophilus gracilis Smirs. 


Station 43. 339 fathoms. 1 specimen. 
OEE ge) ee a” 
ee 48. 533 fs 1 oe 
ee Re a i 


Prionocrangon pectinata, sp. nov. 
Plate Ii. Figs. 4-7. 
Rostrum spiniform, inclined at an angle of 45° to the axis of the body. 
Median dorsal line of the carapace armed with a row of eight spiniform teeth, 


' Proce Roy. Soc. Edinburgh for 1881-82, p. 684; Wood-Mason and Alcock, 
Ann. Mag. Nat. Hist., 6th Series, Vol. VII. p. 197, Fig. 4, 1891. 


158 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


which extends backward nearly to the posterior border of the carapace. Antero- 
lateral margins of the carapace angulated below the orbit. Telson much 
shorter than the appendages of the sixth abdominal somite, broad, with a pair 
of dorsal longitudinal ribs, abruptly contracted a short way beyond the mid- 
dle; tip truncate, setiferous. 

The eyes are absent ; their peduncles are transformed into a pair of closely 
apposed trihedral processes, with acute and somewhat divergent tips. The 
first segment of the antennule is very long, reaching nearly to the end of the 
antennal scale; the second and third segments are, on the other hand, very 
short, the third bearing two flagella, the outer of which is very much shorter 
than the inner. The antennal scale is long and narrow, its outer margin 
lightly concave. 

Length, 28 mm. 

Station 201. Off Martinique. 565 fathoms. 1 Q. 

The rostrum is proportionally smaller than in P. ommatosteres Wood-Mason,} 
while the dorsal teeth of the carapace are larger, more numerous, and extend 
farther back on the cephalothorax ; the telson is shorter; the antennal scale is 
longer than the proximal segment of the antennule. According to Wood- 
Mason, there is no trace of eyes or eye-stalks in P. ommatosteres. In P. pecti- 
nata there are distinct rudiments of the eye-stalks, as above described. 
P. ommatosteres comes from the Andaman Sea, 405 fathoms, and the Bay of 
Bengal, 200-350 fathoms. 


Glyphocrangon aculeata A. M. Epw. 


Station 29. 955 fathoms. 1 specimen. 
a) AGS 769 - 2 
USS INTE ESY Mtey?fs) o 1 «6 
« 190. 542 i 4 &s 
“ 195. 502) « i> 4 
«6265. 576 ‘e 2 ee 
“ VIII. 610 ae 1 - 


Glyphocrangon spinicauda A. M. Epw. 


Station 148. 208 fathoms. 2 specimens. 
“6274. 209 ce 12 * 
275.) 28 oo 6 x 
SOE AOS SEO. 4 gt 7 ys 


1 Ann. Mag. Nat. Hist., 6th ser., Vol. VIII. p. 362, 1891; Journ. Asiatic Soe. 
Bengal, Vol LXIII. p. 152, 1894; Ill. Zool. R. I. M. S. “Investigator,” Crust., Pl. 
IX. Fig. 4, 1895. 


FAXON: NOTES ON THE CRUSTACEA. 159 


Glyphocrangon nobilis A. M. Epw.! 


Station 41. 860 fathoms. 6 specimens. 


“ 130. 451 os 2 et 
a hG2 yy fot a: 2 

ie LAVORO es 4 HS 
oO LIGS4 SO! = 1 young. 
fr VIDS 824 ~ 1 specimen. 
fi 1Sd.. 338 “é 7 =< 
GALI Sta - 1 young. 
“© 222. 422 i ye 
es Die a 1 specimen. 


Glyphocrangon neglecta, sp. nov. 
Plate I. Figs. 5, 6. 


Rostrum longer than the rest of the carapace, trending a little downward for 
the anterior half of its length, then curving gently upward to the tip, which is 
slender and acute; the anterior half of the rostrum is distinctly carinated in the | 
median line, but the carina fades away before attaining the base of the rostrum ; 


1 The Glyphocrangon doubtfully referred to G. nobilis in my Report on the Stalk- 
eyed Crustacea of the “ Albatross ” Expedition of 1891 (Mem. Mus. Comp. Zodl., Vol. 
XVIII. p. 142, 1895) is distinct from G. nobilis, as appears from an examination of a 
larger number of specimens of the latter species. In the “ Albatross ” species, which 
may be called Glyphocrangon vicaria, the upper surface of the rostrum is corrugated 
on each side of the median carina, in front of the anterior pair of lateral spines ; in 
G. nobilis this corrugation does not exist. In G. vicaria the anterior moiety of the 
fourth or lateral crest of the carapace is broken into two parts by a deep notch; 
the part in front of the notch is produced anteriorly to form a strong spine, while 
sthe part behind the notch merely forms a projecting angle or shoulder; in G. nobil 
the anterior moiety of the fourth crest is continuous from the posterior end to the 
anterior spine. The tubercles of the first and second crests are more prominent and 
spiniform in G. vicaria than in G. nobilis. The dorsal carine of the telson are den- 
tate anteriorly in G. vicaria, simple in G. nobilis. G. vicaria is even more closely 
related to G. longirostris Smith, which it represents on the Pacific side of the 
American continent. These are the chief differences between the two species: the 
rostrum, corrugated above in both species, is narrower in front of the anterior lat- 
eral spines in G. vicaria than in G. Jongirostris. The anterior moiety of the fourth 
lateral carina is broken into two distinct parts by a notch in the former, while it is 
merely sinuate in its outline in the latter. The tubercles on the first and second 
crests of the carapace are more prominent and spiny in the former than in the 
latter. The median dorsal crest of the abdomen, moreover, is more prominent. 
These differences, though very small, appear to be constant, and afford another 
instance of a slight divergence between two representative forms on the Atlantic 
and Pacific sides of the American continent. ‘The type specimensof G. vicaria were 
dredged in 1189 fathoms, Lat. 0° 54’ N., Long. 91° 9’ W., “ Albatross ” Station 3411. 


160 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


there are two pairs of lateral rostral spines, one of which lies in advance of the 
eyes, the other just behind the posterior wall of the orbit ; on the lower face 
of the rostrum there appears just the slightest trace of a median longitudinal 
carina. The upper surface of the first or dorsal pair of caring is eroded ; be- 
hind the cervical groove this pair of carine converge towards one another. Just 
in front of this pair of carinze, lying in the median line at the base of the ros- 
trum, isa small tubercle or papilla. In the interval between the first and second 
carine on each side are about four faint tubercles on the cardiac region, and on 
each side of the gastric region are four larger low tubercles, the hindmost of 
which is the largest of all. The anterior moiety of the third carina (adopting 
Wood-Mason’s terminology) is well developed as a backward prolongation 
of the external orbital spine, which is long, acute, and inclined outward and 
upward. The fourth carina is also developed both anteriorly and posteriorly 
to the cervical groove, its anterior moiety being continuous with the antero- 
inferior, or branchiostegian, spine of the carapace. Barring the external orbital 
and branchiostegian spines, the anterior moieties of both the third and fourth 
caring are entire, without a trace of spine or tooth. The trend of the branchi- 
ostegian spine is nearly straight forward, its downward and outward deflection 
being very slight. With the exceptions noted above, the spaces between the 
caring of the carapace are pretty smooth. . 

The abdomen is lightly sculptured for the genus to which this species be- 
longs. Only the first and sixth segments are conspicuously carinated above. 
The pleure of the second abdominal segment are one-toothed. The telson 
exceeds the last pair of abdominal appendages, and is rather abruptly bent: 
upward at the tip. 

Length, 75 mm.; cephalothorax including rostrum, 35 mm.; rostrum, 19 mm.; 
telson, 13 mm. 

Station 261, off Grenada. 340 fathoms. 1 9 with eggs. Type. 

“153, off Montserrat. 303 “ L@. 
“« ~ 260, off Grenada. 291 oe 1 young. 

This species is peculiar in having the anterior moiety of the third and fourth 
carine of the carapace well developed and continuous with the external orbital 
and branchiostegian spines respectively. In G. gilesii Wood-Mason, which 
also has the anterior portion of both the third and fourth crests developed, 
these crests are produced anteriorly into small spines independent of the- 
external orbital and branchiostegian spines. 


Stylodactylus serratus A. M. Epw. 


Station 205. 334 fathoms. 3 specimens. 
Se oles o5G ce it " 


Pantomus parvulus A. M. Epw. 


Station 134 248 fathoms. 2 specimens. 


FAXON: NOTES ON THE CRUSTACEA. 


Pandalus longipes A. M. Epw. 


Station 274. 209 fathoms. 12+ specimens. 


« 291. 200 12+ : 
pew vadaes "tO! 2 
oe SGU) B91) 12+ 


Pandalus ensis A. M. Epw. 


Station 208. 213 fathoms. 


specimen. 
uk 2582 ASD. 9“ : 


we 


Pandalus leptocerus Smiru. 


Station 345. 71 fathoms. 1 specimen. 


Heterocarpus levis A. M. Epw. 


Station XXVI. 297 fathoms. 1 specimen. 


Heterocarpus alexandri A. M. Epw. 


Station 196. 1030 fathoms. 1 specimen. 


Heterocarpus ensifer A. M. Epw. 


Station 146. 245 fathoms. 1 specimen. 
Ory loe., 05 as ] ee 
Se 2Oos, lOo xs 2 as 


Nematocarcinus cursor A. M. Epw. 


Station 151. 356 fathoms. 12+ specimens. 
“« 160. 393 = 2 Bs 
i> 261. 583 1 - 
- S205. (334. 2 “ 
Sees BIS ne 2 “ 
‘<3 O74. 209 * 1 «“ 


Hoplophorus gracilirostris A. M. Epw. 


Station 100. 250-400 fathoms. 1 specimen. 
« 191. 108-250 ce 1 ef 
« 996. 494 Cs es 
23); (464 ‘ 1 < 
“ 958. 159 “ iy ye 
« 971. 458 « Li a 


161 


162 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Acanthephyra affinis, sp. nov. 


Plate If. Fig. 1-3. 


Similar to Acanthephyra (Systellaspis) lanceocaudata Bate, but different in 
the following regards : the apical tooth of the antennal scale projects forward 
far beyond the membranous part of the organ; the telson is shorter than even 
the inner branches of the posterior pair of abdominal appendages, and its dorsal 
surface is flattened, but not grooved. 

The seven teeth that surmount the gastric crest are closely approximated, 
and increase in size successively from the first to the fifth. The sixth is about 
equal to the fifth, the seventh a little smaller. The egg of this species measures 
3X 2mm. 

Length, 100 mm. 

Station 258. 159 fathoms. 19. 

This species belongs to the subgenus Systellaspis, in which the orbit is con- 
tinuous to the first antennal tooth (the orbital tooth being absent), the dorsal 
carina of the sixth abdominal somite is wanting, and a prominent angle or tooth 
projects from each side of the anterior border of the first abdominal somite, 
overlapping the posterior margin of the carapace. The eggs, moreover, are of 


oo? 


large size, indicating a protracted period of intra-oval development. 


Acanthephyra debilis A. M. Epw. 


Station 107. 428 fathoms. 1 specimen. 


Acanthephyra armata A. M. Epw. 


Station 135. 450 fathoms. 1 specimen. 
a 151. 356 ae 2 . 


Sicyonia edwardsii Miers. 


Station 142. 27 fathoms. 1 specimen. 


Sicyonia brevirostris Srimps. 


Station 38. 20 fathoms. 1 specimen. 


Peneus brasiliensis Larr. 


Station 37. 35 fathoms. 2 specimens. 
cic! Maem! 55 od 3 young. 


FAXON: NOTES ON THE CRUSTACEA. 


Parapeneus megaiops SmItH. 


Station 147. 250 fathoms. 4 specimens. 


oo etaci 208) 4 Zi 
eesngetoon» Bhs 
ceo. 21s) + 4 « 
EE ogi 12 eee 
Seas Uae y ane 


Parapeneus politus Sirus. 


Station 36. 84 fathoms. 27 specimens. 


Haliporus debilis (Smita). 


Station 47. 321 fathoms. 1 specimen. 


Plesiopeneus armatus (Bare). 


Station 31. 1,920 fathoms. 2 specimens. 
es iijc 411 ee 1 oe 


Hemipeneus triton Fax. 


Station 227. 573 fathoms. 1 specimen. 


Benthesicymus bartletti Smita. 


Station 29. 955 fathoms. 1 specimen. 
ES 33. 1400-1568 xe 1 e 
elGs: 769-878 eS 2 
lyse 824 oe 1 es 
SS P90: 542 ce 1 
LS PE 573 “: 2 ae 
Ae 1058 ce 1 <¢ 
C9 Aa, 576 i 1 ee 
e288. 399 2 2 re 


Sergestes robustus Smiru. 


Station 205. 334 fathoms. 1 specimen. 
bee oanl. van, 
SSS OU O2O iN. ce 
mer 2G4. ALG. 1S 
tt) 265,576 -; “ 
pase eO2G, 9) °* 


a On a 
n 


163 


164 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Sergestes mollis Smiru. 


Station 30. 968 fathoms. 2 specimens. 


SCHIZOPODA. 


Lophogaster longirostris, sp. nov. 
Plate. II. Figs. 8-10. 


Similar to L. typicus Sars, but different in the great length of the median 
spine of the rostrum, which far surpasses the antennular peduncle, and almost 


attains to the tips of the antennal scales. There are six teeth along the outer 
edge of the antennal scale. Length, 27-mm. 


Station 50. 119 fathoms. 20 specimens. 


Gnathophausia zoéa W.-Supm. 


Station 185. 333 fathoms. 2 specimens. 


ec 201. 565 ee ce 
ce 221. 423 ce se 
“e Ad 573 ce ia3 


S| Yrs Thslay He 
«230. «464 o 
<= 284. 347 a 
4 P4sish GEN, y 


(JCI NO 
a 
a 


Eucopia sculpticauda Fax. 


Station 30. 968 fathoms. 1 specimen. 


Petalophthalmus armiger W.-Sunm. 


Station 29. 955 fathoms. 1 Q.. 


This is the specimen figured in my Report on the Stalk-eyed Crustacea of 


the “ Albatross” Expedition of 1891, Pl. LIII. Fig. 2 (Mem. Mus. Comp. Zodl., 
Vol. XVIII.). , 


FAXON: NOTES ON THE CRUSTACEA. 165 


STOMATOPODA. 


Squilla empusa Say. 


Station 36. 84 fathoms. 1 specimen (young). 


Pseudosquilla ciliata (Fasr.). 


Martinique. 1 specimen. 


ISOPODA. 


Bathynomus giganteus A. M. Epw. 


Station 179. 824 fathoms. 1 specimen, 157 & 80 mm. 
VII. 610 = 1 5 1077 49) 

According to Wood-Mason and Alcock (Ann. Mag. Nat. Hist., 6th Series, 
Vol. VII. p. 270, 1891), this remarkable Isopod was captured in the Bay of 
Bengal at a depth of 740 fathoms. Dr. Arnold Ortmann! has described a 
second species of Bathynomus (B. dederleini), taken on the coast of Japan, 
near Enoshima, Sagarni Bay. The depth is not recorded. 


1 Proc. Acad. Nat. Sci. Phila., 1894, p. 191. 


166 


Fig. 1. 
Fig. 2. 
Fig. 3. 
Fig. 4. 
Fig. 5. 
Fig. 6. 
Fig. 1. 
Fig. 2. 
Fig. 3. 
Fig. 4. 
Fig. 5. 
Fig. 6. 
Big ai. 
Fig. 8. 
Fig. 9. 


Fig. 10. 


BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


EXPLANATION OF THE PLATES. 


PLATE I. 


Iconaxius caribbeus Fax. M. C. Z., No. 4195. Blake Sta. 283. x 54. 

The same. Head, from above. X 4}. 

The same. Right chela, from the outside. X 5}. 

Iconarius caribbeus Fax. Telson and posterior pair of appendages. 
M. C. Z., No. 4147. Blake Sta. 241. Much enlarged. 

Glyphocrangon neglecta Fax. Female, dorsal view. M. C. Z., No. 4454. 
Blake Sta. 261. 14. 

The same. Lateral view. X 1. 


PLATE Ii. 


Acanthephyra affinis Fax. Female. M. C. Z., No. 4410. Blake Sta. 258. 
x 11. 

The same. Telson. X 14. 

The same. Antennal scale. X 13. 

Prionocrangon pectinata Fax. Female. M. C. Z., No. 4436. Blake Sta. 
21. xX 4: 

The same. Carapace, from above. X 4. 

The same. Chela. X 4. 

The same. ‘Telson and posterior pair of abdominal appendages. X 4. 

Lophogaster longirostris Fax. M. C. Z., No. 4580. Blake Sta. 50. X 4. 

The same. Carapace, from above. X 4. 

The same. Telson and posterior pair of abdominal appendages. X 4. 


XON, BLAKE’ CRUSTACEA PLATE |. 


% 


S a. “i ‘e 
\ =r 


a ‘TI 
A 
Aye’ 


4 —— : ass VT : 
T= SN 


H.Emerton del, Heliotype Printing Co. 


Iconaxius caribbeus Fax. 
Glyphocrangon neglecta Fax. 


1-4 
9-6 


a 


J 
(ON, BLAKE” CRUSTACEA 


A 
A 
A 
A 
A 
A 
A 
A 


merton del. 


1-3 Acanthephyra affinis Fax. 
4-7 Prionocrangon pectinata Fax. 
8-10 Lophogaster longirostris Fax. 


Hehotype Printing Co. 


oui? 


W 


as 
‘ 


No. 4.—On the Color and Color-Patterns of Moths and Butter- 


Jlies+ 


By ALFRED GOLDSBOROUGH MAYER. 


This research is an investigation of the general phenomena of Color 
in Lepidoptera, and also a special account of the Color-Patterns of 
the Danaoid and Acraeoid Heliconidae, and of the Papilios of 
Tropical South America, and has been carried out under the direction 
of my friend and instructor, Dr. Charles B. Davenport; and the work 
was done in connection with one of the courses given by him in 
Harvard University in 1894-95, Iam indebted to Dr. Davenport 
not only for suggesting the subject, but also for his kindness in deyot- 
ing much time to a criticism of the results. 

The paper is divided into three parts. Part A contains an ac- 
count of the general phenomena of color in Lepidoptera; Part B 
is devoted to a special discussion of the color-variations in the Heli- 
conidae, with special reference to the phenomena of mimicry; and 
Part C consists of a summary of those results which are believed to 
be new to science. A Table of Contents is given at the end of the 


paper. 


PART A. 


GENERAL PHENOMENA OF COLOR IN LEPIDOPTERA. 
I. CLassIFICATION OF CoLors. 


We follow Poulton (90) in dividing Lepidopterous colors into (1) 
pigmental and (2) structural. 

(1) Pigmental Colors are due to the presence of an actual pig- 
ment within the scales, and although such colors are very common 
in the Lepidoptera, it is frequently very difficult to say off-hand 
whether a given color is due to a pigment or to some structural effect. 
Coste (90-91) and Urech (93) have, however, given criteria for de- 
termining whether acolor is due to a pigment or to some other cause. 
They succeeded, for example, in dissolving out the color in many 


*Contributions from the Zodlogical Laboratory of the Museum of Comparative Zodl- 
ogy at Harvard College, E. L. Mark, Director, No. LX XIV. 


*This paper was written in 1895 essentially as it now stands. 


170 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


cases, leaving the wing white or colorless. Coste used as solvents 
a number of strong acids and alkalis; while Urech confined him- 
self to the use of water, hydrochloric acid, and nitric acid. Their 
results may be conveniently summarized as follows :— 

Black according to Urech is a pigmental color, for it may be dis- 
solved out of the wings by means of hydrochloric or nitric acid, 

Brown is usually insoluble in water, but is soluble in hydrochloric 
or nitric acid. 

The red and orange pigments of the Pieridae, Lycaenidae, 
Nymphalidae, Zygaenidae, and some Papilios are soluble in water. 
They are insoluble in water in the Sphingidae, Arctidae, Bombycidae, 
Saturnidae, and Geometridae. 

Yellow pigment is acted upon by reagents in almost thesame way 
as the red and orange, especially if both red and yellow appear upon 
the same wing. It is soluble in the Pieridae, Lycaenidae, Nym- 
phalidae, Satyridae, and some Papilios, but insoluble in the Sphin- 
gidae, Arctidae, Geometridae, and a few Noctuidae, 

White is usually a structural color, but can be dissolved out 
from the wings of the Pieridae by water, being in this case, of — 
course, due to a pigment. ' 

Green pigment can be dissolved out by water in the cases of the 
Pieridae, Lycaenidae, and Geometridae. In the vast majority of _ 
cases, however, it is a structural color. © ; 

Violet and blue are almost always due to structural causes. In a 
few cases, however, as in Smerinthus ocellatus, a blue pigment can 
be dissolved out. 

We see, then, that black, brown, red, orange, and yellow are 
usually due to pigment, while white, green, violet, and blue are gen- 
erally due to structural effects. 

It is well known that the scales of Lepidoptera are essentially 
hollow, flattened sacs often inclosing pigment, and Burmeister (78) 
arrives at the conclusion, from a study of the scales in various spe- 
cies of Castnia, that the pigment is for the most part attached to the _ 
upper layer of the scale-sac, rendering it opaque, while the lower q 
layer receives less pigment and is, in consequence, a little more 


translucent. ‘ 
(2) Structural Colors owe their origin to the external structure of — 
the scales or wing-membranes and not to the presence of a pigment. — 
They are often caused by diffraction, due to the scales being covered 
with fine, parallel striae. Some of the most splendid colors in the — 


th i a 


MAYER: COLOR AND COLOR-PATTERNS. Jie 


animal kingdom are due to this cause; such are the iridescent and 
opalescent hues of many of the Morphos and Indo- Asiatic Papilios. 
Very often the scales which display such brilliant colors contain no 
pigment whatsoever; for if one will merely soak them in alcohol, 
ether, or water, all color disappears, and the scales become as _ trans- 
parent as glass. This test was devised by Dimmock (’83), who 
used it upon the brilliantly colored scales of many beetles. It 
was first discovered by Burgess (80), and has since been con- 
firmed by Kellogg (’94), that the striae which produce these structural 
colors are all upon the outer surface of the scale, 7. ¢., the surface 
which is away from the wing-membrane and exposed to the light. 
Kellogg (94) has determined the distance apart of the striae upon 
the scales of many species of Lepidoptera. It appears, for example, 
that the striae upon the scales of Danais plexippus are 2u apart, 
those upon the transparent scales of Morpho sp. 1.54, upon the 
pigment-bearing scales of Morpho 0.724, and upon Callidryas 
eubule 0.94 apart. It is very evident, then, that the brilliant color- 
ation of the scales may be due to this fine striation, for the striae 
upon Rowland’s or Rutherfurd’s finest gratings are approximately 
1.5~ apart, which is about the average distance between the ridges 
of the scales. 

Structural colors are, however, not always due to diffraction; in 
the case of white, for example, the color is almost invariably due to a 
reflection of all, or nearly all, the light that impinges upon the scales. 
As long ago as 1855 Leydig pointed out that the silvery white color 
seen in the scales of some spiders, such as Salticus and Tegenaria, 
was due to air contained within them ; and more recently Dimmock 
(83) has shown that silvery white and milk-white colorations are 
due to optical effects produced by reflected light. In the silvery white 
scales, however, such as those of the under surface of the hind wings 
of Argynnis, there must be a polished reflecting surface toward 
the observer, for both silvery and milk-white colors appear simply 
milk-white by reflected light. 

(3) Combination Colors owe their richness and brilliancy to a 
combination of structural and pigmental effects. The geranium-red 
spots upon the hind wings of the Mexican Papilio zeunis Lucas owe 
their red color to pigment, but over this red there plays, in certain 
lights, a beautiful pearly iridescence, which, in combination with the 
red, greatly enhances its charm. Urech (92) has demonstrated that 
in the Vanessas there are scales which have chemical coloring matter 


172 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


and interference colors also. In addition, he points out the interest- 
ing case of certain Lycaenidae where the scales exhibit to the eye 
only interference effects, and yet a pigment can be dissolved out of 
them by the use of water. 

(4) Quantitative Determination of Pigmental Colors. I have 
analyzed the colors of many butterflies by means of the spectroscope, 
and also by Maxwell’s discs. As is well known, Maxwell’s dises are 
colored circular discs of cardboard, perforated at the center and slit 
along a radius so that two or more of them may be slid over each 
other, thus exposing different proportions of each. Then by rapidly 
rotating them the colors become blended, and thus it becomes 
possible to match any color, and to discover its fundamental con- 
stituents. By this means I have determined that the vast majority 
of the colors found in Lepidoptera are impure; that is to say, they 
contain a large percentage of black. 

For example the white of the upper surface of the wings of the 
common Pieris rapae consists of: 17% black, 13% emerald-green, 
10% lemon-yellow, and 60% white. 

Also the so-called “ blacks” found in butterflies are rarely jet-black, 
but, almost always, only deep shades of brown. For instance the 
deep brown color of the under surface of the wings of Heliconius 
melpomene consists of 938% black, 3% lemon-yellow, 3.5% of 


Maxwell’s fundamental red (vermilion), and 0.5% of von Bezold’s 


fundamental blue-violet. 

The purest color I have met with is the canary-yellow ground 
color of the wings of Papilio turnus, which seems to consist of 
white light with the addition of a little yellow. 

Other colors all possess considerable black. Thus the glaucous 
green of Colaenis dido consists of black 29%, vermilion 24%, 
emerald-green 37%, von Bezold’s blue-violet 10%. 

The sepia-brown ground color of Cercyonis alope consists of black 
71%, vermilion 21.5%, emerald-green 7.5%. 

The tawny rufous color of the wings of Mechanitis polymnia, ete., 
is made up of black 46%, vermilion 40%, lemon-yellow 14%. 

The rufous red patch on the upper surface of the fore wings of 
Heliconius melpomene is made up of black 27%, vermilion 66.5%, 
lemon-yellow 6.5%. 

The yellow of the fore wings of Mechanitis polymnia consists of 
lemon-yellow 67%, emerald-green 14%, and white 19%. 


MAYER: COLOR AND COLOR-PATTERNS. 173 


(5) Spectrum Analysis of Colors of Lepidoptera. I have made 
some spectrum analyses of the light reflected from the wings 
of various butterflies, by means of a piece of apparatus most kindly 
suggested for the purpose by Prof. Ogden N. Rood of Columbia 
College. The arrangement is shown in Figs. 1, 2, Plate 1; Fig. 1 
being a perspective view, and Fig. 2 a horizontal section of the 
apparatus, which consists of a rectangular box, blackened upon the 
inside, and having a well-fitting cover. A rectangular slit (O) was 
cut through one of the long sides of the box, near one end, and the 
other end of the same side was perforated in order to allow the 
admission of the direct-vision spectroscope (S). Imagine that we 
wish to examine the yellow spots from a butterfly’s wing. All of 
the yellow spots from the wing are cut out, and pasted upon two 
pieces of cardboard so as to make two large unbroken patches of 
color. The pieces of cardboard are then blackened upon all those 
places where the colored wing was not pasted. One of the card- 
boards is then suitably mounted upon the back of the box at B; the 
other is placed upon a vertical support (F), the plane of which is 
parallel to the back of the box. 

The working of the apparatus is as follows: the sunlight enters 
by the slit (O) and is reflected and diffused three or four times 
between the pieces of colored wing mounted upon the back (B) of 
the box, and the vertical support (F). The manner of this reflec- 
tion and diffusion is shown by the dotted lines of Fig. 2. After 
undergoing several reflections, the light enters the direct-vision 
spectroscope (S). The slit of the spectroscope is wide open, and 
thus the light which enters it may readily be examined. It was 
found that it was necessary that the light be reflected more than 
once from the wing before it enters the spectroscope, for the first 
reflection shows so much white light that it is usually quite impossi- 
ble to analyze the true color of the wing, the predominant colors 
being obscured by a continuous spectrum. In general it was found 
that the colors of the wings are not simple, but compound ; that is 
to say, they are made up of a mixture of several different colors. 

For example, the spectrum of the rufous ground color of the 
upper surface of the wings of Danais plexippus consists of all of the 
red and yellow of the spectrum and about 75% of the green. 

The red spots upon the upper side of the fore wings of Heliconius 
melpomene also consist of the red and yellow and a very faint, 
hardly visible, trace of green. 


174 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


The glaucous green patches on the wings of Colaenis dido are 
composed mainly of green and yellow, but there is also a faint develop- 
ment of about half of the blue and a still fainter trace of red. 

The iridescent blue-green ground color of the upper surface of the 
wings of Morpho menelaus, viewed in such a way that the light 
makes an angle of about 20° with the normal to the surface of the 
wing, gives a spectrum of green and blue about equally developed. 

The yellow ground color found on the upper side of the wings of 
Papilio turnus shows a continuous spectrum, in which the yellow 
seems to be rather more brilliant than in the normal spectrum of 
white light. 

The sepia-brown ground color of the upper surface of the wings 
of Cercyonis alope gives a spectrum which lacks only the blue-green 
and blue. 

(6) Summary of Results. The researches of Coste (90-91) and 
Urech (93) have demonstrated that the colors of butterflies and 
moths may be produced by two causes : by the presence of an actual 
pigment, or by some structural effect. Some colors are due entirely 
to pigment, others to structural causes, and still others to a combina- 
tion of the two. 

Black, brown, red, orange, and yellow are invariably due to 
pigment. 

Green is usually due to a structural effect, but in a few cases there - 
is a green pigment present. 

White, blue, and violet are almost invariably due to structural 
causes. 

In addition to these facts I have found that most of the colors 
which are displayed by Lepidoptera contain a surprisingly large 
percentage of black. Also they are usually not simple colors, but 
composed of a mixture of several different colors. It is remarkable 
that Natural Selection, which is generally assumed to have been one 
of the principal factors in bringing about the wonderful develop- 
ment of colors in Lepidoptera, has not been potent enough to make 
these colors purer than is the case in existing butterflies. 


Il]. THE ESSENTIAL ‘NATURE OF PIGMENTAL COLOR IN 
LEPIDOPTERA. 


(1) Pigments of Larvae. Poulton (’85) showed that the phy- 
tophagous larvae of Lepidoptera “ owe their colour and markings to 


MAYER: COLOR AND COLOR-PATTERNS. L75 


two causes: (1) Pigments derived from their food-plants, chloro- 
phyll and xanthophyll, and probably others; (2) pigments proper 
to the larvae, or larval tissues made use of because of some (merely 
incidental) aid which they lend to the colouring, e. g. fat.” Poulton 
concludes that all green coloration is due to chlorophyll, and 
that nearly all yellows are due to xanthophyll. All other colors, 
including black and white and some yellows, are due to pigments 
proper to the larvae themselves. 

Later, in 1893, Poulton proved that the larvae of Tryphaena 
pronuba could transform both etiolin and chlorophyll into a larval 
coloring matter, which may be either green or brown. It thus 
appears that some brown pigments are derived from food, and are 
merely modified plant pigments. Green larvae have green blood, 
and this color is due te chlorophyll in solution. It is remarkable 
that this chlorophyll solution is stable under the prolonged action of 
light, and in this respect is different from any other known solution 
of chlorophyll. It is worthy of note, further, that the spectrum 
of this green blood shows a great resemblance to that of chlorophyll. 
“In fact the two spectra are far nearer to one another than the 
ordinary spectrum of chlorophyll in alcoholic solution, is to the 
unaltered chlorophyll of leaves.” 

(2) Pigments of Imagines. In 1891, Urech showed that the 
similarity between the color of the urine of butterflies and the 
principal color of their scales is so close that it cannot be considered 
as accidental, but rather must be regarded as physiological. Urech 
compares in a table the color of the urine and that of the scales 
of 29 species of Lepidoptera. In all but two species the resem- 
blance is very close.! 

Urech further shows that the color of the urine (and the corres- 
ponding color of the scales) is not dependent upon the kind of food, 
for one and the same food plant may be differently digested in 
different groups of Lepidoptera. Thus he compares the behavior ofa 
Vanessa with that of one of the Microlepidoptera (leaf-rollers). Both 
of these feed upon the nettle (Urtica). In the larva of the Vanessa the 
contents of the stomach are intensely green, but become red in the 
pupa. In the case of the leaf-roller the contents of the stomach are 
neyer markedly green and become insipid in color during the pupal 
stage. 


1 Likewise, Hopkins (’94) has shown that in the Pieridae the urine is tinged by a yellow 
substance having exactly the color of the wings. 


176 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Poulton has shown that the reddish fluid voided by the Vanessas 
immediately after emergence from the chrysalis contains uric acid, 
and Hopkins (94) says that when the yellow Pieridae emerge, they 
often void from the rectum a large quantity of uric acid. It should 
be borne in mind however, as Urech himself suggests, that the pig- 
ment found within the wings may not be identical in chemical com- 
position with the similarly colored fluid from the alimentary tract. 

Hopkins (’89, 91, °94, °96) has discovered that the white pigment 
found in the scales of Pieridae is uric acid, and that the red and 
yellow pigments of the Pieridae are due to derivatives of uric 
acid. He also says, “these uric acid derivatives used in ornamen- 
tation, are apparently confined to the Pieridae alone among butter- 
flies.” Hence when a Pierid mimics an insect of another family, the 
pigments in the two cases are chemically quite distinct. This is well 
seen in the genera Leptalis (Pieridae) and Mechanitis (Danaidae). 

In addition to this, Griffiths (92) finds that the green pigment 
found in Papilio, Parthenos, Hesperia, Limenitis, Larentia, Ino, and 
Halias is a derivative of uric acid, to which he gives the name of 
“Lepidopteric acid” and assigns the empyrical formula C,, H,, Az, 
Ne Os. 

In a paper published in 1896 in the Bulletin of the Museum of 
Comparative Zodlogy at Harvard College, Vol. 29, I have shown, 
p- 226-230, that the pigments of the scales of Lepidoptera are | 
derived by various chemical processes from the blood, or haemo- 
lymph, of the pupa, and that the haemolymph is a proteid substance 
containing egg-albumen, globulin, fibrin, xanthophyll, orthophos- 
phoric acid, iron, potassium, and sodium. 


III. DEVELOPMENT OF THE VARIOUS COLORS’ iN THE PUPAL 
WINGS. 


A few researches have been carried out upon this interesting 
topic, but as the literature is scattered and has never been brought 
together, it will perhaps not be amiss to present a brief résumé of 
the principal facts which have been already ascertained. 

(1) Historical Account of previous Researches. In 1889 
Schiffer (’89) discussed the question of the order and time of 
appearance of the colors in the pupal wings of several of the 
Vanessas. Unfortunately he apparently did not make his obser- 


MAYER: COLOR AND COLOR-PATTERNS. 177 


vations at sufficiently close intervals of time, and was, therefore, 
led into some misstatements, which have been corrected by van 
Bemmelen (’89) and Urech (91). 

Van Bemmelen carried out an elaborate research upon the 
development of the various spots and colors upon the wings of 
Pyrameis cardui, Vanessa urticae, V. io, Pieris brassicae, and a few 
other forms. He discusses in detail the time and manner of appear- 
ance of all of the different spots upon the wing. Into these details 
we shall not follow him, but shall merely present his general con- 
clusions regarding the development of the various colors. In Pieris 
brassicae it appears that during the first days of the pupal stage the 
wings are colorless and transparent ; after a few days, however, the 
fore wings become opaque, and white; later the hind wing, also, 
goes through the same changes. The wings then remain unaltered 
until about two days before the butterfly issues. Then, very sud- 
denly, the black spots and the yellow ground tone of the under 
sides appear. White is thus the primary color; black and yellow 
secondary. The first color to make its appearance in the case of 
Pyrameis cardui is a brown-yellow ground color, which may be 
observed in pupae four days old. The hind wings are at this time 
somewhat darker than the fore wings. The color then changes from 
darker brown to cinnamon-brown. The black spots appear later upon 
this delicate reddish brown ground color. The three fused spots 
which form the whitish band in the middle of the front edge of the 
fore wing appear during the last days of development, just before 
the completion of the final color-pattern. 

Both van Bemmelen and Urech have shown that in Vanessa 
urticae the order of appearance of the various colors is the same as 
in Pyrameis cardui. The first color to appear in Vanessa urticae is 
a faint reddish tinge ; this deepens and forms the ground color, and 
later the black spots appear upon it. 

Urech (91) has made a careful study of the development of the 
colors upon the pupal wings of Vanessaio. The wings are at first 
wholly white. Then in a restricted area of this white is noticed the 
appearance of a yellow, which forms the yellow of the mature wings. 
Almost contemporaneous with the development of the yellow comes 
the red, which appears in another part of the primitively white field, 
and gradually deepens in color until it forms the brownish red 
ground color of the adult wings. Still later another portion of the 
primitive white changes into the black of the mature wing. The 


178 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


under side of the mature wings of Vanessa io is mainly uniform 
black, and in this case also this color develops from the white at a 
very rapid rate, near the end of the pupal stage. This development 
of the black directly upon the white areas is quite remarkable in 
Vanessa io, and very different from that of both Vanessa urticae 
and Pyrameis cardw, where the black spots develop upon a field 
already tinged with red. Urech points out the fact, that some of 
the white spots seen in the mature wings of the Vanessas represent 
the ‘“ primitive white ” of the pupal wings. 

Finally, the latest paper upon the subject of the development 
of color in the pupa is that of Haase (93), who has examined 
the pupae of a number of Papilios (e. g., philenor, machaon, 
asterias, turnus, and podalirius), and finds that during early pupal 
life the wings are as transparent as glass; after a time, however, 
they change to an impure white, which soon becomes yellowish, and 
then the various colors which are destined to adorn the mature 
wings begin to appear. 

If we are to learn much of fundamental import concerning the 
phylogeny of color in Lepidoptera, the researches should be carried 


out upon the lower moths, and not upon such highly specialized 


forms of Rhopalocera as the Vanessae. 

In my paper on Wing scales, etc. (Mayer, ’96, p. 232), I have 
come to the conclusion that dull ocher-yellow and drabs are, 
phylogenetically speaking, the oldest pigmental colors in the Lepi- 
doptera. The more brilliant colors, such as bright yellows, reds, 
and pigmental greens, are derived by complex chemical processes, 
and are, phylogenetically speaking, of recent appearance. 


I have made a study of the development of the colors and pattern 
in the wings of Callosamia promethea Linn. and of Danais plexip- 
pus Fab. 


(2) Development of Color in the Pupal Wings of Callosamia 
promethea. The cocoons of Callosamia promethea are very abundant 
during the winter months, when they may be found hanging to the 
stems of the food plants of the larvae. The pupal wings remain 
perfectly transparent all through the winter, until about ten days 
before the time when the moth is destined to issue ; they then become 
opaque white. An examination of the wings at this period shows 
that the scales are perfectly formed (Fig. 25, Plate 3), except for the 


MAYER: COLOR AND COLOR-PATTERNS. 179 


lack of pigment, which is developed later. If one treats the scales at 
this stage with oil of cedar-wood or clove oil, they become practically 
invisible under the microscope, thus demonstrating that there is 
no pigment within them. Fig. 26, Plate 3, gives the appearance 
presented by a scale taken from the light drab-colored margin of 
the mature wing. This is about the lightest area upon the wing, 
except the white spots; but it will be seen that this scale is much 
darker in appearance than the unpigmented one shown in Fig. 25. 
The white or unpigmented condition of the wing lasts for about 
four days. The wings then become uniformly tinged with an 
impure yellow or light drab, and very soon after this the colors 
begin to make their appearance. They first appear upon the lower 
surface of the wings. Fig. 28, Plate 3, represents the under 
surface of the fore wing of a female in a very early stage of color 
development; in fact the upper surface shows, as yet, no trace of 
the colors. It will be seen that a few dark red streaks have 
appeared near the central portion of the wing, and it is worthy of 
note that these occupy the interspaces between the nervures. The 
ocellus near the apex of the wing appears faintly outlined upon its 
background of impure yellow. 

Fig. 27, Plate 3, represents the under side of a hind wing of 
a male in about the same stage as Fig. 28. Here, again, the red color 
occupies the interspaces, and indeed it is only later that the nervures 
become clouded over by it. 

Figs. 29 and 30, Plate 3, represent, respectively, the under and 
upper sides of the fore wing of a male about five hours after the 
first appearance of the colors. Upon the upper side (Fig. 50) we 
see two gray streaks near the base of the wing and a light cinnamon- 
brown color extending from the lower edge toward the middle of 
the wing. The ocellus near the apex is now quite apparent, but 
still faint in color. On the under surface (Fig. 29) the red markings 
have developed to a much greater extent than in Fig. 28. The 
outermost of the two white spots which occupy the center of this red 
area becomes the white central spot of the mature wing; the inner- 
most one is soon obliterated owing to its becoming clouded over 
with red. 

Figs. 37 and 36 represent respectively the upper surface of 
the fore wing and the lower surface of the hind wing of a female, 
slightly more advanced than in Fig. 30. Fig. 31 represents a male 
and Fig. 38 a female about twelve hours after the first appearance 


180 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


of the color. It is remarkable that in this stage the male and female 
wings are quite similar in general appearance, except that the ground 
color of the male is now a dusky gray, while that of the female is a 
cinnamon-brown. 

From this time onward, however, the wings of the two sexes begin 
to differ more and more in appearance, for the ground color of the 
male becomes deep black, while that of the female remains cinnamon- 
brown. This change is well exhibited by Figs. 32 and 39, Plate 3, 
which give the appearance of the upper surfaces of the male and 
female wings respectively at about twenty hours after the first appear- 
ance of the colors. Fig. 55 represents the hind wing of the same male 
whose fore wing is shown in Fig. 32. Figs. 34, 35, 40, and 41 give 
the appearance of the pupal wings just before emergence, when 
the colors are completely formed. 

To summarize; Figs. 27, 29, 33, and 35 give successive stages in 
the development of color in the male; and Figs. 28, 36-41 give 
similar stages for the female. It becomes evident, from a comparison 
of these successive developmental stages, that the colors appear first 
upon the central portions of the wings, and that the outer and costal 
edges of the wings and the nervures are the last parts to acquire 
the mature coloration. 

It is worthy of remark that the color-pattern of the mature male 
Callosamia promethea is quite a departure from the type of coloration - 
which is commonly found among the Saturnidae. The jfemale, 
however, conforms very well to the general pattern of the other 
species of the family. It is quite evident that the deep black colora- 
tion of the male is, phylogenetically speaking, a new acquisition, and 
that the coloration of the female represents the less differentiated 
and therefore, more primitive type. 

It is interesting in connection with these facts to observe that the 
color-patterns of both male and female develop in almost identical 
ways up to the twelfth hour after the first appearance of the color ; 
that then, however, the grayish ground color of the male wings 
begins to deepen into the characteristic jet black of the adult, while 
the light cinnamon ground color of the female merely becomes 
slightly darker as the wings mature. 

(3) Development of Color in the Pupal Wings of Danais 
plexippus. Figs, 42-45, Plate 3, are intended to illustrate four 
stages in the development of color in the pupal fore wings of Danais 
plexippus. The pupal stage of this species is of brief duration, last- 


MAYER: COLOR AND COLOR-PATTERNS. 181 


ing from one to two weeks only, according to the temperature to 
which the chrysalis is exposed. For the first few days the wings are 
perfectly transparent, but about five days before the butterfly issues 
they become pure white. An examination of the scales at this 
period shows that they are completely formed and merely lack 
pigment. In about 48 hours after this (see Fig. 42) the ground 
color of the wings changes to a dirty yellow. It is interesting to 
note that the white spots which adorn the mature wings remain 
pure white. Fig. 43 illustrates the next stage, where the black has 
begun to appear in the region beyond the cell. The nervures them- 
selves, however, remain white. Fig. 44 shows a still later condi- 
tion, where the dirty yellow ground color has deepened into rufous, 
and the black has deepened and increased in area and has also 
begun to appear along the edges of the nervures. In Fig. 45 the 
black has finally suffused the nervures, the base of the wing and 
the submedian nervure being the only parts that still remain dull 
yellow.’ It is apparent that in Danais plexippus, as in Callosamia 
promethea, the central areas of the wings are the first to exhibit the 
mature colors, and that the nervures and costal edges of the wings 
are the last to be suffused. 


TV. Tue Laws wHiIcH GOVERN THE CoLor-PATTERNS OF BUTTER- 


FLIES AND Morus. 


(1) Historical Account of previous Researches. The earliest 
paper upon this subject is by Higgins (’68). He came to the 
conclusion, that “the simplest type of color presents itself in the 
plain uniform tint exhibited when the scales are all exactly alike.” 
He also thought it probable that “the scales growing on the mem- 
brane upon or near the veins would be distinguished from the 
scales growing on other parts of the membrane by a freer develop- 
ment of pigmentary matter, and that in this manner would arise 
a kind of primary or fundamental color-pattern, namely, a pale 
ground with darker linear markings following the course of the veins, 
é. g. Pieris crataegi.” He also attempted to explain the formation of 
eye-spots by assuming that crescent-shaped markings migrate out- 
wards from the sides of the nervures and meet so as to inclose a 
space. 


182 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


It is, however, untrue that there is a freer development of pig- 
ment within the scales lying upon the nervures; in fact, the reverse 
is the case, as we have seen, in both Danais plexippus and Callo- 
samia promethea. Higgins’s explanation of the formation of eye- 
spots is also viaccess 

Darwin (71, Vol. 2, p. 135) published four excellent figures from 
a drawing by Trimen, illustrating two simple ways in which eye- 
spots are actually formed, both diametrically opposed to Higgins’s 
hypothesis. Darwin says that in the South African butterfly, Cyllo 
leda, “in some specimens, large spaces on the upper surface of the 
wings are coloured black, and include irregular white marks, and 
from this state a complete gradation can be traced into a tolerably 
perfect ocellus, and this results from the contraction of the irregular 
blotches of colour. In another series of specimens a gradation can 
be followed from excessively minute white dots, surrounded by a 
scarcely visible black line, into perfectly symmetrical = large 
ocelli” with several rings. 

Scudder (’88—89) and, afterwards, Bateson (’94) have shown 
that the ordinary eye-spots, such as those found in Morpho and the 
Satyridae, are invariably placed in the interspaces between the longi- 
tudinal veins of the wings, and also that they are often found repeated 
upon homologous places of both pairs of wings. Bateson says that 


ocelli are often seen upon both surfaces of the wing, the centers of. 


the upper and lower ocelli coinciding. In the majority of cases, 
however, the upper and lower ocelli, although coincident, have quite 
different colors. The simpler sort of ocelli, such as those seen in the 
Satyridae or in Morpho, have their centers on the line of the fold- 
marks or creases of the wing. It sometimes happens that these 
creases seem to begin from the center of an ocellus. As these 
creases commonly run midway between two nervures, it usually re- 
sults that the center of the eye-spot is exactly half way between two 
nervures. The large eye-spots of Parnassius apollo are an exception 
to thisrule. In some Morphos, Satyridae, etc., in cell I® of the hind 
wing there are often two creases and two eye-spots, one for each 
crease; but if there be only one eye-spot present, its center does not 
correspond with the middle of the cell, “but is exactly upon the 
anterior of the two creases.” I have observed the same law for the 


white marginal spots in cell I* in Ceratinia vallonia, C. fimbria, and 


Mechanitis polymnia. 
In 1889 Scudder, in his work upon the Butterflies of New 


MAYER: COLOR AND COLOR-PATTERNS. 183 


England, called attention to the following facts: the transverse 
series of dark spots so often seen in the body of the wings of 
Lepidoptera are invariably placed in the interspaces between the 
longitudinal veins, never upon the veins themselves, excepting 
only in rare instances, where the spots occur at the extreme margin. 
He also pointed out that in many types of moths all differentiation 
in coloring has been greatly retarded, so far as the hind wings are 
concerned, by their almost universal concealment by day beneath 
the overlapping front wings. In these cases “ the simplest departure 
from uniformity consists of a deepening of the tint next the outer 
margin of the wing.” It is but a step from this condition to a 
band of dark color or a row of spots parallel with the margin. This 
explains why the transverse style of markings, for the hind wings 
at least, is so common. Scudder showed that “the number of 
instances, in butterflies, in which similar markings appear in the 
same areas of the two wings, and in the same relative position 
in these areas, is far too common to be a mere coincidence. It is 
most readily traced in the disposition of the ocelli, which are very 
apt to be similar in size and perfection, and to be situated between 
the same branches of homologous veins.” 

(2) Laws of Color-Patterns. As aresult of my own study of 
the wings of moths and butterflies, | am prepared to propose the 
following additional laws of color-patterns. (@) Any spot found 
upon the wings of a moth or buttertly tends to be bilaterally symmet- 
rical both as regards form and color, the axis of symmetry being 
a line passing through the center of the interspace in which the 
spot is found, and parallel to the direction of the longitudinal 
neryures. For example, in Figs. 6 and 7, Plate 2, each spot is 
bilaterally symmetrical about the axis HH. The same law holds 
for the spots represented in Figs. 8-14 and 16. 

(6) Spots tend to appear not in one interspace only, but as a row 
occupying homologous places in successive interspaces. Indeed we 
almost always find similar spots arranged in linear series, each sim- 
ilar in shape and color to the others and occupying the center of its 
interspace. The rows of spots represented in Figs. 8-14 and 16 
will suffice to illustrate this law. 

It is interesting to notice that bands of color are often made by 
the fusion of a row of adjacent spots; and, conversely, chains of 
spots are often formed by the breaking up of bands, leaving 
a row of spots occupying the interspaces. Many instances of this 


184 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


are to be seen in certain specimens of various species of the 
Heliconidae. For example, in Heliconius eucrate (Fig. 58, Plate 
4) I have observed that certain specimens show a row of distinct 
spots in place of the, usually entire, band which crosses the middle 
of the hind wing. In fact, the vast majority of bands can be 
analyzed into a series of similar elements, each element occupying an 
interspace. Thus, in Plate 2, Fig. 17, which represents a wing of 
Saturnia spini, the band seen crossing the wing parallel with its 
margin is made up of a series of fused crescents, each crescent 
occupying an interspace. 

If, on the other hand, this band were to break away from the 
nervures, the result would be a series of crescent-shaped spots each 
occupying the center of an interspace. It is very interesting to 
observe the manner in which bands degenerate and disappear. 
Numerous opportunities for doing this may be had among the Heli- 
conidae. In some species, as in Melinaea parallelis, hardly any two 
specimens are alike in the condition of the black band across the 
middle of the hind wings. The most common method of disappear- 
anceis a shrinking away of the band at one end. This is wellillus- 
trated in Figs. 84-87, Plate 7, which represent a sort of “* Mercator’s 
Projection ” of the wings of Mechanitis isthmia (for explanation of the 
plan of projection see page 207.) Fig. 84 represents a male, showing 


a well-marked band of hardly separated spots extending across the 


middle of the hind wing. Fig. 87 shows a female in which the 
spots are thinner and more crescentic and the separations much 
more marked. Fig. 85 is also drawn from a female, in which it will 
be seen that the band has shrunk away leaving only a portion of it 
at the right, and in Fig. 86, which represents another female speci- 
men, only one faint spot is left. 

It is very common to find bands shrinking away at one end. 
Sometimes, however, they shrink away at both ends, and very often 
they break up into a row of spots, which may then contract into the 
centers of their interspaces and finally disappear. It is worthy of 
note that it is very rare to find a band breaking at the middle of its 
length and each half receding from the other. Such a case is, how- 
ever, shown by Melinaea parallelis (see Fig. 82, Plate 7), where one 
sometimes finds specimens in which the black band across the middle 
of the hind wings is complete and unbroken; whereas in other 
specimens, as in Fig. 82, it is partially broken in the middle, and in 
still others the break has become a wide gap by the drawing away of 
the halves of the band from each other. 


MAYER: COLOR AND COLOR-PATTERNS. 185 


We see, then, that it is very common to find bands shrinking 
away from either end, but very rare to find them broken in the 
middle region. This, however, is only a special case of the law 
enunciated by Bateson (794), that the ends of a linera series are more 
variable than the middle. Almost any row of spots also exhibits 
the same law, in that the spots occupying the middle portions of 
the row are similar one to another, while those at the ends of the 
series depart more or less from the type. (See Figs. 10-13, 
Plate 2.) 

The position of spots which are situated near the edge of the 
wing is largely controlled by the wing-folds or creases. In Meli- 
naea egina (Fig. 96, Plate 8) there is a row of white spots near the 
outer edges of the wings, and each of these spots is cut in two by a 
narrow black line which extends along the wing-fold. Also in Cera- 
tinia vallonia (Fig. 81, Plate '7) and in many other forms of the 
Danaoid Heliconidae one often finds two creases in a cell, and in 
this case there are two marginal spots, one on each crease. In 
many other cases, however, the marginal spots are double in each 
cell, although there is but a single wing-fold; the spots in these 
cases are situated at some distance on either side of the fold. (See 
Figs. 95, 96, Plate 8.) Another very common condition is exem- 
plified in Fig. 83, Plate 7, where there is a single marginal spot 
situated upon the wing-fold in each cell. 

(3) Detailed Discussion of the Laws of Color-Patterns. Figs. 
6-14 and 16, Plate 2, are taken from special cases which serve 
to illustrate the two chief laws of color-pattern, 7. ¢., that spots tend 
to be bilaterally symmetrical about an axis (HH, Figs. 6, 7) passing 
through the center of the cell parallel with the nervures; and 
also, that spots of similar shape and color tend to be repeated in 
a row of adjacent cells. 

In Fig. 7 the spots are separated in the middle, but still incline 
outward symmetrically from the center ; indeed, instances of double 
spots are very common. In such cases, however, each half spot is a 
reflection of its mate on the other side of the axis passing through 
the center of the cell. 

Fig. 8 represents various eye-spots found in the Morphos, and 
will serve to illustrate the laws of eye-spots which have been enunci- 
ated by Scudder (’89) and Bateson (94). These spots occupy the 
center of the cells in which they are found. In cell II, for example, 
is a large eye-spot with a crescent in its center, and it will be 


186 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


observed that this crescent follows the general law and is bilaterally 
symmetrical about the usual axis." 

Fig. 9 shows the law of repetition of some very complex spots, each 
being bilaterally symmetrical. It is found in Parthenos gambrisius. 

Figs. 10 and 11 represent Ornithoptera urvilliana and O. priamus 
respectively. In Fig. 10 we see an instance of a spot within a spot, 
and in Fig. 11 an even more complex case, for here there are three 
systems of spots one within another. 

Fig. 12 represents the marginal markings found in Hestia jasonia 
and Fig. 13 Hestia leuconoe var. clara. These two examples are — 
intended to illustrate the fact, that, although the markings are 
situated upon the nervures, they are bilaterally symmetrical not 
about the nervures as axes, but about the usual axis passing midway 
between the nervures. In Fig. 12 it will be seen that the two 
curved markings situated upon nervures 1° and 2, and projecting 
into cell I*, are bilaterally symmetrical only in reference to the axis 
through the middle of the cell. 

In allied species the spot situated upon nervure 1° is often absent. 
The system of markings is therefore undergoing degeneration at this 
end (cf. Fig. 13, cell It). The curved mark upon nervure 5 (Fig. 
12) projecting into cell V is plainly symmetrical with respect to its. 
fellow in the opposite side of cell V, and not with its near companion 
which projects into cell IV. The sameis also true in the case of the 
spots in cell VI. 

In Fig. 13 the spots appear at the first glance to be bilaterally 
symmetrical about both nervures and centers of cells, but in cell IV 
the marking situated on nervure + does not quite reach to the cen- 
ter, and it is interesting to observe that its fellow on nervure 5 also 
falls short of reaching the center and is therefore symmetrical with 
respect to the other curved spot in cell [V. This case also furnishes — 
an instance of a break in the middle of a linear series. 

Fig. 14 is taken from the under surface of the hind wing of - 
Papilio emalthion. It serves to illustrate the fusion of two orig-— 
inally separate rows of spots. In this case the crescent-shaped spots 
above have fused with the rectangular ones below, so as to inclose 
a portion of the ground color of the wing. Sometimes two rows of 


1 A very beautiful exception (Fig. 19, Plate 2) to thisrule for the crescents found in eye- 
spots is seen in the under surface of the fore wing of Missanga patinia Moore. It will be 
noticed that the large black crescent found in this beautiful eye-spot is 90° away from its 
usual position. This is the only exception of the sort known to me. 


MAYER: COLOR AND COLOR-PATTERNS. 187 


spots of different colors fuse, giving a chain of spots which are of 
one color above and another below. 

In Fig. 16 the spots composing the row BB are blue (dark) 
above, and red (light) below. It will be observed that the color is 
bilaterally symmetrical, as usual, about the axis through the middle 
of the cell. Such bicolored spots are often due to a simple fusion, 
as before stated; but sometimes they may, perhaps, be intrinsically 
bicolored. 

Fig. 15 is a beautiful instance of an exception to the general rule 
that spots are bilateral about the axis through the center of the cell. 
It is taken from Ornithoptera trojana Staudinger.’ The light spots 
represented near the outer edge of the wing are of a brilliant irides- 
cent green. It is evident that they are distinctly bilateral with 
respect to the nervures ; especially is this true of the pair adjacent to 
nervure 1. Ornithoptera brookiana Wallace illustrates another 
exception, though in a less marked degree.? Other allied species of 
Ornithoptera, however, would seem to show that these apparent 
exceptions may have been derived from forms which exhibited two 
spots in each cell and followed the usual rule. These are the only 
instances of such exceptions known to me. I do not doubt, how- 
ever, that further study would reveal others. 

In Fig. 17 an example is given of the peculiar kind of eye-spots 
found in the Saturnidae. The species from which the figure was 
taken is Saturnia spini. It will be seen that this so-called eye-spot 
is quite different in formation from the ocelli of butterflies. It is 
simply a series of curved cross-bands between nervures, arranged 
symmetrically on both sides of the cross vein CC. The “eye- 
spots” upon the wings of Attacus luna and in the genus Telea are 
also of this sort. True eye-spots, however, similar to those found 
among the Morphos and Satyridae, occur in moths, as in the apex 
of the fore wing of Samia cecropia, Callosamia promethea, ete. 
“False” eye-spots are also found on the wings of butterflies; in 
Vanessa io, for example, the so-called eye-spot of the fore wing has 
been shown by Dixey (’90) to be made up of a series of fused 
spots. It will be remembered that Merrifield (94, Plate 9, Fig. 4) 
caused this “ ocellus ” to break up into its constituents by subjecting 
the pupa to a temperature of 1° C. The ocellus upon the hind wing 
of Vanessa io is no doubt a true eye-spot; the only evidence which 

1 See Watkins, ’91, Plate 4. 


2See Hewitson, ’5€~76, Vol. 1. 


188 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


might lead one to infer that the ocellus of the fore wing was of the 
same character is, that an aberrant form is sometimes found in 
nature having the “eye-spots” on both fore and hind wings 
obliterated, thus indicating a possible connection between the two 
(see South, *89). 

Fig. 18 is intended to illustrate the process of degeneration occur- 
ring in bands. Band BB is represented as breaking down by the rare 
method of parting in the middle. Example, Melinaea parallelis. 
Band EE is degenerating at one end; this is a very common 
method. 

Figs. 20-23 represent hypothetical conditions not found in nature ; 
all being contrary to the conditions of the laws which have just been 
stated. 

In Fig. 20 row RR presents three spots for each celi. I believe 
this has not been found in nature, but I should not be surprised if 
it were discovered, for it is not contrary to any of the laws. 

Row CC, on the other hand, is contrary to the law of bilaterality, 
the crescents not being bilateral about axes passing through the 
middle of the interspaces parallel with the longitudinal nervures. 

Fig, 21 is intended to show a series of spots arranged side by 


side in twos in each cell, and of different colors. This, I believe, is 


impossible, for it is contrary to the law of bilaterality of color 
arrangement about the usual axis (HH, Figs. 6, 7). 

In Fig. 22 there are several conditions which are impossible; e. 7., 
an eye-spot situated upon a nervure is never seen in nature, also 
two spots originally side by side, as in cell IIT, never rotate around 
each other so as to come to lie one above the other. Spots often 
move, however, as shown by the arrows in cell IV, thus giving 
rise to fusions; or they may move away from each other, causing a 
wider gap between the rows. In cell I® are shown two looped 
spots. One form (A) is quite usual, being found indeed in Cymo- 
thoe caenis Drury.!. The other form of spot (D) is an impossibility, 
not being bilaterally symmetrical. 

Fig. 23 illustrates other impossibilities in color-pattern, none of 
them, of course, being found in nature. For example, one never 
finds a row of slanting spots such as SS. Also one never sees a 
row of similar spots in a/ternate interspaces, such as is shown in 
DD, for this would be contrary to the law that similar spots are 
repeated in a row of adjacent interspaces. These last four diagrams 


1 See Cramer (1779-82), Vol. 2, Plate 146. 


MAYER: COLOR AND COLOR-PATTERNS. 189 


(Figs. 20-23) have been introduced merely to give an idea of the 
curiously strict limitations which nature has imposed upon the differ- 
entiation of the color-pattern. Many beautiful effects might have 
been produced, such for example as that of alternate interspaces 
showing different colors, but this is not seen in nature. 

It is interesting to recall the fact, that the colors themselves are 
impure and by no means so brilliant as they, perhaps, might have 
been, had Natural Selection been more severe in regard to color. 

There is doubtless some physiological reason why spots almost 
invariably appear and disappear in the middle of the interspaces, and 
when we know more of the anatomical and histological conditions 
of the wing during the development of the colors, we may be able to 
discover it. It will be remembered that in the developing pupal 
wings of Callosamia promethea and Danais plexippus I found that 
the colors first made their appearance in the interspaces, and finally 
spread out so as to tinge over the nervures. 


(4) Origin of Color- Variations. There is every reason to 
believe that all kinds of spots and bands, which are essentially 
only fused spots, may appear or disappear in any individual 
specimen without going through a long course of Natural Selection 
and slow phylogenetic differentiation. Darwin and Trimen (71) 
and Bateson (94) have demonstrated that this is true for eye-spots. 
In the Heliconidae I have found that bands and rows of spots are 
very variable in different specimens of the same species (see Plate 7, 
Figs. 84-87). 

There is a large and widely scattered literature recording the 
appearance and disappearance of colors and markings upon the 
wings of Lepidoptera. Limits of time and space prohibit my doing 
justice to it here, but it may be well to call attention to a very few 
of the more recent papers upon the subject. Many of the color- 
aberrations recorded in this list of papers may be due to the direct 
influence of environmental conditions upon the individual, but others 
are no doubt true sports or, to speak crudely, “ congenital” variations, 
and might under favorable conditions of life become the ancestors 
of new varieties or species. It seems highly probable that new 
Species often arise from just such sports in the manner so frequently 
and ably expounded by Bateson. 


190 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


PartTiaAL BIBLIOGRAPHY OF REMARKABLE COLOR-ABERRATIONS IN 


LEPIDOPTERA. 


Bairstow, S. D.’77. Ent. Mo. Mag., Vol. 14, p. 67. (Zygaena filipendulae. ) 

Bean, T. E. 95. Can. Ent., Vol. 27, p. 87-93, Plate 2. (Nemeophila petrosa 
and varieties. ) 

Benson, E. F. ’*83. Entomologist, Vol. 16, p. 210. (Arge galathea. ) 

Buewse, L. ’89. Feuille jeun. Natural., 19 Ann., p. 142-145. 

Breienet, F. 90. Bull. Soc. Ent. France, (6), Tome 10, p. 29-50. (Thecla 
rubi; Melithaea athalia.) 

CarrinGton, J.’78. Entomologist, Vol. 11, p. 97, Fig. (Cidaria suffumata.) 

Carrincton, J. 83. Entomologist, Vol. 16, p.1, Fig. (Callimorpha dominula.) 

Carrineton, J.’88. Entomologist, Vol. 21, p. 73, Fig. Editorial Note. (Arctia 
caia.) And numerous other papers in the Entomologist. 

CrarK, J. A. ’89. Entomologist, Vol. 22, p. 145-147, Plate 6. (Triphaena 
comes. ) 

CockERELL, T. D. A. ’86. Entomologist, Vol. 19, p. 250-251. (Epinephele 
tithonus. ) 

CocKkERELL, T. D. A. ’88. Entomologist, Vol. 21, p. 189. (Pieridae.) 

CocKk®rRELL, T. D. A. ’89. Entomologist, Vol. 22, p. 1-6, 13, 20-21, 26-29, 54— 
56, 98-100, 125-180, 147-149, 185-186, 245-245. 

Dewirz, H.’85. Berlin. Ent. Zeitschr., Bd. 29, p. 142, Taf.2. (Precis amestris.) 

Epirors oF Enromo.tocist, 778. Entomologist, Vol. 11, p. 169-170, Plate 2. 
(Vanessa atalanta and several Lepidoptera.) 

Epwarps, W. H.’68. Butterflies of North America. (Numerous plates.) 

Fertic, F. J. ’89. Feuille jeun. Natural., 19 Ann., p. 84. (Variations of 
Lepidoptera in Alsace.) 

Frircu, E. A. ’78. Entomologist, Vol. 11, p. 50-61, Plate. (Colias edusa. } 

Goss, H.’78. Entomologist, Vol. 11, p. 73-74, Fig. (Chelonia villica.) 

Opertuir, C.°89. Bull. Soc. Ent. France, (6), Tome 9, p. 74-76. 

Opertutr, C. 93. Feuille jeun. Natural., 24 Ann., p. 2-4. 

PousapE, G. A. 791. Ann. Soc. Ent. France, (6), Tome 11, p. 597-8598, 
Pl. 16. (Thais rumina. ) 

Ricuarpson, N. M.’89. Ent. Mo. Mag., Vol. 25, p. 289-291. (Zygaena filipen- 
dulae.) 

Scupper, S. H. ’89. Butterflies of New England, p. 1218. (Bibliography of 
variations of Pieris rapae.) 

Sourn, R.’89. Entomologist, Vol. 22, p. 218-221, Plate 8. (Various Vanessidae.) 

Spryer, A.’74. Stettiner Ent. Zeitung, Bd. 35, p. 98-103. 

Tuipie, H. ’84. Berlin. Ent. Zeitschr., Bd. 28, p. 161-162, Fig. (Apatura iris.) 

Turr, J. W. ’89. Entomologist, Vol. 22, p. 15, 160-161. (Melanie Agrotis 
corticea and pale variety of Lycaena bellargus.) 


(5) Climate and Melanism. Lord Walsingham (85), in his 
presidential address before the Yorkshire Naturalists’ Union, brought 
forward the idea, that, although Arctic insects might be perfectly 


. 


MAYER: COLOR AND COLOR-PATTERNS. 191 


able to withstand the most severe cold while in hibernation during 
the winter, it is of great importance for them to absorb as much heat 
as possible during the short summer. He placed several species of 
lepidopterous larvae upon a snow surface exposed to bright sunshine. 
The snow melted at different rates under the various larvae, and 
in two hours the darkest insect had sunk by far the deepest into the 
snow, proving that it was the best absorber of heat. This ingeni- 
ous experiment of Lord Walsingham should be made the beginning 
of an extensive and careful research. 

Chapman (’88) has shown that it may be of advantage to moths 
inhabiting wet regions to display dark colors, or become melanic. 
His observations were made upon Diamea flagella, and he says that 
upon one showery afternoon he observed that one side of the tree 
trunks was wet and dark in color; the other side being dry was 
paler. “As a consequence, the dark specimens of flagella were very 
conspicuous upon the dry portions, hardly visible on the wet, whilst 
with the ordinary form the conditions were reversed, those on the 
wet bark were conspicuous, those on the dry much less so.”  Per- 
haps the dull coloration of Arctic moths may be partially due to the 
effect of the somber background of rocks in the regions which they 
inhabit. 

(6) Relation between Climate and Colors of Papilios. It is well 
known that the Lepidoptera in the Tropics display the richest 
variety and greatest number of colors. I have counted the colors 
exhibited by the 22 species of Papilio enumerated by Edwards as 
inhabiting North America north of Mexico, and also those which 
are displayed by the 200 species of Papilio named in Schatz’s list as 
found in South America. The “colors” were determined by com- 
parison with the colored plates in Ridgway (786). 

In this manner it was determined that the North American 
Papilios exhibit 17 colors, viz., black, brown, primrose-yellow, canary- 
yellow, sulphur-yellow, orange, white, greenish white, apple-green, 
cream-color, azure-blue, sage-green, rufous, pearl-gray, indigo-blue, 
iridescent blue, iridescent green. 

On the other hand the South American Papilios exhibited 36 
colors, viz., black, translucent black, brown, white, canary-yellow, 
citron-yellow, olive-yellow, primrose-yellow, chrome-yellow, straw- 
yellow, gamboge-yellow, cream-color, greenish white, apple-green, 
malachite-green, emerald-green, sage-green, slaty green iridescence, 
pea-green, azure-blue, iridescent Berlin-blue, indigo, pearl-blue, 


192 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


glaucous blue, salmon-buff, écru-drab, flesh-color, coral-red, rose-red, 
geranium-red, geranium-pink, olive-buff, iridescent 
geranium-pink (as in P. zeuxis), and transparent areas. 

As 200 species in South America display but 36 colors, while 22 
in North America show 17, it follows that, while the number of 
species in South America is 9 times as great as in North America, 
the number of colors displayed is only a little more than twice as great. 
The richer display of colors in the Tropics, therefore, may be due 
simply to the far greater number of species, which gives a better oppor- 
tunity for color-sports to arise, and not to any direct influence of the 
climate, The number of broods, also, which occur in a year is much 
greater in the Tropics than in the Temperate Zones, so that the Trop- 
ical species must possess a correspondingly greater opportunity to 


vermilion, rufous, 


vary. 


V. Tue CAusES WHICH HAVE LED TO THE DEVELOPMENT AND 
PRESERVATION OF THE SCALES OF THE LEPIDOPTERA. 


(1) Experiments and Theory. It is well known that the scales 
of Lepidoptera are morphologically identical with hairs. Indeed, a 
graded series from simple hairs, such as are found covering the | 
body-surface of most Arthropods, up to perfectly developed flat 
scales bearing well differentiated striae may usually be found upon 
one and the same insect. 

It is also remarkable that the color-bearing scales of beetles have 
been developed in the same mamner as those of moths and butterflies, 
and that in this case also hairs have become differentiated into scales 
which are precisely similar in appearance to those of the Lepidoptera 
(see Dimmock, ’83). 

This is only another of the numerous instances met with in nature 
where similar conditions of selection have developed complex organs 
which are similar in appearance, though found in widely separated 
groups. A list of papers relating to the development of scales has 
been given by Dimmock (’83, p. tbr): 

Most of the hairs which cover the body-surface in Arthropods are 
true sensory structures, the axis of each of which is a protoplasmic 
process from a single cell of the hypodermis, which lies below the 
cuticula. They have probably been developed because the cuticula, 


MAYER: COLOR AND COLOR-PATTERNS. 193 


being hard, chitinous, and inflexible, would serve but poorly as a 
tactile or sensory surface. 

Of course no one would venture to ascribe any sensory function to 
the scales which cover the wing-membranes of the Lepidoptera. 
We may, however, make several more or less reasonable hypotheses 
concerning the probable uses of the scales, and by testing these sup- 
positions arrive perhaps at some plausible explanation of their reten- 
tion and the complex development which they have undergone. 

(1) They may have caused the wings of the ancestors of the 
Lepidoptera to become more perfect as organs of flight, by causing 
the frictional resistance between the air and the wing-surface to 
become more nearly an optimum. 

(2) The appearance and development of the scales may have 
served, as Kellogg (94) has suggested, “to protect and to strengthen 
the wing-membranes.” 

(3) The present development of the scales may be due to the 
fact that they displayed colors which were in various ways advan- 
tageous to the insects. 

Concerning the first of these three hypotheses, the wing has, 
broadly speaking, two chief functions to perform in flight. It must 
beat more or less downward against the air, and must, in addition, 
glide or cut through the air, supporting the insect in its flight. For 
the mere beating against the air a relatively /arge co-efficient of 
friction between the air and the wing might be advantageous; but 
for gliding and cutting through the air a small co-efficient of friction 
would certainly be an advantage. There must therefore be an 
optimum co-eflicient of friction, which lies somewhere between these 
two. 

In order to determine the co-efficient of friction between the wing 
and the air, use was made of a method which, in one form or 
another, has long been known to engineers; that is, of observing the 
ratio of damping of the vibrations of a pendulum. 

It is well known that when a pendulum is swinging free, and 
uninfluenced by any frictional resistances, the law of its motion is 
expressed by the formula, 

») 

(1) d=A sin Ft 
where d is the displacement of the pendulum from its middle 
position after the interval of time t, A is the maximum displace- 
ment and T the time of a complete vibration, back and forth. If, 


194 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


however, frictional resistances interfere, the formula becomes, 
») 


V2 
(2) d= ae. sin Tt . 
1 
estes K —log dT, =: 
(3) Hence, K = 4 loge sin 2m #2? OF 1 t= 
—log d 


me a AT, log e 


where K is a constant dependent upon the friction, e is the base of 
the Napierian system of logarithms and T, is the time of a complete 
vibration, which may be different from the T, representing the time 
of vibration when not under the influence of friction. 

The plan was, then, to attach the wing of some large butterfly or 
moth to the end of a short, light pendulum in such a way that it 
would either fan against the air, or cut through it, and then to 
observe the ratio of damping of the pendulum’s vibrations. A 
drawing of the pendulum with a wing attached is given in Plate 1, 
Fig.3. The wing is here shown in the position for “cutting or elid- 
ing” through the air. It would be in the position for fanning against 
the air, if it were rotated 90°. The pendulum was made of brass 
and steel, the ends being of brass and the slender middle portion of 
steel. Its vibrations were read off upon an are graduated in milli 
meters. ‘The readings were certainly accurate down to 0.5 mm. 
The pendulum was hung upon a steel knife edge (N,N, Fig. 3), 
which rested upon firm level glass bearings. The pendulum was 
24.21 cm. long, and weighed 19.61 grams. Its time of vibration 
(T,) was 0.877 seconds. This rate of vibration was practically 
unaltered when a wing was fastened to the end of the pendulum, 
the reason being that the wings were very light, the heaviest, that of 
Samia cecropia, weighing only 0.038 grams. The wing to be expert- 
mented upon was fitted into a deep, narrow slot at the free end of 
the pendulum, and then cemented in by means of a little melted 
beeswax. It thus became a perfectly rigid part of the pendulum 
itself. 

The pendulum with wing attached was deflected through a known 
are, read off upon the millimeter scale, and its reading at the end of 
the first swing carefully observed. Then if A be the initial deflec- 
tion, which we may call unity, and if d be the reading after the first 
swing, the ratio of damping is given by the expression a In experi- 


menting with a fore wing of Samia cecropia “fanning the air,” It 


MAYER: COLOR AND COLOR-PATTERNS. 195 


was found, as the mean of many trials, that this ratio of damping 
was 0.919, that is to say, the amplitude of the 2d swing was 0.919 
as great as the amplitude of the Ist, that of the 3d only 0.919 as 
great as that of the 2d, and soon. The scales were then carefully 
removed from the wing-membranes, by means of a camel’s hair brush, 
and by again testing the vibrations it was found that the new ratio of 
damping was 0.917. This is so near the value of the ratio of damp- 
ing with the scales on (0.919), that it may be considered identical, 
the difference being due to errors of experimentation. 

Hence we must conclude that the presence of the scales upon the 
wing-membrane has not altered, appreciably, the co-efficient of fric- 
tion which would exist between scaleless wing-membranes and the 
air. The results indicate rather, that when the scales appeared upon 
the wings of the scaleless, clear-winged ancestors of the Lepidoptera, 
the co-efficient of friction remained unaltered. This tempts one to 
the further conclusions, that the co-efficient of friction between the 
air and the wings was already an optimum in these clear-winged an- 
cestors before the appearance of the scales, and therefore that Natural 
Selection would operate to keep it unaltered. 

A wing of Samia cecropia cut so as to give it the same shape and 
dimensions as one of Morpho menelaus, gave an identical damping 
ratio. I conclude that the co-efficient of friction may be the same 
for both moths and butterflies, at least for those which move their 
wings at about the same rate in flight. 

It was found in the case of the Samia cecropia wing, that when 
it was vibrated in the position for “ cutting through ” the air, the ratio 
of damping was 0.991. It will be remembered that, when the wing 
“fanned” the air, this ratio was 0.917. We may find the ratio be- 
tween the resistance encountered in “ fanning” and that encountered 
in “gliding” through the air by substituting these values in equa- 
—logd 


tion (4), K= AT, loge’ 


d 
Thus for fanning, A= 0.917 and T, — 0.877. Making A unity, 


Kk ales. 0.917 


0.877 loge — ae 


d 
In cutting through the air, A =0.991 and T, as before = 0.877. 


L 


—log 0.991 


0.877 loge — °°! 


Hence in this case K — 


196 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


The wing, then, encounters at least 10 times the resistance in fan- 
ning that it does in gliding through the air. It should be said that 
this last experiment is somewhat crude, for the wing necessarily 
could not be made to cut the air with that delicate precision which 
is probably realized by the insect in flight. I should not be 
surprised, if in nature the insects encountered at least 20 times 
the resistance in beating the air, that they do in merely gliding 
through it. 

Concerning Mr. Kellogg’s supposition, that the scales were devel- 
oped to “protect and to strengthen the wing-membranes,” I will 
admit that they may serve in some slight degree to protect the wing- 
membranes from scratches, etc.; but I am unable to accept his con- 
clusion, that they strengthen the wing-membranes, any more than 
that the shingles upon a roof serve to add strength to it. The 
wing-membranes themselves are tough, elastic, and not easily torn or 
scratched, and the scaleless wings of the Neuroptera and Hyme- 
noptera are very rarely found torn or scratched in nature. 

In 1858 Mr. Alexander Agassiz called attention (759) to the fact, 
that “the nervures of the wings of butterflies are so arranged as to 
give the greatest lightness and strength; they are hollow, with their 
greatest diameter at the base of the wing, the point of greatest 
strain, their diameter gradually diminishing to the edge of the 
membrane. If a section be made across such a wing parallel to the 
axis of the body, we find very much the arrangement which has 
been experimentally proved by Fairbain and Stephenson as giving 
the greatest strength of beams, as exemplified in the tubular bridge. 
We find the strongest nervure placed either on or near the anterior 
edge of the upper wing; there is no such nervure on the lower 
wing, all being of nearly the same size, as such a one would have 
prevented the elasticity of the wing from assisting the flight to 
any considerable extent.” Mr. Agassiz has informed me that he 
carried out an extensive series of experiments upon the rigidity of 
the wings of various species of Lepidoptera. He placed little 
platinum strips upon the wings and observed the extent of the 
bending produced. His results demonstrated that the Sphinx moths 
possess by far the strongest wings, and that the Danaoid and 
Acraeoid Heliconidae have very weak wings. The reason for this 
probably lies in the fact, that the Sphinx moths move their wings 
with great rapidity, while, according to Bates (762) and all sub- 
sequent observers, the Heliconidae have a slow flight. 


MAYER: COLOR AND COLOR-PATTERNS. 197 


As the scales have been developed not because they aided the 
insects in flight or strengthened the wings, their retention must 
have been due to some other cause, probably to their display- 
ing colors which were advantageous to their possessors in various 
ways. As Dimmock (’83) says, “it is only in insects where certain 
kinds of brilliant coloration have been developed that one finds 
seales.” Indeed, I believe that the vast majority of the scales 
found in Lepidoptera are merely color-bearing organs. They prob- 
ably first made their appearance upon small areas of the wings, 
perhaps adjacent to the body, and were merely colored hairs, sim- 
ilar to those of the surface of the body, which had grown out upon 
the wings. In this position they displayed some color which was 
of advantage to the insect; perhaps serving to render it less con- 
spicuous than formerly. Under these circumstances they would 
naturally be preserved through the operation of selection until 
finally they became modified into true scales; just as the hairs in 
the Coleoptera have undergone a similar modification. If this 
be true, it is easy to see how they might spread out over 
the surfaces of the wings until the whole wing became covered 
with scales. 

(2) Summary of Conclusions. The scales do not aid the insects 
in flight, for the wings have precisely the same efficiency as organs 
of flight when the scalesare removed. The phylogenetic appearance 
and development of the scales upon the scaleless ancestors of the 
Lepidoptera did not in the least alter the efficiency of their wings as 
organs of flight. This efficiency of their wing surfaces was probably, 
therefore, already an optimum before the scales appeared. The 
scales do not appreciably strengthen the wing-membranes, that 
function being performed by the nervures. The majority of the 
scales are merely color-bearing organs, which have been developed 
under the influence of Natural Selection. 


PART B. 
COLOR-VARIATIONS IN THE HELICONIDAE. 


I. Genrrat CaAusES WHICH DETERMINE COLORATION IN THE 
HELiconipak. 


In 1861, after eleven years of study within the forests of South 
America, Bates read his, now classic, paper upon the life and habits 


198 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


of the Heliconidae of the Amazon region. In it he first brought 
forward his ingenious theory of Mimicry—a theory which, under 
the able interpretations of Wallace and Fritz Miiller, and in more 
recent times, under the impetus of the zeal of their numerous disci- 
ples, has yielded so much that is of interest to scientific men. 

The Heliconidae are, above all, creatures of the forest, and Bates 
found that the number of species increases as one travels inland 
from the Lower Amazons towards the eastern slopes of the Andes, 
so that the hot Andean valleys near Bogota, or in Ecuador, contain 
perhaps the greatest number. In their range they are restricted to 
the Tropics of the New World. Only two species, Dircenna klugii 
and Heliconius charitonius, extend so far north as the extreme South- 
ern States of the United States, and none of them are found much 
further south than 50° 8S. Lat. 

Bates and Felder first saw that the Heliconidae were naturally 
divided into two distinct groups. One, the Danaoid Heliconidae, 
consists of about twenty genera, all more or less closely related, and 
evidently an offshoot from the great universal family, the Danaidae, 
members of which are found in both Hemispheres. The other group, 
the true Heliconidae, is composed of two closely related genera, Heli- 
conius and Eueides. They are allied in structure to the Acrae- 
idae and hence their name, Acraeoid Heliconidae. Schatz and Réber — 
(78592, p. 105) say of the Acraeoid Heliconidae:— They are an ~ 
offshoot of the great family Nymphalidae, which have undergone a 
remarkable development in the length of the fore wing, and in this 
respect have been developed in a direction parallel with the Danaoid 
Heliconidae. In their structure, however, they are quite distinct 
from the Danaoid group. 

Schatz has proposed a new classification for the Heliconidae. He 
finds that the genera Lycorea and Ituna, which Bates included among 
the Danaoid Heliconidae, are very closely allied to the Danaidae, he 
therefore says that Lycorea should be placed among the Danaidae, 
while Ituna is clearly midway between the Danaidae and the Dana- 
oid Heliconidae. Schatz proposes the name “‘ Neotropidae ” for the 
Danaoid Heliconidae. However, I think the name “ Danaoid Heli- 
conidae,” being older and more descriptive of their relationship, 
should by all means be retained. In this paper I shall follow Bates’s 
classification, and include among the Danaoid Heliconidae the twenty 
genera: Lycorea, Ituna, Athesis, Thyridia, Athyrtis, Olyras, Eutre- 
sis, Aprotopos, Dircenna, Callithomia, Epithomia, Ceratinia, Sais, 


MAYER: COLOR AND COLOR-PATTERNS. £99 


Seada, Mechanitis, Napeogenes, Ithomia, Aeria, Melinaea, and 
Tithorea. The Acraeoid Heliconidae will then consist of the two 
remaining genera, Heliconius and Eueides. 

Staudinger (*84—°88) records 455 species belonging to the Danaoid 
group, and 150 belonging to the Acraeoid group. 

Nearly all that we know concerning the early stages of the 
Heliconidae is due to Wilhelm Miiller (’86). He gives figures and 
more or less complete descriptions of the early stages of Dircenna 
xantho, Ceratinia eupompe, Ithomia neglecta, Thyridia themisto, 
Mechanitis lysimnia, and also of Heliconius apseudes, H. eucrate, H. 
doris, Eueides isabella, E. aliphera, and E. pavana. Bates (°62, p. 
596) says that he raised the larvae of Heliconius erato and Eueides 
lybia. Schatz and Rober (’85—92) figure the larva and pupa of 
Ceratinia euryanassa. Edwards has given a detailed account of the 
early stages of H. charitonius. 

Miiller found that the larvae of the Danaoid group feed on various 
species of Solanum, while the genera Heliconius and Eueides feed 
upon the Passifloreae. The larvae are conspicuously colored, and 
often gregarious; they seem to take but little pains to hide them- 
selves during the chrysalis stage, for Miiller says that he has seen 
the silver-spotted, white chrysalids of Heliconius doris hanging in 
great numbers in the near neighborhood of the larval food plant. 
The mature insects also furnish a good example of what Wallace 
(67) designated as “warning coloration,” for their tawny orange 
and black wings are very conspicuous as they sail slowly around in 
circles, settling at frequent intervals in their lazy irregular flight. 

Bates was the first to call attention to the circumstance that they 
often possess a rather strong and disagreeable odor, and in 1878 
Fritz Miiller confirmed this observation for a number of the Heli- 
conidae. He found, for example, that the genera Ituna and Ilione 
have a pair of finger-like processes near the end of the abdomen, 
which can be protruded and then emit a rather disagreeable odor ; 
and he also found that the Acraeoid Heliconidae, especially the 
females, possess a disgusting odor. Seitz (89), however, examined 
about fifty species of Heliconidae and found that many of them 
appear to have no odor. For example, he says that Heliconius 
eucrate and Eueide dianasa have no odor, but that some specimens 
of Heliconius beskei, and Eueides aliphera have a horrid odor, 

Whether they are odorous or not, it would seem that the Heli- 
conidae have but few enemies to fear, for not one of the many 


200 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


skilled observers who have studied them in their native haunts has 
ever seen a bird attack them, and the only ground for believing that 
they are attacked rests upon the rather dubious evidence of a few 
specimens found by Fritz Miiller having symmetrical pieces 
apparently bitten out of the hind wings. Belt (74) observed that a 
pair of birds which were bringing large numbers of dragon-flies and 
butterflies to their young never brought any of the Heliconidae, 
although these were abundant in the neighborhood. In fact, Belt 
was able to discover only one enemy of these butterflies, and that 
was a yellow and black wasp, which caught them and stored them 
up in its nest to feed its young. The Heliconidae then, in spite of 
their weak structure, conspicuous colors, and slow flight, enjoy a 
peculiar immunity. 

As is well known, Bates (62) first called attention to the fact that 
the Heliconidae were “ mimicked ” or imitated both in color-pattern 
and shape of wings by a number of other genera of butterflies and 
even moths. Bates had no difficulty in showing that this mimicry 
might easily be explained upon the ground that the Heliconidae, on 
account of their bad taste and smell, were immune from the attacks 
of birds and other insectivorous animals, and that therefore it gave a 
peculiar advantage to a butterfly belonging to any other group not 
thus protected, to assume the shape and coloration of the Heliconidae ; 


for then the birds could not perceive any difference between it and ~ 


the true Heliconidae. Bates found that fifteen species of Pieridae 
belonging to the genera Leptalis and Euterpe, four Papilios, seven 
Erycinidae, and among diurnal moths three Castnias and fourteen 
Bombycidae imitate each some distinct species of the Heliconidae 
occupying the same district. Healso found that all of these insects 
were much rarer than the Heliconidae which they imitated. In some 
cases, indeed, he estimated the proportion to be less than one to a 
thousand. Wallace (’89, p. 265), who has added so much to our 
knowledge of this subject, aptly defines this kind of mimicry as an 
“exceptional form of protective resemblance.” 

But by far the most remarkable discovery made by Bates was the 
fact, that species belonging to different genera of the Heliconidae 
themselves mimic one another. Neither Bates nor Wallace was 
able to give any satisfactory explanation of the cause ot this latter 
form of mimicry, for all of the genera of the Heliconidae are 
immune. They therefore supposed it to be due to “ unknown local 
causes,” or similarity of environment and conditions of life. 


MAYER: COLOR AND COLOR-PATTERNS. 201 


Thus the matter rested until 1879, when Fritz Miiller brought out 
his well-known paper upon “Ituna and Thyridia, a remarkable 
example of mimicry,” in which he showed that both of these genera 
are protected, yet they mimic each other. He also showed that this 
mimicry might be due to Natural Selection brought about in the 
following manner. It is possible that young birds, upon leaving the - 
nest, are not furnished with an unalterable instinct which tells them 
exactly what they should and should not eat; so they may try 
experiments, and would then in all probability taste a few of the 
Heliconidae before finding out that they were unfit to eat. Miiiler 
then demonstrated that, if this supposition be true, it becomes a 
decided advantage to the various species of Heliconidae to resemble 
one another. His reasoning was as follows: Let it be supposed 
that the young and inexperienced birds of a region must destroy 
1,200 specimens of any distasteful species of butterfly before it 
becomes recognized as such, and let us assume further that there are 
in existence 2,000 specimens of species A, and 10,000 of species B ; 
then, if these species are different in appearance, each will lose 1,200 
individuals, but if they resemble each other so closely that they can- 
not be distinguished apart, the loss will be divided pro rata between 
them, and A will lose 200, and B 1,000; therefore A saves 1,000 or 
- 50% and B saves only 200 or 2% of the total number of individuals 
in the species ; hence, while the relative numbers of the two species 
are as | to 5, the relative advantage derived from the resemblance 
is as 25 to 1. 

Blackiston and Alexander (’84) have given a complete mathe- 
matical statement of Miiller’s law, and have come to the conclusion 
that, if the number of individuals destroyed is small compared with 
the number constituting the species, the relative advantage is 
inversely as the square of the original numbers; but if the number 
destroyed is large compared with the original number, the ratio of 
advantage is much greater than the inverse squares of the original 
numbers. Their deduction may be briefly stated as follows : — 


202 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Designation of Species A B 
(1) Original number . . - - : - a> b 
(2) Number lost without imitation. . e — =e 
(3) Remainders without imitation . . (a—e) (b—e) 

a b 


Nianiber Tack waehiamentiemie Se 
(4) Number lost with imitation aa, oe 


Au | 
(5) 2emainders with imitation a (1 = sh) b (1 pod =) 


+b a+b 
(6) Excess of remainders due a 
: ae oe e 
to imitation, or ‘‘abso- arihee ae 
E 2/2 z atb a+b 
lute advantage” (3)—(9) 


28ib b | e 
ab ae | ab Se 
a? Te 


a 
e 

__ & (a—e) 1 =) 

= b (tye (1— +) 


without imitation (6): (3), 
—proportional advantage. 


(7) Ratio of excess to mor ent 


tage of B to proportional 


(8) Ratio of proportional sional 
advantage of A. 


b 


It is evident, then, if e be small compared with a and b, that the 
proportional advantage of B is to the proportional advantage of A 
as a2 is to b?. If, however, the loss (e) is great compared with a or. 
b, the relative gain for the weaker species becomes even greater than - 
the ratio of the squares of b and a. 

If it be true, then, that young birds, when they leave the nest, do 
not possess a directing instinct telling them what they should and 
should not eat, but actually do experiment to some extent upon 
various insects which they meet with, Miiller’s law is amply sufficient. 
to account for the numerous cases of mimicry and remarkably close 
resemblances which are found among the species of the Heliconidae 
themselves. 

Unfortunately no direct experiments have ever been made upon 
the feeding-habits of young South American birds, nor have the 
contents of their stomachs been examined. There have been a few 
experiments, however, which seem to support the idea that some 
animals do learn to associate an agreeable or disagreeable taste with 
the coloration and appearance of their prey. It is well known that 
Weismann (’82, p. 336-339) found that the black and yellow 
larvae of Euchelia jacobaeae were refused by the green lizard of | 
Europe. He then introduced some young caterpillars of Lasiocampa — 


MAYER: COLOR AND COLOR-PATTERNS. 203 


rubi, which are very similar in appearance to those of Euchelia. 
The lizards first cautiously examined the larvae, and finally ate them. 
After this Weismann reintroduced the E. jacobaeae larvae and the 
lizards were seen to taste them, apparently mistaking them for the 
edible L. rubi caterpillars. 

Poulton (87) carried out a most careful and well-conducted 
research upon the protective value of color and markings in insects 
in reference to their vertebrate enemies. He experimented upon three 
species of lizards and a tree-frog. Poulton combines his results with 
those of other observers and presents them in the form of a table, 
which certainly supports the suggestion of Wallace (’67), that 
brilliant and conspicuous larvae would be refused as food by some 
at least of their enemies. Poulton also shows that a limit to the 
success of this method of defence (conspicuous larvae having 
unpleasant taste or smell) would result from the hunger which the 
success itself tends to produce. In the Tropics, deed, where 
insectivorous birds and lizards are far more numerous than with us, 
and where competition for food is great among them, “we may feel 
sure that some at least would be sufficiently enterprising to make the 
best of unpleasant food, which has at least the advantage of being 
easily seen and caught.” This last suggestion of Poulton certainly 
seems reasonable ; moreover, it has occurred to me that young birds, 
being but little skilled in the art of obtaining their food, might quite 
often be forced by hunger to try various kinds of insects, and per- 
haps even the Heliconidae themselves. 

Beddard (92, p. 153-167) reports the results of an extensive 
series of experiments carried out by Mr. Finn and himself upon 
marmosets, birds, lizards, and toads. He arrives at conclusions which 
are quite different from those of Poulton and others, but it appears 
to me that his experiments were by no means so critically performed 
as those of Poulton. He frequently threw larvae into a cage con- 
taining many birds and observed them struggle for the prey. It 
may well be, however, that a bird would be quite willing to swallow 
a very unsavory mouthful in order to prevent any of its companions 
from, apparently, enjoying it. However, Beddard found that 
toads will eat any insect without hesitation in spite of brilliant 
coloration, strong odors, or stings. He also found that birds and 
marmosets would often devour “conspicuously colored ” larvae with- 
out any hesitation, and that some “protectively colored” or ineon- 
Spicuous larvae were refused. There can be no doubt that many 


204. BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


insectivorous animals pay but little attention to the colors of their 
prey ; for example, it is well known to anglers that trout and salmon 
will snap at the most gaudily colored “flies,” which may or may not 
have any counterpart in nature. 

The whole question of warning coloration will have to be made 
the subject of an extensive research upon both old and young 
insectivorous animals before we can safely arrive at any certain con- 
clusions respecting it. 


Il. Metruops Pursvep IN STUDYING THE CoLor--PATTERNS OF 
THE HELICONIDAE. 


No comparative study of the color-patterns displayed by the 
Heliconidae has ever been made. In fact, very few such studies 
have been carried out upon any Lepidoptera. The only works I 
know of are those of Eimer (’89) and Haase (792) upon the colora- 
tion of the Papilios, and of Dixey (90) upon the wing-markings of 
certain genera of the Nymphalidae and Pieridae. The family of the 
Heliconidae with its numerous species and comparatively simple 
coloration affords an excellent opportunity for such a research. 

In making this study of the Heliconidae I was permitted through 
the kindness of Mr. Samuel Henshaw to make free use of the collec- 
tion in the Museum of Comparative Zodlogy at Harvard. I also 
found the colored figures in the works of the following authors of 
great service : Hewitson (’56—76), C. und R. Felder (6467), Hiib- 
ner (0625), Humboldt et Bonpland (’33), Cramer (1779-’82), Stau- 
dinger (84°88), Godman and Salvin (7986), and Ménétriés (’63) ; 
likewise the following shorter papers published in various serials : 
Bates (°63, °65),Butler (765, 69, 6974, °77), Druce (’76), Godman 
and Salvin (’80), Hewitson (754), Snellenen van Leeuwen (87), Srnka 
(84, °85), Staudinger (’82), and Weymer (75, ’84). I was thus 
enabled to examine the color-patterns of 400 (89%) of the species 
of the Danaoid group, and of 129 (86%) of the Acraeoid group, 
either from the insects themselves or from figures given by the 
authors named above. The remaining species were either inaccessi- 
ble to me, or were so vaguely described as to be unavailable. A 
list of the species known to me is given in Table 28. 

(1) The Two Types of Coloration in the Danaoid Heliconidae. 
It'is very remarkable that the color-patterns of all of the Heliconidae 


MAYER: COLOR AND COLOR-PATTERNS. 205 


may be grouped into two very closely related types. To the one of 
these I have given the name “ Melinaea type,” for it is characteristic 
of most of the species of the genus Melinaea. It is well represented 
by Figs. 46, 48, 49, 51, and 55-57 (Plate 4). The insects which 
belong to this type possess wings colored with rufous, black, and 
yellow. 

The other type I designate as the “ /thomia type,” for it is very 
characteristic of most of the species of the genus Ithomia. Figs. 47 
and 52 (Plate 4) afford examples of it. This type differs from the 
Melinaea in that the rufous and yellow areas upon the wings have 
become transparent. 

There are, also, many species, found in numerous genera, which 
fall between these two types of coloration, for the yellow and rufous 
spots upon their wings have become translucent, so that one may 
speak of them as “translucent yellow” and “translucent rufous.” 
These spots are, so to speak, in process of becoming transparent, but 
a few yellow or rufous scales still remain dusted over the otherwise 
clear spaces. Most of the Dircennas are good examples of this type 
(Fig. 54, Plate 4). 

Of the 400 species of the Danaoid Heliconidae, about 125 belong 
to the “ Melinaea type.” It is well represented by most of the 
species of the genera Lycorea, Athyrtis, Ceratinia, Mechanitis, and 
Melinaea. About 30 Ithomias and half a dozen Napeogenes also 
belong to it. About 160 species belong to the “ Ithomia type,” and 
of this number fully 120 belong to the genus Ithomia. The others 
are found in the genera Ceratinia, Napeogenes, Ituna, and Thyridia, 
and many of them resemble the Ithomias so closely that they are 
said to mimic them. About 100 species, some of which are found in 
almost all of the genera, are intermediate in their color-patterns 
between the Melinaea and the Ithomia types. The 15 remaining 
Species are represented by Melinaea gazoria (Fig. 53, Plate 4), 
Ceratinia eupompe, and a few Ithomias, such as Ithomia hemixantho. 
In these furms almost all color has disappeared, so that the whole 
wing has become of a uniform dull translucent yellow, bordered on 
the outer edges by a grayish black. 

(2) Detailed Description of the Melinaea Type of Coloration. 
Figs. 46, 48, 49,51, and 55-57 (Plate 4) afford examples of this type 
of coloration. In these insects we find the proximal half of the 
central cell of the fore wing occupied by a rufous-colored area, which 
I call the “inner rufous.” It is marked I in all of the figures. 


206 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Beyond the “inner rufous” we find a black spot, marked II in the 
figures. It usually occupies the middle region of the cell of the fore 
wing, and I have designated it as the “inner black.” Beyond the 
“inner black,” and occupying most of the outer portion of the cell 
of the fore wing, is a light-colored area, marked III in the figures. 
This area is rufous in color in Fig. 49, but it is usually yellow, as in 
Figs. 46, 48, 51, 54-97, and I have called it the “inner yellow.” 
Beyond the “inner yellow,” and occupying the extreme outer 
portion of the cell, lies the “ middle black” (IV). In many species 
it is fused, as in Figs. 46-48, 56, 57, with the large black area, the 
“outer black” (VII), which occupies the greater portion of the outer 
half of the fore wing. Just outside of the cell beyond the “ middle 
black” one finds a well-developed yellow area (V), the “ middle 
yellow,” and there is sometimes still another yellow patch beyond 
this, which is marked VI and called the “ outer yellow.” Finally, 
one often finds a row of white or yellow spots, the “‘ marginal spots” 
(IX), lying very near the outer margin of the fore wing (see Figs. 
47-49, 51, 54, 56). These spots are very well developed in the 
genera Ceratinia, Napeogenes, Ithomia, and Melinaea. One more 
very characteristic marking of the fore wing remains to be noticed ; 
that is the longitudinal black stripe (VIII). It is also worthy of note 
that the front costal edge of the fore wing is almost always tinged 
with black. 

The pattern of the hind wing is quite simple. The ground color 
is usually rufous and a “middle black” band (XI) runs across the 
middle of the wing. The outer edge is bordered by the ‘outer 
black” (XIII). Above the “middle black’? band lies the “ inner 
rufous” (X) of the hind wing, and below the “ middle black ” band 
one finds the “outer rufous” (XII) of the hind wing. One often 
finds a row of white or yellow dots within the outer black border of 
the hind wing, and these I designate as the “ marginal spots” of the 
hind wing. 


The Jthomia type of coloration, it will be remembered, may be 
derived from the Melinaea, by simply imagining the rufous and 
yellow areas to have become transparent. Also the outer black 
usually suffers a reduction so as to become only a rather narrow 
border along the outer margin of the fore wing. Thyridia psidi 
(Fig. 47) is a good example of this type. It will be seen that the 
black areas remain about the same as in the Melinaea type, but that 


MAYER: COLOR AND COLOR-PATTERNS. 207 


the rufous and yellow have become transparent. The middle and 
outer yellow areas have also fused into a large transparent patch. 

Ithomia sao (Fig. 52, Plate 4) is another good example of the 
Ithomia type. In this particular species the “inner black” of the 
fore wing is absent, and the “middle black band” of the hind wing 
has disappeared. When we come to consider the other Ithomias, 
we shall find that in this genus it has probably fused with the 
marginal black of the hind wing. 

I have made a record of the color-variations that affect the 
various characteristic areas just considered, and have recorded them 
for every one of the species of the Danaoid and Acraeoid Heliconidae 
knewn to me. As these records are too extensive for convenient 
inspection, I have condensed the results, and they will be found 
in Tables 1-27 inclusive. Thus, Table 1 gives the variations in 
color of the “inner rufous” area of the fore wing for each genus 
of the Danaoid Heliconidae; Table 2 records the variations of the 
“inner black”; Table 3 the “inner yellow” area, etc. In Table 1 
we find, for example, opposite the genus Ituna, the number 2 in the 
column labeled “transparent.” This indicates that in two species of 
Ituna the “ inner rufous” area is transparent. 

In order to facilitate the study of the color-patterns Dr. Daven- 
port suggested that I make use of the ingenious projection method 
invented by Keeler (93). This method consists in “squaring the 
wing” in the manner shown in Figs. 4 and 5 (Plate 1). In Fig. 4 
the large rectangle (A, B, C, D) just at the right of the figure of the 
hind wing represenisa kind of Mercator’s projection of the wing 
itself. The nervures 1*, 1, 2, 3, etc, are represented by the 
vertical lines 1%, 1», 2, 3, ete., on the rectangle A, B, C, D. In cells 
I, I®, and I*, (bounded by nervures 1*, 1°, and 2,) one finds a 
sinuous line winding across the middle of the cell. This line 
appears in the same relative position upon the rectangle A, B, C, D. 
The same is true of the eye-spot found in the cell bounded by 
neryures 2 and 3, and of all the other markings of the wings. 
The central cell of the wing itself is shown projected in the dotted 
rectangle E, F, G, H. 

In the case of the fore wing (Fig. 5), the central cell of the wing 
is dotted, and is shown projected upon the similarly dotted area 
within the rectangle I, J, K, L. In other respects the method of 
projection is the same as in the case of the hind wing. 

In this manner the colors displayed by various species of Danaoid 


208 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. . 


and Aeraeoid Heliconidae have been represented in color in Plates 
5-8. Each large rectangle upon the left hand side of the Plate 
represents a hind wing, the small middle rectangles show the colors 
of the cell of the hind wing, and the right hand rectangles give the 
fore wings, all being projected in the manner illustrated in Figs. 4 
and 5, Plate 1. The chief advantage in Keeler’s projection method 
lies in the fact, that similar areas in the projection of the wings lie 
vertically under or over one another, and thus by merely glancing 
up or down the plates one may observe the color-variations which 
occur in homologous cells of all the species represented. 


Ill. Generat Discussion oF THE CoLoR-PATTERNS AND OF 


Mniucry in THE GENERA HeEticonius AND EUEIDEs. 


Among the species of the genera Heliconius and Eueides we find 
remarkably little variation in venation, but great diversity in color- 
pattern of the wings, and in this respect they are very different from 
the Danaoid Heliconidae, where, it will be remembered, we find fully 
twenty different types of venation and only two types of color- 
pattern. ; 

(1) The Four Color Types in the Genus Heliconius. Schatz 


and Réber (’85—92) divide the species of the genus Heliconius into j 


four groups based on color differences, as follows:—(1) the 
«Antiochus group” (Plate 4, Fig. 50); (2) the “Erato group” 
(Fig. 60); (3) the “Melpomene group” (Fig. 59); and (4) the 
“Sylvanus group,” a good example of which is Heliconius eucrate 
(Fig. 58, Plate 4). , 

It will become apparent through an inspection of Figs. 50, 60, 59, 
and 58, which represent respectively, Heliconius antiochus, H. erato, 
H. melpomene, and H. eucrate, that the first three are quite closely 
related in color-pattern, while the fourth (H. eucrate) approaches 
very closely to the plan of coloration of the Melinaea type of the 
Danaoid Heliconidae. In fact this resemblance is so close that it 


may be safely said that the members of the “ Sylvanus group,” to 


which H. eucrate belongs, mimic the Danaoid Heliconidae. 

The “Antiochus group” is represented by Heliconius anti- 
ochus (Plate 4, Fig. 50, and Plate 5, Fig 62). H. sara, H. galanthus, 
and H. charitonius (Plate 5, Figs. 61, 63, 64) are also members of 
this group; other examples are H. apseudes, H. cydno, H. chiones, 
H. hahnesi, H. sappho, H. leuce, H. eleusinus, and H. clysonymus. 


1 


4 


: 


f 


MAYER: COLOR AND COLOR-PATTERNS. 209 


These species are characterized by their blue iridescence, and the 
narrow yellow or white bands upon the primaries; the hind wings 
are pointed at the outer apex, and the venation approaches the type 
found in Eueides aliphera. H. ricini (Plate 5, Fig. 66) is a good 
example of a form intermediate in coloration between group 1 and 
the “ Erato group ” (2). 

The type of group 2 is Heliconius erato (Plate 4, Fig. 60, and 
Plate 5, Figs. 67 and 68). This group is closely allied to group 1 in 
its characteristics. A good connecting link between groups 1 and3, 
the “ Melpomene group,” is H. phyllis (Fig. 65). 

The third, or “Melpomene group,” is represented by H. mel- 
pomene, H. callicopis, H. cybele, H: thelxiope, and H. vesta (Plate 6, 
Figs. 70-74, and Plate 4, Fig. 59). H. vulcanus, H. venus, H. 
chestertonii, H. burneyi, and H. pachinus are also examples of this 
group. : 

(2) Mimicry between the Genus Heliconius and the Danaoid 
Group. To Schatz’s group 4, the “ Sylvanus group,” belong all those 
species of Heliconius which have departed widely from the colora- 
tion pattern of the other three groups, and have come to resemble 
various species of the genera Melinaea, Mechanitis, and Tithorea of 
the Danaoid Heliconidae. H. eucoma, H. eucrate, H.dryalus, and H. 
sylvana (Plate 8, Figs. 88, 89, 91, and 95) are good examples of 
group 4. By glancing at the diagrams on Plate 8 it will be seen 
that H. dryalus resembles Melinaea paraiya very closely ; in fact, the 
likeness is so close that it is almost certain that no eye could distin- 
guish between the two insects when they are upon the wing. Another 
startling resemblance is that between H. eucrate and Melinaea thera 
(Plate 8, Figs. 91 and 92); moreover, there is but little difference 
between the color-patterns of H. eucrate, Eueides dianasa, and 
Mechanitis polymnia (Figs. 91, 93, and 94). H. sylvana and 
Melinaea egina (Figs. 95 and 96) are also said to mimic each other. 
The resemblance certainly appears very close at a casual glance, yet 
when the colors are plotted, as in Figs. 95 and 96, the differences be- 
come quite apparent. H. claudia (Plate 5, Fig. 69) is a good con- 
necting link between the Sylvanus group and the Melpomene group. 
In both the Melpomene and Sylvanus groups the venation has departed 
from the Eueides aliphera type, and the contour of the hind wings is 
much more rounded and elliptical than is the case in the Antiochus 
and Erato groups. (Compare Figs. 50 and 60 with Figs. 58 and 59, 
Plate 4.) There are rather less than twenty species which certainly 


210 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


belong to the Sylvanus group; among them may be mentioned, 
‘n addition to those already spoken of, Heliconius numata, which 
resembles Melinaea mneme and Tithorea harmonia ; H. zuleica, which 
resembles a Mechanitis and is a good copy of Melinaea hezia; and 
H. metalilis, which is said to mimic Melinaea lilis; there are also 
striking resemblances between 


H. aurora and Melinaea lucifer ; H. messene and Melinaea mesenina ; 
H. eucrate and Mechanitis lysimnia; _H. hecalesia and Tithorea hecalesina ; 
H. hecuba and Tithorea bonplandii; H. ethra and Mechanitis nesaea ; 

H. formosus and Tithorea penthias ; H. pardalinus and Melinaea pardalis ; 
H. telchina and Melinaea imitata ; H. ismenius and Melinaea messatis. 


Most remarkable of all perhaps is the close resemblance between 
Heliconius aristiona, Mechanitis methone, and Ithomia fallax of 
Staudinger. In fact, Staudinger states in his “ Exotische Schmet- 
terlinge” that he hesitated for some time to describe Ithomia fallax 
on account of its close resemblance to Hewitson’s Mechanitis methone. 
Good lists of the Heliconidae which are said to mimic one another 
are given by Wallace (89, p. 290, 251), and by Haase (’93%, p. 
146, 147). 

(3) The Three Color- Types in the Genus Hueides. In the 
genus Eueides we meet with three color-types represented by 
E. aliphera, E. thales, and E. cleobaea. These insects are dis- 


tinctly smaller than the species of the genus Heliconius, and the 


yellow spots upon their primaries are more ocherous in color than 
in Heliconius. E. aliphera (Plate 6, Fig. 77) represents the most 
highly specialized color-type. Eueides mereaui (Fig. 76), however, 
is a good connecting link between the color-patterns of E. aliphera 
and E. thales (Fig. 75), and E. thales is almost identical in color- 
pattern with Heliconius vesta (Fig. 74). 

The other type of Eueides is represented by E. cleobaea, E. 
dianasa, E. isabella, etc. (Plate 6, Fig. 78, and Plate 8, Fig. 93). 
These resemble the Sylvanus group of Heliconius or various Melinaeas 
and Mechanitis. 

(4) Detailed Discussion of Plates 5-8. Pate 5 is intended 
to illustrate the types of coloration found in the Antiochus and 
Erato groups of the genus Heliconius. In H. sara (Fig. 61) the 
wings are suffused with a dark blue iridescence, and some narrow 
yellow bands of color are found upon the primaries. In H. antiochus 
(Fig. 62) we find similar bands of color upon the primaries, but 


they are changed to white. H. antiochus may have descended © 


MAYER: COLOR AND COLOR-PATTERNS. 211 


from an albinic sport of H. sara. In H. galanthus (Fig. 63) 
the white areas have greatly increased in size, and the iridescent 
blue has become much lighter. In H. charitonius (Fig. 64) we 
find the wings crossed by yellow spots and bands, but in some speci- 
mens this yellow color exhibits a decidedly reddish tinge. The figure 
of H. charitonius in Staudinger’s “ Exotische Schmetterlinge ” illus- 
trates this peculiarity ; indeed, spots whichare commonly yellow are 
often found red, and vice versa. In H. phyllis (Fig. 65) we find 
along the upper part of the diagram of the hind wing a yellow mark- 
ing, and a similarly shaped red mark is found in its near ally, H. 
thelxiope (Fig. 73, Plate 6). The same is also true of H. ricini 
(Fig. 66, Plate 5). 

H. erato (Figs. 67 and 68, Plate 5, and Fig. 60, Plate 4) is very 
remarkable, for there are no less than four distinct color-types 
exhibited by different individuals of this species; one of them (Fig. 
67) shows the basal half of the hind wing marked by six red tongues 
of color edged with iridescent blue, and there is a dark rufous 
suffusion upon some parts of the fore wing. In other specimens 
(Fig. 68) the red tongues of color which characterized the hind wing 
of Fig. 67 are almost absent, and only the blue iridescence is left ; 
also there is no rufous to be seen upon the fore wing. In another 
type the blue iridescence of the hind wing has become green, and in 
still other specimens the yellow stripes upon the fore wing have 
become white. 

As one looks over the diagrams upon Plates 5-8, it becomes evi- 
dent that yellow frequently changes to white, for we often find one 
or two species of a genus which exhibit white spots identical in shape 
and position with spots which are yellow in most of the others. Good 
examples of this are H. antiochus (Plate 5, Fig. 62), Melinaea 
parallelis and Ceratinia leucania (Plate 7, Figs. 82 and 83) ; likewise 
the white spot near the outer apex of the fore wing in H. eucrate 
(Plate 8, Fig. 91), which is yellow in many individuals. Yellow 
areas are also frequently changed to rufous or red ; thus the yellow 
basal half of the hind wing of H. eucrate (Plate 8, Fig. 91) is often 
found of a rufous tinge in individual specimens of the species, and 
among the specimens of this species in the Museum of Comparative 
Zodlogy one can trace a gradation of this area from bright yellow 
to rufous. H. claudia (Plate 5, Fig. 69) is introduced in order to 
exhibit some of the differences between the “Sylvanus” group, to 
which it belongs, and the “ Antiochus” and “ Erato” groups. 


212 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


PLATE 6 is intended to exhibit the characteristic color-patterns 
found in the Melpomene group and in the genus Eueides. Fig. 70 
represents H. melpomene, and Fig. 71 its near ally, H. callycopis, in 
which the red area of the fore wing has become broken up, and some 
red spots have made their appearance near the base of the hind wing. 
In the next variety of H. melpomene, H. cybele (Fig. 72), it is 
remarkable that the pattern of the fore wing has come to resemble 
the Sylvanus type, and is identical in general plan of coloration with 
the fore wings of the Melinaeas or Mechanitis (see Figs. 84 or 85, 
Plate 7, or Figs. 92 or 94, Plate 8). In its close ally, H. thelxiope 
(Fig. 73), a still nearer approach to the Melinaea type has come 
about by the development of a black band across the middle of the 
hind wing, and one has only to imagine a general fusion of the seven 
club-shaped red stripes of the hind wing in Fig. 73, Plate 6, in order 
to produce exactly the Melinaea type as exhibited, for example, by 
Eueides cleobaea (Fig. 78). In this connection it is worthy of note 
that Bates (62) showed that H. thelxiope was derived from H. 
melpomene, there being between the two many intermediate forms. 

H. vesta (Fig. 74) is evidently a close relative of H. thelxiope, 
and what is still more worthy of note is, that it is almost identical in 
the general effect of its color-pattern with Eueides thales (Fig. 75)! 
The yellow spots upon the fore wing of E. thales are, however, duller 
in hue than are those of H. vesta, and the insects are somewhat 
different in size, H. vesta spreading 78 mm., while E. thales spreads 
only 66 mm. It will be noticed that the chief difference between 
the color-patterns of these two species lies in the fact, that, while the 
black stripes of the hind wings in H. vesta lie along the nervures, in 
Eueides thales they occupy the middle of the cells themselves. The 
general resemblance of the two color-patterns may of course be 
merely accidental. An easy explanation, however, is afforded by 
the theory of mimicry, for the two species look very much alike 
until one subjects their color-patterns to close analysis, when 
remarkable differences appear. E. thales (Fig. 75) may have been 
derived from some such form as E, mereaui (Fig. 76), for one has 
merely to imagine a greater development of the black and a general 
deepening of the rufous upon the hind wing of E. mereaui to make 
it resemble E. thales quite closely. Finally, in E. aliphera (Fig. 77) 
the black serrated border of the hind wing is still more reduced, and 
the black stripe which crosses the cell of the fore wing in E. mereaui 
is not present. 


MAYER: COLOR AND COLOR-PATTERNS. US 


Pate 7 is intended to illustrate the peculiarities of color-pattern 
found among the Danaoid Heliconidae. Thyridia psidii (Fig. 79) 
is an example of the transparent type of color-pattern found among 
the Danaoid Heliconidae, and especially prevalent among the 
Ithomias. It will be seen by comparing Fig. 79 with the other 
figures upon Plates 7 and 8, that the chief difference lies in the fact, 
that in this type both the rufous and yellow areas have become 
transparent. The black area of the fore wing has also suffered a 
reduction, especially along the outer margin of the wing. Inci- 
dentally it should be mentioned, that in this particular species the 
middle black band of the hind wing has become tilted up at a sharp 
angle, instead of crossing the wing horizontally. A life-size figure 
of the wings of Thyridia psidii is given on Plate 4, Fig. 47. 

In Napeogenes cyrianassa (Fig. 80) and Ceratinia vallonia (Fig. 
81) portions of the usually yellow and rufous areas have become 
transparent. 

The spots upon the fore wing of the Melinaeas are usually yellow, 
but in Melinaea parallelis (Fig. 82) they are white. It would seem 
that this form may have descended from some albinic sport. 
Ceratinia leucania (Fig. 83) resembles Melinaea parallelis so closely 
in general plan of coloration, that it is very difficult to distinguish 
between them, even when the two insects are seen side by side. 
Ceratinia leucania, however, is somewhat smaller than Melinaea 
parallelis. Both occupy the same region in Central America, and 
the specimens from which the diagrams were drawn came from 
Panama. 

Figs. 84-87 are drawn from various specimens of Mechanitis 
isthmia, all from Panama. They are intended to give some idea of 
the range of individual variation which is met with in this extremely 
variable form. The contraction of the middle black band of the 
hind wing in this form has already been noticed in the general 
discussion of the laws of color-pattern (see page 184). In Fig 87 it 
will be seen that the inner yellow stripe which usually crosses the cell 
of the fore wing has become very narrow and changed to a rufous 
color. However, upon the under surface of the wing it still remains 
as a yellow stripe. Indeed, in most color-changes the upper side of 
the wing seems to take the initiative, the under surface being more 
conservative. This is not true, however, in the Ithomias, where 
the black areas of the under side of the wings often are found to be 
rufous in color, while they still remain of the normal black upon the 


214 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


upper surface. The colors of the under surface are, however, 
usually identical with those of the upper, though they are always 
duller in hue. This may be due to the fact, that the colors of the 
upper surface are more frequently seen than those of the lower, for 
these insects often float lazily along with their wings horizontally 
extended. The operation of Natural Selection would then be more 
severe with the upper surfaces than with the lower. 

Prater 8 gives an analysis of the color-patterns of some of the 
Heliconinae and those Melinaeas, ete., which they resemble. H. 
eucoma (Fig. 88) is a good example of the Sylvanus type, and with 
its rufous, yellow, and black wings it is certainly a wonderfully close 
copy of the color-pattern found so commonly among the species of 
the genus Melinaea of the Danaoid Heliconidae. 

Heliconius dryalus and Melinaea paraiya (Figs. 89, 90) resemble 
each other so closely in size, shape, and coloration, that it must be 
impossible to distinguish between them when the butterflies are in 
flight; yet an analysis of their color-patterns shows that there are 
considerable differences between them. The shape of the yellow 
bands upon the fore wings is quite different; the inner black spot 
within the cell is double in Melinaea paraiya, and there is also a row 
of white spots along the margin of the fore wing. . 

A much closer resemblance is found between H. eucrate and 
Melinaea thera (Figs, 91 and 92), where the Heliconius is almost a 
true copy of the Melinaea. 

The color-patterns of Eueides dianasa (Fig. 93) and Mechanitis 
polymnia (Fig. 94) are also very nearly the same. Both are 
common species in Brazil. 

Heliconius sylvana is said by Bates and by Wallace to mimic 
Melinaea egina. It will be seen by reference to Figs. 95 and 96 
that their color-patterns are quite different in detail, yet the insects 
look very much alike when placed side by side, and may easily be 
mistaken for each other when upon the wing. Melinaea egina is 
much more common than Heliconius sylvana. 


IV. Generat Discussion oF THE CoOLOR—-PATTERNS AND OF 
Mricry AMoNG THE Danaord HELICONIDAE. 


(1) The Origin of the Two Types of Coloration. The character 
of the variation in the Danaoid Heliconidae is very different from 
that of the genera Heliconius and Eueides, for while there is great 


MAYER: COLOR AND COLOR-PATTERNS. QL 


diversity of color-pattern and very little variation in venation among 
the species of the Acraeoid group, exactly the opposite condition is 
met with in the Danaoid group, where we find at least twenty 
different types of venation and only two types of color-pattern. 
One of these types of coloration is well exemplified by most of 
the Melinaeas (Fig. 48, Plate 4), and I have therefore called it the 
“ Melinaea” type. The other type is exemplified by most of the 
Ithomias (Figs. 47 and 52) and has been designated in this paper as 
the “ Jthomia” type. In the Melinaeas, it will be remembered, we 
find the rufous and black wings crossed by bands of yellow; while 
in the Ithomias, on the other hand, the rufous and yellow areas have 
become transparent, often leaving the wing as clear as glass, and the 
black, which is so characteristic of the outer half of the wing in the 
Melinaea type, has shrunk away until it has come to lie along the 
outer margin of the wing only. 

By a study of all the genera of Danaoid Heliconidae we gain light 
upon the question of the origin of the “ Melinaea” and “ Ithomia” 
types of coloration. As we have seen (page 198), the Danaoid 
Heliconidae are an offshoot from the great family Danaidae. Indeed, 
two of the genera, Lycorea and Ituna, are so closely related to the 
Danaidae that Schatz and Réber (’85—92) propose to include them 
within that family. There can be but little doubt that Lycorea and 
Ituna are remnants of the ancestral forms which long ago shot off from 
the Danaidae to form the Danaoid Heliconidae ; and it is interesting to 
note, that in these two patriarchal genera we find the two distinct 
types of color-pattern which are exhibited by the Danaoid Helico- 
nidae, for all of the five known species of Lycorea are good examples 
of the Melinaea type (see Lycorea ceres, Fig. 46, Plate 4), while the 
four known species of Ituna all exhibit the transparent, or Ithomia, 
type of coloration. In fact, in their color-patterns the species of 
Ituna remind one of gigantic Ithomias. The species of Lycorea, 
however, are colored very much after the pattern of the Danaidae, 
and indeed they have departed but little from the type of the 
members of the great family whence they sprang. On this account 
I believe that the Melinaea type of coloration, which is so charac- 
teristic of the species of Lycorea, is phylogenetically older than 
the Ithomia type. 

In order to account for the origin of the Ithomia type, we may 
assume that, shortly after the primeval forms of the Danaoid Heli- 
conidae began to segregate out from the Danaidae, the species were 


216 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


few and probably rare. Under these circumstances any given insect 
would gain but little advantage by resembling merely the general 
type of the coloration of its fellows. For the relative advantage 
gained by such imitation, according to Fritz Miiller’s law, increases 
inversely as the square of the fraction whose numerator is the actual 
number of the imitating form and whose denominator is the actual 
number of the imitated. Therefore when the insects were still rare 
there would be few to imitate and consequently but little advantage 
would be gained by the imitation. Imagine, for example, that a 
single insect happens to imitate the color-pattern of a group of 100, 
and that the advantage gained thereby is represented by the number 
1; it is evident from Fritz Miiller’s law that, if it happened to 
imitate the coloration of a group of 1,000, its relative advantage 
would be 100 instead of 1. We see, then, that mimicry within the 
group of the Danaoid Heliconidae became an important factor only 
after the group was well established and the insects became common. 
During the early history of the race, then, there would be but little 
tendency towards conservatism of color-patterns, and when the 
“Ithomia” and “Melinaea” types of coloration made their appear- 
ance, they both survived and now serve as the patterns for mimicry ; 
and this accounts very well for the remarkable fact, that there 
are no other types of coloration than these two to be found within 
the whole group with its 450 species! 

(2) Mimicry among the Danaoid Heliconidae. The genus 
Ithomia with its 230 species is the dominant genus of the Danaoid 
group, and in nearly all of the other genera individual species are 
found which have departed widely from their generic type of 
coloration and have assumed the clear wings of the Ithomias. A 
good idea of how far these interesting individuals may depart from 
the coloration of their type may be gained by comparing Fig. 53, 
Plate 4, which represents Melinaea gazoria, with Fig. 48, which 
represents a typical Melinaea (M. paratya). It is evident that 
Melinaea gazoria is startlingly like an Ithomia both in size and 
coloration, although it retains the venation and generic charac- 
teristics of a Melinaea., 

In Mechanitis, which is the most independent genus of the 
-Melinaea type of coloration, all of the species are fair examples of 
the Melinaea type, except Mechanitis ortygia Druce, from Peru. 
Druce (’76) in his description of this curious little species states in 
astonishment that it possesses the venation of a Mechanitis, but the 
size and coloration of an Ithomia ! 


MAYER: COLOR AND COLOR-PATTERNS. 217 


It is quite remarkable that although the genera Melinaea and 
Mechanitis serve as models of mimicry for the Acraeoid Heliconidae, 
they should themselves mimic Ithomia. 

The genus Ithomia is, however, the most independent of all the 
genera of the Danaoid group, and I know of remarkably few good 
instances in which an Ithomia has apparently departed from the 
coloration of its type to assume the guise of the Melinaeas. One good 
example of such a change, however, is afforded by Ithomia fallax of 
Southern Peru, which resembles either Mechanitis methone or Heli- 
conius aristiona of Colombia (see page 210). There is apparently 
a difficulty in ascribing this resemblance to mimicry, for the imitator 
and imitated do not occupy the same geographical regions. 

In direct contrast with the independence of the Ithomias stands 
the case of the genus Napeogenes ; for Godman and Salvin (’79—86) 
say of Napeogenes, that nearly every species mimics some Ithomia 
which occupies the same district; and thus almost the very existence 
of the genus would seem to depend upon its mimicry of Ithomia. 

It is not the purpose of this paper to discuss, in detail, the numerous 
interesting cases of mimicry which are believed to exist between 
members of the Danaoid Heliconidae. An excellent discussion of 
such cases, and of the relationships of the various genera, has been 
given by Haase (’93%, p. 116-127). 


V. QUANTITATIVE DETERMINATION OF THE VARIATIONS OF THE 
CHARACTERISTIC WING—MARKINGS IN THE ACRAEOID AND 


Danaotw HEtLiconimpar. 


(1) Variations of “Inner Rufous” Areas of the Fore and 
Hind Wings. Table 1 gives the color-variations which are exhibited 
by the “inner rufous” area of the fore wings in the Danaoid Heli- 
conidae. This area is marked I in all of the figures upon Plate 4. 
We learn from an inspection of Table 1 that this area is rufous in 
color in 124 species of the Danaoid Heliconidae, transparent in 152, 
black in 24, and that in the remainder it is more or less translucent, 
and of either a yellowish or rufous tinge. 

Table 10 shows the variations which come over the “inner 
rufous” area of the hind wings of the Danaoid Heliconidae. This | 
area is marked X in the figures upon Plate 4. It is apparent at 
a glance that the variations which affect the inner rufous areas of 


218 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


both fore and hind wings are very similar. In order to exhibit this 
fact graphically, the color-variations have been laid off upon the 
diagram, Fig. 97, Plate 9. The base line is marked at equal inter- 
vals with the words “rufous,” “translucent rufous,” “translucent, 
slightly rufous,” “ transparent,” etc., and the ordinates show the 
number of species which exhibit the various colors, rufous, trans- 
lucent rufous, ete. For example, at the point “ translucent rufous ” 
we find that the ordinate is 23; this indicates that in 23 species 
the area is translucent rufous in color. The points thus found 
upon the ordinates are successively joined by straight lines form- 
ing a zig-zag figure. The full line represents the fore wing, and 
the dotted line the hind wing, and it becomes clearly evident from 
the closeness of these two zig-zag lines that the color of the inner 
rufous area of the fore wing (area I, Plate 4) is almost always 
sure to be identical with that of the inner rufous area of the hind 
wing (area X, Plate 4). We see, therefore, that whatever color- 
variation affects the inner rufous area of the fore wing, this area 
in the hind wing is almost always affected in the same manner. 

Fig. 99, Plate 9, is derived from Tables 15 and 24, which show 
the color-variations in the fore and hind wings of the genera Helico- 
nius and Eueides. It is seen that here also the colors of these two 
areas in both the fore and hind wings are almost always identical. 
We here meet with one of those interesitng physiological laws. 
which are independent of Natural Selection, and the meaning of 
which remains a mystery, for surely we can see no reason on the 
ground of adaptation why similar areas upon both fore and hind 
wing should bear similar colors. 

(2) The “ Inner Black ” Spot. Table 2 shows the presence or 
absence of the “inner black” spot in the Danaoid Heliconidae. 
This spot is marked II in the figures upon Plate 4. When pres- 
ent, it is always black in color and is usually found occupying the 
middle region of the cell of the fore wing. The table shows that 
it is about an even chance whether it be present or not, for it is 
absent in 210 species and present in 190. In the genus Ithomia, 
however, it is present in only one third of the species. What is 
most worthy of note concerning it, is the fact that it almost always 
appears, when present, as a single spot. Indeed, it appears as a 
double spot in only 7 species, and 5 of these belong to the genus 
Melinaea. A good example of its appearance as a double spot is 
found in Melinaea paraiya (Fig. 48, Plate 4). It will be remem- 


MAYER: COLOR AND COLOR-PATTERNS. 219 


bered that there are 450 species in the Danaoid group; 25 of 
these belong to the genus Melinaea; yet among these 25 we find 
9 exhibiting this marking as a double spot. Assuming that the 
doubling of this spot has arisen in each species as a sport, and that 
such a sport is as likely to appear in one species as in any other of 


the Danaoid group, then the chances against five such sports 


: Z : * 450X449 448 x 447 x 446 
2 T z 
appearing among the 25 Melinaeas is NING. WERENT 


2,830,000 to 1. Indeed, it is probable that all five of the species 
of Melinaea which exhibit the doubling of this spot are descend- 
ants of a single ancestor in which it appeared for the first time 
double, for the mathematical chance that one such ancestor should 
appear among the Melinaeas, rather than in any other genus, is 
evidently 1 in = or one chance in eighteen. The chance against 


two such unrelated ancestors is, however, ae or about 336 to 
= 20 Xf 


1, and the chance against three is oe or 6,560 to 1, ete. 

By reference to Table 16 we find that in the genera Heliconius 
and Eueides the inner black area is black or iridescent blue in all 
of the species of Heliconius, but absent in 5 of the 18 species of 
Eueides known to me. These 5 include Eueides aliphera and its 
allies. Now there are 150 known species of the Acraeoid Helico- 
nidae, and 24 of these belong to the genus Eueides; so it is evident 
that the mathematical chance against the supposition that five sports 
arose independently in the genus Eueides, in which the inner black 
was absent, is given by Os or 13,900 to1. It is there- 
fore probable that the five Eueides lacking the inner black are 
the descendants of a single ancestor. 

(3) Variations of the « Inner Yellow” and “ Middle Yellow” 
Areas. Tables 3 and 5, and diagram Fig. 98, Plate 9, show the 
color-variations of the “inner yellow” and “middle yellow” areas 
in the fore wings of the Danaoid Heliconidae. These areas are 
marked III and V, respectively, in the figures upon Plate 4. The 
“inner yellow” area, it will be remembered, occupies the outer por- 
tion of the cell of the fore wing; while the “ middle yellow” is found 
in the region just beyond the outer limits of the cell. The two areas 
are often fused together as in Figs. 47, 48, 50, 51, and 55, Plate 4. 
The inner yellow area is usually smaller than the middle yellow, 
and a comparison of Tables 3 and 5 will show that it is much more 
frequently obliterated by the encroachment of the rufous or black 


, or about 


220 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


areas which surround it; for example, while the middle yellow is 
rufous in color in only 14 species, the inner yellow is rufous in 56; 
also the inner yellow area, being usually smaller and less conspicuous 
than the middle yellow, is less important in cases of mimicry, and 
the diagram Fig. 98, Plate 9, shows that it is much more variable 
in color than the middle yellow. The full zig-zag line in this figure 
represents the color-variations of the inner yellow, while the dotted 
zig-zag line gives the color-variations of the middle yellow. As 
there are nine color-variations displayed by each of these two areas, 
and as there are 400 species of the Danaoid Heliconidae recorded 
by me, it becomes evident that, if there were no color preferences 
displayed by these areas, there would probably be about 49°, or 44.4, 
species which would display it as rufous, 44.4 translucent, 44.4 yellow, 
etc. The heavy, straight, dotted line (Fig. 98, Plate 9) represents 
this ideal condition, which would be approximately realized were 
one color as likely to occur as another in the respective areas. Now 
it is evident from an inspection of the figure, that the full zig-zag 
line, which represents the color-variations of the “inner yellow,” 
approaches the straight line condition more nearly than does the 
dotted zig-zag line, which represents the middle yellow.! The 
inner yellow is therefore more liable to color-variations than the 
middle yellow; and this is what we should expect on account of 


its comparatively small size and its consequent inconspicuousness © 


as a characteristic marking in cases of mimicry. 

A comparison of Figs. 97 and 98, Plate 9, is interesting, for it 
shows that the color-variations of the inner rufous are quite similar 
to those of the inner yellow and middle yellow. This serves to 
illustrate the close physiological relationship which exists between 
rufous and yellow. The two pigments are probably closely related 
chemically, for every ordinarily rufous area is sometimes found to be 
yellow, and vice versa. Yellow areas also often change to white. 
Rufous, yellow, and white are evidently closely related color-vari- 
ations.” 


1This is not true for one color, white. 


2It may be well to mention here that the black areas upon the wings are subject to ~ 


very little color-variation. In some cases, however, especially upon the under surfaces 
of the wings in Ithomia, the black has changed toarufous or russet color. For example, 
Table 4 shows that the middle black area (IV in the figures upon Plate 4) is rufous in 
only 12 species out of the 400 which are recorded, and all of these 12 are Ithomias. Also 
Tables 7 and 13 show that the outer black of the fore wing, and the outer black of the 
hind wing are russet in 22and 11 species, respectively. Evidently, black is a far more 
conservative color than rufous, yellow, or white. Probably black is also quite different 
from the other pigments chemically. 


MAYER: COLOR AND COLOR-PATTERNS. 221 


Tables 17 and 19 show the color-variations affecting the “ inner 
yellow ” and “ middle yellow” areas of the fore wing in Heliconius 
and Eueides. There is but little difference between the two tables, 
except that in 15 species of Heliconius the inner yellow is suffused 
with black or blue, while the middle yellow is never suffused by the 
outer black which surrounds it. Fig. 100, Plate 10, exhibits 
graphically the color-variation of these two areas. The “inner 
yellow” is represented as a full line, and the “ middle yellow” as 
a dotted zig-zag. It is evident that here also the inner yellow is 
more variable in color than the middle yellow, for not only does the 
inner yellow area display two more colors, but its chart is a flatter 
zig-zag. 

(4) Variations of the « Middle Black” Mark of the Fore Wing. 
Table 4 shows the color-variation of the middle black mark (area 
IV in figures upon Plate 4). This marking lies along the extreme 
outer border of the central cell of the fore wing. It is small in area, 
but is rendered very conspicuous from the fact that it is situated be- 
tween the inner yellow and middle yellow markings. In spite of 
its small size, however, it is a remarkably permanent marking, for 
Table 4 shows that it is absent in only 20 out of 400 Danaoid Heli- 
conidae. In these 20 it has been obliterated by the fusion of the 
inner and middle yellow areas. It is worthy of note that in 12 
Ithomias it has become rufous in color. This change to rufous is 
the only color-change which the black areas of the wings ever 
display. 

Table 18 shows the variations of the middle black area for 
Heliconius and Eueides. 

(5) Variations of the “ Outer Yellow” Area of the Fore Wing. 
Table 6 shows the variations which affect the outer yellow area 
of the fore wings in the Danaoid Heliconidae. This area is marked 
VI in the figures upon Plate 4; it lies beyond the region of the 
middle yellow, but is usually more or less fused with it. Table 6 is 
only approximately correct, owing to the difficulty in many cases of 
deciding whether the middle and outer yellow be really fused or not. 
Tt will be seen that in the genus Ithomia the middle and outer 
yellows are wholly fused in about 200 species. This is one of the 
marked characteristics of this very independent genus. 

Table 20 shows the color-variations of the outer yellow area in 
Heliconius and Eueides. This marking is present in 81 and absent 
in 48 of the Acraeoid group. It is much more widely separated 
from the middle yellow than is the case in the Danaoid group. 


222 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


(6) The relative Permanency of the Black Areas upon the Fore 
and Hind Wings. <A study of the relative permanency of the 
various characteristic black markings upon the wings is of interest, 
for, if the generally accepted idea concerning the prevalence of 
mimicry within the group of the Danaoid Heliconidae be true, we 
should expect the most conspicuous markings to be the most perma- 
nent, for they are evidently of the most importance for mimicry. 
This is, however, not the case for the black markings. A good 
example of this fact is afforded by a comparison of the relative 
permanency of the black streak which extends along the extreme 
costal edge of the fore wing with the inner black spot (II in figures 
on Plate 4). The inner black spot is certainly a-more conspicuous 
marking than this narrow black streak along the costal edge; yet it 
is much more variable, for Table 2 shows that it is present in 210 
and absent in 190 of the 400 Danaoid Heliconidae. In other words, 
it is about as likely to be present as absent. The black streak upon 
the costal edge, on the other hand, is much more permanent, for it is 
absent in only 52 species out of the 400. 

Another good example of the inaccurracy of the supposition that 
large and conspicuously colored areas are always less variable than 
small ones, is derived from a comparison of the relative variability 
of the large outer black of the fore wing with the small outer 
black of the hind wing. Although the outer black area of the fore 
wing is usually much larger and more conspicuous than the outer 
black margin of the hind wing, it is more variable in color, for it is 
rufous in 22 species, while the outer black of the hind wing is 
rufous in only 11, out of the 400. 

In general, however, large colored areas are more permanent than 
small ones, as was found in the case of the inner and middle yellow 
areas (see page 220). Indeed, a good instance of this greater vari- 
ability of small color areas is afforded by the longitudinal black 
stripe marked VIII in the figures of Plate 4, for this is more 
variable than the larger outer black area of the fore wing. 

(7) The “ Middle Black Stripe” of the Hind Wing. In the 
genus Ithomia the middle black stripe (XI, Plate 4) has migrated 
downward, so that in many species it has become fused with the 
outer black margin, as in Ithomia sao (Fig. 52, Plate 4). In other 
cases there is still to be seen a narrow line of rufous color between 
the middle black band and the outer black margin of the hind 
wing. Such is the case in Ithomia nise (Fig. 54, Plate 4). In 


MAYER: COLOR AND COLOR-PATTERNS. 223 


many other cases the outer black and middle black are completely 
fused, so far as the upper surface of the wings is concerned ; but, 
if one examines the under surface of the hind wings, it will be 
found that a narrow rufous streak still persists between the middle 
black band and the outer black margin of the hind wing. 

(8) Variations of the Marginal Spots of the Fore Wing. The 
marginal spots are found very near the outer margin of the fore 
wing; they are usually either yellow or white, but in some few 
eases they are rufous. It appears from Table 9 that they are 
present in 146 and absent in 254 species of the 400 Danaoid Heli- 
conidae known to me. Fig. 101, Plate 10, shows graphically 
the manner in which these spots occur in those species which 
possess them. It is evident from this curve that the number of 
these spots is not determined merely by chance, for they show a 
marked tendency to appear either as 2 or 3, or as 6 or 7 spots. 
It is due to this fact, that there are two maximum points upon 
the diagram Fig. 101, Plate 10. In those species which exhibit the 
“2- or 3-spot” condition, the spots are found near the front apex 
of the fore wing. In the “ 6- or 7-spot” condition they lie all along 
the outer margin of the fore wing, one spot in each cell. In the 
genera Ithomia, Napeogenes, and especially in Ceratinia these mar- 
ginal spots have become large and conspicuous ornaments. (See 
Fig. 49, Plate 4.) 

Table 22 shows the manner of appearance of these spots in the 
genera Heliconius and Eueides. They are found in only 26 species 
of the 129 known to me; and this number is far too small to war- 
rant general conclusions concerning the order of their appearance. 

(9) The Marginal Spots of the Hind Wing. Table 14 illus- 
trates the manner in which the marginal spots of the hind wings 
_make their appearance. They are absent in 279 and present in 
121 of the 400 species of the Danaoid group. Thus they occur 
rather less frequently than the marginal spots of the fore wing. In 
the 121 species in which these spots are found they show a decided 
tendency to appear either as 4 or as 5 spots. Fig. 102, Plate 10, 
is a graphic representation of the distribution of these spots, derived 
from Table 14. It appears that the outline of the figure approaches 
a probability curve, and is approximately symmetrical about the 
mean ordinate (A, B), situated at 4.54. 


224 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


+ 


VI. Comparison OF THE CoLOR-VARIATIONS OF THE PAPILIOS OF 


SoutH AMERICA WITH THOSE OF THE HELICONIDAE. 


In order to emphasize the peculiarities of the coloration of the 
Heliconidae, I will conclude by instituting a comparison between 
their variations and those of the South American Papilios. There 
are about 200 species of Papilio in South America, and these display 
in all 36 distinct colors. The colors have been determined by 
reference to the plates in Ridgway’s “ Nomenclature of color for 
naturalists.” A list of the colors which are displayed by these Papi- 
lios has already been given upon page 191. 

By exercising a very fine discrimination in distinguishing color we 
may count 15 distinct colors which are displayed by the 450 mem- 
bers of the Danaoid Heliconidae, as follows: black, brown, translu- 
cent black, sulphur-yellow, canary-yellow, citron-yellow, primrose- 
yellow, yellow-rufous, reddish rufous, rufous, white, translucent yel- 
low, translucent rufous, transparent areas upon the wings, transpar- 
ent areas which display iridescence. We see, then, that while the 
200 species of Papilio display 36 different colors, the 450 Danaoid 
Heliconidae exhibit only 15. In other words, the nwmbers of the 
species and of the colors are almost in inverse ratio in the two 
groups; for while the Papilios are only -¢ as numerous as the 
Danaoid Heliconidae, they display almost 12 times as many colors; 
and this is all the more remarkable when we remember that the gen- 
eral class of coloration in the Papilios and Danaoid Heliconidae is 
apparently the same. That is to say, in both groups we find all of 
the species displaying decidedly conspicuous colors, the coloration 
of the upper surfaces of the wings being in both rather more bril- 
liant than that of the lower surfaces, but without essential differences 
in color-pattern. Nor is there an attempt in either case at protective 
resemblances, such as the imitation of the coloration of bark, leaves, 
etc. The color-patterns of the Papilios are, moreover, extremely 
complex, and upon comparing the different species, there are seen 
to be frequent fusions and obliterations of the characteristic mark- 
ings, so that Haase (’93), who has made an extensive study of their 
color-patterns, is forced to divide them into many small groups 
of a few species each. The variation in the form of the wings is 
also very great among the Papilios, for while P. protesilaus pos- 
sesses upon its hind wings, long taillike appendages, the hind 
wings of P. hahneli are rounded off and without marked appendages. 


MAYER: COLOR AND COLOR-PATTERNS. 295 


There is, apparently, but one important respect in which the 
Danaoid Heliconidae are more variable than the Papilios, and that 
is size. For example, Lycorea ceres, which is probably the largest 
of the Danaoid group, has 2.2 times the spread of wing of Ithomia 
nise, which is one of the smallest (see Plate 4, Figs. 46 and 54). 
The largest Papilio, P. androgea, on the other hand, spreads only 
2.16 times as much as the smallest, P. triopas. 

There is another minor respect in which the color-patterns of the 
Papilios are different from those of the Heliconidae. In the Heli- 
conidae the fore wing slightly overlaps the hind wing, and that por- 
tion of the hind wing which is hidden from view is always dull in 
color (see Plates 5-8). In the Papilios, however, the fore wing 
does not overlap the hind wing to such an extent as in the Helico- 
nidae, and it is worthy of note that the costal edges of the hind 
wings in the Papilios are as brilliantly colored as are any other por- 
tions of the wings. 

It is difficult to account for the remarkable conservatism in respect 
to color-variations among the Heliconidae, unless we resort to the 
explanation afforded by the theory of mimicry; for, while there is 
Such remarkable simplicity and uniformity of color-pattern through- 
out the whole group of the Heliconidae, individual variations 
are very common. In the collection at the Museum of Compara- 
tive Zodlogy, for example, one finds a regularly graded series of 
specimens of Heliconius eucrate; at one end of this series the 
“inner rufous” area of the hind wing is bright yellow, and at 
the other end it is rufous; intermediate specimens are found in 
which this area is yellow, but dusted over with rufous scales. Also 
the “ middle black band” of the hind wings in Melinaea parallelis 
is very variable, some specimens showing it broken in the middle 
(Plate 7, Fig. 82), and others having it as an entire band. I have 
also seen one specimen of H. burneyi in which the commonly 
yellow spots upon the under surface of the wings were changed to 
white. Another good instance of individual variability is afforded 
by H. phyllis (Plate 5, Fig. 65), for in this species the series of 
small red spots sometimes found just below the yellow band upon 
the hind wing is very variable, and more often absent than present. 
Still other instances of individual variability are seen in the yellow 
Stripes upon the wings of H. charitonius (Plate 5, Fig. 64), which 
are often found tinged with rufous. Also the remarkable diversity 
in Mechanitis polymnia, and M. isthmia (Plate 7, Figs. 84-87) are 


226 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


other examples which show that there is no lack of imdividual vari- 
ability among the Heliconidae. Yet the Danaoid species as a whole 
vary but little from the two great types of coloration represented 
by Ithomia and Melinaea, and in this respect they are very differ- 
ent from the Papilios, where we find a great many color-types and 
great diversity in shape of wings. Surely there must be some cause 
for the remarkable fact that the Danaoid Heliconidae with their 
453 species should display but two types of color-pattern. I can 
think of but one explanation, which is afforded by Fritz Miiller’s 
theory of mimicry. 


In conclusion it gives me pleasure to thank those friends whose 
generous aid and kindness have done so much to render the prose- 
cution of this research a pleasure to me. I wish to express my 
gratitude to Dr. Charles B. Davenport, who is the real instigator 
and promoter of this research; to Mr. Samuel Henshaw, to whom 
I am indebted for numerous kindnesses, and who placed at my dis- 
posal the extensive entomologicai collections and library of the 
Museum of Comparative Zoélogy at Harvard; to Prof. Edward L. 


Mark for his kindness in revising the manuscript of this paper, and 


for the numerous valuable suggestions which he has made; to Dr. 
Samuel H. Scudder, to whom I am grateful for much kind advice 
and for the use of rare works in his library; to Prof. Ogden N. 
Rood for his valuable suggestions in regard to the spectroscopic 
apparatus; to Dr. Alpheus Hyatt for his valued and kind advice, 
and to my father, Prof. Alfred M. Mayer, for the use of Maxwell’s 
dises and the direct-vision spectroscope. 


PART ©. 


GENERAL SUMMARY OF RESULTS BELIEVED TO 
BE NEW TO SCIENCE. 


(1) The great majority of the colors of Lepidoptera contain a 
surprisingly large percentage of black (p. 172). 

(2) The colors displayed by the scales are not simple, but com- 
pounded of several different colors (p. 173). 

(3) The pigments of the scales of Lepidoptera are derived by 
various chemical processes from the blood, or haemolymph, of the 


MAYER: COLOR AND COLOR-PATTERNS. 227 


pupa. The pupal blood of the Saturnidae is a proteid substance 
containing egg albumen, globulin, fibrin, xanthophyll, orthophos- 
phoric acid, iron, potassium, and sodium (p. 176). 

(4) In Callosamia promethea and Danais plexippus the pupal 
wings are at first perfectly transparent, then white, then impure 
yellow, excepting upon those portions which are destined to remain 
white in the mature wing. The mature colors then begin to appear 
near the central areas of the wings and Jetween the nervures. Last 
of all, the nervures themselves become tinged with the mature 
colors. The central portions of the wings acquire their mature 
colors before the outer and costal edges, or the root of the wing 
adjacent to the body (p. 178, Plate 3). 

(5) The white stage in the development of color in the pupal 
wings represents the condition in which the scales are perfectly 
formed but lack the pigment which is destined to be introduced 
later (p. 178). (See, also, Mayer, ’96, p. 230.) 

(6) Dull ocher-yellows and drabs are, phylogenetically speaking, 
the oldest pigmental colors in the Lepidoptera. The more brilliant 
colors, such as bright yellows, reds, and pigmental greens, are 
derived by complex chemical processes and are, phylogenetically 
speaking, of recent appearance (p. 178). (See, also, Mayer, ’96, p. 
232.) 

(7) While the number of species of Papilio in South America 
is 9 times as great as in North America, the number of colors which 
they display is only twice as great. Hence the greater number of 
colors displayed by the tropical forms may be due simply to the 
far greater number of the species, and not to any direct influence 
of the climate (p. 191). 

(8) The following laws control the color-patterns of butterflies 
and moths: (a) Any spot found upon the wing of a butterfly or 
moth tends to be bilaterally symmetrical, both as regards form and 
color; and the axis of symmetry is a line passing through the center 
of the interspace in which the spot is found, parallel to the longi- 
tudinal nervures (p. 183). (b) Spots tend to appear not in one 
interspace only, but in homologous places in a row of adjacent 
interspaces (p. 183). (c) Bands of color are often made by the 
fusion of a row of adjacent spots, and, conversely, chains of spots 
are often formed by the breaking up of bands (p. 183). (d) When 
in process of disappearance, bands of color usually shrink away at 
one end (p. 184). (e) The ends of a series of spots are more 


228 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


variable than the middle. This is only a special case of Bateson’s 
(94) law (p. 185). (f) The position of spots situated near the 
outer edges of the wing is largely controlled by the wing-folds or 
creases (p. 185). 

(9) The scales in Lepidoptera do not strengthen the wings or 
aid the insects in flight. The vast majority of the scales are merely 
color-bearing organs, which have been developed under the influence 
of Natural Selection. The phylogenetic appearance and develop- 
ment of scales upon the originally scaleless ancestors of the Lepi- 
doptera did not alter the efficiency of their wings as organs of flight. 
It is probable, therefore, that this efficiency was an optimum before 
the scales appeared (p. 197). 

(10) A systematic study of the Danaoid Heliconidae demon- 
strates that their color-patterns can be placed in two types. Type 1, 
the more complex, is closely related to the coloration of the Danaidae 
from which the Danaoid Heliconidae sprang, and is therefore, 
phylogenetically speaking, the older type of coloration. This type 
is characteristic of the genera Lycorea, Melinaea, and Mechanitis, 
and I have called it the “ Melinaea” type. It is characterized by 
the fact that the wings are rufous and black in color, and crossed by 
a definite system of yellow bands. Type 2, the “ Ithomia ” type, is 
characteristic of the genera Ithomia, Ituna, and Thyridia. The 
“ Tthomia” type has been derived from the “ Melinaea” by the 
originally rufous and yellow areas upon the wings having become 
transparent (p. 204). 

(11) The phylogenetic origin of the “ Melinaea” and “ Ithomia” 
types of coloration can be accounted for upon the supposition, that 
when the species of the Danaoid Heliconidae began to segregate ~ 
out from the Danaidae they were for a time rare (p. 215). 

(12) A record of the characteristic markings upon the wings of 
the Danaoid and Acraeoid Heliconidae shows that, physiologically 
speaking, the colors red, rufous, yellow, and white are closely 
related, and that black is quite distinct from these, being the least 
variable color of all (p. 220). 

(13) In both the Danaoid and Acraeoid Heliconidae, whatever 
color-variation affects that part of the fore wing which is adjacent 
to the body of the insect, almost always the same color-variation 
affects the homologous area of the Aind wing in a similar manner 
(p. 218, and Fig. 99). 

(14) The smaller yellow spots upon the wings of the Heliconi- 


i 


MAYER: COLOR AND COLOR-PATTERNS. 229 


dae are more liable to color-variations than are the larger ones. 
This is what we should expect, if the theory of mimicry be true; 
for large spots are more conspicuous, and therefore their preservation 
is more important (p. 220). This rule, however, does not hold for 
the black markings of the wing (p. 222). 

(15) The mathematical chance against five similar and inde- 
pendent color-sports arising in the genus Melinaea is as 2,830,000 
to 1. Hence, the five Melinaeas which display the “inner black” 
as a double spot are probably descended from a single ancestor (p. 
219). 

(16) The marginal spots of the fore wing in the Danaoid Heli- 
conidae show a marked tendency to appear either as 2 or 3, or else 
as 6 or 7 spots (p. 223, Fig. 101). The marginal spots of the hind 
wing show a marked tendency to appear either as 4 or 5 spots 
(p. 223, and Fig. 102). 

(17) The 200 species of Papilio in South America display 36 
distinct colors, while the 450 species of Danaoid Heliconidae exhibit 
only 15. Hence the numbers of the species and of the colors are 
almost in inverse ratio in the two groups. This may be explained 
by the fact, that the Danaoid Heliconidae mimic one another, while 
the Papilios do not (p. 224). 

(18) The colors are dull upon those portions of the hind wing 
which are hidden from view by the overlapping fore wing (p. 225). 

(19) There is no lack of individual variability among the species 
of the Danaoid Heliconidae; yet the species as a whole vary but 
little from the two ‘great types of color-pattern represented by 
Melinaea and Ithomia. In order to account for this remarkable 
fact I am forced to resort to Fritz Miiller’s theory of mimicry (p. 
225). 


230 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


TABLE 1. 


Showing the Variations in Color of the “ Inner Rufous” (Area 
I in Figures on Plate 4) of the fore wing in the Danaoid Heli- 


conidae. 
Sey elaine 
se/s* | & les | 8 | 
= 5 =O |Sm S | sa =): : ; m 
= | ah |geg| & lan | fs | S| =] 2 
@)8 |eH8|.8 |e" >) eee eee 
| } 
yecorea 6) 2, 5 
Ttuna Es eee 2 | 1 
AGNES) fost ites ty Ee 3 1 | 
Thyridia 5 | | 
Athyrtis 2 
Olyras A Sigal + 1 1 iP | | ie 
WLEESIS eee 2 
Aprotopos. )\..72 = "= il de] 
Dircenna i et a A ee 1 Le 
Callithomia 1 2 | 
Epithomia . 1 1 
Ceratinia 28 1 2 7 Al 2 | 
Sais Raa Pe 5) 
Seagal ee SR | | 
Mechanitis | 18 eek: a) 1 40 
Napeogenes 7 3 2 Ned | ead 3 | 
Ithomia 99'| 43.) 15") 120} “26 pees 3. | a 
Aeria | | 
Melinaea 22 | |e ia | 
Tithorea i | | 6 | 
Total 124 | 23] 28 | 152] 30] 22) 1) 24) Gf 
Excluding Ithomia 95 | 10 S| 32 | 4 | 17 | 1.) 28 
I 


Note: The costal edge of the fore wing is usually black; it is rufous or brown, how- 
ever, in 47 Ithomias and dull yellow in one; it is rufous in two species of Sais, in one 
species of Ceratinia, and in one species of Athesis. Hence it is black in 348 species and 
light colored in 52. : 


oe ’ 


MAYER: COLOR AND COLOR-PATTERNS. 


TABLE 2. 


231 


Showing the Variation (presence or absence) of the “Inner 


Black” (Area IT) of the fore wing in the Danaoid Heliconidae. 


bere) ae 
GENERA. lose Z Remarks. 
o = 
= <{ 
| 
Lycorea 5 | 
Ttuna 3 
Athesis Zila 
Thyridia 5 | 
Athyrtis 2 
Olyras . 4 
Eutresis 2 | 
Aprotopos 2 
Dircenna . a) 4 
Callithomia 3 
Bipihemia . . . . 2 
Meranda. 2 . . - 29 | 12 
Sais . 4 1 
Scada Z 
Mechanitis By) al 
Napeogenes 13 | 17 | Appears as 2 spots in 1 species. 
Ithomia 72 |140 | Appears as 2 spots in 1 species. 
/Aeria 4 
Melinaea 21 3 | Appears as 2 spots in 5 species. 
Tithorea 10 
Total /210 |190 
| 


Excluding Ithomia 


232 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


TABLE 3. 


Showing the Variations in Color of the “Inner Yellow” (Area 
III) of the fore wing in the Danaoid Heliconidae. 


| > = > 5 
|e. |#8 | & lee 2 He 
| 83/88.) § 18h.) se $23 
GENERA. 2 | 32 less) & |ese) as) E | 4 |28e 
S|ae\|ge| 8 lam |a>| 2 | 2 |Faa 
Ble jee |.46 jes |e > | s 
Lycorea | 5 
Ituna 3 
Athesis | 2 1 
Thyridia 5 
Athyrtis 1 1 
Olyras . | 4 
Eutresis ieee! 1 
Aprotopos 1 1 
Dircenna . 1| 2 3 6 
Callithomia 3 
Epithomia i 1 
@eratniay = i.) ce) 16 1 1 7 7 2 6 1 
SSIS Me aoe fot Bie Bas 2 2 i 
Scada tah Oa Seer: 7 
Mechanitis ... . 11 15 
Napeogenes . .. . 2 | 4.) 44 1 4 5 
eihomiia, \-. (2 wtues « 10] ;Ae | 342) 194 es 9| 138 1 ¥ 
Ysa ti ge SCeerae Cah Poe eae a 4 
Melmnaen ». 26.5" 14 4 4 2 
Withorea =: 2 <= < | | 6 3 i 
Pabale Sega hong el) sts 13 | 24 | 158 38 | 31] 64| 6] 10 
Excluding Ithomia . 46 21 10 | 24 15 | 29 | 51 | 5 3 


MAYER: COLOR AND COLOR-PATTERNS. Zan 


TABLE 4. 


Showing the presence or absence of the “ Middle Black ” Mark 
(Area IV) of the fore wing in the Danaoid Heliconidae. 


GENERA. Present. Absent. es ial pee faa ae 

5 
3 
3 
5 
: 2 
resis / : 2 
topos 2 

C e 1 alt| i 
lithomia 3 
thomia 2 

ratinia 34 7 
Pe te 5 
4 a 
nitis” 24 

eogenes 25 5 

194 6 12 rufous or brown. 

, 4 

23 1 
10 

366 20 ie 12 
é uding Ithomia . 172 14 


>= 


—?. 


234 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Showing the Variation in Color of the 
(Area V) of the fore wing in the Danaoid 


TABLE 5. 


“ Middle Yellow” Band 


Heliconidae. 


: Shee | WSR ES = = =| | cet g= 8 
GENERA. 2 SON ace eae lee z =55 
& | #2 |2e8| # | 254) 2S] 6 | 3 lees 
= SSF G0 | a = D a Ses 
= = ies = 3 = = = £3 
Lycorea & 
Ituna 3 
Athesis 2 1 
Thyridia 5 
Athyrtis 1 1 
Olyras 1 3 
Eutresis 2 
Aprotopos 1 1 
Dircenna 2 3 2 2 3 
Callithomia 3 
Epithomia 1 if 
Ceratinia 2 4 5 24 
Sais . 5 
Seada 6 1 
Mechanitis 4 1 ; 19 
Napeogenes 2 5) 11 4 4) 6 
Ithomia 5 6 12 | 123 24 8 14 20 
Aeria 4 
Melinaea 2 3 2 15 2 
Tithorea 7 1 2 
Total 14 6 2) | 159 a5 | 31 | 108 2 24 
9 0 9 36 94) 2 4 


Excluding Ithomia 


te~ 


MAYER: COLOR AND COLOR-PATTERNS. 


TABLE 6. 


235 


Showing approximately the Number of Species in which the 
“Outer Yellow” (Area VI) of the fore wing in the Danaoid Heli- 
conidae appears as a separated Marking. It is usually fused with 
the “ Middle Yellow” Area. 


Wholly fused Partially fused 


GENERA. with middle with middle Separate. 
yellow. yellow. 
Lycorea 1 4 
Ituna 2 a 
Athesis 3 
Thyridia 5 
Athyrtis 2 
Olyras 1 3 
Eutresis 2 
Aprotopos 2 
Dircenna 7? 5? 
Callithomia 1 2 
Epithomia 1 1 
Ceratinia . 22 16 about 3 about 
Sais Zi 
Scada 4 
Mechanitis 1 20 
Napeogenes 6 24 about 
Ithomia 200 about 
Aeria 4 
Melinaea 1 17 
Tithorea j 10 
— 
Total about 250 | about 30 
| 


“about 90 | perhaps 20 


Absent. 


me 
We ee 


236 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


TABLE 7. 


Showing the Degree of Development of the “ Outer Black” (Area 
VII) of the fore wing in the Danaoid Heliconidae. 


| Well devel- Reduced 

Guana oped over a_ to the outer Present, but changed to 
ea ; large area of margin of the another color. 
_theforewing. fore wing. 


LyGorea sce 5) ees a 5 

Ituna 3 

Athesis 2 1 

Thyridia 5 

Athyrtis 2 

Olyras 5 1 

Eutresis 2 

ASPEOLOPOS) © es) = = ya | 1 1 

IDineennagey e062 tae il 5 7 

Callithomia 5 

Epithomia 2 

Ceratinia eye 28 15 2 partly rufous. 

SAIS Ei) ee Ree a aan ae eee 2 | 3 partly rufous. 

Beadage.), ema | 3 4 | 

Mechanitis 22 2 

Napeogenes 26 t 

Ithomia Be 161 54 _ 16 rufous or brown. 

WACTIdigs 52 os Beksp onet al 4 

Melingeaeg a4) 24 1 partly rufous. 

Tithorea 10 

Nh) 7 Ree oe 305 95 | 22 partly rufous. 
| | 


MAYER: COLOR AND COLOR-PATTERNS. Zot 


TABLE 8. 


Showing the presence or absence of the “ Longitudinal Black 
Stripe” (Area VIII) which runs parallel with the lower Edge of 
the fore wing in the Danaoid Heliconidae. 


| Present and ivereetn Whole area 
GENERA. well developed nodaeed Absent. suffused with 
as a stripe. black. 
| 
MMEGTEA = . 2 . | 5 
Ttuna ere. *| 3 
OJUNESTS: 3 
Thyridia | 5 
Athyrtis eA 4 | 2 
nt 5 3 1 
Eutresis : 2 
Aprotopos il 1 
Dircenna ee 7 3 2 
Sallithomia . .~. . | 1 2 
Mpithomia. . .. . il 1 
Beramige. . . . . 37 2 2 
SIS 6 9) 3a 3 | 1 1 
c'TL2 6.3 ee 6 1 
Mechanitis . .. . ily 2 5 
Napeogenes . .. . 20 | 6 4 
Juche 200 | 6 + 2 
2) 2 4 
Mielaea. - .. . 14 | 5 5 
Mihorea . =... et 5 5 
| 

GL ae 338 24 14 24 


238 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


TABLE 9. 


Showing the Manner of Occurrence of the Marginal Spots (Area 
IX) of the fore wing in the Danaoid Heliconidae. 


{ | } 
With- 1 2 3 4B 6 i 8 - 9 


GENERA. snes | spot. |spots. spots, spots. spots. spots. spots.) spots. spots. 
| 

Lycorea 5 
Ituna 3 | | 
Athesis 3 | 
Thyridia . 4 be | | | 
Athyrtis 1 | 1 | 
Olyras 3 f | 
Eutresis 2 
Aprotopos Lis ye | 
Direcenna. . 11 1 
Callithomia . i oe 2 | | 
Epithomia 2 | 
Ceratinia 13) OE Gan ok 4} 12 | U | 
Sais = i) ! 
Scada . 4 | 1 | 1 1 | 
Mechanitis . . 17 | Le See 1 3 
Napeogenes . . 14 | Fao! 39s) ang oa 5 3 3 
Ithomia- . . | 187} 2) 44) 14) % ) de) ee 
Weria. 25) 2h 40 
Melinaea. . . 1B 1 L | .2 |) Gees 
Tithorea . . . 5 | 1 | are: 2 1 
Batali (= bos | 4 2 20| 18| 19 29| 33|/ 5| 1 


MAYER: COLOR 


TABLE 10. 


AND COLOR-PATTERNS. 


239 


Showing the Color-Variations affecting the “* Inner Rufous” (Area 


X) of the hind wing in the Danaoid Heliconidae. 


| Black, 


SN eos ie |e ccon alles 
GENERA. z zs eae e Sze ae 2 
ez = ie = ied = => 

Lycorea 5 
Ituna u 2 
Athesis . 2 1 
Thyridia 4 i 
Athyrtis 2 
Olyras 3 1 
Eutresis 2 
Aprotopos . 1 
Dircenna 8 2 2 
Callithomia 3 
Epithomia . 2 
Ceratinia 23, 3 6 6 1 2 
Sais 5 
Scada 7 
Mechanitis 16 4 
Napeogenes 7 6 6 6 5 
Ithomia 31 14 12 133 14 5 1 
Aeria 4 
Melinaea 19 4 1 
Tithorea 6 
Total 123 23 25 155 29 23 12 


240 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


TABLE, 11. 


Showing the Variations of the “ Middle Black Stripe” (Area XT) _ 
of the hind wing in the Danaoid Heliconidae. 


Fused with 

GENERA. Present. Absent. the oe inal |) Changed color, 
Lycorea 5 
Ituna 2 1 
Athesis 2 1 
Thyridia 2 2 1 partially 
Athyrtis 2 

1 changed 
Olyras . ul 2 to translu- 
cent yellow? ~ 

IDI, ES SF Bab As 2 
Aprotopos 2 
Dircenna 4 if il 
@alithomiar fn es e 3 
Bpithomia ~2 = = = Z 
Geratiniatiee le kee 21 20 
Sais . cea went os Ue 1 4 
Scadagees. (eee et Fi 
Mechanitis) 27) 22": 22 2 
Napeogenes . . . - 15 15 
Kthomia % ) cae 45 1 168 
Aeria am mar ee 4 
Melinaiea &) 5 2s ee 15 9 
Mithorea, eee 8 2 
4015-7) MSS ony ere ey a 146 45 212 1? 


MAYER: 


COLOR AND COLOR-PATTERNS. 


TABLE 12. 


Showing the Color-Variations of the “ Outer Rufous ” 
of the hind wing in the Danaoid Heliconidae. 


(Area XIT) 


GENERA. eo | S = = = . |z z= 
3 Z Z ZS = a See 
eg = = 4 = mes 
Lycorea 5 
Ituna 1 2 
Athesis 2 1 
Thyridia 4 1 
Athyrtis 2 
Olyras 3 | 1 
Eutresis 1 1 
- Aprotopos 1 1 
Dircenna 2 4 3 3 
Callithomia 1 2 
Epithomia 2 
Ceratinia 29 1 1 7 1 2 
Sais . 5 
Seada 7 
Mechanitis 21 5 
Napeogenes 15 15 
Ithomia 44 & 1 165 
Aeria | 4 
Melinaea ily 1 6 
Tithorea 5 PA i? 1 
Total fe) Te 6) | 20.) 9S) 8 OR roe at 
Excluding Ithomia 109 7 Gal ets 2) 8 3 | 34 1 


242 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


TABLE 13. 


Showing the presence or absence, and Color-Changes of the 
“Outer Black” (Area XIII) of the hind wing in the Danaoid 
Heliconidae. . 


GENERA. | Present. Absent. Changed color. 
sas | 

Lycorea | 5 

Ituna | 3 

Athesis 3 

Thyridia 5 

Athyrtis 2 

Olyras . 4 

Eutresis 2 

Aprotopos 2 

Dircenna Bee Oh eee | 12 

Gallithomian] #5 eal 3 

Epithomian yi | 2 

Ceratinia 41 

Sais . 5 

Scada Baer) aie 7 

Mechanitis . .. . 24 

Napeogenes . Saat 30 

Tthomiag, 4). 2... se 210 1 11 changed to rufous or brown. 
Aeria 4 

Melinaea 24 

Tithorea ! 10 

otal ie eee eh al U8 1 11 


MAYER: COLOR AND COLOR-PATTERNS. 


TABLE 14. 


243 


Showing the Number of the Marginal Spots of the hind wing in 
the Danaoid Heliconidae. 


] : 
avabR=| 1 2 


so 
o 


+ 


6 


Seailleeng 


es. Lanors | spot. spots. spots. spots. spots. spots.) spots.) spots.) spots. 
| | 

Lycorea 1 3 1) | | 
Ituna 2 1 } 
Athesis ts ies 1 
Thyridia call eam! 
Athyrtis 1 1 
Olyras . 5 1 
Eutresis 1 1 
Aprotopos 2 
Dircenna. . 10 1 1 
Callithomia . 2 | 1 | 
Epithomia 1 1 | 

_ Ceratinia . 18 sale well 2 5 5 2 Ales 
Sais . 4 | 1 
Scada 3 1 1 2 
Mechanitis . 18 ne i 2 2 | | 
Napeogenes . 21 1 Ll tale dL INY eats 1 
Ithomia 164 8 Cle el2 9 6 6 1 
Aeria 4 
Melinaea . 19 1 5 1 
Tithorea 4 | 2 2 2 
Total 279 HeeMay | 134) $26 2a LT 1S 4 1 


244 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
TABLE £5. 


Showing the Color-Variations of the “ Inner Rufous” Area of the 
fore wing in Heliconius and Eueides. 


ddist Trides- 
Rufous. or Yellow. | Ocher. White. | Black. cent 
\ blue. 
2 | 

Heliconius . . | 35 8 11 2 29 26 

| | | 

Eueides 14 if >) 2 || 
Total... 45%) 49 8 11 1 2 ee 26 


Note: The costal edge of the fore wing is always black. 


TABLE 16. 


Showing the Variations affecting the ‘Inner Black” Area of the 
fore wing in Heliconius and Eueides. 


Black. Tridescent blue. Rufous. 
Heliconius 85 26 
Eueides 13 5 
Total 98 | 26 5 


Note: In 54 species of Heliconius the inner rufous is entirely suffused with black. 


TABLE 17. 


Showing the Color-Variations of the “ Inner Yellow” Area of the 
fore wing in Heliconius and Eueides. 


Irides- 
Rufous. Red. Yellow. | Ocher. White. | Black. cent 
blue. 
Heliconius . . | Il 12 54 20 12 3 
Eueides . 6 12 
Boil) ea a ae 12 | 54 | 12 | 20 12 | 3 w 


MAYER: COLOR AND COLOR-PATTERNS. 245 


TABLE 18. 


The “ Middle Black” Area in the fore wing in Heliconius is 
present as a Black or Blue Marking in 99 Species and absent in 12. 
It is present as a Black Mark in all 18 Species of Eueides. 


TABLE 19. 


Showing the Color-Variation of the “ Middle Yellow ” Area of the 
fore wing in Heliconius and Eneides. 


| Reddish 


Rufous. | yufous. Yellow. Ocher. White. Black. 
Heliconius . Lin -} 12 65 23 
Eueides .. 5 12 1 
Maat . |... 16 12 65 1251) 24 


TABLE 20. 


Showing the Color-Variations of the “ Outer Yellow ” Area of the 
Sore wing in Heliconius and Eueides. 


Reddish | Irides- 


Rufous. | pyufous. | Yellow. Ocher. | White. | Black. cent 

; | blue. 

Heliconius . . 2 1 47 24 | 33 4 
Eueides . . . | 6 Labeda 

Motal. . . | 2 1 47 6 2 | 44 bee 


TABLE 21. 


The “ Outer Black” Area of the fore wing in all the 111 species 
of Heliconius known to me is Black or Iridescent Blue. 
It is Black in all 18 Eueides. 


246 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


TABLE 22. 


Showing the Manner of Occurrence of the Marginal Spots of the 
fore wing in Heliconius and Eueides. 


i 4 5 6. ha 8 9 
| out 
pues spot. spots. spots. spots. spots.| spots. spots. spots. spots. 
= — | = | le: 
Heliconius 89 5 2 i) o 3 A) alte 1 
j | ‘ 

Eueides | 14 | 3 1 
ae e ; > Wise = : ae 
Motale. | eee 103 | 8 | 2 5 3) 5 2 1 


TABLE 23. 


Showing the Variations affecting the “ Longitudinal Black Stripe” 
ot the fore wing in Heliconius and Eueides. 


Whole area suf- Well developed as Absent (area suf- 
fused with black. | .a black stripe. fused with rufous). 
= =| | 
Heliconius 75 25 13 
UCIGeS I, Wht) or 2 16 
LOtaltie tet. oe Le i 39 13 


TABLE 24. 


Showing the Color-Variations of the “ Inner tufous’”’ Area of the 
hind wing in Heliconius and Eueides. 


= | S i] is) cS 
; | Oo 5 a. =e ee! 
wi | = =A of 2 Bay as Se aS2= 
= S 5 ae = ne | MO | wo | Mae 
= = s | on 2 joes | Sa | se see 
s 2 = = of = ree =e? BSE |soe 
= | re So; | #4 a |S" (eae 
Heliconius . . 42 | 7 3 L | (26) | 226 siao 1 5 
. | ad ¢ 
Eueides 15 2 1 
! | 
Total 2 kes oe al 57 | 7 3 | 2 ea he 26 10 1 5 


MAYER: COLOR AND COLOR-PATTERNS. 247 


TABLE 25. 


Showing the Variations of the “ Middle Black Stripe ” of the hind. 
wing in Heliconius and Eueides. 


eS : Well devel- 1 ae Absent Absent 
opedasamcere 4 ace (suf- (place taken | (place taken 
or less distinct fase with | py redorru- | by ocher 
stripe. black). fous). color). 
Heliconius . .. . AT 59 5 
Eueides 6 10 1 
a ee 53 59 | 15 1 


TABLE 26. 


Showing the Color-Variations of the “ Outer Rufous” Area of the 
hind wing in Heliconius and Eueides. 


~ } Irides- 
| Rufous. pee | Yellow. White. | Ocher. | Black. cent 
af] | blue. 
Heliconius . . | 30 4 19 3 49 6 
Eueides . . . 17 1 
mrotal. ... 47 4 19 5 50 6 


TABLE 27. 


The “ Outer Black ” Area of the hind wing is Black in 106 species 
of Heliconius, White in 4, and Yellow in 1. It is Black in allthe 18 
species of Eueides known to me. 


248 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


TABLE 28. 


Showing the Number of Species in each Genus of the Heliconidae 
examined, and also the Number known according to the Enumera- 
tion of Staudinger (84—’88). 


ee Number of species ex- |, Number of species 

GENERA. amined by me. known to Staudinger 
Lycorea. . . .. . . . | 4speciesand1 var. | 4 species and 1 var. 
Ituna . 3 a 
Athesis 3 4 
Thyridia . 5 | 4 
Athyrtis . 2 | 2 
Olyras 4 3) 
Eutresis 2 2 
Aprotopos 2 4 
Dircenna 12 20-++ 
Callithomia . 3 8 
Epithomia 2 | 2 
Ceratinia 41 50+ 
Sais 5 4 
Sceada . ee are eat Pee iy 7 9 
NGC EHS Gp a Sse) oe alte 10 species, 14 var. ——-10 species, 13 var. 
Napeogenes oe seragt Be 30 30+ 
TEOMA: ney eo esi te 212 250+ 
Aeria . Se ee nae 4 4 
Melinaeae (75 Mi-se-net ee 24 | 25 
Tithorea . : 10 18 
Total of Danaoid Heliconidae. | 400 453+ 

— | 

Heliconiuss!.) = ee ee 111 130 


IUGId eS = i- eae 18 24 


PGi eee ee 529 | 607+ 


7 


MAYER: COLOR AND COLOR-PATTERNS. 2AY 


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— 


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Godman, F. D., and Salvin, O. 

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92. Recherches sur les Couleurs de quelques Insectes. Comptes Rendus 

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95. Untersuchungen iiber die Mimicry auf Grundlage eines natiirlichen 
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Hopkins, F. G. 
89. Urie Acid Derivatives functioning as Pigments in Butterflies. Proc. 
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94. The Pigments of the Pieridae. A Contribution to the Study of Excre- 
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MAYER: COLOR AND COLOR-PATTERNS. 253 


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254 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Walsingham. 
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MAYER: COLOR AND COLOR-PATTERNS. 255 


TABLE OF CONTENTS. 


PART A. 
GENERAL PHENOMENA OF CoLor IN LEPIDOPTERA. 


I. CLASSIFICATION OF CoLorRs. 


PAGE 
(1) Pigmental Colors : 3 ; ; 5 ‘ 3 : ; : 169 
(2) Structural Colors : : é : : : ; : ; 170 
(8) Combination Colors . : ; : é : 171 
(4) Quantitative Determination of Eismental Goloxs ; ; ‘ 172 
(5) Spectrum Analysis of Colors of Lepidoptera. ; : ; ; 175 
(6) Summary of Results 174 

Il. THE EssentiaAL Nature OF PIGMENTAL COLOR IN LEPIDOPTERA. 

(1) Pigments of Larvae. : : : ; E , ; : ‘ 174 
(2) Pigments of Imagines : : : ; : 175 
Il. DEVELOPMENT OF THE VARIOUS CoLors IN THE PuPAL WINGs. . 

(1) Historical Account of previous Researches : 176 


(2) Development of Color in the Pupal Wings of Callosamia nromerhen 178 
(3) Development of Color in the Pupal Wings of Danais plexippus 


(archippus) . : : s 3 : : : : : : 180 

IV. THe Laws WHICH GOVERN THE CoLoR—PATTERNS OF BUTTERFLIES AND 
Morus. 

(1) Historical Account of previous Researches ; , : ‘ ; 181 
(2) Laws of Color-Patterns . : 183 
(8) Detailed Discussion of the Tare of Color- Patterns : 185 
(4) Origin of Color-Variations : : : ; 189 

Bibliography of Geipe pecntions ; : : : : 190 
(5) Climate and Melanism : : : : ; 190 
(6) Relation between Climate and Colors of papilies Z : : : 191 


V. THe Causes WHICH HAYE LED TO THE DEVELOPMENT AND PRESERVATION 
OF THE SCALES OF THE LEPIDOPTERA. 


(1) Experiments and Theory . : : : A : ; : : 192 
(2) Summary of Conclusions . : : ; : : : : 197 


PART, 3B: 
CoLoR—VARIATIONS IN THE HELICONIDAE. 
I. GENERAL CAUSES WHICH DETERMINE COLORATION IN THE HELICONIDAR. 


Il. MeruHops PURSUED IN STUDYING THE COLOR—PATTERNS OF THE 
HELICONIDAE. 


(1) The Two Types of Coloration in the Danaoid Heliconidae 204 
(2) Detailed Description of the Melinaea Type of Coloration 205 


256 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Ill. GenerAt Discussion OF THE COLOR—PATTERNS AND OF MIMICRY IN 
THE GENERA HELICONIUS AND EUEIDES. 


(1) The Four Color-Types in the Genus Heliconius . : 208 
(2) Mimicry between the genus Heliconius and the Danaeid: 

Group : . : : 209 
(8) The Three Color- Pees in the (enna meides : ; A 210 
(4) Detailed Discussion of Plates 5-8 : : : - : 210 


TV. GENERAL Discussion OF THE COLOR—PATTERNS AND OF MImicry 
AMONG THE DaNnaotrp HELICONIDAR. 


(1) The Origin of the two Types of Coloration ‘ : : 214 
(2) Mimicry among the Danaoid Heliconidae  . : ‘ 4 216 


V. QUANTITATIVE DETERMINATION OF THE VARIATIONS OF THE CHAR— 
ACTERISTIC WING—MARKINGS IN THE ACRAEOID AND DANAOID 
HELICONIDAE. 


(1) Variations of the ‘‘ Inner Rufous” Areas of the Fore and 


Hind Wings . : : ; : : ; : 217 
(2) The “ Inner Black” Spot : : : 5 : : : 218 
(8) Variations of the ‘‘Inner Yellow” and “ Middle Yellow ” 

Areas : : 219 


(4) Variations of the “ Middle Black 2 Mark ot ae Race Wine 221 
(5) Variations of the “ Outer Yellow ” Area of the Fore Wing 221 
(6) The Relative Permanency of the Black Areas upon the Fore 


and Hind Wings : : ; : 222 
(7) The “ Middle Black Stripe ” of as Hind Wing 4 : 222 
(8) Variations of the Marginal Spots of the Fore Wing . ‘ 223 
(9) The Marginal Spots of the Hind Wing : : : : 223 


VI. ComparRIsSON OF THE COLOR—VARIATIONS OF THE PAPILIOS OF 
Soutn AMERICA WITH THOSE OF THE HELICONIDAE. 


PART Cc: 


GENERAL SUMMARY OF RESULTS BELIEVED TO BE NEW TO SCIENCE. 


Tables : : : : ‘ ; d : : : : 5 : 250 
Bibliography . ; : ‘ z : : : : : : : 249 


Explanation of Plates 


Birt eo ms 
> cated Ogee ’ : 2 ee Sy a b i 
! arent. VT en one | Pudi? if 


MayveErR, — Color and Color-Patterns. 


PLATE I. 


ABBREVIATIONS. 


B. Back surface covered with wings. O. Orifice for admission of light. 
F. Front surface covered with wings. S. Spectroscope. 
Arrow indicates directions of rays of light. 


Fig. 1. Perspective view of spectroscopic apparatus used in determining the 
composition of the colors of Lepidoptera. 
. Horizontal section of same. See p. 175. 
Fig. 3. Pendulum used in determination of the frictional resistance between 
the air and the wings of Lepidoptera. See p. 193. 
Figs. 4,5. Diagrams to illustrate Keeler’s method of projection, as applied to 
Lepidoptera. See p. 207. 


=| 
i 

ge 
bo 


B Meisel lith Basten 
| AGM. del = 


fi 


MAYER, — Color and Color-Patterns, 


PLATE 2. 


Diagrams to illustrate the laws which govern the Color-Patterns of Lepidoptera. 


Fig. 6. Euthalia bellata (W. L. Distant, °82—86, Plate 43, Fig. 12). Tllustrates 
the law of bilaterality of spots. See p. 183. 

Zethera musa (G. Semper, ’86-’92, Taf. 7, Fig. 10). Bilaterality of 
double spots. See p. 183. 

Fig. 8. Eye-spotsin Morpho. See p. 182, 185. 

Fig. 9. Parthenos gambrisius (W. L. Distant, ’82-’86, Plate 11, Fig.7). A series 
of complex spots, each one being similar to the rest, and bilaterally 
symmetrical. 

Figs. 10, 11. Ornithoptera urvillana and O. priamus (R. H. F. Ripon, ’89—93). 
Spots within spots, all being bilaterally symmetrical. 

Figs. 12,13. Hestia jasonia and H. leuconoe. Axis of lateral symmetry 
(H,H), for spots passes through center of interspfce. HH. jasonia — 
(F. Moore, *90-96, Plate 3, Fig. 1). H. leuconoe (G. Semper, 
86-92, Taf. 1, Fig. 3). 

Fig. 14. Papilio emalthion, to illustrate fusion of two rows of spots. 

Fig. 15. Ornithoptera trojana, an apparent exception to the law of bilaterality. 
See p. 187. 

Fig. 16. Limenitis proserpina (S. H. Scudder, ’88~’89, Plate 2, Fig. 9), showing 
fusion of two rows of differently colored spots. See p. 187. 

Fig. 17. Saturnia spini, false eye-spot. See p. 187. 

Fig. 18. Cases of degeneration of bands of color. See p. 184. 

Fig. 19. Missanga patina (F. Moore, ’90-’96, Plate 72, Fig. 2°). Exceptional 
form of eye-spot. See foot note p. 186. 

Figs. 20-23. Hypothetical conditions of coloration, not found in nature, being 
contrary to the laws of color-pattern. See p. 188. 


tx 
en 
JQ 


MayeR- COLOR AND COLOR PATTERNS. PRAT Ce: 


B Meisel lith, Boston 


Butu.Mus. Comp ZOOL.VOL. XXX. 


Mayer. — Color and Color-Patterns 


PLATE 3. 


To illustrate color-development in Callosamia promethea and Danais plexippus. 


Fig. 24. Enlarged view of pupal wing of C. promethea in the ‘‘ white stage.” 
See p. 178. 

Fig. 25. Scale from wing of C. promethea in white stage of color-development, 
showing the total absence of pigment in the scale. See p. i178. 

Fig. 26. Scale from light drab-colored area of mature wing of C. promethea. 

Figs. 27, 36, and 33. Successive stages in the formation of color in pupal hind | 
wing of C. promethea. 

Figs. 28, 37-40. Successive stages in the formation of color in the pupal fore 
wing of 9 C. promethea. See p. 179, 180. 

Figs. 29, 30-35. Successive stages in the formation of color in the pupal wings 
of g C. promethea. (Figs. 29, 30-33, 35 fore wing; Fig. 54 hind 
wing.) See p. 179, 180. 

Figs. 34 and 41. Pupal hind wings of C. promethea, respectively mature g 
and 9. 

Figs. 42-45. Successive stages in the color-development of D. plexippus. See 
p. 180-181. 


= ‘ 
ee 
Pa 
7 een 
— 
: 
« 


AN 
oo 
i as Pal 
, Pat 5 ; rs 


a Ya! fret . oP 


MAYER. — Color and Color-Patterns. 


PLATE 4. 


Systematic analysis of the characteristic markings upon the wings of the Heli- 


conidae. 
TAT. ete: 


Fig. 46. 


Fig. 47. 


Fig. 48. 
Fig. 49. 
Fig. 50. 
Fig. 51. 
Fig. 52. 
Fig. 53. 
Fig. 54. 
Fig. 55. 
Fig. 56. 
Fig. 57. 
Fig. 58. 
Fig. 59. 
Fig. 60. 


Homologous markings are designated by the same numerals, I, I, 


Lycorea ceres; an example of the “ Melinae& type” of coloration. 
See p. 205. 

Thyridia psidii; an example of the “Ithomia type” of coloration. 
See p. 206 and Plate 7, Fig. 79. 

Melinaea paraiya. 

Ceratinia ninonia. 

Heliconius antiochus. 

Napeogenes duessa. 

Ithomia sao. 

Melinaea gazoria. 

Ithomia nise. 

Mechanitis polymnia. 

Eueides cleobaea. 

Tithorea furia var. 

Heliconius eucrate. 

Heliconius melpomene. 

Heliconius erato. 


7 


ty 


MAYER. — Color and Color-Patterns. 


PLATE 5. 
Color-patterns of the Antiochus and Erato groups of the Heliconidae projected 
by Keeler’s method. See p. 207. 


Figs. 61, 62. Heliconius saraand H. antiochus, to show variation of yellow to 
white. See p. 210, and Fig. 50, Plate 4. 


Fig. 63. H. galanthus, showing development of white. 
Fig. 64. H. charitonia, rows of double spots. 

Fig. 65. H. phyllis, close relation between yellow and red. 
Fig. 66. H. ricini. 


Figs. 67, 68. H. erato: two color-types. 
Fig. 69. H. claudia; an example of the Sylvanus group. 


- eFP ef® Peewee * @ 


| 


MAYER. — Color and Color-Patterns. 


PLATE 6. 


Color-patterns of the Melpomene group of Heliconius and of the genus 
Eueides. 


Fig. 70. H. melpomene, the type of the Melpomene group. See p. 212, and 
Fig. 59, Plate 4. 

Fig. 71. H. melpomene var. callicopis, showing the breaking up of the red 
area of the primaries. 

Fig. 72. H. melpomene var. eybele ; the fore wing has assumed a color-pattern 
which recalls the ‘‘Melinaea type” of coloration found in the 
Danaoid Heliconidae. ; 


Fig. 73. H. thelxiope, derived phylogenetically from H. melpomene, and 
showing a rather close approach to the “ Melinaea type” of color- 
ation. See p. 212. 

Fig. 74. H. vesta. 

Fig. 75. Eueides thales g ; represented to show the close resemblance of its 


color-pattern to H. vesta. See p. 212. 
Figs. 76,77. E. mereaui and E. aliphera. E. mereaui is intermediate in color- 
pattern betwee, E. thales and E. aliphera. : 
Fig. 78. Eueides cleobaea, to show the close approach of this insect to the 
‘« Melinaea type” of coloration. 


MAYER. — Color and Color-Patterns. 


PLATE 7. 


Intended to show some types of coloration which are found in the Danaoid 
Heliconidae, and also the remarkable individual variation in Mechanitis isthmia. 


Fig. 79. Thyridia psidii, an example of the “Ithomia” type of coloration. 
See p. 213, and Plate 4, Fig. 47. 

Fig. 80. Napeogenes cyrianassa, showing semi-translucent condition of wings. 
See p. 215. 

Fig. 81. Ceratinia vallonia. 

Fig. 82. Melinaea parallelis; albinism of spots on primaries ; black band of hind 
wing broken in the middle. See p. 188, 213. 

Fig. 83. Ceratinia leucania, which probably mimics M. parallelis. 

Figs. 84-87. Mechanitis isthmia, showing remarkable individual variation in 
the black stripe of the hind wings, and also in the “ inner yellow” 
spot of the fore wings. See p. 184, 215. 


i Vape OGD C/1E8 


VTUUNASSA 


Doubl. &Hew. 


Coratinia 


vallonia Hew: 


Nelhnaca paralleis 
= < = sar E 

, # Buil. 
F of if a) & fi ut 


lCeratinialeucania 


Bates. 


Necharutis 


ithinia Bates: 


Nechanitis 


isthinia Bates. 


Necharuitis 


isthinia Bates. 


Nechaniiis 


isthmia Bates 


2"2U 


MAyeER.— Color and Color-Patterns. 


. PLATE 8. 


Illustrates the mimicry between members of the « Sylvanus” group of the genus 
Heliconius and various Melinaeas, etc. 


Fig. 88. Heliconius eucoma, an example of the “Sylvanus” type of coloration 
in genus Heliconius. See p. 214. 

Figs. 89,90. Heliconius dryalus and Melinaea paraiya; close resemblance of 
their color-patterns. See p. 214. 

Figs. 91, 92, 93,94. Respectively Heliconius eucrate, Melinaea thera, Eueides 
dianasa, and Mechanitis polymnia; showing close resemblance 
between color-patterns. See p. 214. 

Figs. 95, 96.. Heliconius sylvana and Melinaea egina; these two forms are said 
by Bates to mimic each other. See p. 214. 


MAYER- COLOR AND GOLOR PATTERNS 


HIND WING 


ID 


per 
wind dpa 


—~ © eee be as awe ww 


I WW . | ‘ VI 
> ap 
ai 

® | dad 
mm ae 


Heliconius 


aucomma Tlibrn. 


oe oe 


Heliconius 


arvalius Hoprtr 


SY elinac Zh 


paraiya Leak. 


Heltcontins 


Guucrale [hibr 


Melinaca 


thera Feld. 


Lueides dianasa 


Libya 


96 


Mechanitis 


polviinia Linn. 


Heliconiiis 


SVlVan Crane, 


Nelowea 


Cyt Cram 


5 hoge = 
» 


Mayver.— Color and Color-Patterns. 


PLATE 9. 


Diagrams to illustrate color-variations. 

The various colors are laid off at definite intervals along the axis of abscissae, 
and the ordinates represent the number of species which exhibit the various 
colors. 


Fig 


Fig. 


, OG: 


Represents the color-variations of the “inner rufous ”’ area of the fore 


and hind wings in the Danaoid Heliconidae. ‘The full line represents 
the variations of the fore wing. The dotted line those of the hind 
wing. The closeness of these two lines shows the intimate relations 
between the color-variations of the “inner rufous” areas upon fore 
and hind wing. See p. 218. 

The full line represents color-variations of “inner yellow” spot of fore 
wings in Danaoid Heliconidae. The dotted line represents same for 
“middle yellow.” It is apparent that the “inner yellow” is more 
variable than the *‘ outer yellow,’ and also that the variations of 
both are quite similar to those of the “inner rufous.” See p. 219. 


Color-variations of “inner rufous” areas of Acraeoid Heliconidae, 


The full line represents the fore wing and the dotted line the hind 
wing. See p. 218. 


PLATE. 


- GOLOR AND GOLOR PATTERNS. 


MAYER 


ID 
ith, Bosten. 


B Meisel 


wovig N3au9 


MOT1T3AA ania 


MO119A 
LNAONISNVUL | 


Pee eee 
MOTI9A Cecct i) la 
ATLHOI1S ' {~|—t—f | I 
ANIONISNVYL FO 


ANDYVASNV YL 


Ssnoany 
AILHDIIS = | 
INSONISNYUL | 


ButL.Mus. Comp ZoOO.u.VOL. XXX. 


MOT13A 


snoind 


INSONMISNWYL 7 aau 


He Hain 
ek Ze sete 


~snoiny 


Peery 


i E “E+E serereeee. seca 


F 


Mayer. — Color and Color-Patterns. 


ig. 100. 


Fig 


- 102. 


PLATE 10. 


Color-variations of “inner yellow” spots on fore wings of Acraeoid 
Heliconidae. The full line represents the “ inner yellow,” the dotted 
line the “ middle yellow.” See p. 221. 

Variations of marginal spots upon fore wing in Danaoid Heliconidae. — 
These spots tend to appear either as 2 or 5, or as 6 or 7 spots. See 
p- 223. 

Variations of marginal spots of hind wings in Danaoid Heliconidae. 
These spots tend to appear either as 4 or as 5 spots. See p. 223. 


MayerR- GOLOR AND COLOR PATTERNS. 


L100 


39N39S3q14} 
ania 
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JLIHM 
acest aS A 3YHOO 
gscepiasss c cecastastl 
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Beeuseeond 
Eeecaeae 
HEH H 
(aL a 
PEE 5 
to iat a Sw ba N ie 
PTI CAPER oe UREA qaiuw 
i=) b—] °o oO 
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7 IES 


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eel 


B Meisel lith, Basten. 


XXX. 


1 
da 


Buti.Mus. ComMP ZOOL.VoO 


No. 5.— The Mesenteries and Siphonoglyphs in Metridium mar- 
ginatum Milne-Kdwards. By G. H. Parker.! 


Introduction. — Since the publication of the Hertwigs’ (’79) paper on 
the anatomy of the actinians, the attention of investigators has been 
more and more directed toward the details of the internal structure of 
these organisms. This new departure has been conducted in the main on 
the lines of systematic zodlogy, and, though its advocates in the begin- 
ning may have been somewhat Utopian in their expectations, it has 
certainly carried our understanding of the natural relations of this group 
of animals a long step forward. The new features thus introduced into 
the classification have, however, been subject to frequent modification, 
and every actinian newly investigated may be expected to exert some 
influence on the classification finally adopted. It is to be regretted that 
much of this kind of investigation has been of necessity carried out on a 
limited, often a very limited, number of specimens, so that the possible 
error of regarding individual variations as characteristic of large groups 
is not always eliminated. 

The following pages contain a record of certain structural peculi- 
arities in a single species of actinian, the common Metridium marginatum 
Milne-Edwards of our coast, as represented by a considerable number 
of specimens. As this record shows, uniformity of structure is by no 
means a general characteristic of this species; hence these observa- 
tions are to some extent a contribution to the study of the variability 
of this animal. 

The material on which the following observations were made consisted 
of 131 adult specimens of Metridium marginatum. These were collected 
in part by myself and in part by my laboratory assistant, Mr. J. I. 
Hamaker, to whom I am under obligations for this kindness. All the 
specimens came from the neighborhood of Newport, R. I., and were 
prepared and to some extent studied in Mr. Alexander Agassiz’s Labora- 
tory at that place. I here wish to express my thanks to Mr. Agassiz for 
the privilege of carrying on this work at the Newport Laboratory. 


1 Contributions from the Zodlogical Laboratory of the Museum of Comparative 
Zoology at Harvard College, E. L. Mark, Director, No. LXXYV. 
VOL. xxx.— No. 5. 1 


260 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


The specimens were prepared by the Tullberg (’91) method, which 
consists in stupefaction by the gradual introduction of magnesic sulphate 
into the water containing the actinians, and in subsequent hardening by 
means of chromic acid. This method, when properly employed, yields 
beautifully expanded and thoroughly hardened specimens, and my expe- 
rience with it has been such that I can fully indorse the recommendations 
given it by Tullberg (91), Carlgren (93, p. 7), and others. Speci- 
mens prepared in this way were cut transversely with a common razor, 
and the number and arrangement of the mesenteries and siphonoglyphs ? 
were recorded. Owing to the large size of the specimens, this could be 
easily done under the magnification of an ordinary hand lens. 

Siphonoglyphs.— The Hexactinia, to which Metridium belongs, were 
until recently supposed to possess always two siphonoglyphs; but this 
surmise has been shown to be not well grounded, and, in the species 
under consideration, as McMurrich (91, p. 131) has already pointed 
out, either one or two siphonoglyphs may be present. In the 131 
specimens that I examined, 77 (or about 59 per cent) had only one 
siphonoglyph (Fig. 3), 53 (or about 41 per cent) had to siphonoglyphs 
(Fig. 1), and a single specimen possessed three such organs (Fig.6). In 
no instance was a specimen found without a siphonoglyph. The smooth 


surface of the siphonoglyph is so strongly contrasted with the longitu- 


dinally ribbed surface of the rest of the esophagus that in none of the 
specimens examined was there any uncertainty as to the number of 
siphonoglyphs present. The striking difference between these two kinds 
of surface cannot be made to appear so clearly in the figures as it did 
in the actual specimen, where, in addition to the cut face, the natural face 
of the cesophagus could also be inspected. MeMurrich (’91, p. 131) 
remarks that in the individuals examined by him, those with one sipho- 
noglyph were almost, if not quite, as frequent in occurrence as those 
with two, but in my enumeration it will be seen that they were really 
somewhat more numerous. 

Since only one of the 131 specimens possessed three siphonoglyphs, it 


1 The term “siphonoglyphe” was first introduced into zodlogical nomenclature 
by Hickson (’84, p. 694), and has since been widely accepted. Professor Hickson 
kindly informs me that the last syllable of this term is derived from the Greek 
word yAu@is, which in the plural form, yAu¢ides, has been used to signify the grooves 
on an arrow for the insertion of the feathers. The root of this word appears to 
call for no final e, and since in making English words it is best, as Professor 
Hickson remarks, to use only roots, I therefore propose to change the spelling 
of the term in question by omitting the final e, and to this Professor Hickson 
assents. 


li, ee 


Y= es 


PARKER: METRIDIUM MARGINATUM. 261 


is obvious that this condition may be set aside as distinctly exceptional, 
and, further, since the other specimens were almost equally divided be- 
tween those with one and those with two siphonoglyphs, these conditions 
may fairly be considered typical. It will be convenient in the subse- 
quent discussion to designate these two types by special names, and I 
shall call that characterized by one siphonoglyph the monoglyphic type, 
and that by two the diglyphic type. 

Variations in the number of siphonoglyphs have already been recorded 
in other actinians. Thus, besides the observations of McMurrich already 
alluded to, Thorell (59, Tab. I. Figs. 1 and 2) figured and described 
specimens of Metridium dianthus either with one or with two siphono- 
glyphs. The monoglyphic condition was also recognized for this species 
by Gosse (’60, p. 12), who, in ignorance of Thorell’s observations, sup- 
posed this condition to be characteristic of the species, a mistake after- 
wards corrected by Foot (’63, p. 64). The presence in some specimens 
of one, and in others of two siphonoglyphs in Jf. dianthus, as first as- 
serted by Thorell, has recently been confirmed by G. Y. and A. F. 
Dixon (91, p. 19), and by Carlgren (’93, p. 104). Furthermore, the 
Dixons and Carlgren agree in stating that, though two siphonoglyphs 
may be present in this species, one is the rule. G. Y. and A. F. Dixon 
(91, p. 20), moreover, have recorded one specimen of M. dianthus with 
three siphonoglyphs. 

Representatives of the genus Sagartia also show variations in the 
number of their siphonoglyphs; thus G. Y. Dixon (’88, p. 120) ob- 
served that in Sagartia venusta, S. nivea, and S. mineata, either one or 
two siphonoglyphs may be present. The same is probably true of 
S. rosea (cf. F. Dixon, ’88, p. 139), and of S. lactea (cf. McMurrich, 
94, p. 177). In specimens of Bunodes thallia, studied by ’G. Y. and 
A. F. Dixon (89, p. 318), one, two, three, and even four siphonoglyphs 
were observed, although in each of twenty-three adult specimens of 
B. verrucosa the same authors (’89, p. 322) found regularly two siphono- 
glyphs. Finally Blochmann and Hilger (’88, p. 391) described a specimen 
of Gonactinia in which traces of a third siphonoglyph seem to have 
been present. 

It is evident from the foregoing account that in several actinians besides 
Metridium a variation in the number of siphonoglyphs is not unusual, 
though this variation may not be so pronounced as to constitute a struc- 
tural type. The importance of these peculiarities from a systematic 
standpoint has already been appreciated, and in the more recent defini- 
tions of the Hexactinia the statement is made that these actinians possess 


262 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


two siphonoglyphs (occasionally one), thereby recognizing the mono- 
glyphic and diglyphic types as normal. 

Mesenteries.— In Metridium marginatum the pairs of mesenteries are 
attached lengthwise to the wall of the column, and either reach the 
cesophagus and unite with it (complete mesenteries) or fall short of 
that structure (incomplete mesenteries). Of the pairs of complete 
mesenteries the two usual kinds can be distinguished: those whose 
longitudinal muscles face the exocels (directive mesenteries) and 
those whose longitudinal muscles face the endocels (non-directive 
mesenteries). 

The directive mesenteries are remarkable for the constancy of their 
relations to the siphonoglyphs. To each siphonoglyph is attached a 
single pair of directives, and in no instance among the 131 specimens 
examined was an exception to this rule found. In the monoglyphic 
type (Figs. 3, 4, 5, 7, and 8) one pair, and only one pair, of directives 
was present; in the diglyphic type (Figs. 1 and 2) two pairs were 
invariably observed ; and even the single specimen with three siphono- 
glyphs (Fig. 6) formed no exception, but exhibited three pairs of 
directives. 

This exact correlation between the number of siphonoglyphs and of 
directives, which probably also obtains in other species of Metridium 
(cf. Carlgren, ’93, p. 106), as well as in the allied genus Sagartia (cf. 
F. Dixon, ’88, p. 136), is rather striking, because the two sets of 
structures concerned are not invariably thus associated in all actinians. 
For instance, in Peachia and Oractis (cf. MeMurrich, ’91, pp. 139, 137)> 
though two pairs of directives are present, only one siphonoglyph 
occurs ; and in Ptychodactis (cf. Appellof, 94, pp. 5, 7), though two 
pairs of directives can be seen, no siphonoglyphs are observable. These 
instances serve to show that in some actinians directive mesenteries 
may occur without siphonoglyphs, and thus they render more striking 
the correlation between the variations of the directives and of the 
siphonoglyphs in Metrid ium marginatum. 

The non-directive mesenteries vary so much in their number and ar- 
rangement that they can best be considered in connection with the par- 
ticular types with which they occur. In the diglyphie type (53 speci- 
mens), in addition to the two pairs of directives, there may be from 
four to ten pairs of non-directives. The frequency of the occurrence 
of the different numbers of pairs is indicated in the following table :-— 


PARKER: METRIDIUM MARGINATUM. 263 
Dictypuic Tyre (two siphonoglyphs and two pairs of directives). 


Pairs of Non-directives . 


Number of Cases observed 


In this type (Fig. 1) the two pairs of directives of course divide the 
non-directives into two groups. I regret that, before it occurred to me 
to determine the number of mesenteries in each of these two groups for 
the 53 specimens of this type, some of the specimens were so far dis- 
sected as to render them no longer serviceable for this enumeration. I 
can therefore make a statement concerning this division in the cases of 
only twenty specimens. 


Groups of Non-directives . 


Number of Cases observed . 1 10 | 2 2 


In comparing these results, it will be observed, first, that the great 
majority of individuals (40 in 53) possess four pairs of non-directives, 
and, next, that the arrangement of these non-directives in ten cases out 
of twenty is in two groups of two pairs each. This symmetrical 
arrangement of the four pairs of non-directives in the diglyphic type 
reproduces the assumed typical Hexactinian arrangement, and, since 
the representatives of the other variations are comparatively so few in 
numbers, this may be taken to be the only characteristic condition of 
the diglyphic type. 

In the monoglyphic type (77 specimens), in addition to the one pair 
of directives, there were from three to fourteen pairs of non-directives. 
The frequency of their occurrence is shown in the following table : — 


Monoctypuic Type (one siphonoglyph and one pair of directives). 


eax 
i s|e|7 8 


9 |} 10] 11 | 12) 13) 14 


| 


L\4 saat 1 


Pairs of Non-directives . .| 3 


Number of Cases observed .| 1 | 4 | 20 | 19 | 21) 5 


| | 


Admitting the monoglyphic type to be derived from the diglyphic by 
the conversion of a pair of directives into a pair of non-directives, 


264 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


one would expect the monoglyphic type with five pairs of directives to 
be most often met with. Such, however, is not the case, for specimens 
with six or seven pairs of non-directives are about as numerous as those 
with five. Since any of the three groups with five, six, or seven pairs 
of non-directives is represented by a greater number of individuals than 
all the other minor groups of variations taken collectively (cf. Table of 
Monoglyphic Type), it is clear that in the monoglyphic type there are 
three structural subtypes characterized respectively by five, six, and 
seven pairs of non-directives, instead of only a single such subtype, as in 
the diglyphic condition. These relations indicate a certain degree of 
distinctness between the diglyphic and the monoglyphic type ; for the 
monoglyphic has obviously a greater range in variation, as shown in 
its three subtypes, than the diglyphic with only a single one. It is an 
interesting fact in this connection, that the monoglyphic subtype with 
six pairs of non-directives often repeats (Fig. 4), so far as its com- 
plete mesenteries are concerned, the arrangement of mesenteries found 
in Seytophorus, for which R. Hertwig (’82, p. 104) constructed a sep- 
arate family, the Monaulee. 

It might at first be suspected that the three monoglyphic subtypes 
pointed out above, and in fact all the variations in the number of com- 
plete mesenteries, could be explained on the assumption that certain 
incomplete mesenteries by excessive growth had become complete, or 
that complete ones had become incomplete, thus introducing a varia- 


tion in the number of complete mesenteries, without, however, altering . 


the total number of all kinds of mesenteries; but in the individuals ex- 
amined the relative development of the incomplete mesenteries was 
found to be subject to so much variation that the satisfactory deter- 
mination of the total number of mesenteries as a basis of comparison 
was practically impossible, and all attempts to carry through interpre- 
tations such as that suggested above resulted in such ambiguous and 
strained results that the unnaturalness of the method condemned it. 
Moreover, in the monoglyphic type with s¢x pairs of non-directives 
(Fig. 4), incomplete as well as complete mesenteries are sometimes so 
symmetrically placed that no attempt to readjust them is warranted. 
What may be said of such cases is, that, in place of the usual five pairs 
of non-directives, six pairs are present, and this increase cannot be 
ascribed to reinforcement from the ranks of incomplete mesenteries. 
Such cases as these are so frequent, and instances that may be inter- 
preted as the conversion of complete into incomplete mesenteries or 
the reverse are so few, that it must be admitted, I believe, that these 


a a 


rey 2 


— 


PARKER: METRIDIUM MARGINATUM. 265 


differences are due much more frequently to fundamental differences in 
the plans on which the mesenteries of different individuals are laid 
down than to the more easily conceived relation between complete and 
incomplete mesenteries. 

The incomplete mesentertes have not been exhaustively investigated. 
Their great number, variability in size, and the frequent difficulty met 
with in attempting to classify them, render such a task nearly impos- 
sible. In what are generally assumed to be the more typical specimens 
of Metridium (Fig. 1), an exoccel may contain one pair of secondary 
mesenteries, two pairs of tertiaries, four pairs of quarternaries, and 
evidences (ridges) of eight pairs of quinaries. Though this condition 
was occasionally realized, in the great majority of cases irregularities in 
what are presumably secondaries and tertiaries, not to mention higher 
orders, were so numerous that consistent tabulation was out of the 
question. So far.as size and position were concerned, what seemed to 
be secondaries showed such variations that no two specimens in which 
the arrangement of the complete mesenteries agreed, had similar arrange- 
ments of the secondaries, except in six instances of the 40 typical 
diglyphic specimens ; and each of these six instances showed variations 
in the tertiaries characteristic of it as an individual. So far, then, as 
the incomplete mesenteries are concerned, we soon reach groups of vari- 
ations by which individuals may be characterized ; in other words, if the 
variations of the primaries (complete mesenteries), secondaries, and 
tertiaries be considered together, it will be seen that no two of the 
131 specimens examined were alike, each one having a combination 
of variations peculiar to itself. This is, perhaps, the most important 
feature in the variations of the incomplete mesenteries. 

That variations in the number of mesenteries, such as have been 
pointed out in the preceding paragraphs, occur in other actinians is 
well known. Thus Carlgren (’93, p. 106) states that in Metridium 
dianthus, in addition to a single pair of directives, six, seven, or even 
nine pairs of non-directives may occur, and F. Dixon (88) has shown 
that in several species of Sagartia the number of non-directives may 
reach twelve or even sixteen pairs. Further, in four specimens of 
Bunodes thallia, G. Y. and A. F. Dixon (’89, pp. 317, 318) found 
respectively 15, 19, 21, and 26 pairs of non-directives. These citations 
suffice to show that extensive variations in the mesenteries may occur 
in other actinians than Metridium marginatum, but the cases recorded 
for any one species are so few that generalizations cannot be drawn 
from them. 


266 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


As a rule, variations in the mesenteries occur in both members of a 
pair in the same way, but not infrequently one finds pairs in which the 
two members are not equally developed. When this occurs amongst 
the complete or nearly complete mesenteries, it may result in the forma- 
tion of a pair one member of which is complete and the other incom- 
plete (Fig. 2). The 131 specimens of Metridium examined possessed 
in all 739 pairs of non-directives, and, of these, 17 pairs (or about 2.4 
per cent), distributed through thirteen individuals, possessed each an 
incomplete member. Of the thirteen individuals exhibiting this vari- 
ation, ten were of the monoglyphic type, and three of the diglyphic 
type. In the monoglyphic type it is customary to assume that the 
single siphonoglyph present corresponds to the so-called ventral one 
of the diglyphic condition. This assumption is at least convenient, 
for it allows us to distinguish in each pair of lateral non-directives a 
dorsal and a ventral member. Admitting this distinction for the sake 
of description, it may be said that seven of the ten monoglyphic 
specimens had each a single pair of non-directives in which one mem- 
ber was incomplete, and of these incomplete mesenteries four were 
dorsal, two were ventral, and one was indeterminable (Fig. 8); and 
that the three remaining monoglyphic specimens had each two such 
pairs, of which in one instance both the incomplete mesenteries were 
dorsal, and in two instances one was a dorsal and the other a ventral 
mesentery (Fig. 7). Thus in the ten monoglyphic specimens, this 
variation was observed in thirteen pairs of mesenteries, of which eight 
presented incomplete dorsal members, four incomplete ventral members, 
and one was indeterminable. It is evident that this variation is not 
limited to either dorsal or ventral members, and is not correlated with 
the fact that in many actinians ventral members, as a rule, develop later 
than dorsal] ones ; in other words, this variation is probably not to be 
regarded as atavistic. 

In the adult condition of the diglyphic type, I see no way of distin- 
guishing dorsal from ventral, and the most that can be said of the three 
cases of variation met with under this type is that in two of them only 
one mesentery each was incomplete (Fig. 2), while in the third two were 
incomplete. In the latter case the two mesenteries (as in Fig. 7) were 
not on corresponding sides ; hence one of them must have been dorsal 
and the other ventral, but exact determination could not be made. The 
variations in the diglyphic type, then, present no essential features 
not already met with in the monoglyphic type. 

Many pairs of mesenteries in which both members are incomplete 


PARKER: METRIDIUM MARGINATUM. 267 


show variation of the kind indicated above, in that one member is 
larger than the other (Fig. 4), but because of the extreme variability of 
these parts no record has been kept of such variations. 

In a few cases single mesenteries have been observed (Fig. 2). These, 
as the arrangement of the longitudinal muscles of their neighbors shows, 
have absolutely no trace of a mate. In the instance figured, it is diffi- 
cult to decide which of the two mesenteries, the complete (y) or the 
incomplete (x), is the single one. One or other must be. Single mesen- 
teries as exceptions have already been recorded by F. Dixon (’88, p. 138) 
in Sagartia, and by Carlgren (’93, p. 106) in Metridium. 

Among the complete mesenteries, two cases of union by what would 
have been the median margins of the participants have been observed 
(Fig.8). An instance of this kind has already been recorded by R. 
Hertwig (82, p. 37) in Tealia, and in this, as in Metridium, the united 
mesenteries were not members of the same pair, but of adjacent pairs. 

No instances of the occurrence of longitudinal muscles on both the 
exoceel and the endoccel face of the same mesentery, as observed by 
McMurrich (’89, p. 30) in Aulactinia, have been noticed. 

So far as the mutual arrangement of complete and incomplete 
mesenteries is concerned, the monoglyphic and diglyphic types show 
rather characteristic differences. In the diglyphic type the complete 
mesenteries usually show no special tendency to collect at one pole or 
the other of the animal (cf. Fig. 1). In the monoglyphic type there is 
often a marked tendency for all but two pairs of the non-directives 
to collect opposite the directives (cf. Fig. 5); consequently the half of 
the animal centering about the directives has an arrangement of parts 
like that found in the corresponding half of a diglyphic animal, while 
the other half contains a more or less crowded group of non-directives. 
In this respect Metridium seems to differ from Sagartia, in which, 
according to the figures given by F. Dixon (’88, Plate I.), such a 
crowding of non-directives is not noticeable, This condition recalls in 
a superficial way that found in Cerianthus, in which an active growth 
of mesenteries takes place opposite the siphonoglyph. 

The characteristic arrangements of the mesenteries in connection with 
the monoglyphic and diglyphic types probably recur under similar con- 
ditions in M. dianthus; for such arrangements have been figured by 
Thorell (59, Tab, I. Figs. 1 and 2) and briefly described by Carlgren 
(93, p. 106). 

While the crowding of the mesenteries occurs as a rule only in mono- 
glyphic specimens of Metridium, one instance of it has been observed 


268 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


in a diglyphic specimen (Fig. 2), and here the general resemblance to 
the specimen «with three siphonoglyphs (Fig. 6) is so striking that 
I have felt almost justified in interpreting this specimen as a triglyphic 
animal, at one pole of which the directives, with the loss of the siphono- 
glyph, had given place to a group of non-directives. 


In the preceding account I have intentionally avoided, as far as pos- 
sible, the use of the terms dorsal and ventral as applied to the two poles 
of the actinian’s body. This has not been because of objections that 
might well have been raised against these terms in themselves, as 
Haddon (89, p. 300) has done, but because of the more fundamental 
question of whether dorsal and ventral can really be distinguished in an 
adult Metridium. These terms, as is well known, may be applied with 
perfect precision to the adults of forms like Edwardsia, where the longi- 
tudinal muscles bear very unlike relations to the two poles of the animal ; 
but in forms like the diglyphic type of Metridium (Fig. 1), where the 
muscles of the pairs of non-directives are similarly related to both poles, 
this means of distinguishing dorsal and ventral is lost. It has been 
suggested that even in cases of this kind dorsal and ventral may still be 
distinguished, either by the conditions of the siphonoglyphs, — the ven- 
tral being better developed than the dorsal (Faurot, 95, p. 62),—or 
by the condition of the subsidiary mesenteries, —the more dorsal pairs, 
because of their earlier development, remaining larger than the ventral 
ones (Carlgren, ’93, p. 100). Unfortunately, these criteria, even sup- 
posing them to be true, which is by no means certain, cannot be em- 
ployed on the diglyphic type of Metridium because of the similarity of 
its two poles. So far as the adult diglyphic Metridium is concerned, I 
am obliged to confess that I can find no satisfactory criteria for the 
determination of dorsal and ventral relations. 

With the monoglyphic type the case seems simpler. It is generally 
stated that, when only one siphonoglyph is present, it is the ventral one ; 
but, as Carlgren (’93, p. 100) remarks, so far as Sagartia is concerned, 
this statement has never been accompanied with any direct proof; nor, 
I may also add, has it been proved for Metridium. The argument used 
by MeMurrich (91, p. 133) to show that the single siphonoglyph in the 
monoglyphic Metridium is the ventral one may be used with equal accu- 
racy to show that this siphonoglyph is the dorsal one, for the argument 
advanced rests upon the sequence of the development of the mesen- 
teries, which, being unknown in Metridium, has simply been assumed by 
MeMurrich. The case of Metridium seems to be precisely like that of 


PARKER: METRIDIUM MARGINATUM. 269 


Sagartia, in which, as Haddon (’89, p. 300) remarks, it seems impossible, 
in our present state of knowledge, to determine dorsal and ventral rela- 
tions. It is probable that this determination can be made only after 
the sequence of development of these mesenteries has been discovered. 
In the four types of sequence thus far known (cf. Fowler, ’94, p. 470), 
the ventral directives are always the third pair of mesenteries to form, 
and the dorsal directives either the second or fourth. It is probable 
that, when the developmental sequence of the mesenteries is discovered 
for the two types of Metridium, the determination of dorsal and ventral 
in this actinian will be made with as much‘certainty as in any other, 
and we shall probably then know whether in the monoglyphic type the 
single siphonoglyph is a dorsal one, a ventral one, or in some specimens 
one and in others the other. 

Before concluding this account of the mesenteries in Metridium, I 
wish to consider briefly some other aspects of the monoglyphic and 
diglyphic types. When I first perceived that there were two structural 
types in Metridinm, I suspected that they might be correlated with 
sexual differences. To test this question, I determined the sexes of a 
number of individuals of each type. In ten monoglyphic specimens, 
five were females and five were males; in twenty-seven diglyphic speci- 
mens, fifteen were females and twelve were males. Evidently the two 
types are not correlated with difference in sex. 

The fact that the two sexes occur in about equal numbers under both 
types suggests that these two types may in reality be two varieties of 
the species Metridium marginatum. In support of this opinion, it may 
be mentioned that the two types show a difference in the degree of their 
variability, the diglyphic type having only one subtype, the monoglyphic 
three ; and, further, that, while the diglyphic type presents usually a 
rather typical Hexactinian arrangement of mesenteries, the monoglyphic 
type shows a general tendency to crowd the non-directive mesenteries to 
the region opposite the one siphonoglyph. 

These differences, however, fairly marked as they are, are insufficient 
in my opinion to warrant the assumption that the two types are really 
varieties, and the determination of this question must wait, I believe, 
till more is known of the breeding habits of Metridium. If it can be 
shown that in the offspring of one animal representatives of both types 
occur, the idea that we are dealing with varieties could not be main- 
tained, and the species could at most be said to be dimorphic. If, how- 
ever, the types could be shown to breed true, they might with justice 
be described as varieties. 


270 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Should these types prove not to be of the value of varieties, they may 
still possibly be correlated with the methods of reproduction. Besides 
the sexual method, Metridium marginatum reproduces non-sexually by 
small buds cut off from the margin of the animal between its aboral 
disk and its column. This method of reproduction, long ago hinted at 
by Verrill (’69, p. 257), can usually be seen taking place in any large 
specimen. Similar conditions have been observed in Metridium dianthus 
by G. Y. and A. F. Dixon (91, p. 20), and by Carlgren (93, p. 108). 
Possibly the two types here described are the products, one of the 
sexual, the other of the non-sexual, method of reproduction. The solu- 
tion of this question, however, must be left to future investigation. 


CAMBRIDGE, January 9, 1897. 


PARKER: METRIDIUM MARGINATUM. 271 


PAPERS CITED. 


Appellof, A. 
94. Ptychodactis patula, n. g. & sp. der Reprasentant einer neuen Hex- 
actinien-Familie. Bergens Museums Aarbog for 1893, No. LV. pp. 1-22, 
Pls. I-III. 


Blochmann, F., und Hilger, C. 
88. Ueber Gonactinia prolifera, Sars. Morph. Jahrb., Bd. XIII. pp. 385- 


401, Taf. XIV., XV. 


Carlgren, O. 
’93. Studien iiber nordische Actinien. Kongl. svenska Vet-Akad. Hdlgr. 
N. F., Bd. XXV. No. 10, pp. 1-148, Taf. I-X. 


Dixon, F. 
"88. Onthe Arrangement of the Mesenteries in the Genus Sagartia, Gosse. 
Sci. Proc. Roy. Dublin Soc., N. S., Vol. VI. pp. 186-142, Pls. L, IT. 


Dixon, G. Y. 
’88. Remarks on Sagartia venusta and Sagartianivea. Sci. Proc. Roy. Dub- 
lin Soc., N. S., Vol. VI. pp. 111-127. 


Dixon, G. Y., and Dixon, A. F. 
’89. Notes on Bunodes thallia, Bunodes verrucosa, and Tealia crassicornis. 
Sci. Proc. Roy. Dublin Soc., N. S., Vol. VI. pp. 310-326, Pls. IV., V. 


Dixon, G. Y., and Dixon, A. F. 
"91. Report on the Marine Invertebrate Fauna near Dublin. Proc. Roy. 
Trish Acad., 3d Ser., Vol. II. pp. 19-33. 


Faurot, L. 
95. Etudes sur l’Anatomie, l’Histologie et le Développement des Actinies. 
Arch. Zool. expér. et gén., 3e Ser., Tome III. pp. 43-262, Pls. 
I-XII. 


Foot, F. J. 
’63. Notes on the Astreacea of the Coast of Clare. Proc. Nat. Hist. Soc. 
Dublin, Vol. III. pp. 63-69. 


272 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Fowler, G. H. 
94. Octineon Lindahli (W. B. Carpenter): an Undescribed Anthozoon of 
Novel Structure. Quart. Journ. Micr. Sci., Vol. XXXV. pp. 461-480, 
Pls. 29, 30. 
Gosse, P. H. 
60. A History of the British Sea-anemones and Corals. London, 1860. 
viii + 362 pp., Pls. 1—XI1. 


Haddon, A. C. 
’89. A Revision of the British Actinie. Part I. Sci. Trans. Roy. Dublin 
Soc., 2d Ser., Vol. IV. pp. 297-361, Pls. XXXI-XXXVII. 


Hertwig, O. und R. 
°79. Die Actinien anatomisch und histologisch mit besonderer Beriicksichti- 
gung des Nervensystems untersucht. Jena. Zeitschr., Bd. XIII. pp. 457- 
640, Taf. XVIL-XXVI. 


Hertwig, R. 
82. Report on the Actiniaria dredged by H. M. S. Challenger during the 
years 1873-1876. The Voyage of H. M.S. Challenger, Zoology, Vol. 
VI. pp. 1-136, Pls. I—XIV. 


Hickson, S. J. 
84. On the Ciliated Groove (Siphonoglyphe) in the Stomodzum of the 
Aleyonarians. Phil. Trans. Roy. Soc., London, Vol. 174, pp. 693-705, 
Rise 50a we 
McMurrich, J. P. 
’89. The Actiniaria of the Bahama Islands, W. I. Journ. Morph., Vol. III. 
pp- 1-74, Pls. I-IV. 
McMurrich, J. P. 
91. Contributions on the Morphology of the Actinozoa. III. The Phylo- 
geny of the Actinozoa. Journ. Morph., Vol. V. pp. 125-164, Pl. IX. 


McMurrich, J. P. 
'94. Report on the Actinix collected by the United States Fish Commission 
Steamer Albatross during the Winter of 1887-1888. Proc. U. 8. Nat. 
Mus., Vol. XVI. pp. 119-216, Pls. XIX.-XXXYV. 


Thorell, T. 4 
59. Om den inre byggnaden af Actinia plumosa Mill. Ofversigt kong]. 


Vet-Akad. Forhandl., Vol. XV. pp. 7-25, Tab. I. 
Tullberg, T. 
'91. Ueber Konservierung von Evertebraten in ausgedehntem Zustand. Biol. 
Foren, Forhdlgr., Bd. [V. pp. 4-9. 


Verrill, A. E. 
’69. Our Sea-Anemones. Amer. Naturalist, Vol. II. pp. 251-262. 


PARKER. — Metridium marginatum. 


EXPLANATION OF PLATE. 


All figures represent transverse sections of the column of Metridium marginatum 
Milne-Edwards. In each case all the complete mesenteries have been drawn, and 
in some instances a few of the incomplete ones. All figures are magnified about 
1.5 diameters. 


Fig. 1. 


Fig. 2. 


Diglyphic specimen, showing typical hexamerous arrangement of the 
complete mesenteries, and a regular group of subordinate mesen- 
teries in a primary exoceel. 


Diglyphic specimen, showing a very irregular arrangement of the com- 
plete mesenteries, one group of which resembles the crowded group 
of mesenteries found opposite the directive pole in the monoglyphic 
type. In this crowded group occurs an unpaired mesentery (z or y). 


Monoglyphic specimen, with five pairs of non-directives, showing the 
regular arrangement of the subordinate mesenteries about the pair of 
non-directives opposite the directive pole. 

Monoglyphie specimen, with six pairs of non-directives, showing the 
regular arrangement of the subordinate mesenteries about the two 
pairs of non-directives opposite the directive pole. 


Monoglyphic specimen, with ten pairs of non-directives, showing the 
crowding of the non-directives in the region opposite the directive 
pole. 


Triglyphic specimen. 


Monoglyphic specimen, showing pairs of mesenteries in which dorsal as 
well as ventral components are incomplete. 


Monoglyphic specimen, showing the union of two primary mesenteries. 


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No. 6. — Photomechanical Changes in the Retinal Pigment Cells of 
Palemonetes, and their Relation to the Central Nervous System. 
By G. H. ParKker.! 


ConTENTS. 
Page Page 
Introduction .. . 275} 6. Summary of Changes in Nor- 
Structure of the Eye in Paleroncted 276 malwRetinawe - 288 
Photomechanical Changes in Nor- Sympathetic Phoumecnaniead 
maleketinaey, = = . 2. .« 218 Changes .. . . 289 
i Methods . . . . . 278) Localized PHotomiectiantoal Ghanees 290 
2. General Changes in Rating . 279] Photomechanical Changes in Ex- 
3. Changes in Proximal Retinu- cised Eyes and Retinas . . . 291 
Jari@ells . . . . 2 219) (Generally Summaryee nee eo 
4, Changes in Accessory Cells s QS Note 4) Ue en ele Parner mmc 
5. Changes in Distal Retinular Papers) Cited= 20) eae ee eZ OO 
Cells .... =... . 284| Explanation of Plate. 
INTRODUCTION. 


THE present paper is a record of a series of experiments on the photo- 
mechanical changes in the pigment cells of the retina in Palemonetes 
vulgaris Stimp. This species is especially favorable for such work, 
since its retina exhibits in a marked degree all the kinds of pigment 
changes that have thus far been observed in the eyes of crustaceans. 

Those that have worked upon this subject have, in the main, fol- 
lowed in the lines laid down by Boll, Engelmann, and others in their 
studies on the eyes of vertebrates. Although the pigment changes in 
vertebrates are relatively simple, they are, even now, far from being 
satisfactorily understood, and it is therefore not surprising that in 
the arthropods, where the pigment changes of the retina are probably 
more complex than in any other group of animals, much still remains 
to be done. There has been a tendency, moreover, among some of 
those that have studied such phenomena, to generalize on observations 
taken from eyes of totally different types, such as the compound eyes 
of insects and the simple eyes of arachnids; and this tendency, though 

1 Contributions from the Zoological Laboratory of the Museum of Comparative 


Zodlogy at Harvard College, E. L. Mark, Director, No. LXXVI. 
VOL. Xxx. — NO. 6. 1 


276 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


in a measure justifiable, has led, I believe, to a want of attention to 
the characteristic differences of the pigment changes in each given type 
of eye, a matter that, in my opinion, lies at the foundation of any 
satisfactory understanding of these changes. What is most needed at 
present, therefore, seems to be a thorough and exhaustive study of the 
pigment changes of each of the more important types, rather than an 
inspection, necessarily more or less superficial, of the various arthropod 
eyes that have not as yet been examined. The following studies have 
been made with the hope that they would contribute in this respect to 
a more complete understanding of the pigment changes in the com- 
pound eye, especially in crustaceans. 

The earliest paper on the pigment changes in compound eyes, so far 
as Iam aware, was published in 1889 by Exner (’89), and contains in 
a condensed form the essential peculiarities of the pigment changes 
in the compound eyes of certain insects. In 1890 Stefanowska (’90) 
published an account in which this subject was again considered, but 
with a wider range of material. In the next year three contributions 
appeared: Exner’s (’91) brilliant and important essay on the physi- 
ology of compound eyes, of which his former publication had been in 
the nature of a partial preliminary notice ; Szczawinska’s (’91) article 
on the pigment changes in the eyes of crustaceans and arachnids ; and 
Herrick’s (’91) account of similar changes in the eyes of Paleemonetes, 
contained in his monograph on the development of Alpheus. Three 
years later Kiesel (94) described some very noteworthy observations on 
the pigment changes in the eyes of insects. The following year the writer 
(Parker, ’95) published, in connection with other matters, an account 
of the retinal pigment changes in Astacus, and a preliminary state- 
ment of the results given in full in this paper was published last year 
(Parker, 96). These, I believe, are all the publications in which the 
questions here raised have been considered. Critical comments on their 
contents will be found in the following pages. 


STRUCTURE OF THE EYE IN PAL#MONETES. 


Before describing the pigment changes in the retina of Palemonetes, 
it will be necessary to outline briefly the structure of the eye in this 
animal. The eye may be said to be that portion of the optic apparatus 
contained in the optic stalk. It consists of a retina, at the distal end 
of the stalk, and a series of four optic ganglia, which extend through 
the axial portion of the stalk. The retina is connected with the first 


PARKER: RETINAL PIGMENT CELLS OF PALASMONETES. 277 


optic ganglion by the retinal nerve fibres. Nerve fibres connect the 
first optic ganglion with the second, the second with the third, and the 
third with the fourth. From the fourth optic ganglion, which is sit- 
uated near the proximal end of the stalk, the optic nerve extends to the 
brain. The finer structure of the optic ganglia in Paleemonetes is in all 
probability essentially the same as in Astacus, where, as I have already 
shown (Parker, ’95, Taf. 3, Fig. 59), each optic ganglion represents a 
region of interruption for the great majority of the nerve fibres that 
intervene between the retina and the brain. 

The retina in Palemonetes is composed of ommatidia, the structure of 
which has already been described at length (Parker, ’91, p. 108, Pl. IX.). 
For convenience I add a brief summary of this description. Each omma- 
tidium is composed of five kinds of cells. Immediately under the cor- 
neal facet (Fig. 1, ern.) are two corneal hypodermal cells (n/. ern.). The 
distal portion of the axis of the ommatidium is occupied by the cone (con.), 
which, as seen in transverse sections (Fig. 3, el. con.), is composed of four 
parts. Each part contains near its distal end a nucleus (Fig. 1, nl. con.) 
and represents a cell. The four cone cells are closely applied to one an- 
other in the region of the cone proper (Fig. 3). Proximally they taper 
off as thick, more or less independent fibres. (Compare Figs. 1, 4, and 5.) 
These fibres separate and apparently terminate near the distal end of 
the rhabdome (Fig. 1, rhd.). I have been unable to trace them further, 
though I suspected that they might end, as in Homarus (Parker, ’90, 
p- 14), on the basement membrane. ‘The distal retinular cells either 
apply themselves to the lower portion of the sides of the cone (Fig. 2, 
el. dst.), in which case they are so closely packed that their outlines 
cannot be distinguished (Fig. 3, el. dst.), or they occupy a more prox- 
imal position (Fig. 1, el. dst.), forming a ring around the attenuated ends 
of the cone cells (Fig. 5). There is, of course, one ring for each om- 
matidium. Each ring contains six distal retinular cells, but these rings 
are so constituted that each cell is at the same time a member of three 
rings; hence there are in reality only twice as many distal retinular 
cells as there are ommatidia. The proximal portion of the axis of the 
ommatidium is occupied by the rhabdome (Fig. 1, rhd.), which is sur- 
rounded by seven functional proximal retinular cells (Fig. 6, el. pr.), 
in addition to which an eighth rudimentary one is present (Parker, 791, 
p- 111). Each functional cell ends distally in a somewbat swollen knob 
containing its nucleus (Fig. 1, nl. px.). From this swollen end the 
cell extends proximally over the rhabdome, beyond which it becomes 
slightly attenuated, and, as a retinal nerve fibre (Figs. 1 and 7, for. r.), 


278 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


pierces the basement membrane (mb. ba.) and extends to the first optic 
ganglion. Here it probably terminates in a fibrillation, as has already 
been shown to be the case in Astacus (Parker, 795, p. 41). The acces- 
sory pigment cells (Fig. 1, el. sn.) occupy the space in the deeper part 
of the retina. ‘The number of these cells is not constant, but, judging 
from their nuclei, it is not more than one or two for each ommatidium. 
Proximal processes extend from these cells through the apertures in the 
basement membrane to the distal surface of the first optic ganglion, and 
distal processes may extend forward to the front faces of the distal 
retinular cells. Each ommatidium in Palemonetes, then, is composed 
of the following cells: two corneal hypodermal cells, four cone cells, two 
distal retinular cells, eight proximal retinular cells (one of which is 
rudimentary), and a variable but small number of accessory pigment. 
cells. Black pigment granules are contained in both the distal and the 
proximal retinular cells, and are limited to these cells; the whitish pig- 
ment lies exclusively in the accessory pigment cells. The seven func- 
tional proximal retinular cells are the only elements of the ommatidium 
that are known to have nervous connections. These brief anatomical 
statements may suffice as an introduction to the consideration of the 
pigment changes in the retina. 


PHOTOMECHANICAL CHANGES IN NorMAL RETINA. 


The general method by which the normal photomechanical action of 
the retinal pigment cells in Paleamonetes was determined consisted in 
the examination of eyes that had been kept in the light or in the dark 
known periods of time. For a dark chamber I used a box with a tight- 
fitting cover. From time to time during the course of the experiments 
this box was tested for its light-progf qualities by exposing in it a very 
sensitive bromide paper, such as is used by photographers. In all my 
experiments this showed complete absence of light. The top of the 
box was pierced by a hole, through which a piece of rubber tubing was 
introduced so that fluids could be poured into the box without exposing 
its contents to light. Two or three turns in this tube were found suff- 
cient to prevent such light as entered the outer end of the tube from 
reaching the interior of the box. Living shrimps in a vessel of water 
were placed in the box, and the cover was carefully closed. After the 
expiration of the required interval, hot water was run in through the 
tube, and the animals were thus killed in the dark. Other killing re- 
agents, such as corrosive sublimate, picric acid, etc., were tried, but 


PARKER: RETINAL PIGMENT CELLS OF PALZMONETES. 279 


none proved so satisfactory as water at about 80° C. The periods of 
exposure to dark in the first set of experiments were as follows: 1 min., 
5 min., 10 min., 15 min., 30 min., 45 min., 60 min., and then at intervals 
of an hour up to 8 hours. It was found subsequently that the experi- 
ments need not have extended over a maximum period of more than 
two hours, and that intervals of about fifteen minutes were all that 
were needed to observe the steps of the change. From each lot of ani- 
mals prepared in this way, the optic stalks were cut into sections for 
examination under the microscope. In a similar way, the eyes of ani- 
mals that had been kept some four hours in the dark were exposed to 
the light for given intervals, killed, cut, and examined. In cases where 
it was necessary to make very accurate comparisons, the eyes of the 
same animal were used for the two conditions; thus, after keeping 
the animal a given time in the light, one optic stalk was removed, and the 
animal kept in the dark. At the expiration of the second interval, the 
second optic stalk was removed and prepared. To guard against indi- 
vidual varietions, in every experiment the eyes of at least three animals 
were examined. 

The only general changes shown by retinas subjected to light or dark 
were changes in the arrangement of the pigment. In other respects 
they were not noticeably altered. Thus, no change in thickness was 
observable ; in one case, a left retina that had been kept in the dark 
measured in its middle region from the corneal cuticula to the basement 
membrane 263 uw, while the right retina from the same animal exposed 
to light measured 270 u. Ina second case, a dark left retina measured 
240 uw, the light right one measuring 233 yp. The cones likewise showed 
no significant differences. By analogy with the perceptive elements in 
the vertebrate eye, one might have expected the rhabdomes, the termi- 
nal nervous organs of the crustacean eye, to shorten in the light and 
lengthen in the dark. I was unable to obtain evidence of such a change 
in Paleemonetes, and yet the conditions for the exact measurement of 
the rhabdomes are so unfavorable in this animal that I am by no means 
certain that these changes may not occur. If, however, they do take 
place, they must be relatively small. The observable changes induced 
in the retina by the absence or presence of light affect the three kinds 
of pigment cells, —the proximal retinular cells, the accessory cells, and 
the distal retinular cells. These will be considered in the order given. 

The pigment in the proximal retinular cells forms at the base of the 
retina a band, called by Exner (’91, p. 62) the retinal pigment. The 
photomechanical changes that this pigment undergoes have already been 


280 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


observed in various crustaceans by Exner (’91), Szczawinska (’91), and 
myself (Parker, ’95); and, so far as the chief facts of these changes 
are concerned, the accounts given by these writers are in substantial 
agreement. In no case, however, has the precise character of these 
changes been followed, nor the time needed for their completion been 
recorded, 

In a proximal retinular cell that shows the full effect of light (Fig. 1), 
the black pigment granules are almost uniformly scattered from the 
distal end of the cell backward through its whole length, including the 
retinal nerve fibre, to the region of the first optic ganglion. In the body 
of the cell proper (Fig. 6), as well as in the retinal nerve fibre (Fig. 7), 
it will be observed that the pigment granules lie entirely within the 
limits of these structures; in other words, the black pigment of this 
portion of the retina is contained entirely within the proximal retinular 
cells. This pigment, though in the main uniformly distributed through 
the cell, shows regularly two slight concentrations, — one at the swollen 
distal end of the cell (Fig. 1), and another on the sides of the rhabdome. 
Small irregular concentrations may also occur in the body of the cell. 
In the eye subjected to light, the only part of the cell except the nu- 
cleus that is entirely free from pigment granules is a transparent axis 
that can be traced from the region of the rhabdome down through the 
body of the cell, and through the whole length of the retinal nerve 
fibre. This is undoubtedly the axis cylinder of the nerve fibre, which, in 
its passage to the rhabdome, extends through the body of the cell. 

In crustaceans like Cancer (Parker, 91, p. 116, Plate X. Fig. 131), 
in which the proximal retinular cells are more fully provided with pig- 
ment granules than in Palemonetes, this axis is more conspicuous. 

In an eye that has been kept in the dark for several hours, the bodies 
of the proximal retinular cells are without trace of pigment (Fig. 2), 
the whole mass of black pigment being concentrated in the retinal nerve 
fibres, i. e. proximal to the basement membrane. Here, as in the for- 
mer case, the pigment lies entirely within the limits of the retinular 
cell. 

The transition from the dark condition to the light condition of the 
eye was accomplished by the following steps. In an eye that had been 
kept some four hours in the dark and then exposed for five minutes to 
the light, the arrangement of the pigment in the proximal retinular 
cells was indistinguishable from that characteristic for full darkness. 
After ten minutes’ exposure to light, the pigment was found to have 
moved forwards to the level of the basement membrane. After fifteen” 


PARKER: RETINAL PIGMENT CELLS OF PALAMONETES. 281 


minutes, it was found throughout the bodies of the cells; and, at thirty 
minutes, well marked concentrations had appeared about the rhabdome 
and at the distal end of each cell. At forty-five minutes, these concen- 
trations were somewhat more pronounced, but after that time no further 
changes were observable. 

The reverse change, which takes place in the dark, is accomplished in 
the following manner. After the animal has been in the dark fifteen 
minutes, the concentrations of pigment about the rhabdomes and at 
the distal ends of the cells have almost disappeared, though the bodies 
of the cells still contain an almost uniform amount of pigment through- 
out their whole length, After thirty minutes, much more pigment is to 
be found proximal to the basement membrane than distal to it, and after 
forty-five minutes almost all the pigment is proximal in position. At the 
end of an hour, the condition characteristic of darkness is fully realized. 

The changes just recorded occur entirely within the limits of each 
proximal retinular cell. There is no reason for believing that the 
changes are the results of a process of pigment production in one part 
of the cell, and of pigment destruction in another. The observed facts, 
on the contrary, suggest that the pigment granules of one region in the 
cell are moved to another. The movement, however, is not accompanied 
by any noticeable change in the position or even the form of the contain- 
ing cell. The pigment granules seem to be carried up and down through 
the cell, as though by a streaming of the cell protoplasm. A similar 
stability of form, accompanied with an internal movement of pigment, 
has been described by Ballowitz (’93, Taf. XXXVI. Fig. 12, and 793%, 
p- 629) in the pigment cells of the skin of fishes. 

Through the kindness of Professor F. H. Herrick, I have had the priv- 
ilege of examining an interesting series of eyes taken from specimens of 
Palemonetes that had been kept living in a dark chamber thirty-eight 
days. The pigment in the proximal retinular cells of such animals 
showed the condition characteristic of the dark. In an animal that had 
been kept in the dark for this period and then exposed to light for four 
hours and three quarters, the pigment returned partially to the position 
characteristic of the light. The greater part of it remained proximal to 
the basement membrane, and from that which moved into the bodies of 
the cells no marked concentrations were formed, either about the rhab- 
domes or at the distal ends of the cells. Long confinement in the dark, 
then, seems to interfere somewhat with the mechanism by which the 
pigment of these cells is normally moved. 

The accessory pigment cells ave located in the base of the retina, and 


282 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


send a few processes distad to the outer surfaces of the distal retinular 
cells, and many proximad through the apertures in the basement mem- 
brane to the distal surface of the first optic ganglion. The pigment 
with which these cells seem to be almost entirely filled is yellowish by 
transmitted light, and white by reflected light. It is especially remark- 
able for its powers of reflecting light, and this quality led Exner (91, 
p- 97) to designate the layer formed from it by the name of the tape- 
tum. Whether this pigment is influenced by the presence or absence 
of light is a matter of some uncertainty. Szczawinska (91, p. 552) 
states that in Astacus, under the influence of light, the cells containing 
it enlarge slightly. Exner (91, p. 105), though at first inclined to re- 
gard the accessory cells as influenced by the light, was finally led to 
abandon this view, and to explain their two apparent conditions by the 
greater or less degree with which they were covered by the migrating 
pigment of the proximal retinular cells. In a preparation from an eye 
kept in the dark, the retinular pigment, as already mentioned, is entirely 
below the basement membrane, and the accessory pigment is almost en- 
tirely exposed, and consequently conspicuous. In the light it is some- 
what covered by the black pigment, which under these circumstances 
fills the bodies of the proximal retinular cells, and it thus becomes less 
noticeable than before. My own studies on the retina of Astacus (Par- 
ker 795, p. 25) led me to agree with Exner that the accessory pigment 
showed only an apparent change. If, however, any change did occur, it 
was certainly not an increase in the size (conspicuousness ?) of the acces- 
sory cells under the action of light, as maintained by Szczawinska, but 
rather the reverse. 

Although in respect to Astacus I am still in doubt as to whether or not 
the accessory cells show any photomechanical changes, I have not the 
least hesitancy in stating that in Paleemonetes such a change does occur. 
The principal difficulty in demonstrating this change comes from the dis- 
turbing influence produced by the migration of the pigment in the proxi- 
mal retinular cells. This difficulty, however, can be overcome by the 
employment of a depigmenting reagent that will remove the retinular 
pigment without affecting the accessory pigment. Such a reagent is 
the depigmenting fluid recommended by Grenacher (’86, p. 214). In 
preparations representing the dark and the light condition, and depig- 
mented by this means, the differences in the distribution of the pigment 
in the accessory pigment cells is so striking that no one would question 
for a moment the photomechanical activities of these cells. In the light 
(Fig. 1) the accessory pigment forms two concentrations, one in the base of 


PARKER: RETINAL PIGMENT CELLS OF PALZMONETES. 285 


the retina, and the other near the distal surface of the first optic ganglion. 
These two concentrations are connected by irregular bands of pigment. 
In the dark (Fig. 2) almost all the accessory pigment is in the base of 
the retina, the concentration near the ganglion as well as the interme- 
diate pigmented bands being represented by only a few small pigmented 
patches. 

The change from the condition produced by the light to that produced 
in the dark is indicated in the following steps. After the animal has 
been about thirty minutes in the dark, the concentration of pigment for- 
merly near the optic ganglion is appreciably nearer the retina.. After 
forty-five minutes, this concentration as such has disappeared, and that 
in the retina has considerably increased. Finally, after two hours, almost 
all the accessory pigment lies in the base of the retina, there being only 
a few small strands proximal to the basement membrane. 

In the reverse change under the influence of light, the intermediate 
pigment strands show a perceptible thickening between ten and fifteen 
minutes after the eye has been placed in the light, and the full concen- 
tration at the level of the ganglion is completed within the period extend- 
ing from forty-five minutes to an hour after that event. 

I have never been able to discover any outlines to the accessory pig- 
ment cells except those indicated by the pigment mass itself. Judging 
from these, the photomechanical changes in the accessory cells involve so 
radical an alteration in the forms of the cells that the latter may be said 
to have assumed a different position. In this respect, then, the pigment 
changes in these cells involve much more active movements than in the 
case of the proximal retinular cells, and possess something of a locomotor 
character. So far as I have observed them, they may be compared with 
perfect propriety to the more or less circumscribed movements of an 
amoeba. When the retina is placed in the light, the cells with their 
contained pigment creep slowly backward through the apertures in the 
basement membrane toward the optic ganglion. When the retina is in 
the dark, they reverse this movement and creep out into the base of the 
retina. The one particular in which this movement differs from that of 
an amceba is that of its limitations in direction. Thus the cells always 
creep either outward or inward. Moreover, in darkness they do not 
creep indefinitely outward, but after about two hours reach a maximum 
limit ; the same is true of their inward course. These limitations may 
be due either to the structure of the regions into which the cells creep, 
or to the intrinsic qualities of the cells themselves; but I have been 
unable to get conclusive evidence as to which it is. 


284 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


In the interesting series of eyes of Paleemonetes loaned me by Professor 
Herrick, the accessory pigment cells of the eyes that had been kept in 
the dark thirty-eight days presented a condition normal for exposure to 
the dark. In those eyes that had afterwards been exposed to the light 
for four hours and three quarters, this pigment had apparently resumed 
the position normal for exposure to light. The mechanism by which the 
accessory pigment changes are brought about, unlike that for the proxi- 
mal retinular pigment changes, is therefore apparently not interfered 
with by prolonged retention in the dark. 

The distal retinular cells present photomechanical changes more com- 
plex than those in the two kinds of cells already considered. These 
changes have been described by Exner (’89 and ’91), Szczawinska (91), 
Herrick (91), and myself (Parker, 95). All investigators are agreed, 
I believe, in stating that in the dark these cells occupy a more distal 
position than in the light. Their probable influence on the amount of 
effective light that enters the retina led Exner (91, p. 63) to call them 
the iris pigment. In Palemonetes, as I have already shown, there are 
two distal retinular cells for each ommatidium. 

In an animal that has been subjected to the full action of light, the 
distal retinular cells (Fig. 1, el. dst.) are plump ovoid bodies in contact 
with the outer ends of the proximal retinular cells. The body of each 
distal cell has the length of about 30. From its outer end a single 
process usually extends to, or at least toward, the corneal hypodermis. 
The whole distal retinular cell, excepting its nucleus and sometimes a 
portion of its distal process, is filled with black pigment. The whitish 
pigment that often occurs on the outer surface of these cells repre- 
sents, as already mentioned, a distal process from the accessory pigment 
cells. 

In animals kept a sufficient time in the dark, the bodies of the distal 
retinular cells (Figs. 2 and 8, el. dst.) are flattened, and applied to 
the sides of the cones. They measure about 70 w in length and pos- 
sess, in addition to their distal processes, shorter proximal ones, which 
extend backward to the outer ends of the proximal retinular cells. As 
before, the cytoplasm is largely filled with black pigment granules, 
which, however, are often more concentrated in the body of the cell 
than elsewhere. 

It must be obvious from this brief description that in considering the 
photomechanical changes of the distal retinular cells two factors are to 
be kept distinct : first, the lengthening and the shortening of the cell 
body, and, secondly, the distal and the proximal migration of the cell as 


PARKER: RETINAL PIGMENT CELLS OF PALZSMONETES. 285 


awhole. That these two elements are distinct can be seen from the fact 
that in certain insects they are related to each other in a way just the 
reverse of that which occurs in Palemonetes; thus, in Lasiocampa, as 
figured by Exner (’91, Taf. IV. Figs. 28, 29), the distal cells are short 
in their distal position, and dong in their proximal one. 

The average length of the bodies of the distal retinular cells from the 
left eye of a given animal prepared in light was about 30 u. The aver- 
age length of the corresponding cells from the right eye of the same 
animal prepared in the dark was about 70 uw. Ina series of preparations 
taken from animals of approximately the same size as that just described, 
the lengthening of the distal cells in the dark took place at a rate indi- 
cated in the first of the following tables. 

In a second series the shortening of the cells under the influence of 
light was shown to take place as indicated in the second table. 

The distal and the proximal migration of the cells are difficult to define, 
because they are accompanied by the lengthening and the shortening of 
the cells. Taking the nucleus as a fixed point in the cell, the maximum 
distance of migration is about 50 uw. In the migration from the proxi- 
mal to the distal extreme, made in the dark, the cell traverses this 
distance in about two hours. The migration in the reverse direction, 
under the influence of light, is completed in about one hour and three 
quarters. 

As the cells move outward, their distal processes shorten and their 
proximal ones form and lengthen. As they move inward, their distal 
ones elongate and their proximal ones shorten and finally disappear. 
The rates of these changes, as well as of those given in the preceding 
paragraphs, are indicated in the following tables of summaries, in which 
the varying lengths of the parts of the cells are given for successive 
periods. 


Migration in the Distal Direction (in the Dark). 


Time. Ohr. | $hr. | hr. | 3 br.|1 hr.| 1} aa 1} hr. | 1} hr.| 2 hr. 

Length of distal process . | 130!| 125 | 110 | 95 | 85 | 65 | 55 | 40 | 30 

Length of cell body . . . 30 | 385 | 80 | 55} 60] 70 | 70 | 70 | 70 

Length of proximal process 0 0 0| 10) 15} 25 | 35 | 50 | 60 
| 


1 All measurements of length are expressed in mikra (thousandths of a milli- 
meter). 


286 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Migration in the Proximal Direction (in the Light). 


1 hr, | 1} hr.| 13 hr.| 13 hr.| 2 hr. 


Length of distal process . 80 | 100} 115} 125) 130} 180 


Length of cell body . 


Length of proximal process 


The changes induced in the distal retinular cells by the light are 
completed, then, in a period between an hour and a half and an hour 
and three quarters long. The changes that take place in the dark require 
for their completion from an hour and three quarters to two hours. 

Rough estimates of the time necessary for the completion of these 
changes in different arthropods have been made by various investigators. 
Szczawinska (91, p. 552) states that in Astacus the condition charac- 
teristic for the dark is reached in six hours, that for the light in two 
hours. Exner (’91, p. 70) states that in an insect, Lasiocampa, the 
changes require about half an hour, and Kiesel (’94, p. 105) gives the 
same time for Plusia. Herrick (’91, p. 455) believes that in Palemo- 
netes the changes are accomplished in about twenty-five minutes, an 
estimate that I should regard as rather too low. 

Exner (91, p. 70) has suggested that muscle fibres might be con- 
cerned in the migration of the distal retinular cells, an idea that gains 
some support from the fact that in the eyes of some insects structures 
like muscle fibres have been seen and described. In the crustacean 
retina, however, Exner was unable to find anything like muscles. At 
first sight it might seem probable that what I have described as the 
proximal and distal processes of the distal retinular cells might be mus- 
cular in nature. But the facts that the proximal process disappears 
entirely during the proximal migration of the cell, and that the distal 
one seems never to be firmly attached near the periphery of the retina, 
are opposed to this view. Moreover, in the distal process, which, on the 
whole, is the more muscle-like of the two, I have been unable to discover 
any evidence of transverse enlargement in the shortened condition, such 
as a contracted muscle exhibits. The cell in its distal migration seems to 
move over the fibre rather than to be drawn onward by a contraction of 
the fibre. Further evidence against the muscular nature of the motor 
mechanism of these cells is to be found in the rate at which the move- 
ment takes place; 50 uw in two hours is exceptionally slow for the action 


PARKER: RETINAL PIGMENT CELLS OF PALZMONETES. 287 


of any kind of muscle. These observations have led me to conclude 
that muscular action, as ordinarily understood, has nothing to do with 
the migration of the distal retinular cells. Obviously, ciliary action is 
in no way connected with the movements of these cells, and there is left 
then only ameeboid movement as a means of explaining these changes. 
Each distal cell might be compared to an amceba, which in its migrations 
outward and inward uses its processes to guide its general motion. The 
rate and general character of the movement agree well with this expla- 
nation. In one respect, however, there is disagreement. Herrick (’91, 
p- 455), in his account of the action of the distal cells in Paleemonetes, 
states that, on contracting, these cells fold together somewhat as a ribbon 
might be folded transversely to its length (cf. Fig. 10), and he believes 
that, on expanding, they unfold again. This condition is one not easily 
reconciled with amceboid movement. 

Through the kindness of Professor Herrick I have had the privilege 
of studying his preparations, and I can confirm his statement that in the 
contracted condition (Figs. 9 and 10) the cells exhibit a series of trans- 
verse folds, which are entirely absent from the expanded form. These 
folds, however, occur, so far as I am aware, only in eyes which have been 
kept an exceptionally long time in the dark (thirty-eight days in the 
case of Professor Herrick’s specimens), and are then exposed to the light. 
In my own preparations, none of which had been kept in the dark more 
than twelve hours, no trace of such folding could be discovered, and I 
have therefore been led to regard these folds as abnormalities induced 
by protracted retention in the dark. Notwithstanding this interpreta- 
tion of the folds, they throw important light, I believe, upon the normal 
action of the distal retinular cells. 

The exact form of these folds is not so simple as might at first be 
supposed. The body of each cell in its contracted condition consists of 
an elongated thickened axial portion and two lateral wing-like expan- 
sions, each of which terminates in a rather sharp edge. In other 
words, these distal cells, when contracted, instead of assuming the 
usual ovoid form, retain more or less the shape that they had when 
expanded (Fig. 8). In a longitudinal section through the axial portion 
of the body of one of these cells (Fig. 9) slight folds are observable. 
In similar sections through the edge of the lateral wings (Fig. 10) the 
folding is seen to be much more pronounced. The folds are most con- 
spicuous at the edges of the wings, and lose in prominence toward 
the axial part of the cell. Another peculiarity of these folded cells, 
as compared with those kept a shorter time in the dark, is that 


288 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


they shorten only to about five sevenths of their original length in- 
stead of to three sevenths. Thus their long retention in the dark 
seems to have prevented a return to the more completely contracted 
condition normal for the light. The fact that the more peripheral parts 
of the cells in the contracted condition are the more wrinkled indicates 
that the axial part has retained its contractile nature more completely 
than the peripheral parts, and suggests the idea that, since this axis 
contracts in a definite direction, it must possess something of the nature 
of a muscular core. It seems to me probable that, whilst the periphery 
of these cells may be characterized by amceboid movements, the core 
acts in a more circumscribed way, much as a muscle would. If this is 
so, the distal and proximal migrations, as well as the expansion of the 
cell body, are probably manifestations of its amoeboid movements, 
while its shortening is probably due in the main to the muscle-like con- 
traction of its central core. Objection might be raised to this com- 
bination of different modes of motion, were it not generally admitted 
that muscular action is, after all, only a more circumscribed form of 
amoeboid movement. 

The presence of a contractile axis in the distal retinular cells is farther 
rendered probable by the fact that in Mysis (Parker, 791, p. 120) an 
axial core free from pigment has been observed in each distal retinular 
cell. At the time I first noticed these cores I suspected that they might 
be the remains of nervous axes, but I now believe there are stronger 
reasons for suspecting them of being contractile bodies. 

The following table gives by way of summary the periods required 
for the completion of the various photomechanical changes in the retina 
of Paleemonetes. 


| From dark to light. From light to dark. 


Proximal retinular cells . . .. . Zhr.to #hr. 2 hr. to 1 hr. 
Distal'retmular’cells = 2 = 3 = 13 hr. to 13 hr. 12 hr. to 2 hr. 
Accessory pigmentcells . ... - 3 hr. to 1 hr. 13 hr. to 2 hr. 


It is a noteworthy fact, that of these changes those that take place 
in the light (positive stimulus) are always accomplished more rapidly 
than the corresponding reversals in the dark. To this statement an 
apparent exception may be found in the tables on pages 285 and 286, in 
which are recorded the periods for the contraction and expansion of the 


PARKER: RETINAL PIGMENT CELLS OF PALZMONETES. 289 


distal retinular cells. The body of the distal cell contracts in the light 
in an interval between 90 and 105 minutes, and expands in the dark 
in between 60 and 75 minutes, thus apparently accomplishing a change 
more rapidly in the dark than in the light. The expansion and con- 
traction are, however, not simple operations, but are complicated by the 
simultaneous production or absorption of the large proximal processes, 
and it is possible that the discrepancy just pointed out is to be accounted 
for by this complication. 

Before leaving this subject I wish to call attention to the compara- 
tive slowness with which all the photomechanical changes of the retina, 
but particularly those of the distal retinular cells, take place. Exner 
has shown that the amount of effective light that enters the eye is, in 
all probability, largely controlled by the action of the distal cells, and 
has therefore called them the iris pigment. The slowness with which 
they respond, however, shows clearly that in their action they have 
little resemblance to the iris of the vertebrate eye, and that their changes 
correspond only to the more general changes in the amount of light in 
their surroundings. The name iris pigment seems to me, therefore, 
somewhat misleading, and hence I prefer to retain the name of distal 
retinular cells, which indicates at once the present position and the 
probable origin of thesg cells, namely, from cells that once formed a 
part of the retinula itself (Parker, ’95, p. 64). 


SYMPATHETIC PHOTOMECHANICAL CHANGES. 


To ascertain whether the retinas in the two eyes of Palemonetes 
were sympathetic toward each other in the same sense that Engelmann 
believed the retinas in the eyes of vertebrates were, I carried out two 
sets of experiments, in both of which animals were so placed that one 
eye was in the dark while the other was exposed to the light. Aftera 
sufficient period both eyes were prepared and examined. The two sets 
of experiments differed only in that I used different means to accom- 
plish the exposure. In one set I tied a living shrimp to the inside of a 
light-proof box, in which a small hole was made so as to allow one optic 
stalk of the animal to project into the lighted exterior. Care was taken 
that the small space between the optic stalk and the edge of the hole 
should be filled with an opaque material (a mixture of thick Canada 
balsam and lampblack). After several hours the animal was killed, and 
its eyes prepared. In the other set of experiments one optic stalk of a 
living animal was covered with a considerable quantity of the mixture 

VOL. XXx.— NO. 6. 2 


290 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


of balsam and lampblack, and, after allowing the animal to swim in a 
brightly illuminated dish for several hours, it was killed, and both its 
eyes prepared and examined. 

The two sets of experiments yielded essentially the same results, 
namely, the eyes exposed to the light always presented the condition 
normal for the light, and those kept in the dark always showed an ap- 
proach, more or less incomplete, to the condition characteristic for the 
dark. This incompleteness might be taken as evidence of a partial 
sympathetic relation between the two retinas; but I believe it is to be 
explained otherwise. In both sets of experiments the eyes supposed to 
be blinded were in reality only partially cut off from the light. In the 
experiment with the light-proof box, I know by actual observation that 
more or less light made its way through the optic stalk that projected out- 
ward to the exterior, and thus gained access to the interior of the box. 
If this is true of the experiment with the box, it is very probable that in 
the second experiment light passed up through the base of the blinded 
stalk, and thus reached at least the proximal part of the retina. 

These experiments, then, are not wholly conclusive, but, so far as 
they go, indicate considerable independence in the relations of the 
two optic stalks. For reasons to be given later, in connection with the 
experiments on excised stalks, I believe I am justified in concluding that 
the two retinas are, in reality, wholly independent of each other. 


LocaLizED PHOTOMECHANICAL CHANGES. 


Another question that naturally presents itself is, whether different 
parts of the same retina are sympathetic toward one another, or whether 
they are entirely independent, i.e. whether or not a retina responds 
locally to stimulus. 

To test this matter, I put minute drops of the mixture of balsam and 
lampblack on the corneal cuticula of the eyes of several shrimps, and 
let them swim for a few hours in well illuminated basins. On examin- 
ing sections of their eyes later, it was found that under each mass of 
applied pigment the retinal cells showed a condition characteristic for the 
dark. This was most pronounced in the distal retinular cells, but was 
also observable in the proximal retinular cells, as well as in the acces- 
sory pigment cells. This experiment shows beyond a doubt that the 
elements of the retina act locally, and respond to differences of light 
and dark independently of one another. This independence furthermore 
explains what is not infrequently seen in sections of otherwise normal 


PARKER: RETINAL PIGMENT CELLS OF PALHZMONETES. 291 


eyes that have been kept in the dark, namely, occasional single proxi- 
mal retinular cells which, instead of having their pigment granules 
transported to the retinal fibres, still hold them in their bodies. Such 
cells have probably suffered some pathological change by which their 
individual photomechanical functions have been interfered with. This 
independence in the action of parts of the retina has already been 
affirmed by Exner (’91, p. 66) for the compound eyes of insects. 


PHOTOMECHANICAL CHANGES IN ExcisepD Eyres AND RetrINas. 


The extent to which the photomechanical changes in the retina are 
influenced by the central nervous organs has never been determined, | 
believe, for any arthropod. That some such influence is exerted is im- 
plied by several investigators; thus Stefanowska (90, p. 156) states 
that, in preparing insects’ eyes, she cut the heads of the animals in two 
so as to prevent the nervous centres from affecting the retinal pigment 
cells, and Szczawinska (’91, p. 531) recommends as a fixing reagent a 
hot solution of corrosive sublimate, because the action is so rapid that 
it is not necessary to use other means of intercepting the central ner- 
vous influences. This belief, that the central nervous organs can exert 
an influence on the retinal pigment cells, is not to my knowledge the 
result of direct experiment, but is the application to other groups of 
animals of a generalization first made by Engelmann and his followers for 
vertebrates. As is well known, Engelmann showed that, when one eye 
of a frog was protected from the light, the illumination of the other 
eye, or even of a portion of the surface of the body, sufficed to produce 
in the pigment cells of the protected eye a condition characteristic for 
the light. This observation naturally led to the conclusion that the 
pigment cells of the retina were controlled in their movements by the 
central nervous organs, and that the optic nerve transmitted impulses 
centrifugally as well as centripetally. Fick (95, pp. 77 and 81), how- 
ever, has recently demonstrated that the same changes occur in a frog’s 
eye even after the optic nerve and sympathetic nerves have been cut, 
and that therefore the central nervous organs take no part in these 
changes. 

Before turning to the experimental evidence obtained from Palzmone- 
tes, it will be well to consider some of the consequences of this question. 
In order that the central nervous organs should have any influence on 
the retinal pigment cells, the two sets of structures must be in nervous 
connection. So far as is known, the only structures in the retina of 


bo 


92 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY 


Paleemonetes that have nervous connections are the proximal retinular 
cells, the accessory cells and the distal cells not being supplied with 
nerves. Since photomechanical changes occur in both the accessory and 
the distal cells, the inference might be drawn that in these instances the 
changes were necessarily independent of the central nervous organs. 
But it might also be argued that these very changes indicate nervous 
connections that have escaped the eye of the anatomist. To this it 
might be replied that, as the nervous connections of the proximal cells 
are so very obvious, it is highly improbable that the distal and accessory 
cells have a hitherto undescribed nerve supply. So far, then, as the 
purely anatomical relations are concerned, they indicate that the photo- 
mechanical changes, in the accessory and distal cells at least, are inde- 
pendent of central nervous influences. 

In the case of the proximal retinular cells, where each cell possesses a 
single nerve fibre, the central nervous organs might control the pigment 
changes. However, if they do, the retinal fibres afford, so far as I know, 
the first good instance of normal double conduction. Since each retinal 
nerve fibre is the one nervous process from some proximal retinular cell, 
and since all these cells show photomechanical changes, it follows that, 
if these changes are controlled by the central nervous organs, all retinal 
fibres must transmit central impulses peripherad. As these same fibres 
are the only nervous connections between the retina and the central ner- 
vous organs, some at least must also transmit retinal impulses centrad. 
Therefore, if it can be shown that the central nervous organs influence 
the photomechanical changes in the proximal retinular cells, it is like- 
wise demonstrated that double conduction is a natural occurrence. As 
Fick (95, p. 73) justly remarks, the solution of this problem involves 
one of the most fundamental principles concerning the transmission of 
nervous impulses. 

The method by which I proceeded to test this matter in Paleemonetes 
consisted in examining the changes that went on in eyes after their con- 
nection with the central nervous organs had been severed. These con- 
nections were conveniently cut in one of two places; either the whole 
optic stalk was excised, in which case the optic nerve was cut between 
the brain and the optic ganglia, leaving the latter in normal connection 
with the retina, or the retinal end of the stalk was cut off, thus separat- 
ing-the retina in the region of the retinal nerve fibres from the optic 
ganglia as well as from the brain. 

To ascertain whether the brain had any influence over the retinal pig- 
ment, the following experiments were tried. Four live shrimps, whose 


PARKER: RETINAL PIGMENT CELLS OF PALZMONETES. 293 


eyes were in the condition characteristic for the dark, were decapitated ; 
their right optic stalks were cut off and put in a light moist chamber, 
and their left stalks, likewise cut off, were placed, as a check on the 
results of the experiment, in a dark moist chamber. After an interval 
of two hours both sets of stalks were hardened and afterwards cut and 
examined. Ina corresponding way optic stalks in the condition charac- 
teristic for the light were cut off and subjected to the dark. 
The results of these experiments are shown in the following tables. 


Four Right Optie Stalks in Dark Condition cut off and placed in the 
Light two Hours. 


Complete change. Partial change. No change. 


Proximal retinular cells . . . 0 3 1 
Distal retinular cells . . . . 0 3 1 
Accessory cells . 0 2 2 


The four left optic stalks cut from the same animals and retained, as 
a check, in a dark chamber, all presented on examination the condition 
typical for the dark. 


Four Right Optie Stalks in Light Condition cut off and placed in the 
Dark two Hours. 


Complete change. Partial change. No change. 


Proximal retinular cells . 


Distal retinular cells . 


Accessory cells . 


The four left optic stalks cut from the same animals and retained, as 
a check, in a light chamber, all presented on examination the condition 
typical for the light. 

It is obvious from these observations that, after the excision of an 
optic stalk, the photomechanical changes may still take place, if not com- 
pletely, at least partially, and it might be inferred from this that the 
brain exerted at least a partial influence over these changes. This con- 
clusion, however, is invalidated by the fact that in one case recorded in the 


294. BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


second table the photomechanical changes were carried out completely, thus 
demonstrating that the brain is not in any way essential to these changes. 
Why this completeness was not seen in other cases I am unable to state 
positively, though I believe it was owing to the changes that gradually 
appear in the tissue of the stalk after its severance from the body of the 
animal. When an optic stalk is excised, the blood in it soon coagulates 
and other alterations doubtless start up, which finally result in the com- 
plete death of the tissues of the stalk. It is these alterations, I believe, 
that overtake and bring to a standstill the slowly progressing photome- 
chanical movements. But, whatever may be the true explanation of the 
incompleteness of the changes in excised stalks, the general conclusion 
remains unaffected, that in Paleemonetes the brain is not essential to the 
photomechanical changes in the retina. 

This conclusion has an important bearing on the question of the sym- 
pathetic relations of the two retinas in a given animal. Since the two 
retinas are nervously connected only through the brain, and since the 
retinas are not influenced from the brain, it follows that the two retinas 
cannot be sympathetically related, a conclusion to which observations 
already recorded have likewise pointed. 

If the photomechanical changes are not dependent in any degree on 
the brain, it may still be asked whether they are not influenced by the 
optic ganglia. To answer this question, I carried out on excised retinas 
a series of experiments similar to those just described for the optic 
stalks. It is much more difficult to separate the retina from the optic 
ganglia than it is to separate the optic stalk from the brain, but with 
careful manipulation it can be done, and the following tables give the 
results of experiments carried out upon such retinas. 


Four Right Retinas in Dark Condition cut off and placed in the 
Light about two Hours. 


Complete change. Partial change. No change. 
Proximal retinular cells . . . 0 4 0 
Distal retinular cells . . . . 0 3 1 
Accessory cells!) = 27.0.) 0 4 0 


The four left retinas kept in the dark as checks on the experiment 
exhibited the normal condition for the dark. 


PARKER: RETINAL PIGMENT CELLS OF PALZMONETES. 295 


Four Right Retinas in Light Condition cut off and placed in the 
Dark about two Hours. 


Complete change. Partial change. No change. 


Proximal retinular cells . . . 0 2 2 
Distal retinular cells . . . . 0 4 0 
Accessory cells) 7 0. 0 3 1 


The four right retinas kept in the light as checks on the experiment 
exhibited the normal condition for the light. 

Although, as the tables show, no case of excised retina with complete 
photomechanical changes has been observed, several of the cases were 
so nearly complete that I have no hesitancy in stating that, in my 
opinion, the photomechanical changes in the retina are as little influ- 
enced by the optic ganglia as by the brain. 

These experiments, then, lead to two conclusions: first, the brain of 
Palzmonetes is not essential to the complete photomechanical changes 
of the retinal pigment cells; and, secondly, the optic ganglia are like- 
wise unessential to these changes. In the latter case, however, the pos- 
sibility of a slight influence must be admitted. The photomechanical 
changes of the retinal pigment cells are, in my opinion, induced by the 
direct influence of the presence or absence of light on these cells. Each 
cell, then, so far as its mode of action is concerned, is not comparable 
to a muscle controlled by an efferent nerve, but to a more or less inde- 
pendent organism, which receives a direct stimulus from the exterior, 
and responds appropriately. The uniformity usually shown by the 
photomechanical movements in the retina as a whole is to be under- 
stood as an individual but uniform reaction of many separate elements 
to a uniform stimulus. There is nothing in the action of the retinal 
pigment cells of Palemonetes that supports the idea of normal double 
conduction of nervous impulses. 


GENERAL SUMMARY. 
1. The only parts of the retina in Palemonetes that exhibit photo- 


mechanical changes are the three kinds of pigment cells. 


2. The proximal retinular cells contain black pigment granules. Jn 
the light these are scattered more or less uniformly throughout the whole 


296 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


length of the cell, including the retinal nerve fibre. There are slight 
concentrations of pigment at the distal end of the cell and around the 
rhabdome. Jn the dark the pigment is limited to the retinal nerve 
fibres. 

3. The change from the dark condition to the light one is accom- 
plished in from 30 to 45 minutes. The reverse change requires 45 to 
60 minutes. 

4, These changes are probably due to internal protoplasmic move- 
ments, by which the pigment granules in the cells are moved in one 
or other direction. 


5. The accesssory pigment cells contain a yellowish white pigment. 
In the light this is massed partly in the base of the retina, and partly 
near the distal surface of the first optic ganglion. The two pigment 
masses are connected by pigmented strands. Jn the dark the pigment is 
almost entirely in the base of the retina. 

6. The change from the dark condition to the light one is accom- 
plished in from 45 to 60 minutes; the reverse change, in from 105 to 
120 minutes. 

7. These changes are probably produced by amaboid movements of 
the cells. 


8. The distal retinular cells contain black pigment granules. Jn the 
light they are contracted, and occupy a proximal position in the retina 
surrounding the axis of the ommatidium near the outer ends of the 
proximal retinular cells. Jn the dark they are expanded (flattened), 
and occupy a distal position in the retina, surrounding more or less 
completely the sides of the cone. 

9. The change from the dark condition to the light one is accom- 
plished in from 90 to 105 minutes; the reverse change requires from 
105 to 120 minutes. 

10. These changes are produced in part by an amceboid movement of 
the cell, and probably in part by a muscle-like contraction of its axial 
portion. 


11. Each set of photomechanical changes carried out in the light is 
completed in less time than the corresponding set of reverse changes 
carried out in the dark. 

12. The photomechanical condition of the retina in one eye has no 
effect upon that in the other eye; i. e. the retinas are not sympathetic. 


PARKER: RETINAL PIGMENT CELLS OF PALZMONETES. 297 


13. The photomechanical action within the retina is localized, small 
groups of pigment cells responding to local stimulation. 

14. In excised eyes (optic nerve cut), complete photomechanical 
changes may occur, thus proving that the brain is not essential to these 
changes. 

15. In excised retinas (retinal nerve fibres cut), nearly complete 
photomechanical changes may occur, thus showing that the optic 
ganglia are probably not essential to these changes. 

16, The incompleteness of the changes in either the excised eyes or 
excised retinas is probably due to the death of the retinal tissues before 
the photomechanical changes have been completed. 

17. The three kinds of retinal pigment cells probably respond to 
direct stimulation from without, and are not influenced by nervous im- 
pulses from within. There is no good evidence in favor of normal double 
conduction of nervous impulses. 


Norte. 


Since the preceding pages were written, Rosenstadt’s (96) paper on 
the structure of the compound eyes in Decapods has been published. 
This contains a brief general account of the migration of the retinal 
pigment in these crustaceans, and calls for a word of comment. In his 
description of the directions of motion shown by the pigment under 
various conditions of light, Rosenstadt agrees with Exner and later 
investigators, but in his account of how this movement is accomplished 
he stands entirely alone. His conception of the process can best be put 
in his own words (Rosenstadt, ’96, p. 759): “ Beim Uebergange des 
Lichtauges in ein Dunkelauge gehen mit dem Pigmente volgende Verin- 
derungen vor sich: Das Pigment tritt aus dem vorderen Ende der 
Retinulazellen [= proximal retinular cells] und wohl auch aus den 
Retinapigmentzellen [= rudimentary retinular cells] aus. Dasselbe 
wird von den Fortsitzen der Irispigmentzellen [= distal retinular cells ] 
aufgenommen, die, wie wir gesehen haben, mit dem im Vorderende der 
Retinulae angesammelten Pigmente im Contact stehen. An diesen 
Fortsitzen kriecht nun das Pigment hinauf; es findet eine Art Pig- 
mentinfiltration der Irispigmentzellen statt. Gleichzeitig wandert das 
Pigment nach hinten zu aus den Retinulazellen aus und gelangt hinter 
die Membrana fenestrata [== basement membrane], wo es von den mit 
Auslaufern versehenen Zellen aufgenommen wird.” This idea that the 


298 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


pigment migrates from one cell to another is, so far as Rosenstadt’s ac- 
count goes, entirely unsupported by direct evidence, and seems to me 
an unwarranted assumption. The proximal movement of the pigment 
from the distal end of the retinula to the opposite side of the basement 
membrane is certainly accomplished within the limits of one set of cells, 
for, as I have shown in this paper, the pigment even when entirely 
proximal to the basement membrane lies in the thick retinal nerve 
fibres, which are merely processes from the proximal retinular cells. 
Although it cannot be stated with certainty that there is no exchange of 
pigment between the distal and the proximal retinular cells in Pale- 
monetes, for in this crustacean in bright light these two kinds of cells 
are closely applied to each other, it is perfectly certain that in other 
decapods, as for instance Paleemon, no such exchange is possible ; for, 
as Exner (91, Taf. V. Fig. 51) has shown, and I can confirm his obser- 
vations, the pigmented parts of the distal and the proximal retinular 
cells never touch, even under full light. These reasons, together with 
the facts set down in the present paper, confirm me in the belief that 
Rosenstadt’s explanation of the migration of the pigment is erroneous, 
and that the one presented in the foregoing account is correct. 


PARKER: RETINAL PIGMENT CELLS OF PALZMONETES. 299 


PAPERS CITED. 


Ballowitz, E. 
93. Die Nervenendigungen der Pigmentzellen. Zeit. wiss. Zool., Bd. LVI. 


pp- 671-706, Taf. XXXV.-XXXIX. 
Ballowitz, E. 
93. Ueber die Bewegungserscheinungen der Pigmentzellen. Biol. Centralb., 
Bd. XIII. pp. 625-630. 


Exner, S. 
’89. Durch Licht bedingte Verschiebungen des Pigmentes im Insectenauge 
und deren physiologische Bedeutung. Sitzungsb. Acad. Wiss. math.- 
naturw. Cl., Bd. XCVIII. Abt. ILI. pp. 143-151, 1 Taf. 


Exner, S. 
91. Die Physiologie der facettriten Augen von Krebsen und Insecten. 


Deuticke, Leipzig und Wien. vi-+ 206 pp., 7 Taf. 
Fick, E. A. 

'95. Ueber die Frage, ob zwischen den Netzhauten eines Augenpaares ein 
sympathischer Zusammenhang besteht. Vierteljahrsschr. naturf. Gesell. 
Zurich, Jahrg. XL. pp. 71-83. 

Grenacher, H. 

’86. Abhandlungen zur vergleichenden Anatomie des Auges. I. Die Retina 
der Cephalopoden. Abhandl. naturf. Gesell. Halle, Bd. XVI. pp. 207- 
256, 1 Taf. - 

Herrick, F. H. 

91. Alpheus: A Study in the Development of Crustacea. Memoirs Nat. 

Acad. Sci., Vol. V. pp. 370-461, 38 Pls. 


Kiesel, A. 
94. Untersuchungen zur Physiologie des facettirten Auges. Sitzungsb. 


Acad. Wiss. math.-naturw. Cl., Bd. CII. Abt. III. pp. 97-1389, 1 Taf. 


Parker, G: H. 
90. The Histology and Development of the Eye in the Lobster. Bull. Mus. 


Comp. Zod]. Harvard Coll., Vol. XX. pp. 1-60, Pls. L-IV. 


300 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Parker, G. H. 
‘91. The Compound Eyes in Crustaceans. Bull. Mus. Comp. Zodl. Harvard 
Coll., Vol. XXI. pp. 45-140, Pls. 1-X. 


Parker, G. H. 
95. The Retina and Optic Ganglia in Decapods, especially in Astacus. Mitt. 
Zool. Stat. Neapel, Bd. XII. pp. 1-73, Taf. 1-3. 


Parker, G. H. 
96. Pigment Migration in the Eyes of Palemonetes. A Preliminary Notice. 
Zool. Anzeiger, Jahrg. XIX. pp. 281-284. 


Rosenstadt, B. 
96. Beitriige zur Kenntniss des Baues der zusammengesetzten Augen bei den 
Dekapoden. Arch. mikr. Anat., Bd. XLVII. pp. 748-770, Taf. XXXIX., 
XXXX. 


Stefanowska, M. 
90. La Disposition histologique du Pigment dans les Yeux des Arthropodes 
sous ’Influence de la Lumiére directe et de l’Obscurité complete. Recueil 
Zool. Suisse, Tome V. pp. 151-200, Pls. VIII., IX. 


Szczawinska, W. 

’91. Contribution a l’étude des yeux de quelques Crustacés et Recherches ex- 
périmentales sur les mouvements du pigment granuleux et des cellules 
pigmentaires sous l’influence de la lumiére et de l’obscurité dans les yeux 
des Crustacés et des Arachnides. Arch. de Biol., Tome X. pp. 523-566, 
PIS; SCV. Se Vit, 


el 


PARKER. — Retinal Pigment. 


EXPLANATION OF PLATE. 


All the figures were taken from he aid A of the eyes of Palcemonetes vul- 


garis Stimp. They were drawn with the aid o 


n Abbé camera, and are all mag- 


nified 335 diameters. 


cl. con 
cl. dst. 
cl. px. 
el. sn. 
con. 
crn. 
Fig. 1 
Fig. 2 
Fig. 3 
Fig. 4 
Fig. 5 
Fig. 6 
Fig. 7 
Fig. 8 
Fig. 9 
Fig. 10. 


ABBREVIATIONS. 
Cone cell. fbr.r. Retinal nerve fibre. 
Distal retinular cell. mb. ba. Basement membrane. 
Proximal retinular cell. nl. con. Nucleus of cone cell. 
Accessory pigment cell. | nl. ern. Nucleus of corneal hypodermis cell. 
Cone. nl. px. Nucleus of proximal retinular cell. 
Corneal cuticula. rhb. Rhabdome. 


Longitudinal section of an ommatidium, showing the arrangement of 
pigment characteristic for the light. 

Longitudinal section of an ommatidium, showing the arrangement of 
pigment characteristic for the dark. 

Transverse section of a cone from an ommatidium, such as is shown in 
Fig. 2 (dark). 

Transverse section through the proximal processes of the distal retinular 
cells in an ommatidium such as that shown in Fig. 2 (dark). 

Transverse section through the distal retinular cells of an ommatidium 
such as that shown in Fig. 1 (light). 

Transverse section through the retinula (rhabdome and proximal retinular 
cells) of an ommatidium like that shown in Fig. 1 (light). 

Transverse section through two groups of retinal nerve fibres. 

Lateral view of a cone with one of its two distal retinular cells still 
attached. The distal retinular cell shows the condition characteristic 
for the dark. The preparation was isolated from a retina macerated 
in Miiller’s fluid. 

Longitudinal section through the bodies of two distal retinular cells, 
which show slight foldings accompanying their shortening. The 
preparation was made from an animal which had been kept in the 
dark thirty-eight days and then exposed to light for four hours and 
three quarters. The figure was drawn from preparations made by 
Professor F. H. Herrick, who kindly granted the author the privilege 
of studying them and making drawings from them. 

Longitudinal section through the edges of two distal retinular cells (see 
p. 287), from the same set of sections as that from which Fig. 9 was 
drawn. 


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LO. 


BULLETIN 


OF THE 


MUSEUM OF COMPARATIVE ZOOLOGY 


AT 


HARVARD COLLEGE, IN CAMBRIDGE. 


VOL. XXXI. 


CAMBRIDGE, MASS., U.S. A. 
1897 -1898. 


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University Press: 
Joun Witson anv Son, Campripce, U.S.A. 


CONTENTS: 


No. 1.— Contributions from the Zodlogical Laboratory. LXXXIII. Contri- 
butions to the MorpHotoey of the TurBettarta. II. On some TuRBEL- 
LARIA from Illinois. By W. McM. Woopwortu. (1 Plate.) October, 1897 

No. 2.— On the Relations of certain PLates in the DintcutTHyips, with De- 
scriptions of New Species. By C. R. Eastman. (5 Plates.) October, 1897 

No. 3.— Contributions from the Zodlogical Laboratory. LXXXIV. TricHo- 
NympuHA, and other Parasites of Termes Fravipres. By J. F. Porter. 
(6 Plates.) October, 1897 . 

No. 4.— Contributions from the Zodlogical Laboratory. LXXXV. Varia- 
tions in the Bracur1aL and Lumpro-Sacrat PLexi of Necturus Mactto- 
sus RaFinesquE. By F.C. Waite. (2 Plates.) November, 1897 

No. 5.— Reports on the Dredging Operations in the ‘“ Albatross” in 1891. 
XXII. The Isopopa. By H.J. Hansen. (6 Plates and Chart.) Decem- 
Team, LR S65, S's! “a 2 

No. 6.— Contributions from the Zodélogical Laboratory. LXXXVII. The 
Thoracic Derivatives in the PostcarpINaL VEINS in Swine. By G. H. 
ParRKeER and C. H. Tozier. March, 1898 

No. 7.— Contributions from the Zodlogical Laboratory. LXXXIX. The 
SEGMENTATION of the Nervous System in Squatus AcanTHIAs. A Con- 
tribution to the MorrHotoey of the VERTEBRATE Heap. By H. V. Neat. 
(2) Eley et5)) IVE ae os rr 


PAGE 


17 


45 


69 


93 


145 


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No. 1. — Contributions to the Morphology of the Turbellaria. 


Il. 


On some Turbellaria from Illinois." By W. McM. Woopwortu. 


Tue material upon which this paper is based was collected by the 
staff of the Biological Experiment Station of the Illinois State Labora- 
tory of Natural History at Havana, Illinois, and came from the Ilinois 
River and the streams and lakes tributary to it.” The material was 
collected between the first of July and the last of October, 1894, and 
the first of March and the last of September, 1895. I am indebted to 
Prof. S. A. Forbes, Director of the Illinois State Laboratory, for the 
privilege of studying the collections, and am under obligations to Dr. 
C. A. Kofoid, Superintendent of the Experiment Station, and to Prof. 
Frank Smith for their patience and kindness in answering my many 
inquiries, and for sending me living material from time to time. I must 
thank these gentlemen also for the excellent state of preservation of all 
the material that came into my hands. 

Although I have to report but two species that are probably new, it is 
hoped that these pages will help to clear away some of the confusion 
that exists in the published accounts of the North American fresh-water 
Turbellaria. Although the figures accompanying this paper do not, with 
two exceptions, represent new forms, they are, I believe, the first pub- 
lished illustrations, in the natural colors of the living animals, of some 
of our common fresh-water Turbellaria. é 

I have not entered into the finer anatomy of the species to any extent, 
as that would have been beyond the scope of the paper. Unfortunately, 
too, not all of the species were represented by sexually mature indi- 
viduals, and as the characters of the sexual organs are of the highest 
systematic importance, the unavoidable lack of data upon this subject 
must necessarily leave my work more or less incomplete. It is to be 


1 Contributions from the Zodlogical Laboratory of the Museum of Comparative 
Zodlogy at Harvard College, E. L. Mark, Director, No. LX XXIII. 

2 For a description of the localities under investigation by the Experiment 
Station, see C. A. Hart, On the Entomology of the Illinois River and adjacent 
Waters. Bull. Ill. State Lab. Nat. Hist., Vol. IV. p. 150, 1895. 

VOL. XXXI.—No. 1. 1 


2 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


regretted also that living material could not be had of all the species 
treated of, as descriptions based solely upon alcoholic material are, to 
say the least, unsatisfactory, owing especially to the effect of reagents 
upon the pigments. 

The collections comprise five (/) species of Triclads and two Rhab- 
doceels. Three of these I hope to show are cosmopolitan in their 
distribution. 


Dendroccelum lacteum Orrstep. 
Figures 1, 9-15. 


Dendrocelum lacteum OERSTED, 1844, p. 52. Woopwortu, 1896°, p. 1048. 

Dendrocelum superbum G1RaRD, 1851, p. 265; 18518, p. 2. 

Dendrocelum superbum Leipy (non Girard), 1852, p. 288. 

Dendrocelum pulcherrimum G1RaRD, 1851, p. 265; 18514, p. 2. 

Procotyla fluviatilis Stimpson (Leidy MS.), 1857, p. 23. Dresine, 1862, p. 517. 
Lerpy, 1885, p. 51. Hatiez, 1890, p. 105. Girarp, 1893, p. 164. Woop- 
worTH, 1896, p. 95; 18962, p. 241.1 

Procotyla leidyit GiRARD, 1898, p. 166. 


Station C; 13,166, “small pool on sandy margin of Thompson’s Lake near 
Station G.” 

Color, milk white; creamy, yellowish, or, in larger, older specimens, roseate ; 
no pigment except in eye-spots. Very translucent ; intestine in all its ramifi- 
cations easily seen, grayish to brown or black according to the character of the 
contents, more deeply colored in larger specimens. A slight constriction 
immediately behind the plane of the eyes, marking off an anterior head end and 
producing the lateral projecting rounded cephalic appendages. Gradually 
widening posteriorly from constricted or neck region to a point about one 
sixth the total length from the anterior end, then margins are parallel to about 
one fifth the distance from the posterior end, tapering gradually from this point 
to the rounded posterior extremity. When in active motion and fully extended, 
the lateral margins are smooth and nearly parallel; but when partly contracted 
or at rest, the margins are very sinuous or crenate and thrown into folds, like 
the margins of many marine Planarians. A well marked median adhesive 
disk or sucker at the anterior end, the diameter of which is about one third the 
broadest diameter of the head. When the animal is in motion the shape of 
the anterior end is continually changing, owing to the protrusive and retract- 
ive movements of the disk, which can be protruded for a considerable distance, 
though projecting only slightly when in the retracted condition. Eyes usually 
two, but frequently accessory eye-spots from one to six in number. Distance 
between the eyes a little greater than from eyes to margin of head. Mouth 
opening (in preserved material) slightly posterior to a point midway between 


1 By a gross error I have in the above paper also referred to Keller und Tiede- 
mann’s Nordamerik. Monatsber. for 1851. 


aa a 


WOODWORTH: ILLINOIS TURBELLARIA. 3 


anterior and posterior ends. Gonopore (in preserved material) two sevenths of 
the total length from the posterior end. Length from 10 to 22 mm.; greatest 
breadth from 2 to 3 mm. 

Dendrocelum lactewm, which is one of the commonest and most widely dis- 
tributed of European fresh-water Planarians, has often been the object of study, 
and its structure is better known, perhaps, than that of any other Triclad. 
Chief among the papers dealing with this species is that of Iijima (1884) ; our 
American form agrees so closely with his account that a discussion of the finer 
anatomy of the species wil! not be entered into here, except in so far as regards 
the anterior adhesive disk, and some points in connection with the male sexual 
organs ; for other details the reader is referred to Iijima. 

It is remarkable that lijima should have overlooked the adhesive disk of 
D. lactewm. Though he states (p. 362) that he did not find the organ, two of 
his Figures (Taf. XXII. Figs. 9 and 10) are very suggestive, and coincide 
with similar preparations of my own. The organ was seen by von Baer (1827, 
p. 715), who describes it asa “kleine Pupille” ; Dugés (1828, p. 150) also 
speaks of it as “un renflement qui peut aisément se creuser en cupule, en ven- 
touse semblable a celle de la queue des sangsues et la face inférieure des 
Douves,” and he shows it in his Fig. 7, Pl. 4. Leydig (1864) figures it (Taf. 
I. Fig. 2), and in his account of its structure mentions the absence of rhabditi 
and cilia in the cells lining the depression. The organ is characteristic of the 
genus Dendrocelum, and occurs in every other species of the genus whose struc- 
ture has been investigated. The failure of lijima to find the organ is explained 
by Weltner (1887, p. 800) as being due to the great variability of the organ 
itself in the same individual, depending upon different phases of contraction 
and expansion, it being at times difficult to recognize it. The variability in 
the shape of the disk was also referred to by Leydig (Joc. cit.), and Girard (1893, 
Pl. 4) figures many phases of its activity. I also can testify to the great 
mobility of the adhesive organ, and to its varying prominence in the same and 
in different individuals. 

As a rule, it is more prominent in the largest, oldest individuals; in those 
10mm. and under in length, I have often had considerable difficulty in 
recognizing it even in sections, there being nothing to replace it but a shallow 
groove. Figures 13-15 are from sections of an individual of the largest size, and 
represent the organ in an exceptionally prominent condition. The organ can- 
not be compared to the sucker of cotyligerous Turbellaria, or to the muscular 
sucking disks of other Platyhelminths, nor to that of the leech; it lacks the 
special musculature often so elaborately developed in these. It is simply a 
depression at the anterior end of the animal into which open the numerous 
mucous glands which in most Triclads are found in this region. It is compara- 
ble, rather, to the frontal organ of the Accela, and more particularly to the 
organ existing at the anterior end of Mesostoma lingua, which has been de- 
scribed and figured by Graff (1882, p. 288). In this species the pit is formed 
by the inversion of that part of the anterior end where the two great tracts of 
rhabditi (“Stabchenstrassen”) open to the exterior, and according to Graff is 


4 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


not permanent or constant. I have elsewhere (1891, p. 23) expressed my 
belief in the homology of the “ Stiibchenstrassen” of the Rhabdoccels with the 
cephalic slime glands of Triclads, and a comparison of the sagittal section of 
M. lingua (Graff, 1882, Taf. VI. Fig. 3) with Figure 15 of this paper is strikingly 
suggestive in this regard. Kennel (1888, p. 455) has also shown that in many 
fresh-water Triclads there appears in preserved material a more or less shallow 
groove, or depression, on the ventral surface at the anterior end corresponding 
to aregion by which the living animals are able to attach themselves. It is 
the region where the cephalic slime glands open to the exterior. He suggests 
for the ridges at the sides of these ventral depressions the names “ Haftwul- 
sten” and “ Haftlappen.” Grube (1872) hag also described similar pits or 
grooves in several fresh-water Planarians. 

The organ in D. lactewm is not a true sucker, nor does the animal employ its 
anterior end for the purposes of attachment to any greater degree than the 
posterior or lateral margins of its body, along the ventral surface of which 
numerous mucous glands have their openings. In truth, it is the margins and 
posterior end that adhere more firmly to a support ; often when the animal is 
forcibly removed from the side of the aquarium the parts of the margin or the 
posterior end will adhere so firmly to the glass that the points of attachment 
are drawn out into digitate processes. Figures 13 and 14 are from transverse 
sections through the adhesive pit of D. lactewm, Figure 13 being through the 
almost extreme anterior part, while Figure 14is somewhat more posterior. Both 
sections are from the same individual, — one in which the organ was unusually 
well developed. Figure 15 represents a sagittal section through another indi- 
vidual, and Figure 9 is a surface drawing of an individual killed in hot corro- 
sive sublimate. It will be seen that the region involved in the organ embraces 
all that portion of the hypodermis occupied by the openings of the mucous 
glands. As already shown by Leydig (1864), the hypodermis which lines the 
depression is devoid of cilia and rhabditi, the latter being replaced by the 
mucous glands, and the transition from one to the other being gradual. The 
exact histological character of the hypodermis lining the depression could not 
be ascertained, the terminal ducts of the glands being so closely compacted as 
to mask all details in this region. In Figures 14 and 15 are seen cross and 
longitudinal sections of the retractor muscle of the organ. Nothing in the 
shape of a protractor muscle could be discovered, this function possibly being 
assumed by the circular muscles of the dermo-muscular sac. 

The variation in the number of the eye-spots is not a feature peculiar to the 
American form of D. lactewm, as I stated in a previous note (1896°, p. 1048), 
for accessory eyes, “ Nebenaugen,’ have been observed in this species by Car- 
ritre (1882, p. 164) and lijima (1884, p. 488), according to whom they are not 
rare. According to my observations there is in such cases usually one pair of 
eye-spots, corresponding to the normal single pair, which is more prominent 
than the accessory eye-spots. In what appears to bea closely allied form, Soro- 
celis gutata Grube (1872), there exist two series of eye-spots arranged in the 
form of two arcs ; the number of eyes in each are is usually seven, but there 


WOODWORTH: ILLINOIS TURBELLARIA. 5 


may be as few as two, nor is the number always the same on both sides. I 
have never observed more than a total of eight eye-spots in D. lactewm, there 
being from one to six accessory eyes, either equally or unequally distributed on 
the two sides of the head. Girard (1893, p. 165) describes and figures varia- 
tions in the number of the eyes, and also shows the accessory eyes to be smaller 
than the pair of normal eye-spots. Leidy (1885, p. 50) also speaks of the 
accessory eyes. Of the many Illinois specimens of D. lactewm that have come 
under my observation, 33 per cent of the individuals exhibited variations in 
the number of the eyes. 

Anastomoses in the branches of the intestine exist in most of the specimens 
examined ; these are usually confined to the branches of the posterior trunks, 
which are often united by transverse commissures. In one case there were three 
such transverse commissures. Girard (1893, Pl. 4) figures an individual in 
which the posterior trunks can no longer be recognized as such, the digestive 
tract existing as a meshwork or reticulum. Leidy (1885, p. 50) and Lijima 
(1884, p. 390) also mention anastomoses, and the latter figures a commissure 
uniting the posterior trunks, while Wheeler (1894, p. 176), in a species 
which he believed to be D. lactewm, failed to detect any such connections. 
Hallez (1892) also figures such connections for this species. 

I have little to add to the account of the sexual organs given by Iijima, 
but do not find the penis to be so nearly spherical in shape as that figured by 
him. The shape of the male organ is more like that figured for D. lactewm 
by Schmidt (1861, Taf. 1V.), longer and more cylindrical. Nor isthe cavity of 
the penis so large as that figured by lijima, the muscular walls being much 
thicker (Figs. 10-12). The cavity of the penis is lined with a glandular epi- 
thelium, which projects into the lumen of the organ in folds, thus producing a 
large secreting surface. It is possible, of course, that the cavity varies at 
different periods of sexual activity. In one important particular only do my 
observations on the sexual organs differ from those of Iijima. According to 
that author the vasa deferentia open separately and directly into the cavity of 
the penis and at considerable distance from each other. My observations do 
not confirm the existence of such conditions, but show that the vasa defer- 
eutia unite into a slender ductus ejaculatorius, which extends in the longi- 
tudinal muscles of the organ along its ventral surface to that point where 
the penis begins to taper off to form the slender distal free intromittent part of 
the organ (Fig. 10). At this point, which may be designated as the root of 
the penis proper, and corresponds with the posterior limit of the glandular 
cavity, the duct becomes confluent with the cavity of the penis. In other 
words, the glandular cavity of the penis may be said to pour its secretions at 
this point into the seminal duct, and the greater mass of the penis can be com- 
pared to a prostate gland, the Kérnerdriise of Polyclads. In transverse sec- 
tion it is often difficult to follow the course of the duct, owing to its small size 
and to the fact that the lumen is often obliterated by the approximation of its 
walls, and frequently lies in one of the glandular folds projecting into the cavity 
of the organ (Fig. 12). The sheath of the penis in its deeper portions is thrown 


6 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


into many folds to provide a greater surface for the glands that secrete the shell 
of the cocoon (Fig. 11). These glands are spoken of by Iijima, but are not 
figured by him. 

In a recent paper I (1896*, p. 1048) have discussed the synonymy of Leidy’s 
Procotyla fluviatilis and expressed my belief in the identity of this species with 
Dendrocelum lactewm ; upon the evidence presented above, I wish to emphasize 
my conviction in this regard. The apparent differences between the forms are 
in the American form the possible greater prominence of the frontal adhesive 
organ, the more frequent variation in the number of the eyes, and the separation 
of the seminal duct from the glandular part of the male sexual organ. The 
first two of these differences, as | have endeavored to show, are differences in 
degree only, and have no systematic value. The discrepancy between my 
account of the male sexual organs and that of Iijima alone stands as a real 
difference. Although I can find no account of anything similar in other Tri- 
clads, there are conditions comparable to these in the male organs of Polyclads 
and Rhabdoceels (Lang, 1884, and Graff, 1882), and it is possible that this 
detail has hitherto been overlooked in Triclads. 

Neither have I any hesitancy in placing under D. lactewm Girard’s D. pul- 
cherrimum and D. superbum. The former differs, he says (1851, p. 265), from 
D. lactewm “by having three pairs of eyes instead of two,” while his D. su- 
perbum (18518, p. 2) “gleicht vielleicht noch mehr als die vorgehende Art 
[D. pulcherrimum] den D. lacteum, wovon es sich unterscheidet durch kleinere 
diinnere Gestalt, zwei Paar Augen, vorstehendere Horner und rothe oder milch 
weiss Farbe.” It is significant, too, that Leidy (1852°, p. 288) should have first 
ascribed his Procotyla fluviatilis to Girard’s D. superbum (see Woodworth, 1896°, 
p- 1048), and that Girard, in his recent extended monograph of North American 
Turbellaria, makes no mention of these two old species of his. 


Planaria gonocephala Ducts. 
Figure 5. 


Planaria gonocephala Ducks, 1830, p. 83. 

Planaria gonocephaloides Stimpson, 1857, p. 23. D1Es1ne, 1862, p. 498.  SrL.xI- 
MAN, 1885, p. 69. 

Dugesia gonocephaloides GIRARD, 1851, p. 265; 1851*, p. 2; 1891, p. 183. 


Sides parallel, tapering gradually posteriorly to a rounded point. Anterior 
end obtusely pointed, angular, the sides of the head making an angle of about 
60°. Two angular cephalic appendages. In alcoholic material the auricles are 
not prominent, scarcely showing at all in some specimens. Length of head 
about 31; of the total length of the animal. Eyes two, in a plane joining the 
apices of the auricles. Clear area surrounding the eyes sometimes elongated 
in the antero-posterior direction. Color, dark reddish brown to grayish brown, 
uniform. Posterior margins of auricular appendages free from pigment. 
Length 9 to 15mm., greatest breadth 4 to2 mm. (Two alcoholic specimens, 
which must have measured 20 mm. or more in length when alive.) 


WOODWORTH: ILLINOIS TURBELLARIA. i 


11,6646, Station C; 13,069, Station C. 

Only one of the specimens exhibited sexual organs. There is no copulatory 
bursa and the oviducts open separately into the vagina immediately before it 
enters the genital atrium. The species in every way agrees with descriptions 
of the European form. There can be no doubt that it is the species described 
by Girard as Dugesia gonocephaloides. According to this author the latter differs 
from P. gonocephala only in the elongated form of the clear areas surrounding 
the eye-spots, and upon this meagre difference is founded the genus which 
Girard afterwards (1891) extended to include all forms bearing angular cephalic 
appendages. Stimpson and Diesing retained the specific name, but placed it 
under the genus Planaria, a fact that was apparently unknown to Silliman, who 
renamed it Planaria gonocephaloides. Hallez (1890, p. 78) has discussed the 
value of Girard’s genus Dugesia, and places D. gonocephala as a synonym of 
P. gonocephala, in which species the elongated shape of the periocular areas is 
not uncommon. P. gonocephala has been shown by [ijima (1890, p. 338) to be 
cosmopolitan in its distribution, as it was found by him in Japan. 

The color of the American representatives of P. gonocephala differs from that 
of the European in being of a deeper hue. The European forms vary from a 
gray toa brownish green, while the Illinois specimens are of a deep brown, 
which is well reproduced in Figure 5. Some specimens from France that I 
have received through the kindness of Professor Hallez, of Lille, are almost 
white in the alcoholic condition. Girard (1893, p. 183) USGais the color of 
Dugesia gonocephaloides as often being a blackish brown. 


Planaria dorotocephala, sp. nov. 
Figures 4, 7. 


Planaria maculata var. a GIRARD, 1893, p. 182. 


Sides parallel, tapering gradually to a point posteriorly. Anterior end large, 
sharply pointed, the sides of the head making an angle of about 45° with each 
other. Two long sharply pointed, very prominent auricular appendages, 
slightly posterior to the plane occupied by the eyes. Auricles always prom- 
inent in preserved material. Length of head about 4 of the total length of the 
animal. Width of the head at its junction with the auricular appendages 
greater than the diameter of body anywhere posterior to the appendages. Color, 
reddish to yellowish brown, uniform. Posterior margins of auricular append- 
ages free from pigment. Occasionally a narrow light median streak extending 
caudad from just back of the eye-spots. Length 8 to 13mm., greatest breadth 
4 to 1} mm. 

22,081; Station H, Matanzas Lake; Station C; Havana. 

A very active, restless, rampant form. When in motion the head is elevated 
and moved from side to side, the long auricular appendages being elevated above 
the head. After being disturbed, it does not come to rest for a long time as 
compared with P. gonocephala and P. maculata, the latter being particularly 
sluggish. A peculiarity of this species is the frequent occurrence of accessory 


8 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


posterior intestinal trunks, a condition that was present in fifty out of seventy- 
one specimens examined. In place of the two posterior trunks of the intes- 
tine, which exist in the ordinary Triclad type, there are often as many as three 
parallel trunks on each side of the pharyngeal chamber (Fig. 7). The acces- 
sory trunks either take their origin at the root of the pharynx, like the two 
normal posterior trunks, or exist as parallel branches of the latter, and 
usually unite with it and with each other close to their posterior termina- 
tions. I have never seen any anastomoses of the trunks of one side of the 
body with those of the other side, as in D. lacteum. When accessory trunks 
are present, they bear no lateral branches, but in place of these possess 
slight projections or buds, the lateral branches probably being suppressed 
through lack of space. The anterior portion of the intestinal tract, in almost 
every case where the specimens were not too heavily pigmented to be studied, 
exhibited anastomoses of the lateral branches, the intestine in some instances 
existing as a network (Fig. 7). (See under D. lacteum, ante, p. 2.) Unfortu- 
nately, I can say nothing in regard to the sexual organs of this species. In 
over one hundred individuals examined, not one was sexually mature or showed 
any signs of sexual organs. 

Girard (1893, p. 181) describes three varieties of Dugesia maculata. The 
description he gives of his first variety, var. a, agrees closely with the form 
described above in size and shape, in color, in the frequent occurrence of 
the light median streak, in the greater length of the auricular appendages, 
and the more pointed shape of these and the head end. His Figure 56 also 
shows a third posterior trunk of the intestine, but median in position and 
anastomosing with the lateral trunks. There can be little doubt that var. a 
of Girard’s D. maculata and P. dorotocephala are the same. 

There is also a striking resemblance between P. dorotocephala and Kennel’s 
figure of P. auriia (1888, Fig. 3). 


Planaria maculata Lerpy. 
Figures 2, 3. 


Planaria maculata Lerpy, 1848, p. 251; 1848*, p. 78; 1852, p. 225; 1852%, p. 289; 
1885, p.50. Dresrne, 1850, Vol. L. p. 205; 1862, p. 499. Simpson, 1857, 
p. 23. Si~tman, 1885, p. 70. Woopworrs, 1896, p. 94; 1896%, p. 240. 
Dugesia maculata Girard, 1851, p. 264; 1851*, p. 2; 1893, p. 181, var. bande. 
Planaria tigrina GirarRD, 1851, p. 264; 1851*, p. 2; 1891, p. 179. 


Anterior end pointed, sides of the head making an angle of about 60°. Two 
angular cephalic appendages. Length of head about ;; the total length of the 
animal. Eyes two. Sides parallel to about 3 of the total length from the 
anterior end, then tapering gradually to a rounded point. Color, blackish to 
purplish by reflected light ; blackish or gray by transmitted light. In smaller 
young specimens, the pigment occurs in isolated patches and spots; in older 
specimens the pigment patches become more confluent, chiefly in the median 
region leaving clear irregular areas scattered over the surface of the animal. 


7 _ a ee 


WOODWORTH: ILLINOIS TURBELLARIA. 9 


Smaller spots of deep brown or black occur scattered over the surface among 
the larger patches. In the largest, oldest specimens there are very few or none 
of the clear areas. Frequently a light median streak more or less free from 
pigment occurs, extending backward from between the eye-spots (Fig. 3). 
Length 4 to 9 mm., greatest breadth $ to 13 mm. 

Station D; 13,011, Station C; 18,113, Illinois R. at Havana; 22,050a, Sta- 
tion E; 13,521, Station L; 22,020, Station K ; 22,033d, Station L; 22,011c, 
Station L; 22,053, Station H. 

Planaria maculata is the commonest of our fresh-water Planarians and was 
the first one to be described (Leidy, 1848); however, nothing has been pub- 
lished regarding its sexual organs, and I am unable to offer anything in this 
regard ; not one of the hundreds of specimens that came under my observation 
was sexually mature. The species, as already mentioned, is sluggish, seldom 
being in motion in the aquaria, and when stirred up in company with P. gono- 
cephala and P. dorotocephala it is the first to come to rest. It is possible, as 
also suggested by Leidy (1885, p. 50), that it is nocturnal in its habits. It is 
usually found on the protected sides of stones, of empty Unio shells, or of 
aquatic plants, and often huddled together in large numbers. 

About 40% of the Illinois individuals that I examined exhibited mutilations 
at the anterior or posterior ends, either by the absence of a head, or by being 
truncated posteriorly. In the cases where the posterior end is lacking the 
pharynx, instead of occupying a position midway between the anterior and 
posterior ends, extends almost to the posterior limits of the animal. ‘There is 
no pigmentation about such scars. I have elsewhere (1896, p. 240) referred to 
the mutilations in Planaria maculata, and suggested that they were the result 
of reproduction by transverse division. I have since learned from Dr. Harriet 
Randolph that the species divides spontaneously, and that small fragments from 
any part of the body will regenerate into a new worm.} 


1 Through the courtesy of Dr. Randolph I am able to present the following 
table. The material upon which the experiments were made was collected on the 
island of Naushon, Vineyard Sound, Mass. 


PLANARIA MACULATA. CASES OF FISSION. 


Speci ; . Kind of Date of First Date of Sec- 
pecimen Date of Isolation. Water. Raeaiond Interval. ond Fission. 


July 24 Rain July 30 

July 24 Rain August 1 
July 24 (2) Rain August 3 | 14 days | August 17 
August 17 * Laket | August 21 
August 17 (?) Laket | August 25 
August 18 Spring | August 20 
August 17 or 18 | Spring | August 26 
August 17 or 18 | Spring | August 28 


1 
2 
3 
4 
5 
6 
7 
8 


* Whole number isolated on August 17 and 18 was forty individuals. 
+ From which the Planarians came. 


10 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Van Duyne (1896) experimented with a form which he states “ war die Pla- 
naria torva von Wood’s Holl.” <A study of his figures leads me to believe that 
the species used by him was P. maculata. I have found P- maculata to be 
very common at Wood’s Holl and elsewhere on Cape Cod, where I have col- 
lected and where I have never met with a species that resembles in any way 
the P. torva of Europe ; nor has that species ever been described from the 
United States. In P. torva the head is not defined from the body by lateral 
cephalic appendages, the anterior end of the animal is simply rounded. The 
contrary is true of the species shown in Van Duyne’s figures, where there is 
indicated a sharply marked angular head and well marked projecting cephalic 
appendages. 

Many of the specimens showed dark spheroidal bodies scattered through the 
parenchyma, as many as fifteen of the bodies occurring in a single individual. 
In sections they proved to be small encysted Trematodes. The cysts measured 
from 0.025 to 1.1 mm. in diameter. I have also met with Trematodes in the 
mesenchyma of bdelloura parasitica. 

Borelli (1895) has called attention to the resemblance of his Paraguayan 
P. dubia to our P. maculata as described by Girard, and has named a variety 
of it P. dubia var. maculata. 


Planaria unionicola, sp. nov. 
Figure 8. 


It is with considerable reserve that I offer a new species upon the meagre 
evidence at my disposal, but such data as there are cannot be reconciled with 
descriptions of any known species. There was but one specimen (138,646), 
which was much contracted and shrivelled. It was found creeping on the 
mantle of Unio alatus, dredged from deep water in the Illinois River near 
Havana, August 30, 1895. According to the collector’s notes the general color 
of the animal was ‘‘ brownish red ... mottled with purplish dots.” On an out- 
line drawing evidently made from the living animal, and which is reproduced 
in Figure 8, the color is also indicated as “light brick red” and the “ purple 
dots” as occurring “all over the surface in masses, except at the margins,” the 
dotted line in the figure no doubt representing the limits. The red color is 
also noted as being absent over an elongated posterior median area extending 
nearly to the posterior end of the animal (Fig. 8). The head end hasa sinuous 
outline, producing three lobes or rounded projections, a median anterior one 
and two lateral cephalic appendages. There are two eyes occupying the inner 
margins of large circular periocular spaces. The sides are nearly parallel, and 
the posterior end is abruptly rounded, so blunt, indeed, as to suggest an injury 
or that transverse division had taken place. If the clear median elongated 
space indicates the position of the pharynx, its extreme posterior position is 
also indicative of an injury or a division. The color of the alcoholic specimen 
was a deep rusty red. Owing to the crumpled condition of the specimen nothing 
of any internal organs could be recognized, even when subjected to the most 


WOODWORTH: ILLINOIS TURBELLARIA. Fi 


powerful clearing reagents. The length of the specimen was 2.8 mm., the 
greatest breadth 1.8 mm. When alive the worm probably must have attained 
a length of from 8 to 10 mm. 

Whether the occurrence of the animal on the mantle of Unio is an indica- 
tion of a parasitic mode of life, like that of the Triclads of the genera Bdelloura 
(Leidy, 1852*) and Synccelidium (Wheeler, 1894), or whether its occurrence 
was purely accidental, can only be determined by careful and extended search. 


Mesostoma ehrenbergii O. Scumipr. 
Figure 6. 


Mesostoma ehrenbergii O. Scumrpt, 1848, p. 47. 
Mesostoma wardii WoopworTH, 1896, p. 95; 1896+, p. 241. 


No. 13,521, Station L; No. 13,626, Illinois River. 

In a report on the Turbellaria collected by the Michigan Fish Commission I 
(1896, 1896*) described what I believed to be anew species of Mesostoma under 
the name of M. wardii, basing the species chiefly upon the uniformly small 
size and the absence of cephalic tracts of rhabditi or ‘‘ Sttibchenstrassen.” A 
comparison, however, with a larger number of Illinois Mesostoma has convinced 
me of the identity of both the Michigan and Illinois forms with M. ehrenbergit 
of Europe, and I must hereby cancel the species established by me in 1896. 
The Illinois specimens range from 14 mm. to 6 mm. in length, while the 
Michigan specimens measured only from 2 to 3 mm. In all of the small 
Illinois specimens the cephalic tracts of rhabditi are lacking, and are prominent 
only in the largest individuals. The largest of the specimens, 6 mm. in length, 
contained eight young worms in the left uterus, the right uterus being empty. 

The viviparity of M. ehrenbergit was known to Focke (1836), and the same 
author has described the differences between the brown hard-shelled winter 
eggs and the smaller translucent summer eggs, and pointed out the fact that 
in the viviparous condition the young ones arise from the thin-shelled summer 
eggs. The simultaneous occurrence of both summer and winter eggs in the 
same individual was observed by Leuckart (1852). Four of the Illinois speci- 
mens contained the characteristic large opaque brown hard-shelled winter 
eggs, and therefore agree with the European form in producing both kinds of 
eggs at the same season, though in no case were there both kinds of eggs actually 
present in the same individual. About 40% of the individuals contained the 
translucent summer eggs, the smaller specimens showing no signs of sexual 
organs. The winter eggs have the same shape as those of the European spe- 
cies, such as would result if one half of a hollow sphere were infolded into 
the other half. As in the European species, too, the diameter of the winter 
eggs considerably exceeds that of the summer eggs, the former measuring 
0.525 and the latter 0.350 mm. in diameter. The occurrence of M. chrenbergii 


in the United States also gives to this well known species a cosmopolitan 
distribution. 


12 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


Stenostoma leucops O. Scumipr. 


Stenostoma leucops O. Scumivt, 1848, p. 59; Srrtrman, 1885, p.55; Orr, 1892. 
Stenostoma neoboracense GIRARD, 1893, p. 220. 


Said to have been abundant in the aquaria of the laboratory during May, 
1896. Of this species I received only some stained mounted specimens. The 
following account is from notes made by Dr. Kofoid upon living material. 

When extended, in motion, the worms measured 1 to 1} mm. in length. 
Margins transparent, central portion brownish, posterior third of the animal 
often with a faint tinge of pink or maroon, and in one specimen this color 
extended throughout the length of the animal. Several lobe-like projections 
on the surface near the mouth when the animal is contracted. The anterior 
end, when extended, exhibits two lateral depressions, —the ciliated pits. 
The projecting anterior end is very mobile, and from its activity undoubtedly 
highly sensitive. No trace of sexual organs. Cilia very prominent on the 
general surface of the body, and always directed forward when the animal 
is quiescent. Locomotion is sometimes in a posterior direction, and is ac- 
companied by waves of constriction progressing from in front backwards. 
Forward motion, however, is apparently due wholly to ciliary action. Repro- 
duction by fission was indicated in some specimens by the existence of well 
marked constrictions at about one third the total length from the posterior end ; 
the constriction involved both the bypodermis and the digestive tract. 


WOODWORTH: ILLINOIS TURBELLARIA. 13 


LIST OF WORKS REFERRED TO IN THE TEXT. 


Baer, K. E. von. 
1827. Beitrage zur Kenntniss der niedern Thiere. Nova Acta Acad. Ces. 
Leop. Car., Tom. XIII. Pars 2, pp. 523-762, Taf. XXVIII.-XXXIII, 
(VI. Ueber Planarien, pp. 690-730, Taf. XX XITI.] 
Borelli, A. 
1895. Viaggio del dott. Alfredo Borelli nella Republica Argentina e nel 
Paraguay. Planarie d’acqua dolce. Boll. Mus. Zool. e Anat. Comp. 
R. Univ. Torino, Vol. X. No. 202. 
Carriere, J. 
1882. Die Augen von Planaria polychroa Schmidt und Polycelis nigra Ehrb. 
Arch. mikr. Anat., Bd. XX. pp. 160-174, Taf. IX. 
Diesing, K. M. 
1850. Systema Helminthum. Vol. I. Vindobone. 
Diesing, K. M. 
1862. Revision der Turbellarien. Sitszungsb. Akad. Wiss. Wien, Bd. XLIV. 
Abth. 1, pp. 485-578; Bd. XLV. Abth. 1, pp. 191-318. 
Dugés, A. 
1828. Recherches sur l’organisation et les meurs des Planariées. Ann. Sci. 
Nat., Tom. XV. pp. 189-183, Pl. 4, 5. 
Dugés, A. : 
1830. Apercu de quelques observations nouvelles sur les Planaries et plu- 
sieurs genres voisins. Ann. Sci. Nat., Tom. XXI. pp. 72-90, Pl. 2. 
Duyne, J. van. : 
1896. Ueber Heteromorphose bei Planarien. Arch. ges. Physiol., Bd. LXIV. 
pp- 569-574, Taf. X. 
Focke, G. W. 
1836. Planaria Ehrenbergii. Ann. Wiener Mus., Bd. I. pp. 193-206, 
Taf. XVII. 
Girard, C. 
1851. Brief Account of the Fresh Water Planarie of the United States. 
Proc. Bost. Soc. Nat. Hist., Vol. Ii. pp. 264, 265. 
Girard, C. 
1851*. Die Planarien und Nemertinen Nord-Amerika’s. Nord-Amerik. Mo- 
natsbericht Natur- u. Heilk., Bd. IL. pp. 1-5. 
Girard, C. 
1893. Recherches sur les Planariés et les Némertiens de l’ Amérique du Nord. 
Ann. Sci. Nat.; Zool., Tom. XV. pp. 145-310, Pl. 3-6. 
%e, 


14 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Graff, L. von. 
1882. Monographie der Turbellarien. I. Rhabdocelen. Leipzig. 
Grube, E. 
1872. Beschreibung von Planarien des Baikalgebietes. Arch. Naturg., 
Jahrg. 38, Bd. I. pp. 273-292, Taf. XI., XII. 
Hallez, P. 
1890. Catalogue des Turbellariés (Rhadoccelides, Triclades et Polyclades) 
du Nord de la France, etc. Lille. Also in Rev. Biol. Nord de la Frauce, 
Tom. IL., [V., V., 1890-93. 
Hallez, P. 
1892. Morphogénie générale et affinités des Turbellariés. Lille. 
Iijima, I. 
1884. Untersuchungen iiber den Bau und die Entwicklungeshichte der 
Siisswasser Dendrocelen (Tricladen). Zeitschr. wiss. Zool., Bd. XL. 
pp- 359-464, Taf. XX.-XXIII. 
lijima, I. 
1887. Ueber einige ‘Tricladen Europa’s. Jour. Roy. Coll. Sci., Imp. Univ. 
Japan, Vol. I. pp. 337-358, Plate XXV. 
Kennel, J. von. 
1888. Untersuchungen an neuen Turbellarien. Zool. Jahrb., Abth. Anat. 
u. Ont., Bd. III. Heft 3, pp. 447-486, Taf. XVIII., XIX. 
Lang, A. 
1884. Die Polycladen. Fauna u. Flora d. Golfes v. Neapel, XI. Leipzig. 
Leidy, J. 
1848. Descriptions of Two New Species of Planaria. Proc. Acad. Nat. Sei 
Philad., Vol. III. pp. 251, 252. 
Leidy, J. 
1848*. Descriptions of Two New Species of Planaria. Ann. Mag. Nat. 
Hist., Ser. 2, Vol. I. p. 78. 
Leidy, J. 
1852, Helminthological Contributions, No. 2. Proc. Acad. Nat. Sci. 
Philad., Vol. V. pp. 224-227. 
Leidy, J. 
18522. Helminthological Contributions, No. 3. Proc. Acad. Nat. Sci. 
Philad., Vol. V. pp. 289-244. 
Leidy, J. 
1852>. Corrections and Additions to former Papers on Helminthology. 
Proc. Acad. Nat. Sci. Philad., Vol. V. pp. 284-290. 


Leidy, J. 
1885. Planarians. The Museum, Vol. I. No. 4, pp. 49-52. 
Leuckart, R. 


1852. Mesostoma Ehrenbergii anatomisch dargestellt. Arch. Naturg., Jahrg- 
18, Bd. I. pp. 234-250, Taf. IX. 


WOODWORTH: ILLINOIS TURBELLARIA. 15 


Leydig, F. 
1864. Tafeln zur vergleichenden Anatomie. Tubingen. 
Oersted, A. 8. 
1844. Entwurf einer systematischen Hintheilung und speciellen Beschrei- 
bung der Plattwiirmer, ete. Copenhagen. 
Ott, H. N. 
1892. A Study of Stenostoma leucops O. Schm. Jour. Morph., Vol. VII. 
No. 8, pp. 263-304, Plates XIV.-XVII. 
Schmidt, O. 
1848. Die rhabdoccelen Strudelwiirmer (Turbellaria rhadocela) des siissen 
Wassers. Jena. 
Schmidt, O. 
1862. Untersuchungen iiber Turbellarien von Corfu und Cephalonia. 
Zeitschr. wiss. Zool., Bd. XI. pp. 1-30, Taf. I-IV. 
Silliman, W. A. 
1885. Beobachtungen iiber den Siisswasserturbellarien Nordamerikas. 
Zeitschr. wiss. Zool., Bd. XLI. pp. 48-78, Taf. III, 1V. 
Stimpson, W. 
1857. Prodromus descriptionis animalium evertebratorum quae in Expedi- 
tione ad Oceanum Pacificum, etc. Pars 1. Turbellaria dendrocela. Proc. 
Acad. Nat. Sci. Philad., Vol. IX. pp. 19-31. 
Weltner, W. 
1887. Dendrocelum punctatum Pallas bei Berlin. Sitzungsb. k. Preuss. 
Akad. Wiss., Bd. XXX VIII. pp. 795-804, Taf. VI. 
Wheeler, W. M. 
1894. Syncelidium pellucidum, a New Marine Triclad. Jour. Morph., 
Vol. IX. pp. 167-194. 
Woodworth, W. McM. 
1891. Contributions to the Morphology of the Turbellaria. I. On the 
Structure of Phagocata gracilis Leidy. Bull. Mus. Comp. Zodl., Vol. 
XXI. No. 1, pp. 1-42. 
Woodworth, W. McM. 
1896. Preliminary Report on Collections of Turbellaria from Lake St. Clair 
and Charlevoix, Michigan. Bull. Mich. Fish Commission, No. 5, pp. 94, 
95. 
Woodworth, W. McM. 
1896*. Report on the Turbellaria collected by the Michigan State Fish 


Commission during the Summers of 1893 and 1894. Bull. Mus. Comp. 
Zool., Vol. XXIX. No. 6, pp. 239-243. 
Woodworth, W. McM. 


1896". Notes on Turbellaria. American Naturalist, Vol. XXX. pp. 1046- 
1049. 


16 


BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


EXPLANATION OF THE PLATE. 


ABBREVIATIONS. 
brs. Copulatory bursa. ov dt. Oviduct. 
dt. ej. Ductus ejaculatorius. pe. Penis. 
gl. sh. Shell gland. ut. Uterus. 
gl. pr’st. Prostate gland. va. df. Vas deferens. 
go’po. Gonopore. vag. Vagina. 
mu. ret. Retractor muscle. ae Vesicule seminales. 


The lines adjacent to the Figures 1, 2, and 47 indicate the natural size of the 


object. 
Fig. 1. 
Fig. 2. 
Fig. 3. 
Fig. 4. 
Fig. 5. 
Fig. 6. 
Fig. .7. 
Fig. . 8. 
Fig. 9. 
Fig. 10. 
Fig. 11. 
Fig. 12. 
Fig. 13. 
Fig. 14. 
Fig. 15. 


Dendrocelum lacteum. From life, x 4. 

Planaria maculata. From life, x 6. 

P. maculata. Head end, to show the occurrence of the light median streak. 
From life. 

Planaria dorotocephala, sp.nov. From life, X 7. The line at the left of 
the figure indicates the natural size of the colored part of the figure 
only. 

Planaria gonocephala. From life, x 5. 

Mesostoma ehrenbergii, showing young worms in the left uterus. From 
a specimen in clove oil, X 14. 

P. dorotocephala, sp. nov., to show intestinal tract. A reconstruction from 
a series of frontal sections, X 6. 

Planaria unionicola, sp. nov. From a drawing, apparently from life, 
accompanying the specimen. 

D. lacteum. Head end, to show the adhesive organ. From an alcoholic 
specimen killed in corrosive sublimate, 5. 

D. lacteum. Diagram of sexual organs, X 20. 

D. lacteum. Longitudinal section to show the sexual organs, X 50. 

D. lacteum. Transverse section through penis, X 40. 

D. lacteum. Transverse section through extreme anterior end, to show 
the adhesive organ, X 80. 

D. lacteum. ‘Transverse section from same individual as last, somewhat 
posterior to it, X 80. 

D. lacteum. Median longitudinal section through the anterior end and 
adhesive organ, X 80. 


Gp 4 


an 
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No. 2.— On the Relations of Certain Plates in the Dinichthyids, 
with Descriptions of New Species. By C. R. EASTMAN. 


THE present contribution may be regarded as a continuation and 
enlargement of two previous articles on the Dinichthyids,’ one of which 
discussed the relationships of certain detached and little known plates, 
and the other endeavored to trace the ancestry of the group. Some of 
the plates mentioned in the first paper are now illustrated and more 
fully described, together with others which afford additional evidence 
regarding the osteology of Dinichthys; and the views set forth in the 
second paper are now considered more in detail. In addition, descrip- 
tions are offered of several new species, and restorations are given of 
the dorsal and ventral aspects of Dinichthys. 

Unless otherwise stated, the material upon which all of the following 
descriptions are based is preserved in the Museum of Comparative Zodl- 
ogy at Cambridge, Mass. ‘To Mr. Alexander Agassiz, Director of the 
Museum, the most cordial and grateful thanks of the writer are due for 
the opportunity to study the collection, and to publish the results herein 
set forth. 

Dorsal Plates. — It is proposed to consider first the system of. plates 
covering the dorsal surface of the body in Dinichthys. These plates are 
shown in their natural arrangement, as known to exist in D. intermedius 
and D. terrelli, in Plate 1, Fig. 1; their correspondence with homologous 
elements in Coccosteus and related genera will be obvious from an inspec- 
tion of the diagrams. The restoration here given may seem to call for 
a word of explanation, since it differs in certain respects from the familiar 
ones of Newberry and others.” The cranial osteology is based upon one 
of the most perfect heads of Déinichthys intermedius ever discovered, 
now the property of the Cambridge Museum. A full description of the 


1 Amer. Journ. Science, [4], Vol. II. pp. 46-50, July, 1896. Proc. Amer. Assoc. 
Adv. Science, Buffalo Meeting, August, 1896 (Abstract in Amer. Geol., Vol. XVIII. 
pp. 222, 223). 

2 Newberry, J. S., Palwozoic Fishes of North America (Monograph U. S. Geol. 
Survey, Vol. XVI. Plate LII. Fig. 2), 1889. Dean, B., Fishes, Living and Fossil, 
1895, p. 134, Fig. 134. 

VOL. XXXI.— NO. 2. 


20 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


same specimen has already been published by E. W. Claypole.! Inas- 
much as this cranium lacks the marginal and suborbital plates, these 
have been supplied in the diagram from Newberry’s restoration. The 
fact that they are shown more in projection than perspective imparts a 
wider and more flattened appearance to the cranium than is strictly nat- 
ural ; the dorso-laterals are likewise drawn as if flattened out, instead of 
conforming to the curvature of the body. The outline of the dorso-median 
has been reduced to scale from a photograph of an exceptionally perfect 
plate obtained from Dr. William Clark by the British Museum; its exact 
position as regards the dorso-laterals has been ascertained from speci- 
mens in the Museum of Comparative Zodlogy. Hence the restoration 
can be considered as such only in the sense that the parts are now 
brought together in their completeness and proper relationships, and 
are shown on the same scale. 

The earlier restorations already referred to are subject to the following 
criticisms. First, the anterior portion of the dorso-median is produced 
in imagination so as to cover the exposed space behind the occipital 
region ; secondly, the conditions of overlap and underlap are represented 
on only one side of the antero-dorso-lateral, instead of on three sides ; 
thirdly, the postero-dorso-lateral is not shown at all. 

Hitherto the postero-dorso-lateral has never been found in direct 
association with other plates, and its position has accordingly remained 
in doubt. It has long been known under Newberry’s designation of 
“post-clavicular,” and is a plate of not uncommon occurrence in the 
detached condition. Its triangular form, the markings impressed upon 
it by overlying plates, and the course of the sensory canal system 
across it, appeared to the writer? sufficient evidence for assigning the 
plate theoretically to the position indicated in the diagram; and it is 
therefore interesting to record the discovery of a specimen which estab- 
lishes the entire correctness of this inference. The new specimen repre- 
sents the right antero- and postero-dorso-lateral plates of D. terrelli, 
firmly articulated together, as shown in Plate 2, Fig. 1. It is from 
the Cleveland Shale, and was found in the vicinity of Lindale, Ohio, by 
Mr. Prentis Clark. The inner surface of the plates is alone visible, the 
external side being embedded in the matrix. The mode of articulation 
between the two plates is by pegs and sockets, the position of which is 
fairly constant among the specimens that have been observed. The lar- 


1 Claypole, E. W., The Head of Dinichthys (Amer. Geol., Vol. X. p. 199), 
October, 1892. 
2 Amer. Journ. Science, [4], Vol. II. p. 48, July, 1896. 


EASTMAN: THE DINICHTHYIDS. rail 


gest and most perfect plate that the writer has seen is preserved in the 
Museum of Comparative Zoology, and measures 65 cm. in length (Cata- 
logue No. 1325). The corresponding element in D. intermedius is hardly 
to be distinguished except for its smaller size. An excellent example of 
the latter species belonging to the School of Mines Cabinet of Columbia 
University shows the postero-dorso-laterals of either side of the body 
commingled with other plates pertaining to the same individual; it is 
valuable for furnishing comparative measurements of the different bones, 
and deserves further study. 

The orientation of the plate in question may be readily determined, 
either by an inspection of the overlapped area, or by noting the course 
of the sensory canals. These arise at the anterior border, where they 
meet the single straight furrow that traverses the antero-dorsal-lateral ; 
and from this point they sweep inwardly, sometimes as a single and 
sometimes as a double channel, as far as about the middle of the exposed 
portion of the plate, where they cease. In this respect the genus differs 
from Coccosteus, which has the canals continued on to, and in some 
cases entirely across, the dorso-median. The insunken area formed by 
the overlap of the latter plate stands in marked contrast to the irregular 
depressions produced by the overlap of the antero-dorso-lateral. The 
graceful curve forming the postero-lateral boundary of the dorso-median 
is projected upon the underlying plate, and shallow depressions are left 
where the transverse ridge on the under surface of the dorso-median 
rested on the subjacent plate. This ridge, it should be noted, occupies 
the same relative position as its homologue in Coccosteus. 

The upper boundary of the lateral plates is indicated by a deeply in- 
sunken area on the antero-dorso-lateral, and a slight indentation on the 
free margin of the postero-dorso-lateral. Below, these as yet undiscov- 
ered plates must have been connected with the ventral armoring, either 
directly, or more probably through the intervention of the “ claviculars.” 
The curvature of the ascending arm of the latter furnishes us at the same 
time with the curvature of the missing laterals, and we can also form 
an approximate estimate of their height and length. It is to be hoped 
that the laterals may yet be identified as such, when the entire dermal 
covering of Dinichthys can be compared plate for plate with its European 
congeners. 

Ventral Plates. — Grave difficulties have been encountered in the 
attempt to reconstruct the ventral armor of Dinichthys, owing to the 
detached condition in which the plates have invariably been found. It is 
perhaps but natural that the views which were originally entertained 


viv BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


regarding the structure of the plastron should have received important 
modifications in consequence of later discoveries. Thus, Newberry’s 
supposed posterior ventrals were afterwards identified as the suborbitals, 
and his so called “jugulars” have since been demonstrated by Wright? 
to be in reality the posterior ventrals. 

The restorations of Wright and Dean? (the latter being somewhat 
modified after Wright’s figures) leave the median plate or plates un- 
accounted for, and it remained for Dean in a subsequent publication ? 
to reconstruct the ventral covering afresh, with the addition of a single 
element along the median line. But as pointed out by the present 
writer in a review of Dean’s article,* the evidence is not entirely con- 
clusive that a distinct antero-median ventral was not present in advance 
of the posterior element and overlying it, although now obscured on the 
specimen by weathering. The writer has since had an opportunity for 
examining the original, which is referred by Dr. Dean with some hesita- 
tion to Dinichthys gouldi. Although it is badly fractured precisely at 
the spot where we should expect a suture to exist, and therefore in- 
capable of affording positive proof on this point, nevertheless the fact 
that the two plates we know were at least potentially present should 
have retained their normal position with respect to each other, while 
the adjacent plates have become displaced, points strongly toward a 
union of some kind between them. 

For an undoubted example of fusion of the mid-ventrals we must turn to 
the specimen of Dinichthys terrelli figured by Newberry on Chart VI. 
(F igure A), accompanying the second volume of the Ohio Geological Sur- 
vey Report. The original is still preserved in the School of Mines Cab- 
inet, and has been recently refigured by Dr. Dean.’ The resemblance of 
the anterior and posterior portions to plates presently to be described, 
and occurring as distinct elements, is sufficiently obvious. In this speci- 
men, and the statement doubtless applies to all adult individuals of the 
same species, fusion exists between the mid-ventrals ; in D. gould: fusion 
probably likewise exists. These two instances are sufficient, in Dr. Dean’s 
estimation, to compel us “to accept the thesis that the median ventral 


1 Wright, A. A., The Ventral Armor of Dinichthys (Amer. Geol., Vol. XIV. 
pp. 313-820), 1894. Report Ohio Geol. Survey, Vol. VII. pp. 620-626, 1893. 

2 Dean, B., Fishes, Living and Fossil, 1895, Fig. 135, p. 154. 

3 Dean, B., The Ventral Armoring of Dinichthys, etc. (Trans. N. Y. Acad. 
Sci., Vol. XV. pp. 157-163, May, 1896). 

4 Amer. Geol., Vol. XVIII. pp. 316, 317, 1896. 

5 Dean, B., Trans. N. Y. Acad. Sci., Vol. XVI. Plate III., 1897. 


EASTMAN: THE DINICHTHYIDS. 23 


plates of Dinichthys must be separate or fused in all members of the 
genus.” 

Under ordinary circumstances, such an interpretation would appear 
most logical, since we should expect, a priort, marked differences in the 
mode of union of the mid-ventrals to be indicative of different genera. 
We might reasonably infer that these differences were accompanied by 
variations in the dentition and other parts of the body, although this is a 
point which could only be determined empirically. Should it be ascer- 
tained, however, that forms existed having a like dentition, a like con- 
figuration and arrangement of plates as in Dinichthys, yet differing among 
themselves as respects the mode in which the median ventrals were 
united, there would be difficulty in estimating the value of this latter 
character. Ought it to be regarded as a valid generic distinction, or, 
other things being equal, merely as an adaptive variation affecting 
different species indiscriminately? From present indications it would 
appear highly probable that diverse conditions existed in the ventral 
plates of forms which agree in their remaining characters, so far as 
known, with Dinichthys. It must be noted, also, that amongst the 
species of this genus the paired ventral plates are exceedingly variable 
in their characters, more so in fact than any other plates of the body. 
Not only do they vary in form, relative proportions, and mode of union 
among different species, but there are considerable differences.to be ob- 
served within one and the same species; one class of variations within 
specific limits will be referred to later under the head of ventro-lateral 
plates. 

To sum up these difficulties briefly, we must admit on the one hand 
that theoretical considerations are opposed to the view that species of 
one and the same genus should differ widely as respects the number and 
arrangement of the median ventrals; but on the other hand, evidence 
is wanting to show that the forms they represent differed in any respects 
further than this from Dinichthys. And until positive evidence is forth- 
coming, such as finding the plates naturally associated with the denti- 
tion, it is impracticable to employ characters of the ventral plates as a 
test of generic rank. In our opinion, both prudence and convenience 
dictate that plates which resemble the known elements of Dinichthys, 
when found in the detached condition, are to be referred to that genus 
until criteria are at hand for determining them otherwise. Accordingly, 
certain isolated plates, whose description follows, will be referred to 
Dinichthys by virtue of their obvious affinities with that genus. And 
it will be assumed, provisionally at least, that in this genus the median 


24 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


ventrals may exist in three different conditions of union; they may 
simply overlap, as in Coccosteus ; they may be fused into a single elon- 
gated piece; or they may be interlocked with one another. Examples 
of these three modes of union will now be considered.? 

Interlocking Median Ventrals.— Two instances have been recorded 
where the median ventral plates of Dinichthys are articulated with one 
another ; the first was made known by E. W. Claypole in 1893,? the 
second by the present writer in 1896. In both cases the plates oc- 
curred in the detached condition, and were referred provisionally to 
the genus Titanichthys. Further investigation has since shown this to 
have been an erroneous determination, and the only genus that they 
can be certainly referred to in the present state of our knowledge is 
Dinichthys. The original of Professor Claypole’s figure is preserved in 
the Museum of the Ohio State University at Columbus. It is a very 
large and heavy postero-ventro-median, and with it were associated the 
greater part of the postero-ventro-laterals. The proportions indicate a 
considerably larger species than either D. terrelli or D. hertzeri, and 
accordingly the name D. ingens* has been suggested for it by A. A. 
Wright. As a detailed description of these remains is in course of 
preparation by Professor Wright, it is sufficient for our purpose merely 
to cite this as an illustration of a particular mode of union between the 
median ventrals. 

The other example of articulation or dovetailing is furnished by a speci- 
men in the Museum of Comparative Zodlogy, now figured for the first 
time (Plate 2, Fig. 2). It is broadly lozenge-shaped, and its diagonals 
measure 20 by 31 cm. The resemblance of this plate to the posterior 
part of the single element in D. terre/li, already referred to, as figured 
by Newberry and Dean, is obvious. Its size, thickness, and markings im- 
pressed upon it by the paired ventrals, are also in substantial agreement. 
In these particulars it is seen to be closely allied to D. terrelli ; but on 
the other hand the articulation with the antero-ventro-median is precisely 
the same as in D. ingens. The plate in question was collected by Mr. 
Terrell, in the Cleveland Shale of Lorain County, Ohio; but whether 


1 See abstract of a preliminary paper by A. A. Wright, entitled, “New Evi- 
dence upon the Structure of Dinichthys” (5th Ann. Rep. Ohio State Acad. Sci., 
1897, pp. 59, 60). 

2 Report Geol. Survey of Ohio, Vol. VII. p. 611, Plate XL. Fig. 1. 

3 Amer. Journ. Science, [4], Vol. II. p. 47. 

4 Should an identity be established between these plates and the mandible 
described by Claypole as D. kepleri, the latter name is entitled to priority. 


EASTMAN: THE DINICHTHYIDS. 25 


associated or not with other remains cannot now be ascertained. Theo- 
retical considerations are certainly opposed to the idea that this plate 
pertained to either D. terrelli or D. ingens ; and we are compelled to 
regard it as indicating an as yet unknown Dinichthyid species. 

Fused Median Ventrals. — Under this head must be placed the two 
examples already referred to, that have been described by Newberry * 
and Dean.? The originals are preserved in the School of Mines Cabinet 
at Columbia University, and have been determined as WD. terrelli and 
D. (2) gouldi. Whether fusion took place as a strictly adaptive charac- 
ter in forms having a thin plastron, whether it occurred only in adult 
individuals, or whether it characterized all the individuals belonging to 
particular species, are questions for future discoveries to determine. 
That fusion did not exist in all species of Dinichthys appears, however, 
extremely probable. 

Overlapping Median Ventrals. — Species which have the postero-ventro- 
median overlapped by the anterior element represent the normal or 
primitive condition, as exemplified by the genus Coccosteus. Three 
specimens of the detached antero-ventro-median and two of the postero- 
ventro-median are preserved in the Cambridge collection, whose relations 
to contiguous plates were plainly those of overlap and underlap. The 
bone shown in Plate 2, Figs. 5, 6, exhibits such a striking resemblance 
to its homologue in D. terrelli, that there can be no doubt as to its iden- 
tity. It is evident that the plate under discussion is entire, since its mar- 
gins taper gradually to a thin edge, and show no signs of having been 
broken away from a lower portion. Hence, the only important difference 
that is to be observed between this specimen and D. terrelli relates to the 
mode of union with the posterior element ; in the present case it overlaps, 
in D. terrelli it is fused with the hinder piece. As we know of no other 
species to which it can be referred, we must include it, provisionally at 
least, under the last named species. 

The special characters of this plate have been described elsewhere, 
although at that time the specimen was supposed to belong to Zitan- 
ichthys. It may be remarked in passing that the semicircular flange 
forming the anterior margin (seen best on the ventral aspect) is contin- 
uous with similar compressed borders on the antero-ventro-laterals. 
None of these margins reveal any trace of plates overlapping them in 


Report Geol. Survey of Ohio, Vol. Il. Part. II. (Paleontology), pp. 10, 31, 
and Chart VI. Fig. A. 

? Trans. N. Y. Acad. Science, Vol. XV. pp. 157-163, 1896; Ibid., Vol. XVI. 
pp. 57-60, 1897. 


26 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


front ; so that an interlateral element, such as is present in Cocco- 
steus, cannot be said to exist. We are therefore limited to assigning a 
strictly lateral (external) position for the so called “claviculars” or 
coracoids. 

A second specimen of the ventro-median preserved in the Cambridge 
collection (Catalogue No. 1299) shows the longitudinal ridge on the vis- 
ceral surface more strongly developed than the first, and is both thicker 
and wider towards its posterior extremity. 

There is yet a third specimen, which is smaller and of somewhat 
different configuration from the preceding; this is shown in Plate 3, 
Fig. 1. The visceral surface is embedded in the matrix, so that its 
character cannot be made out. In form it is somewhat suggestive of 
the parasphenoid bone of Ctenodus, but its structure as seen under the 
lens proves it to be Dinichthyid. The plate was obtained by Dr. Clark 
in the Cleveland Shale, near Lindale, Ohio. Owing to its smaller size, 
it may be referred with some reservation to D. intermedius. 

From the same locality as the preceding, Dr. Clark has also obtained 
two unique plates, one of which is preserved in counterpart, and is 
shown in Plate 5, Fig. 1. Lanceolate in outline, and perfectly sym- 
metrical, it presents a very graceful appearance ; its length is 29 cm., 
and its maximum width 12.5 em. Only the visceral aspect is exposed, 
and this is marked by two slightly oblique ridges, such as occur also in 
the corresponding position of D. terrelli. The plate is abruptly trun- 
cated in front, and bears indications of overlap by the antero-ventro- 
median. We shall find that additional light is thrown upon these 
relationships when we consider the plastron immediately to be described. 
The specimen is somewhat thinner than other ventral plates that have 
been noticed thus far, and it differs also in form. For the present, it 
must be regarded as representing an unknown Dinichthyid. 


OVERLAPPING MEDIAN VENTRALS PRESERVED IN SITU. 


So far, but two instances have been reported where the ventral plates 
were retained in their natural relations with respect to one another. 
The less perfect of these was described very briefly by von Koenen,* by 
whom jt is doubtfully referred to D. minor. Only the left half of the 
plastron is preserved in this case ; its entire length is assumed to have 
been about 16 cm., and its width 6 or 7 cm. The condition of the 


1 Koenen, A. von, Ueber einige Fischreste, etc. (Abhandl. Gesellsch. Wis- 
sen. Gottingen, Vol. XL. p. 18), 18965. 


ford 


EASTMAN: THE DINICHTHYIDS. ai 


specimen is too imperfect to admit of a precise determination of the 
several elements, as the author has informed us by letter. 

The only other instance recorded where the plastron has been pre- 
served 7m situ, is that made known by the writer at the Buffalo Meeting 
of the American Association for the Advancement of Science. For the 
discovery of this interesting fossil, science is indebted to Mr. F. K. 
Mixer, Curator of the Buffalo Society of Natural Sciences, who found 
the slab in place at the bottom of a small stream bed near Sturgeon 
Point, on the lake shore, twenty miles west of Buffalo, N. Y. The 
horizon at this point is the black Portage Shale, which has already 
yielded a considerable number of fish remains.’ The plates were cor- 
rectly determined by Mr. Mixer to be of Dinichthyid nature, and were 
so labelled by him and placed on exhibition in the Museum of the 
Buffalo Society. To this enthusiastic collector the writer is greatly in- 
debted for the privilege of studying the specimen, and of presenting the 
following description of it. 

Although the fossil has suffered considerably from aqueous and 
atmospheric erosion, the salient features have been so far preserved as to 
furnish points of control sufficient for reconstructing almost the entire 
topography. The slight extent to which the diagram given in Plate 1 
has been reconstructed may be seen from a comparison with a photo- 
graph of the actual fossil, reproduced in Plate 4. In most cases the 
sutural indications are so distinct, and continuous over such an area, 
that we have only to produce them in the same general direction across 
breaks in the surface until they meet, in order to complete the small 
portions that are interrupted. Thus, among the prominent landmarks 
that are left may be mentioned the terminal angles of the antero- 
ventro-laterals, which overlie the postero-ventro-laterals in their natural 
position. Half way between these points gives us the median line of the 
body ; and as all the plates are arranged symmetrically with reference to 
it, it is clear that the fossil has been in no wise distorted. A knowledge 
of this fact permits us to supply the contours of one side from informa- 
tion derived from the other, and fortunately the two sides supplement 
each other to a remarkable degree. The only boundary lines that are 
not tolerably distinct are the forward portions of the antero-ventro- 
laterals. We will consider the relations of the different plates in order. 

Ventro-Median Plates. —The first question that arises concerning the 
median ventrals is whether they are represented by one element or by 


1 Mixer, F. K., Amer. Geol., Vol. XVIII. p. 223, October, 1896. Williams, 
H. U., Bull. Buffalo Soc. Nat. Sci., Vol. V. pp. 81-84, 1886. 


28 ULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


two? And if two, what is theit mode of union? We have no hesita- 
tion in answering that two median plates are present, and that the an- 
terior overlaps the posterior, as in Coccosteus. The evidence appears 
perfectly decisive, and is of twofold nature; it depends upon a prom- 
inent surface elevation over the very region where we should expect the 
boundary between two median ventrals to be, and upon the fact that 
two centres of ossification are discernible. 

The surface elevation referred to is palpably of the same nature as 
those prominences which are formed by the hinder extremities of the 
antero-ventro-laterals where they are superimposed upon the posterior 
pair of ventro-laterals. All of these elevations are more or less eroded 
in the specimen, but the one under consideration is scarcely more so 
than the others. If it were a purely fortuitous bulge of the surface, we 
should expect similar ones to occur elsewhere, whereas the prevailing 
aspect of the plates is flat and smooth. But inasmuch as the only re- 
maining elevations are found at those places where we know for certain 
that boundaries occur, and as this occurs at the only place in the median 
line where we should expect to find a boundary, we are compelled to 
look upon this as a significant, not an accidental feature. Moreover, 
the shape of the elevation corresponds with the tapering extremity of 
the antero-ventro-median, superimposed upon the posterior element ; 
and the outline of the latter is seen to be perfectly normal as compared 
with homologous plates, when we cut it off at this point. In fact, it is 
noteworthy that the shape of the postero-ventro-median bears a marked 
similarity to the bone last described (supra, p. 26), and shown in 
Plate 5, Fig. 1. 

But still more pertinent evidence as to the existence of two median 
ventrals is furnished by the structure of the plates themselves. It is 
apparent at a glance that in the postero-ventro-median ossification pro- 
ceeded from a single centre, which was nearly coincident with the centre 
of the plate itself. On holding the slab so as to reflect light at a proper 
angle, the course of vascular (Haversian) canals can be seen very dis- 
tinctly, especially at the right anterior boundary; and all of these radiate 
toward the centre of the plate. Vascular canals are likewise apparent 
on the antero-ventro-laterals, but are only faintly perceptible on the 
antero-ventro-median. If the latter plate were articulated or fused with 
the posterior element, as in D. terrelli, it would be difficult to account 
for the significant elevation already referred to; and considering the 
relative thinness of the plates, such a mode of union could hardly have 
proved advantageous. It is more natural to suppose that the connection 


EASTMAN: THE DINICHTHYIDS. 29 


among all plates of the ventral armoring was one of simple overlap, as 
in Coccosteus and other forms. 

Ventro-Laterul Plates. —The inner margins of the antero-ventro- 
laterals are traceable with certainty throughout the greater portion of 
their length, but with a lesser degree of probability for the remaining 
(anterior) portion, where they are not only much abraded, but in part 
covered over by extraneous fragments, as will be noted presently. The 
boundaries of these plates are more sinuous than in any other known 
species, and their proportions witb respect to the posterior pair are also 
different. But, as already remarked, the ventrals exhibit a greater range 
of variation, even within specific limits, than all the other plates of the 
body. 

One class of variations that deserves notice here is the relative length 
of the two sets of ventro-laterals. Sometimes the anterior pair is the 
longer, and again, apparently within the limits of the same species, the 
posterior pair exceeds them in length. 

Possibly these differences may have been correlated with sex, a greater 
portion of the abdomen having been protected in the one case than in 
the other ;1 but however this may be, we are obliged to recognize the 
existence of these two patterns or varieties of the plastron. The pres- 
ent specimen, therefore, belongs to that type of plastron which has the 
anterior ventro-laterals longer than the posterior. 

The external margin of the postero-ventro-laterals appears to have 
been evenly rounded. Unfortunately, the central portion of the plates 
has been eroded away, so that the contour of the inner margins can 
only be postulated. It is probable, however, owing to the tenuity of 
these plates, that the condition of their union was one of simple overlap ; 
hence Dean’s figures of D. gouldi (2) have been followed in restoring 
their inner boundaries. Of the anterior borders of these plates, no 
trace whatever remains. There may be some significance attached to 
the fact that the antero-ventro-laterals are symmetrically worn away, 
their present eroded margins forming a regular curve from the ventro- 
median outward. Whether this symmetrical wearing away was in any 
respect influenced by the anterior margins of the hinder pair of plates 
may perhaps be questioned ; but at all events we must conclude that the 
former anterior boundary of these plates was not far from, and was prob- 
ably parallel with, the interrupted edges of the antero-ventro-laterals. 
That the plates in question were separated for a considerable distance 
posteriorly, is witnessed by an impression of the visceral surface of the 


1 Amer. Geol., Vol. XVIII. p. 817. 


30 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


ventro-median, which is preserved as far as its posterior apex on the 
slab. 

We have now to determine what species of Dinichthys is represented 
by the ventral armor just described. In the absence of the dentition, 
we must either associate the remains theoretically with mandibles of 
corresponding size that occur in the same horizon, or must regard the 
plastron as belonging to a new species. Fortunately, the proportions 
between the different body plates are well known in D. terrelli and 
D. intermedius, aud from them we can readily compute the length of 
mandible and size of dorsal shield to which the present specimen would 
correspond. Thus, the ratio between the length of mandible and length 
of the antero-ventro-laterals in D. terrelli1 is 1.14, and, assuming that 
about the same proportion held true for the species now under dis- 
cussion, we should attribute it with a mandible 24 or 25 cm. long. 
Now, from the Genesee Shales near Bristol Center, New York, J. M. 
Clarke has described under the title of D. newberryi a mandible meas- 
uring 28} cm. in length.2 In the same horizon are also found de- 
tached dorsal shields which are considered by this writer as belonging 
to D. newberryi, although their dimensions correspond almost precisely 
with those of D. minor. In fact, Dr. Clarke’s tables (pp. 22, 23) show 
that, while the mandibles of D. newberryi are about one half as large 
as in D. hertzert and D. terrelli, the dorsal shields are less than one 
fifth the size of those in either species. Such a marked discrepancy of 
ratio appears incredible in the light of comparison with other species ; 
and the measurements of the plastron now under discussion militate 
with the assumption that they, the mandibles of D. newberryi, and the 
dorsal shields from the same horizon as the last, all belonged to a single 
species. The correspondence of parts is such as to permit of a theo- 
retical association of the plastron with the mandibles of D. newberryi, 
but not with the dorsal shields that are referred by Dr. Clarke to this 
species ; these latter being more properly assignable to D. minor, or a 
species of equal size with D. minor. 

It must be borne in mind, however, that these conclusions depend 
entirely upon empirical formulas ; they are therefore more or less tenta- 
tive and provisional. It may be presumed from the general nature of 
things, and in the absence of any contrary evidence, that the propor- 
tions existing between parts of the derm skeleton were fairly constant 
within the limits of one and the same genus. But the correspondence 


1 Wright, A. A., Report Geol. Surv. Ohio, Vol. VII. p. 626. 
2 Clarke, J. M., Bull. U. S. Geol. Survey, No. 16, p. 17, 1885. 


EASTMAN: THE DINICHTHYIDS. 31 


of parts as known in Dinichthys does not hold true by any means for 
other genera (Zrachosteus, Mylostoma, etc.) belonging to the same fam- 
ily; and this fact admonishes us not to press hypothetical correlations 
too far, even within specific limits. Caution is enjoined in this particu- 
lar case by yet another consideration. From the same locality and 
formation, Mr. Mixer has obtained a pair of mandibles associated with 
fragmentary Dinichthyid plates. The condition of these remains does 
not warrant a precise specific determination, but their affinities are 
probably with D. minor. The length of each ramus is about 17 cm., 
and the maximum height 5cm. Either, therefore, these remains and 
the plastron represent together but a single species (D. ? minor), or 
we have evidence of two medium-sized species (V.? minor and D.? new- 
berry?) in the Portage Shale. 

Under these circumstances it is apparent that a positive identification 
of the species is impossible. For the sake of convenience, we might 
follow Dr. Clarke’s example, and refer all the detached plates occurring 
in the Genesee Shales to D. newberry?, and all those from Portage Shale 
to D. minor. But there is no reason for supposing that each of these 
horizons contains but a solitary species ; the indications point rather to 
the presence of more than one species in both horizons. And there is 
no reason why the doctrine of correlation of parts should not be applied 
to all the species of Dinichthys until experience has shown it to be in- 
valid for some of them. Provisionally, therefore, we are in favor of refer- 
ring the Portage plastron to the species with which it most closely agrees 
in measurement and geological horizon, that is to say, with D. newberryz. 
On the other hand, the Portage mandibles that have just been men- 
tioned, and the detached dorso-median plates from the Genesee, may be 
referred provisionally to D. minor. 

Comparative measurements of certain derm-plates for several species 
of Dinichthys are exhibited by the table on the following page. 

Besides the plastron just described, there are several other interesting 
structures preserved on the same slab. In advance of the plastron are 
a number of badly weathered fragments, which evidently represent the 
dorsal plates of the body. The forward portions of both antero-ventro- 
laterals are covered over, and their proper boundaries obscured, by some 
of these fragments; but none of them are identifiable with certainty 
unless it be the antero-lateral tip of the dorso-median, which rests upon 
the angle of the right antero-ventro-lateral. This concealment of the 
underlying plates along their margins is unfortunate, since the restored 
anterior boundary has not such a clear basis of fact as one could wish 


32 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


Comparative Measurements of Dinichthys Species. 


| 1 
| 
| 
| 


3 _| 88 4 3 g = 
= z ae = > 5 E es 
S = | Gao = < Ai S| Es 
No. Species, He «| Be |[Asg| Be | es) Bs) Bs lS8e 
a a| 38 = cot = a|" 38 
& a =o = a EI 2 | @& 
3 mi) R = RI 3 
MY ORL (op eeceers are to Scale tera ety ouiies cel (ese 
2|“ hertzeri. ...| 59.0 |675 |. . | 53.0 
8 | “ terrelli . . .| 56.25 | 65.0 | 86.0 | 57.5 
ca Oe . . «| 86.88 | 48.26 | 33.02 | 43.18 | 40.91 | 40.64 | 69.85 | 1.45 
Bi EE Saget woe we hs OS fad SP oo 7 2s PAB 26)) Sees 
6 | “ newherryi . . | 28.83 
7.| Portage plastron, |; ci c-al) a eer ess [hw co | COLO ean 
85 ||: gonlda® tei) 2). LIZOG ak Scans tienen ie elle 8+ 
9 |“ intermedius .| 22.86 | 41.91 | 29.21 | 26.67 | 17.15 ; 22.86 | 35.56 | 0.85 
10] “ minor... = | 10-4 |20.82) 122 (1815) ....| . ~ oe eee 
11 | Detached DM of 
Genesee Shale |. . . |. . | 12.6 | 18.75 


for. At the same time it must be remembered that the front margin 
of the plastron in all species of Dinichthys conforms to a peculiar and 
well marked type. 

To the right of the left antero-ventro-lateral is a small cleaver-shaped 
plate (7.5 cm. long by 3 cm. wide), the like of which is unknown among 
the derm plates of Coccosteids. It certainly does not belong to the 
dental apparatus, and is excluded from the orbital region on account of 
its size. There can be no doubt that the plate is entire, or nearly so; 
but we must confess ignorance as to its position on the body. Just 
behind the unidentified plate is to be seen a small portion of the verte- 
bral axis, very imperfectly preserved, together with supports for the 
dorsal fin. The form of the neural arches is shown with some distinct- 
ness, as well as their articulation with the proximal row of basal carti- 
lages. The outer tips of the distal row of basals appear to have been 
bluntly terminated, or even swollen. 


EASTMAN: THE DINICHTHYIDS. oo 


DESCRIPTIONS OF NEw SPECIES. 


Under this heading are included, besides species altogether new to 
science, certain others which are now demonstrated for the first time to 
belong to the genus Dinichthys. The subject may be properly intro- 
duced by a consideration of the latter forms first. 

As is well known, a large number of genera and species of Arthrodires 
have been founded on detached fragments, which commonly yield but 
little insight into the structure of the fish as a whole. Sometimes our 
knowledge of these forms is increased by the discovery of more perfect 
specimens, or by finding parts in natural association with the dentition 
or with other parts. The dentition obviously yields the most trenchant 
characters that can be employed for the discrimination of species; but 
in Dinichthys scarcely less important characters are furnished by the 
dorso-median plate. Owing to the massiveness of this plate, it is not 
readily subject to fracture or distortion, and is perhaps of more frequent 
occurrence than any other plate in the body. Its configuration varies 
markedly amongst the different species of Dinichthys, but remains 
fairly uniform within the limits of one and the same species; hence its 
systematic importance is very great. 

There is one feature about the dorso-median which appears to be 
peculiar to the Dinichthyids ; or, to put it differently, the Dinichthyids 
are distinguished from remaining Coccosteids by the possession of a cer- 
tain characteristic structure; and this is the large, excavated carinal 
process by which the dorsal shield is terminated posteriorly. (See 
Plate 2, Figs. 3, 4; Plate 3, Figs. 2, 3.) All of the Coccosteide, so 
far as known, have a median longitudinal keel or ridge on the inferior 
surface of the dorso-median ; but it is developed to a different degree, 
and is terminated in a different manner, amongst the several genera. 
In Coccosteus it ends posteriorly in a simple blunt spine ; in Homosteus 
the ridge is stronger, and terminates in a knob at the posterior border 
of the shield; and in Heterosteus the keel is greatly developed, but is 
not produced behind the margin to any great extent, nor is it excavated 
superiorly. This series of Coccosteid genera leads up to the conditions 
that exist in the Dinichthyid group, where the inferior ridge is termi- 
nated posteriorly by a distinct process, such as is unknown in other 
members of the family. If we arrange the Dinichthyid forms in order 
of relative development of the carinal process, we shall have the follow- 


ing series: Coecosteus sp. Pander (hereinafter described as D. livoni- 
VOL. XXXI. — NO. 2. 3 


34 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


cus); a form from the Eifel Devonian, described below as D. pelmensis ; 
Pelecyphorus Trautschold ; Asterolepis bohemica Barrande (hereinafter 
described as D. bohemicus) ; Dinichthys ; and lastly, the genus Titan- 
ichthys, which is so closely allied to Dinichthys as to pass for a mutation 
or modification of the same. Titanichthys is, essentially, a huge Dinich- 
thys with lighter bones and a degenerate dentition. It is presumable 
that when the osteology of Brontichthys, Gorgonichthys, Mylostoma, 
Trachosteus, and related genera, shall have become known as fully as in 
Dinichthys, their affinities with one another will be found to be much 
closer than with the more primitive Coceosteids. Newberry was in- 
clined to regard these forms as constituting a distinct family, the Di- 
nichthyide ; but that would rather overreach the mark. We venture to 
adopt the middle course, and assign to the forms enumerated above the 
rank of a subfamily, known as the Dinichthyine. 

As already remarked, we may regard the presence of a carinal process 
as sufficient ground for referring detached dorso-median plates to the 
Dinichthyine, instead of the Coccosteide in general. For precise gen- 
eric determination, a knowledge of the dentition is of course necessary ; 
but where we are in ignorance of the dentition, we may conveniently 
place all species founded upon such dorsal shields, for the time being at 
least, under the single genus Dinichthys. Precedent for this is already 
furnished by D. precursor, D. ringuebergi, D. tuberculatus, and the plates 
from the Genesee Shale referred to above as D. (‘) minor. To this 
category may now be added the following new species: D. livonicus, 
D. trautscholdi, D. pelmensis, and D. pustulosus. 


Dinichthys livonicus nomen nov. 


1857. Coccosteus aus Livland, C. H. Pander. Ueber die Placodermen des devon- 
ischen Systems, p. 70, Plate B, Fig. 4. : 

1889. Coccosteus, H. Trautschold, Ueber Coccosteus megalopteryx, etc. (Zeitschr. 
deutsch. geol. Gesellsch., Vol. XLI. p. 38). 

1896. Dinichthys livonicus, C. R. Eastman, Observations on the Dorsal Shields 
in the Dinichthyids (Amer. Geol., Vol. XVIL. p. 222). 


The original of Pander’s Plate B, Fig. 4, of his Placodermen des devonischen 
Systems, may be taken as the type of this species, and there may be presum- 
ably associated with it the specimen referred to by A. S. Woodward (Brit. 
Mus. Cat. No. P. 4731), in his Catalogue of Fossil Fishes, Vol. II. p. 293. 
Without doubt this represents one of the smallest and most primitive species 
of Dinichthys, yet its marked development of the carinal process in proportion 
to its size is sufficient reason for excluding it from Coccosteus. It apparently 


EASTMAN: THE DINICHTHYIDS. 35 


has much in common with the type of dorso-median described by Trautschold 
as Pelecyphorus, but may best be considered as representing a distinct species. 

Formation and Locality. — Devonian; Livonia and Government of St. 
Petersburg. 


Dinichthys trautscholdi nomen nov. 


1889. Coccosteus megalopteryx, H. Trautschold, Ueber Coccosteus megalopteryx, 
etc. (Zeitschr. deutsch. geol. Gesellsch., Vol. XLI. pp. 38-45, Plate V. 
Figs. 1-6.) 

1890. Coccosteus megalopteryx, O. Jaekel (Neues Jahrb., Vol. II. p. 145). 

1890. Pelecyphorus, H. Trautschold (Zeitschr. deutsch. geol. Gesellsch., Vol. XLII. 


p. 576). 
1891. Pelecyphorus, G. Giirich, Ueber Placodermen und andere Fischreste im 


Breslauer mineralogischen Museum (Zeitschr. deutsch. geol. Gesellsch., 
Vol. XLIII. p. 906). 

1896. Dinchthys trautscholdi, C. R. Eastman, Observations on the Dorsal Shields 
in the Dinichthyids (Amer. Geol., Vol. X VIII. p. 222). 


The type specimens represented in Plate V. Figs. 1-6 of Trautschold’s paper 
on Coccosteus megalopteryx (loc. cit., 1889), are now preserved in the Breslau 
Museum. They are from the Devonian of the River Ssjass, in Northwest 
Russia, and are apparently very closely related to the foregoing species. The 
principal differences consist in the larger size and less strongly arched condi- 
tion of the dorso-median proper, and the different shape and position of the 
carinal process. The latter is more deeply excavated on its posterior face, 
stands nearly at right angles with the surface of the shield proper, and is 
given off from it slightly in advance of the hinder margin of the same. In 
this last respect we find a resemblance to the dorso-median described by New- 
berry as D. precursor ; and, as in most American species, the process bears dis- 
tinct traces on its inferior surface of the attachment of muscles (Trautschold, 
loc. cit., Plate V. Fig. 6). On the other hand, Coccosteus-like affinities are 
shown by the tuberculated surface of the dorso-median, and by the presence 
upon it of sensory canals. These curve around toward one another posteriorly, 
but are not continued across the middle of the shield. The development of 
the inferior ridge and its terminal process is very pronounced. The dimen- 
sions of the largest process observed by Trautschold are stated to be 6.5 cm. 
in height by 3 cm. in width at the base,— proportions which are eminently 
Dinichthyid. 

This species, which it seems proper to name in honor of its original de- 
seriber, Professor Trautschold, was confused by this author with a Selachian 
ichthyodorulite which he mistook for a swimming appendage of Coccosteus. 
Later, when it was pointed out that Coccosteus could not properly include 
either of these forms, a new generic title was proposed for each, — Megalopteriz 
for the ichthyodorulite (afterwards discovered to be identical with Psam- 
mosteus), and Pelecyphorus for the dorsal shields. Curiously enough, the 


36 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


species were ieft unnamed in both cases, except that provision was made for 
calling them both Megalopteria securigera in the event of their being proved to 
represent but a single species of one and the same genus. The generic title 
Pelecyphorus is preocupied. 

Formation and Locality. — Devonian; River Ssjass, Government of St. 
Petersburg. 


Dinichthys pelmensis sp. nov. 
Plate 2, Fig. 4. 


The type of this species is represented by a specimen in the Schultze Col- 
lection belonging to the Museum of Comparative Zodlogy (Cat. No. 1375). 
It is from the Middle Devonian of Pelm, in the Eifel. 

The greater portion of the left side of the dorso-median is preserved entire, 
but on the right side there remains only an impression of the under surface of 
the bone. The carinal process is admirably preserved, and is of large size in 
proportion to the dorsal shield proper. It is deeply hollowed out posteriorly, 
and stands less nearly perpendicular to the surface of the shield than in the 
two preceding species. The height of the process is 1.2 cm., and its maximum 
breadth 0.5cem. The shield proper is 5.0 cm. long,and rather less than 4.5 cm. 
broad anteriorly. It is slightly arched transversely, and appears to have been 
emarginate in front. The sensory canals are distinctly traceable as far as the 
bone is preserved. That on the left side is seen to begin at a point about 
half way between the antero-posterior extremities of the shield, whence it con- 
tinues nearly parallel with the postero-lateral margin of the same, but stops 
short of the median line shortly in advance of the process. Only the bare 
termination of the canal belonging to the right side is preserved on the present 
specimen. The surface of the plate is covered with fine reticulating ridges, at 
the intersections of which traces of minute tubercles are discernible. The 
effect of weathering, however, has been to reduce these, so that to the unaided 
eye the surface appears to be finely granulated. The thickness of the plate 
does not exceed 2 mm. except in the vicinity of the median longitudinal ridge. 

Formation and Locality. — Middle Devonian ; Eifel District. 


Dinichthys eifeliensis Kayser. 
Plate 3, Fig. 3; Plate 5, Fig. 4. 
1880. Dinichthys eifeliensis, E. Kayser, Zeitschr. deutsch. geol. Gesellsch., Vol. 
SEX, psh7. 
1895. Dinichthys eifeliensis, A. von Koenen, Ueber Fischreste des norddeutschen 


und béhmischen Devons (Abhandl. Ges. Wissensch. Gottingen, Vol. XL. 
pp. 16-18, Plate IV. Figs. 4,5; Plate V. Fig. 1). 


The mandibles of this species are estimated by von Koenen to have meas- 
ured upwards of 50 cm. in length, and as {t is the only Dinichthyid previously 
known with certainty from this locality, we may safely refer to it the speci- 


EASTMAN: THE DINICHTHYIDS. 37 


mens figured in the accompanying plates. That shown in Plate 8, Fig. 3, 
represents without doubt the carinal process of a large dorsal shield, such as 
could well have belonged to a species as large as D. eifeliensis. Two or three 
additional specimens of the process, and several detached plates that are refer- 
able to the same species, also form a part of the Schultze Collection. One 
of these, identifiable as the right antero-ventro-lateral, is shown in Plate 5, 
Fig. 4. 

Formation and Locality. — Middle Devonian; Gerolstein, Berndorf, and else- 
where in the Eifel District. 


Dinichthys bohemicus (BaRrRanDe). 
Plate 2, Fig. 3; Plate 5, Fig. 2. 


1872. Asterolepis bohemica, J. Barrande, Systeme Silurien de la Bohéme, Vol. I. 
Suppl., p. 637, Plate X XIX. Figs. 9-18. 

1880. Asterolepis bohemica, A. von Koenen, Abhandl. Ges. Wissensch. GOttingen, 
Vol. XXX. p. 4. 

1895. Anomalichthys bohemicus, A. von Koenen, Abhandl. Ges. Wissensch. Got- 
tingen, Vol. XL. pp. 8, 21. 


There can be no difficulty in recognizing the form commonly known as 
Asterolepis bohemica Barr., since fossil fishes are not numerous in the Devonian 
of Bohemia, and this one is distinguished by its peculiar ornamentation. The 
tubercles are rather closely set, conical, and their summits, instead of being 
smooth, are finely punctate. The plates are of relatively large size, and usu- 
ally exhibit considerable convexity. 

There are two specimens of the dorso-median preserved in the Schary Col- 
lection, now the property of the Museum of Comparative Zodlogy, besides the 
impression of a third plate supposed to be one of the ventro-laterals. They 
are all from the same horizon, and two are from the identical locality as 
Barrande’s type specimens. As has already been pointed out by von Koenen 
(Joc. cit., 1895, p. 8), it is extremely improbable that the figures given by Bar- 
rande are of the dorso-median. Their lack of bilateral symmetry, and their 
relative thinness, compel us to locate them elsewhere, perhaps on the ventral 
surface. 

Certain it is, however, that the specimens shown in the accompanying 
figures represent the median dorsal plate. Not only do they fulfil the 
requisite conditions of shape, symmetry, and thickness, but both of them 
present fractures on the posterior end, where the carinal process has been 
broken off, leaving a cross-section of the inferior longitudinal ridge. On the 
strength of this evidence we are obliged to assign the species to Dinichthys. 
One of the plates has the inferior ridge much more strongly developed than 
the other, and differs considerably in form. But the ornamentation is essen- 
tially the same, and we are content to refer them both to D. bohemicus, since 
the coinage of new specific titles to include uncharacteristic fragments is 


38 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


greatly to be deprecated. Barrande’s Coccosteus fritscht, as von Koenen has 
already surmised, is probably founded on the dorso-median of Aspidichthys. 
Formation and Locality. — Middle Devonian (Etage Gg1); Bohemia. 


Dinichthys tuberculatus News. 


1888. Dinichthys tuberculatus, J. S. Newberry, On the Fossil Fishes of the Erie 
Shale of Ohio (Trans. N. Y. Acad. Sci., Vol. VII. p. 179). 

1889. Dinichthys tuberculatus, J. S. Newberry, Paleozoic Fishes of North America 
(Monogr. U. S. Geol. Surv., Vol. XVI. pp. 98, 99, Pl. XXXII. Fig. 3). 

1889. Dinichthys pustulosus (errore), M. Lohest, De la découverte d’espéeces améri- 
caines de poissons fossiles dans le Dévonien supérieur de Belgique (Bull. 
Soc. Géol. Belge, Vol. XVI. p. lvii). 

1892. Dinichthys pustulosus (errore), [E. D. Cope], American Devonian Fishes found 
in Belgium (Amer. Naturalist, Vol. XX VI. p. 1025). 


It is proper to record this species in connection with the foregoing, not only 
in order to complete the list of European representatives of the genus, so far as 
they have been described, but also because this is the only species of Dinichthys 
which is known to be common to both continents. This form may be regarded 
as the connecting link between the Old World species and the New; not that 
all the American Dinichthyids were derived from this species, but that this is 
one of the bonds through which the ancestry of the Western fishes can be traced 
backward to its starting point in Northern Europe. This chain of forms leads us 
eastward from Manitoba, through Iowa, Wisconsin, and Ohio, to New York and 
Pennsylvania; from the last named State D. tuberculatus carries us across the 
Atlantic to Belgium; next we meet with D. eifeliensis and D. pelmensis in 
Germany, followed by one species in Bohemia; and finally we come up with 
D. trautscholdi and D. livonicus associated with the ancestral Coccosteus and 
other derivatives from the same stock in the Devonian of Northwest Russia. 

Formation and Locality. — Chemung Group; Pennsylvania. Psammite de 
Condroz ; Belgium. 


It remains only to present a description of certain Dinichthyid remains from 
the Hydraulic Limestone beds of Milwaukee, Wisconsin, a locality from which 
none have hitherto been known. 


Dinichthys pustulosus sp. nov. 
Plate 3, Fig. 4. 


The F. H. Day Collection, purchased by the Museum of Comparative Zodl- 
ogy in 1881, contains a number of fish remains from the Hydraulic Cement 
Quarries near Milwaukee, Wisconsin. Among them are two plates whose 
preservation is such as to warrant description, especially since up to the 
present time but two species (Rhynchodus greenet and Heteracanthus politus) 
have been noticed from this locality. 


EASTMAN: THE DINICHTHYIDS. 39 


The first of these (Plate 3, Fig. 4) is easily recognizable as the left antero- 
dorso-lateral of a new species of Dinichthys, and is chiefly remarkable for its 
finely tuberculated style of ornament. ‘This plate is nearly twice the size of the 
corresponding element described by Newberry as D. tuberculatus, its articulating 
condyle is differently situated, and the tuberculation is entirely dissimilar. 
Of D. tuberculatus, Newberry? speaks as follows: “The tuberculation of the 
surface is relatively coarse, and the tubercles vary much in size and are irregu- 
larly scattered. Most of them seem to be hemispherical and plain, but others 
are more or less pitted, and a few are stellate.” In the present species the 
tubercles are small and closely crowded, and are distinctly stellate at their bases.? 
It is somewhat surprising that there should be so few American species which 
present the characteristic surface ornamentation of the Coccosteide ; the infer- 
ence is that the tuberculated are more primitive than non-tuberculated forms. 

A longitudinal fracture traverses the plate to the left of the sensory canal. 
It is interesting in that it displays very clearly the course of the vascular 
(Haversian) canals, which run essentially parallel with the surface of the plate. 
The canals are also well shown where the articulating condyle has broken off ; 
and from their direction it would appear that the plate had grown by incre- 
ments to the visceral surface only. 

The second specimen in this collection that deserves notice is evidently the 
impression of one of the ventral plates, probably the left antero-ventro-lateral, 
the substance of the bone itself being entirely worn away. The surface orna- 
ment cannot be discovered from this specimen, but several fragments associated 
with it exhibit the same tuberculation as occurs on the antero-dorso-lateral just 
described. The only reason for disassociating the two specimens specifically is 
that they represent individuals of somewhat different size; but the dispropor- 
tion does not appear of itself sufficient ground for separation. The supposed 
antero-ventro-lateral measures 23 cm. in length by 11 cm. in width at about the 
middle of the plate. How much of the anterior portion is wanting cannot be 
accurately determined. Another large specimen from the same locality is 
to be seen on exhibition in the United States National Museum, at Washing- 
ton, D. C., bearing the catalogue number 14,821. 

Fragments of various size, and indistinguishable from this species so far as 
one may judge from the ornamentation, have been collected by the writer in 
the State Quarry fish-bed, near North Liberty, Iowa.8 Other remains have 
been found in the Cedar Valley Limestone of the same State by Professor 
Samuel Calvin. One of the largest of these, which belongs to the State Uni- 


1 Newberry, J. S., Paleozoic Fishes of North America (Monogr. U. S. Geol. Surv., 
Vol. XVI. p. 99), 1889. 

? The artist has represented these somewhat diagrammatically in Figure 4, with 
the result of imparting a rougher aspect to the plate than is natural, although it is 
plain that the original has suffered somewhat from abrasion. 

* See notes “On the Occurrence of Fossil Fishes in the Devonian of Towa,” 
appended to Report on the Geology of Johnson County (pp. 108-116), by Samuel 
Calvin, State Geologist. 1897. 


40 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


versity Museum, shows the posterior portion of the cranium above and below 
very satisfactorily. 

There is good reason for believing that this species also occurs in the Hamil- 
ton of New York State. Mr. F. K. Mixer, who has made a careful search for fish 
remains in the vicinity of Buffalo, has obtained certain fragments from the En- 
crinal Limestone near the mouth of Eighteen Mile Creek, which exhibit almost 
precisely the same style of ornamentation, and agree furthermore in size with 
D. pustulosus. One of these fragments is identifiable as the suborbital plate, 
and shows very distinctly the sensory canals. Another represents about one 
half of one of the ventro-lateral plates, is rabbeted upon the edges, and shows 
some variation in the size of its tubercles. Again we notice that tuberculation 
of the ventral plates bears witness to primitive conditions. The ventro-lateral 
measures 21 em. in maximum width, and is traceable for about the same dis- 
tance in a longitudinal direction, the remaining portion being broken away. 
It is to be hoped that further and better preserved material will be forthcoming 
from this horizon, since by reason of their greater antiquity and primitiveness 
Hamilton Dinichthyids are likely to prove even more interesting than those 
of Upper Devonian age. In the event of these plates being proved by future 
discoveries to belong to a species distinct from D. pustulosus, with which they 
are now provisionally associated, it is but fitting to reserve the name D. mizeri 
for the New York species, in honor of the gentlemen to whom we are indebted 
for our first knowledge of it. 

The title of D. pustulosus, although misapplied by M. Lohest for D. tubercu- 
latus, has never been defined, and we are accordingly at liberty to appropriate 
it for the present species. 

Formation and Locality. — Hamilton Limestone; Wisconsin, Iowa, and New 


York (?). 


In this connection a word may be said concerning another plate discovered 
by Mr. Mixer, near Sturgeon Point on the shore of Lake Erie. The fossil is em- 
bedded in a loose block derived apparently from the Portage Shale, exposures of 
which occur at this locality. It presents the inferior aspect of a small dorso- 
median plate, which is worn away anteriorly in such fashion as to reveal an 
impression of the external surface. This is seen to be finely tuberculated, and a 
few tubercles are left on an impression of a small plate (antero-dorso-lateral ?) 
adjoining the first. The longitudinal carina and its terminal process are both 
indicated, although the latter is partly fractured. The plate is quite thin; 
and this fact, together with its small size, fine tuberculation, and other characters, 
renders it probable that it belonged to an immature individual. It may be 
referred with considerable certainty to D. ringuebergi, a species which until 
the present time has rested upon a solitary dorsal shield from the same locality. 
Mr. Mixer’s specimen is about one fourth smaller than the type, however, 
and is of more slender construction. If properly regarded as a young indi- 
vidual, it is interesting as being one of the few that are known. ° 


EASTMAN: THE DINICHTHYIDS. 41 


It is evident from the figures of the type specimen of D. ringuebergi! that 
the carinal process has been considerably eroded, and the diagram of the infe- 
rior surface is not wholly accurate. If the anterior margin is entire, as repre- 
sented for this species, it covers the region back of the head almost as completely 
- as in Coccosteus. The type specimen is preserved in the private collection of 
its first describer, Mr. E. N. S. Ringueberg, at Lockport, New York. All of 
the specimens discovered by Mr. Mixer that are mentioned in the present 
paper are preserved in the collection of the Buffalo Society of Natural Sciences. 

Another very beautiful example of a young Dinichthyid is preserved in the 
Museum of Oberlin College, and through the courtesy of Professor A. A. 
Wright we have been enabled to reproduce a photograph of it, shown in 
Plate 5, Fig. 3. It is only about 5 cm. long, and 4.5 cm. in maximum width ; 
the external surface is non-tuberculated. Unfortunately the terminal process 
is missing, but the inferior carina is very distinct. It is also seen to be strongly 
emarginate in front. 

The drawings for Plates 1 to 3 have been executed by Messrs. C. A. King 
and J. W. Folsom. Plates 4 and 5 are reproduced from photographs of the 
original specimens, taken by Dr. T. A. Jaggar, Jr., excepting Figure 3 of 
Plate 5. 


1 Amer. Journ. Science, [3], Vol. XX VII. p. 477, June, 1884. 


42 


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BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


/ 


LIST OF AMERICAN 


Sous 


. canadensis Whiteaves, 
. clarki Claypole, 


corrugatus Newberry, 
curtus Newberry, 
gouldi Newberry, 


. gracilis Claypole, 

. hertzeri Newberry, 

. tagens Wright (MS8.), 
. intermedius Newberry, 
. kepleri Claypole, 

. Lincolni Claypole, 

. minor Newberry, 

. newberryi Clarke, 


. precursor Newberry, 

. prentis-clarki Claypole, 
. ringuebergi Newberry, 
. D. 
D. 
. D. pustulosus nobis, 


terrelli Newberry, 
tuberculatus Newberry, 


SPECIES OF DINICHTHYS. 


Upper Devonian, Manitoba. 

Cleveland Shale, Ohio. 

Cleveland Shale, Ohio. 

Cleveland Shale, Ohio. 

Cleveland Shale, Ohio. 

Cleveland Shale, Ohio. 

Huron Shale, Ohio. 

Cleveland Shale, Ohio. 

Cleveland Shale, Ohio. 

Cleveland Shale, Ohio. 

Marcellus Shale, New York. 

Cleveland Shale, Ohio. 

Genesee and (?) Portage Shales, 
New York. 

Corniferous Limestone, Ohio. 

Cleveland Shale, Ohio. 

Portage Shale, New York. 

Cleveland Shale, Ohio. 

Chemung Group, Pennsylvania. 

Hamilton Limestone, Wisconsin, 
Towa, and (?) New York. 


EASTMAN: THE DINICHTHYIDS. 43 


BIBLIOGRAPHY. 


Pander, C. H. 
1857. Ueber die Placodermen des devonischen Systems. St. Petersburg. 


Barrande, J. 
1872. Systéme Silurien du centre de la Bohéme. Vol. I. Supplement. 
’ Newberry, J. S. 
1873. Report Geological Survey of Ohio, Paleontology. Vol. I. 
Newberry, J. S. 
1875. Report Geological Survey of Ohio, Paleontology. Vol. II. 
Kayser, E. 
1880. Dinichthys eifeliensis.  Zeitschr. deutsch. geol. Gesellsch., Vol. 
XXXII. p. 818. 
Koenen, A. von. 
1883. Beitrag zur Kenntniss der Placodermen. Abhandl. kén. Gesell. Wis- 
sensch. Gottingen, Vol. XXX. . 
Clarke, J. M. 
1885. On the higher Devonian Faunas of Ontario County, New York. Bull. 
U.S. Geol. Surv., No. 16, pp. 85-120. 
Lohest, M. 
1888. Recherches sur les Poissons des Terrains Paléozoiques de Belgique. 
Ann. Soc. Géol. Belg., Vol. XV. pp. 112-203. [See especially “ Résul- 
tats Géologiques,” ete., pp. 168-196.] 
Traquair, R. H. 
1889. Homosteus, Asmuss, compared with Coccosteus, Agassiz. Geol. 
Mag., [3]. Vol. VI. pp. 1-8, Plate I. 
Newberry, J. S. 
1889. The Paleozoic Fishes of North America. Monogr. U.S. Geol. Surv., 
Vol. XVI. 
Trautschold, H. 
1889. Ueber Coccosteus megalopteryx Trd., Coccosteus obtusus und Che- 
liophorus verneuili Ag.  Zeitschr. deutsch. geol. Gesellsch., Vol. XLI. 
pp. 35-48, Plates III.-VI. 
Trautschold, H. 
1890. Ueber Megalopteryx und Pelecyphorus.  Zeitschr. deutsch. geol. 
Gesellsch., Vol. XLII. p. 575. 


44 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Jaekel, O. 
1890. Ueber Coccosteus. Zeitschr. deutsch. geol. Gesellsch., Vol. XLII. 
p- 776. 


Traquair, R. H. 
1890. On the Structure of Coccosteus decipiens Ag. Ann. Mag. Nat. 
Hist., [6], Vol. V. pp. 125-136, Plate X. [Literature references, pp. 
135, 136.] 
Woodward, A. S. 
1891. Catalogue of Fossil Fishes in the British Museum. Part II. 
Giirich, G. 
1892. Ueber Placodermen und andere devonische Fischreste im Breslauer 
mineralogischen Museum. Zeitschr deutsch. geol. Gesellsch., Vol. XLII. 
pp- 902-912. 
Wright, A. A. 
1893. On the Ventral Armor of Dinichthys. Report Geol. Surv. Ohio, 
Vol. VIL. pp. 620-626, Plate XLIV. 
Wright, A. A. 
1894. The Ventral Armor of Dinichthys. Amer. Geol., Vol. XIV. pp. 313- 
320, Plate 1X. 
Koenen, A von. 
1895. Ueber einige Fischreste des norddeutschen und boéhmischen Devons. 
Abhandl. k. Gesell. Wissensch. Gottingen, Vol. XL. pp. 1-37, Plates L-V. 
Dean, B. 
1895. Fishes, Living and Fossil. New York, pp. 129-138. 
Dean, B. 
1896. On the Vertebral Column, Fins, and Ventral Armoring of Dinicithys. 
Trans. N. Y. Acad. Sci., Vol. XV. pp. 157-163, Plates VII., VIII. 
Eastman, C. R. 
1896. Preliminary Note on the Relations of certain Body-Plates in the 
Diniclithyids. Amer. Journ. Science, [4], Vol. Il. pp. 46-50. 
Eastman, C. R. 
1896. Observations on the Dorsal Shields in the Dinichthyids. Abstract 
in Amer. Geol., Vol. XVIII. pp. 222, 223. 
Dean, B. 
1897. Additional Note on the Ventral Armoring of Dinichthys. Trans. 
N. Y. Acad. Sci., Vol. XVI. pp. 57-60, Plate III.) 
Wright, A. A. 
1897. New Evidence upon the Structure of Dinichthys. Fifth Ann. Rep. 
Ohio State Acad. Sciences, pp. 59, 60. 


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PLATE l. 


Fig. 1. Dinichthys intermedius Newb. X 3. Projection of cranium and dorsal plates 
in their natural relations with respect to one another. DM, Dorso- 
median; ADJZ, Antero-dorso-lateral; PDZ, Postero-dorso-lateral. 
Sensory canals indicated by double dotted lines, boundaries of plates 
by single lines. The posterior process depends at an angle of about 
60° from the plane of the dorso-median. 

Fig. 2. Dinichthys (?) newberryi Clarke. X 4. Restoration of the Portage plastron 
shown in Plate 4. AVM, Antero-ventro-median; PVJZ, Postero- 
ventro-median; AVZ, Antero-ventro-lateral; PVZ, postero-ventro- 
lateral. Radiating lines show approximately the course of vascular 
canals. Overlapped borders of plates indicated by dotted lines. 


PLATE I. 


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EasTMAN. — Dinichthys. 


Fig. 


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Fig. 


PLATE 2. 


Dinichthys terrelli Newb. X }. Cleveland Shale; Lindale, Ohio. M.C.Z., 
Cat. No. 1879. Fragment showing internal surface of antero- and 
postero-dorso-lateral plates preserved in natural association with each 
other. Their union by pegs and sockets, the sinuous lateral boundary 
of the posterior plate, and the base of articulating condyle of the 
antero-dorso-lateral, are noteworthy features. 

Postero-ventro-median plate of an indetermined Dinichthyid species, from 
the Cleveland Shale of Lorain County, Ohio.  X 4 (approximately). 
M. C. Z., Cat. No. 1300. The external surface, shown here, bears im- 
pressions of overlapping plates, and is notched in front for reception 
of the antero-ventro-median. 

Dinichthys bohemicus (Barr.). X %. Middle Devonian (Etage Gg?); 
Svagerka, Bohemia. M. C. Z., Cat. No. 1377. Tuberculated dorso- 
median plate. The posterior portion of the specimen, which was 
fractured obliquely downward, has been ground smooth and polished, 
so as to show the inferior carina in section. 

Dinichthys pelmensis sp. nov. X 4. Middle Devonian; Pelm, Eifel Dis- 
trict. M.C. Z., Cat. No. 1375. Dorso-median plate with perfectly 
preserved carinal process, and faint indications of sensory canals. 

Dinichthys terrelli Newb. X 4. Cleveland Shale; Lorain County, Ohio. 
M. C. Z., Cat. No. 1301. Antero-ventro-median plate, seen from the 
external surface. Thickness at posterior tip less than 2 mm.; the 
plate has every indication of being entire, or very nearly so. 

Same specimen as shown in Fig. 5, viewed from the internal or visceral 
side. The thickened T-shaped ridge seen on this surface is very 
characteristic. 


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PLATE 3. 


Dinichthys (2) intermedius Newb. X 3. Cleveland Shale ; Lindale, Ohio. 
M. C. Z., Cat. No. 1880. External aspect of supposed antero-ventro- 
median plate. 

Dinichthys terrelli Newb. X 3- Cleveland Shale; Lorain County, Ohio. 
M. C. Z., Cat. No. 1315. Posterior aspect of carinal process belong- 
ing to a large-sized dorso-median, viewed in a vertical position. The 
semicircular incision below, where it overrode the vertebral axis, its 
massive character, and depth of posterior cavity, are remarkable. It 
projects downward and backward at an angle of about 60° with the 
plane of the dorso-median, traced along the median line of the back. 

Dinichthys eifeliensis Kayser. X 1, Middle Devonian ; Berndorf, near 
Hillesheim, Eifel District. M. C. Z., Cat. No. 1374. Carinal process 
detached from dorso-median plate. 

Dinichthys pustulosus sp. NOV. X 1, Hamilton Limestone ; Cement Quar- 
ries, Milwaukee, Wisconsin. M. C. Z., Cat. No. 1381. Slightly 
abraded antero-dorso-lateral plate, showing single sensory canal, and 
relatively fine tuberculation. 


Naa 
j 


Eastman. — Dinichthys. 


PLATE 4. 


Dinichthys (2) newberryi Clarke. X44. Portage Shale; Sturgeon Point, 
near Buffalo, New York. Weathered plastron and associated frag- 
ments. Reproduced from a photograph without retouching. 


Eastman. — Dinichthys. 


Fig. 1. 


Fig. 2. 


Fig. 4. 


PLATE 5. 


Postero-ventro-median plate of an unknown Dinichthyid species. X 2 
(approximately). M. C. Z., Cat. No. 1475. This plate is preserved in 
counterpart, and a portion of the bone adheres to the opposite side. 

Dinichthys (2) bohemicus (Barr.). X 4. Middle Devonian (Etage Gg?) ; 
Chotec, Bohemia. M. C. Z., Cat. No. 1376. Detached dorso-median 
plate, more highly arched and rounded in outline than that shown in 
Plate 2, Fig. 3, but having the same ornamentation. The carinal 
process is slender, and appears only in section where the matrix has 
been ground away. 

Dorso-median plate of a young individual representing an unknown 
Dinichthyid species, seen from the under side. X 2. Cleveland 
Shale ; vicinity of Cleveland, Ohio. Original preserved in Museum 
of Oberlin College. 

Dinichthys eifeliensis Kayser. X $- Middle Devonian; Eifel District. | 
Internal aspect of right antero-ventro-lateral plate. M.C. Z., Cat. 
No. 1474. 


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rMAN. — 


No. 3. — Trichonympha, and other Parasites of Termes flavipes. 
By James F. Porrter.? 


CoNTENTS. 


fcimchonympha agilis. . . . . 48(4 Gregarinida. ..+. . . . =. 65 
eeiyrsonympha vertens . . . - 09|Papers Cited «2... s. . « 67 
3. Dinenympha gracilis. . . . . 63!Explanationof Plates . ... . 68 


NeaRLy twenty years ago Dr. Joseph Leidy (’77) discovered several 
new and quite distinct species of Protozoa living parasitic in the in- 
testine of Termes flavipes.2 A few years later he (’81) published 
an extended account of these strange creatures, accompanied by a 
large number of drawings illustrating the great variety of forms which 
they assume. Some of these parasites, to which he gave the name 
Trichonympha agilis, were so unlike any previously described Protozoa 
that their discoverer was not entirely certain where they ought to be 
classed ; at first (77) he regarded them as “ probably related with the 
Turbellaria on the one hand, and by evolution with the Ciliate Infusoria 
on the other.” Later (’81, p. 529), however, he was disposed to view this 
parasite as having a character intermediate between a ciliate infusorian 
and a gregarine, but (p. 436) more nearly related to the gregarines. 

Since then W. Saville Kent (’85) has made some additional observa- 
tions on the structure and habits of this parasite from material sent 
to him by Dr. Leidy, and has also discovered a very similar parasite, to 
which he gave the name Trichonympha leidyi, in the White Ant of 
Tasmania. 

Besides these studies on Trichonympha, there have appeared descrip- 
tions of some other forms which seem to be closely related to this genus. 
I refer to Joenia, described by Grassi (’85), and to Lydionella, found by 
Frenzel (’91), both parasitic in the intestine of Termites. The paper by 


1 Contributions from the Zodlogical Laboratory of the Museum of Comparative 
Zoodlogy at Harvard College, under the direction of E. L. Mark, No. LXXXIV. 
2 Some of these had been previously seen by Lespés (’56); but he alluded to 
them only briefly (pp. 237, 258) in his memoir. Compare Leidy (’81, pp. 425-428). 
VOL. XXXI. — NO. 3. 


48 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Frenzel is especially important, because it gives a careful account of the 
structural conditions of the genus studied by him, though even he did 
not make use of sections to ascertain the minuter details of structure. 

At the suggestion of Dr. Mark, I began in the autumn of 1895 an 
investigation of this peculiar and interesting parasite of Termes flavipes, 
in the hope of being able, by means of more recent methods, — especially 
sectioning, —to add something to what was already known concerning 
them. An excellent 1.5 mm. homogeneous immersion by Zeiss has been 
of much value to me in studying the minute details of structure. In 
October I captured some of the White Ants, both *‘ workers @@nd 
* soldiers,” in the woods near Cambridge, and on opening the intestine 
of one of them found it swarming with the same kinds of parasites that 
Leidy had discovered in his New Jersey Termites. 


1. Trichonympha agilis. 


Plate l: Plate 2; Plate 3, Figs. 24-26. 


I have represented in Figures | and 2 what seem to me fairly normal 
and characteristic appearances of a quiescent T. agilis, and in Figure 3a 
view of the head end, seen from the anterior pole. My figures should 
be considered, however, simply as in a measure supplementary to those 
of Leidy. His observations were so careful and accurate that I have 
almost no modifications or corrections of his description to suggest ; but 
I shall be able to add something to his account. 

Upon seeing one of these peculiar animals, the question immediately 
arises, Wherefore such a remarkable cloak of cilia? Well developed 
locomotor organs are the last thing one would expect to find in a para- 
site whose food is close at hand and whose field of exploration is so 
limited. With the hope of getting some light on this question, and 
wishing at the same time to discover, if possible, the situation of 
the mouth, I experimented by putting living specimens into various 
fluids; among others, into very much diluted milk. The Tricho- 
nymph immediately on escaping from the intestine began ploughing 
their way through the milk corpuscles, passing across the field of the 
microscope so rapidly that, in order to follow them, it was necessary to 
keep the slide in constant motion. The various courses traversed by 
them were indicated by paths cleared of the oil globules. Upon watch- 
ing their movements, it presently became apparent that the shortest and 
most anterior cilia were to a very large extent responsible for the mo- 
tion. The longer cilia, those extending backwards to about the region 


PORTER: TRICHONYMPHA. 49 


of the posterior end of the body (Figs. 1, 2) vibrated but little, while 
the very long cilia that enveloped the posterior half of the animal and 
reached out far behind (Figs. 1, 2) appeared to be absolutely motionless. 
These last are not quite useless, however, for I noticed that milk cor- 
puscles began to be entangled among them, and soon afterward long 
trains of globules were being dragged behind each animal (Figs. 6 
and 4). At first I thought this was merely accidental, but after longer 
observation I came to the conclusion that the cilia actually enfold 
the globules, for these are gradually drawn in towards the animal, and 
finally come to lie in close contact with the posterior part of its body 
(Figs. 4,5). That the cilia are prehensile in their nature has already 
been observed by Kent, for he says: ‘‘ When placed in diluted milk, the 
animalcules of both the American and Tasmanian species of Tricho- 
nympha have been observed by me to assume a fixed condition that has 
not hitherto been described. An attachment to the surface of organic 
substances or other convenient fulcra is then accomplished through the 
medium of the long fascicle of hair-like cilia that are produced from their 
posterior extremity. These cilia, intersecting one another at a short 
distance from the body, form a sort of hollow cone, the expanded base of 
which grasps the selected fulcrum of support after the manner of an 
acetabulum. This habit of, as it were, anchoring themselves by their 
long caudal cilia was observed in both the adult and the immature 
animalcules.” 

I have observed that Trichonympha may attach itself even to the 
cover glass by means of its cilia. This evident habit of grasping things 
by means of the caudal cilia suggested the idea that these cilia might 
perhaps have the function not only of attaching the animal to the host, 
but also of procuring food ; however, I must acknowledge that it appeared 
highly improbable that these seemingly motionless cilia, which clothe 
and tightly invest the animal, could have a function which ordinarily 
requires so much activity ; furthermore, I had been unable thus far to find 
any mouth opening. Kent in his article says (p. 271): “An important 
point that was left undetermined by Dr. Leidy respecting the structure 
of Trichonympha relates to the precise position of the oral aperture. 
The bodies of the animalcules are almost invariably filled with fragments 
of the woody débris devoured by their hosts, the White Auts, which shows 
that their sustenance is taken into their body in a solid state, and is 
not simply absorbed in the fluid form, as occurs with the group of the 
Opalinide. A prolonged observation of living examples of the American 
species remitted me by Dr. Leidy, and likewise of the Tasmanian type 


~ 


50 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


here introduced, has resulted in my determining that a distinct oral aper- 
ture is developed upon one side of the body at a short distance only 
from the apical extremity. This orifice takes the form of a transverse 
slit, and is followed by a narrow esophageal tract which opens into the 
capacious digestive cavity that occupies one half or two thirds of the 
posterior region of the body. The plan recommended by Dr. Leidy for 
observing the vital phenomena of these animalcules is to empty out the 
intestine of the White Ant containing them into a little white of egg. I 
also have found this material favorable for their observation, but have 
gained an additional insight into their life history by emplgying in a 
like manner thinly diluted milk. In this medium they not ouly live for 
a considerable time, but meet with abundant nutriment, their pharynx 
and digestive cavity being frequently found densely packed with its 
component corpuscles after their immersion in this fluid for a short 
interval.” 

Unfortunately, Kent does not give any figures, and I have been un- 
able to discover in the living animals the mouth he describes. However, 
the existence of such a structure could, I imagined, be easily determined 
by means of sections. These I endeavored to procure by sectioning the 
whole alimentary canal of the Termes, hoping in this way to obtain some 
sections of Trichonympha in a direction favorable for settling this point. 
I was not disappointed. 

To my astonishment I found that the small intestine was completely 
packed with animalcules, and among them hundreds of Trichonympha ; 
these, however, were not promiscuously distributed through the parasitic 
mass, but were, as a rule, excluded from the periphery of the intestine. 
This position, of course, precluded the possibility of their attaching 
themselves directly to the wall of the host’s intestine. 

The sections of Trichonympha thus obtained revealed absolutely no 
trace of such an oral aperture as that described by Kent. They did 
show, however, an interesting condition of affairs, which I think may 
perhaps account for the presence of the numerous fragments of ligneous 
fibre and other food particles so often found within the body, without 
necessarily supposing them to have entered through a persistent mouth. 
In many of the cross sections of the posterior half of Trichonympha I 
observed deep folds of the body wall; these almost invariably contained 
cilia, doubtless engulfed at the time of the folding (Plate 2, Figs. 9, 10, 
and 14). Particles of wood fibre were also seen entangled among the 
cilia in these folds (Fig. 10, for. lig.). I think we have in this condition 
an explanation of the mystery. Cilia entangling particles of woody fibre 


PORTER: TRICHONYMPHA. 51 


become surrounded by a deep fold of the posterior part of the body. 
The most of the cilia are probably afterwards withdrawn from the fold, 
but the lips of the fold become so closely applied to each other (Fig. 14), 
that the ligneous particles are left behind in the depths of the fold. IL 
believe that the lips of the fold afterwards fuse together, that the walls 
of the infolded portion then disappear, and that the food thus finally 
becomes entirely enclosed in the body protoplasm of the parasite. The 
condition of the body wall at a in Figure 14 seems to indicate that the 
wall is there in process of disintegration or dissolution. 

This is certainly a remarkable method of feeding for a ciliate infu- 
sorian ; but I see no escape from the conclusion that solid food material 
never enters the animal in any part of the anterior nucleus-containing 
portion of the body; for of the hundreds of specimens that I have ex- 
amined, both living and dead, none have ever shown solid particles of 
food in the anterior region, whereas the absence of ligneous matter from 
the posterior part of the body is a condition rarely met with. It seems 
highly improbable — to say nothing of the absence of any trace of a 
permanent oral structure —that solid food should pass through this 
anterior region so quickly that not a single case of its passage, or of its 
presence in this part, should have been discovered by any one of those 
who have studied these parasites.t But besides this, it is to be noted 
that the wall of the whole posterior part of the body is thin and ex- 
ceedingly delicate, so that it would not present any great barrier to 
the entrance of foreign particles. It disintegrates so readily that I 
have seen this portion of a living animal go to pieces completely and 
disappear, while the anterior half, bearing the cilia, continued to swim 
around. 

For the sake of convenience in description, I shall recognize in Tricho- 
nympha three regions (compare Plate 2, Figs. 13, 15, 7) : first, an ante- 
rior nipple-like part, which is quite sharply separated from the following 
region; secondly, the middle or bell-shaped part, the axial hinder por- 


1 It is true that Kent speaks of seeing the “pharynx” and digestive cavity 
densely packed with milk corpuscles; but while this is the case in the posterior 
portion of the body, I have never seen milk globules or other particles of food in 
any part of the organism that could be likened to a pharynx. 

Frenzel (’91) likewise was unable to find direct evidence of the existence of a 
mouth opening in Leidyonella. Although he inclined to the opinion that it was 
located at the anterior end of the elongated neck-like region, he admitted the possi- 
bility that it might be located elsewhere. Grassi (’85), too, was equally unsuccess- 
ful in locating the mouth in Joenia, although in this genus likewise the presence of 
particles of woody fibre in the body shows that solid food is taken in. 


52 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


tion of which usually projects a greater or less distance into the follow- 
ing or third part, and always contains the nucleus. These two parts 
together have been called by Leidy the “head.” Thirdly, the posterior 
part, to which Leidy gave the name of “ body.” 

I begin with the anterior region, which I have found to be of rather 
complicated structure. To aid in the description, I have made sev- 
eral large semi-diagrammatic sketches (Plate 2, Figs. 7, 8, 11, 12, 17). 
Figures 7 and 8 are intended to represent sections not thicker than the 
space between two successive longitudinal rows of cilia. 

The nzpple-like part of Trichonympha is, as the name which I have 
given it implies, a conical, more or less elongated protuberance at the 
front end of the parasite. According to its state of contraction it is 
one half, two thirds, or even more than two thirds, as thick at its base 
as itis long. The auterior end is evenly rounded or sometimes more 
pointed. Seen in profile its outline at the posterior end usually passes 
gradually into that of the middle part, but at times it appears sharply 
separated from this part by a deep constriction. Deep focusing and 
especially longitudinal sections show that the separation is more com- 
plete than superficial focusing would lead one to suppose. 

The “nipple” consists of a cylindrical, or slightly tapering, rod-like 
axis (Plate 2, Figs. 7, 8, 18, ax.), composed of finely granular proto- 
plasm, and one or more enveloping layers unlike in structure. The 
anterior end of the axial rod is expanded into a knob-like enlargement 
(Figs. 7, 8, twb.), the anterior surface of which has a conical (Fig. 7), or 
more commonly a hemispherical (Fig. 8) form. The rod is narrowest 
a little in front of its middle, and thence increases in thickness very 
gradually both toward the knob and toward the *‘ bell.” The protoplasm 
of the axial rod is continuous behind with the granular protoplasm 
which occupies the axis of the “bell,” and its surface appears to be 
differentiated into a thin membrane. This protoplasmic axial rod is 
surrounded by two, and possibly sometimes by three, well ei oe 
layers of formed substance (Fig. 7, sé.’, st.” ; Fig. 8, st.’, st”, st”). 
The middle layer often appears as though deste by an eal 
separation between the outer and inner layers, but it is of so common 
occurrence that I do not feel able to declare positively that it is merely 
an artificially produced space. 

The anterior extremity of Trichonympha terminates in a colorless 
translucent cuticular cap (Figs. 7, 8, pil.), which stains only slightly at 
most, and frequently not at all. The centre of the cap covers the an- 
terior expanded end (¢ub.) of the rod-like protoplasmic core, and its 


PORTER: TRICHONYMPHA. 53 


margin rests on the anterior sloping surface of the outer layer (st.’”), 
while the anterior end of the inner layer (s¢.') is covered by the expanded 
knob-like portion (tub.) of the protoplasmic core (Figs. 7 and 8). 

The whole of this portion, forming the nipple-like projection of the 
anterior extremity of the animal, is very active in the living creature ; 
it is constantly in motion, turning from side to side, and, as it were, 
nosing its way through the crowd of its associates. 

The axial rod (Figs. 7, 8, ax.) is apparently the only means of union 
between this nipple-like part of the animal and the bell-shaped region, 
for a fissure (Figs. 7, 8, jis.), encircling the base of the nipple, penetrates 
the layers surrounding the axis, thus severing all other connection. 
Behind the fissure, what seem to be continuations of the outer and 
inner layers of the nipple are carried backward over the bell-shaped 
region, i. e. over a little less than one third the length of the animal. 
A middle layer never appears in this region, a fact which makes it seem 
probable that such an appearance in the nipple region is due merely to 
a space between the inner and outer layers. Since the enveloping layers 
are so much alike in the nipple- and bell-shaped regions, | shall not 
attempt to give a separate description of them in each region, but pro- 
ceed at once to state what I have to say about these layers for the 
whole “head” region. 

The inner layer stains only slightly, if at all. From the fissure back- 
wards, as far as it extends, it is marked by fine lines perpendicular to 
the surface. This appearance is due to the cilia, which penetrate both 
the enveloping layers. The same structural condition probably exists 
in front of the fissure, although it was not possible, owing to the small 
diameter of the nipple-like projection (about 6 w) and the thickness of 
my sections (3.3 uw), to detect any striations. 

The cilia arise from the granular protoplasm, at the deep surface of 
the inner layer, which they traverse, and thus give a striated appear- 
ance (Fig. 16, st.str.). They are of about the same refractive power 
and stainability as the outer layer, and consequently I have not been 
able to trace them through that layer; but I think that their passing 
through it is hardly questionable. The arrangement of the radial stria- 
tions of the inner layer, which I believe to be due to the cilia, is best made 
out from tangential sections of the bell, where their cut ends appear as 
dots. ‘The dots are placed in quincunx order, much as shown in Fig. 
20. This arrangement gives two series of diagonal rows of dots cross- 
ing each other nearly at right angles and making angles of 45 de- 
grees with the longitudinal axis of the animal. Secondary rows, both 


54 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


parallel and perpendicular to the longitudinal axis, though less regular 
than the oblique ones, are easily recognizable. Owing to the surface of 
the bell being curved, the arrangement in quincunx is not as evident as 
I have represented it diagrammatically in Figure 20, but has more the 
appearance shown in Figure 19, where the oblique and transverse rows 
are somewhat curved, and the longitudinal ones diverge slightly as they 
pass backwards. 

The outer layer (Fig. 7, st.’”) is composed of a series of ridges, or, 
more properly speaking, plates. These plates, cut crosswise, are shown 
in Figure 16 (st.’’), which represents a transverse section through the 
middle of the bell-shaped ciliated portion of the body, about in the place 
indicated by the line 16-16 in Figure 15. The plates are seen more 
clearly in the diagrammatic cross section shown in Figure 17, st.’”. 
In surface views of the animal each of the plates of the nipple appears 
to be split into two at the region of the fissure (Fig. 11, jis.) ; this, 
however, is only an optical illusion. Although the plates of the bell- 
shaped part are twice as numerous as those of the nipple-like part, 
there is no continuity between the two sets, the appearance of splitting 
being due to the overlapping of the anterior set over the posterior one. 
The nature of this overlapping (Fig. 12, fis.) may be seen in optical 
longitudinal sections. The plates of the outer layer coincide with the 
longitudinal rows of cilia. Their inner edges are slightly serrated 
(Fig. 8, esp.). owing to a slight inward prolongation of the plate wher- 
ever a cilium in passing through meets it. It is this condition which 
causes the interrupted appearance of the plates seen in tangential sec- 
tions (Fig. 20, csp.). 

The bell-shaped region is generally sharply marked off from the an- 
terior or nipple-like part by a deep constriction (Plate 1, Figs. 1-6) ; 
but sometimes there is a more gradual transition (Plate 2, Figs. 7, 8). 
The boundary between the “bell” and the “body” is ordinarily 
marked by a deep constriction (Plate 1, Figs. 1, 6), or a shoulder-like 
projection of the “body” beyond the outline of the “bell” (Plate 2, 
Figs. 7, 15). The place of this constriction may for convenience be 
called the lip of the bell, though the pendant central mass of the proto- 
plasm of the bell containing the nucleus often projects far behind this 
rim or lip. When the animal is at rest, the bell is of fairly regular aud 
symmetrical form (Figs. 13, 15), but the extreme mobility of the whole 
head region causes it when in action to take a great variety of shapes 
(Plate 1, Figs. 1-6). 

The protoplasm of the bell is differentiated into two distinct kinds, 


al 


PORTER: TRICHONYMPHA. 55 


a coarser and a finer. The coarsely granular protoplasm is situated 
chiefly in the anterior portion of the bell (Fig. 7, pr’pl.), but it is also 
continued backward along the surface, and likewise forms a pendant-like 
structure in the axis of the bell (pr’pl.az., Figs. 7, 13, 15). It stains 
a little darker than the finer protoplasm which occupies the rest of the 
bell, but the transition between the two kinds is not very abrupt. 

Extending backward from the lip of the bell is a coarsely granular 
protoplasmic partition (Fig. 7, st.gran.), continuous with the coarsely 
granular protoplasm of the surface of the bell, and marking the boundary 
between the “ head” and the “ body” of the animal. It has the form 
of a hemispherical bow], at or near the bottom of which is situated the 
nucleus. Although I have been unable to discover in connection with 
this granular partition the existence of any fine membrane serving to 
completely separate head and body, yet the constant relation of the 
nucleus to this granular layer proves the latter to be substantially a 
permanent boundary between these two regions of the animal. In cross 
sections of the animal in the region of the nucleus (Figs. 21, 23; com- 
pare the position of the lines 21-21 and 23-23 in Fig. 15), this par- 
tition gives rise to a kind of radiation, which seems to emanate from the 
nucleus. This appearance might be produced if the bowl-shaped layer 
of granulations were corrugated or thrown into radiating folds; but I 
believe it is due instead to a regular alternation in the thickness of 
radiating portions of the bowl. As seen in cross sections (Figs. 21, 23) 
this bears a slight resemblance to the crown of rods (bastoncelli) figured 
and described by Grassi (’85, p. 236, Figs. 1, 2, 6), as nearly enveloping 
the nucleus in the case of Joenia annectens; but the fact that the bas- 
toncelli have a more precise form, being short club-shaped and curved, 
as well as the fact of their not being limited to a single zone, seems 
to me to preclude the possibility that these two structures are homol- 
ogous. 

The nucleus (Fig. 7, nl.) is situated in the anterior or “head” por- 
tion of the animal, a little posterior to the constriction at the surface 
which marks the transition from the “bell” to the “body.” It lies 
wholly within the bell-shaped portion, however, and is generally sur- 
rounded by a thin sharply defined nuclear membrane. I have almost 
invariably found the chromatin broken up into chromosomes of varying 
size and shape, and without definite arrangement. 

Very frequently the nucleus is invested with an immensely thickened 
membrane (Plate 2, Fig. 22). This nuclear envelope consists of a clear 
homogeneous substance, which stains in eosin, but not in hematoxylin. 


56 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


I have found several stages which I believe lead up to this condition. 
Apparently the substance which is destined to envelop the nucleus first 
collects along the partition separating the body from the head of the 
animal (Fig. 13). Possibly the formation of this thick envelope is pre- 
paratory to the production of the spores. 

The posterior or body half of the animal is principally occupied by a 
highly vacuolated protoplasmic mass (Fig. 7), which has usually reached 
the state of a coarse reticulum. In this reticulum large pieces of wood 
fibre and spores of fungi are frequently found embedded. This central 
protoplasmic reticulum is completely surrounded by a granular proto- 
plasmic wall (st. gran.’, Figs. 7, 13, 15), which is continuous in front 
with the granular partition separating the “ head ” from the “ body.” 

The granular wall is not very thick, and passes on its inner surface 
rather gradually into the protoplasmic network; but it is more sharply 
defined on its outer surface, though even here the transition to a rather 
thick cortical layer (st. ctz., Figs. 7, 13, 15) immediately outside it is not 
very abrupt. This cortical layer is composed of finely granular almost 
homogeneous protoplasm, and is of nearly, though not quite, uniform 
thickness. Its average thickness is about the same as that of the inner 
layer enveloping the bell. 

The surface of the “ body ” appears to be traversed by nearly equidis- 
tant lines, which have a slightly spiral —left-handed or leotropic — 
course (Plate 1, Figs. 1, 2). These lines appear to be continuous in | 
direction with the innermost set of cilia, which cross one another at the 
posterior tip of the body, and in fact the lines are in my opinion due exclu- 
sively to the presence of these cilia, which are closely applied to, but are 
entirely free from, the wall of the “‘body.” Frenzel’s description of the 
condition in Leidyonella is certainly not applicable here. Frenzel (85, 
pp. 306, 307) maintains that in Leidyonella the rigid cilia arise at or near 
the anterior end of the body, but that, instead of being free throughout 
their whole length, they are fused with the peculiar cuticula which covers 
the body, causing ridges, which have a slightly spiral course (he does 
not say whether right or left, and his figures are noncommittal), and 
that they become free only as they project beyond the posterior ex- 
tremity of the body. These ridges are stated to be much more promi- 
nent in the anterior than in the posterior part of their course. But, 
whatever may be the condition in Leidyonella, the spiral markings in 
Trichonympha are not traceable forward any further than the boundary 
between “body ” and “bell,” and they are not due to confluence of cilia 
with the wall of the body of the parasite. 


PORTER: TRICHONYMPHA. Be 


The questions of reproduction and development in Trichonympha, 
though interesting, are very difficult, and I shall not be able to say 
much on them. 

I have chanced upon three individuals, which I at first thought to be 
stages of division (Plate 3, Figs. 24-26), but having seen only these 
three, and these being all from the same host, I am now inclined to 
think that they are simply abnormal forms. But whether these are 
normal or abnormal, I believe that division must be of rare occurrence. 

Leidy has figured several forms (Plate 51, Figs. 11-20), always found 
associated with Trichonympha, which he thinks may be their young ; 
but I believe that not all the forms referred by Leidy to Trichonympha 
can be the young of that genus. Perhaps, indeed, none of them are. 
Those which seem to me to present the best evidence of being the young 
of Trichonympha are shown in Plate 3, Figs. 27-29 (compare also Leidy’s 
Plate 51, Fig. 11). These individuals possess a nucleus situated in about 
the same relative position as that of the adult Trichonympha, and are 
provided with long cilia. The cilia begin at the base of a smooth knob 
(Fig. 29), with which the anterior end of the animal terminates, and 
follow from there the course of deep spiral grooves that have the direc- 
tion of the threads of a right-hand screw, not, as one might infer from 
Leidy’s Figure 11, that of a left-hand screw. The grooves are much 
closer together on the anterior than on the posterior portion of the para- 
site, and consequently, as in the adult Trichonympha, the cilia are much 
concentrated into this region. Again, as in the adult Trichonympha, a 
bunch of long cilia trails out behind the parasites of this type. More- 
over, the anterior cilia alone are used, as in the adult Trichonympha, for 
locomotion, the body often remaining during locomotion perfectly rigid 
and the posterior cilia quiet. 

This spring (1896) I have noticed a great many specimens of this 
parasite with a very much enlarged, apparently swollen, anterior knob 
(Fig. 28); but what this condition signifies I have been unable to 
determine. 

Notwithstanding these several points of agreement, there seem to me 
to be almost insuperable obstacles to assuming that there is a genetic 
relation between the two forms. Chief among these is the pronounced 
dexiotropic course of the spiral groove and accompanying bands of cilia 
in the supposed young; whereas in the adult Trichonympha the bands 
are longitudinal and are limited to the anterior or bell-shaped part. A 
direct conversion of one of these conditions into the other seems to me 
highly improbable, while the obliteration of one and a substitution of 


re 


es ee eee 


OD Orr 


58 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


the other would amount to nothing less than a metamorphosis, none 
of the steps in which have been observed. 

Another point of difficulty is the entire absence in the supposed 
young of any partition separating the nucleus-bearing anterior portion 
of the organism from the posterior portion. 

The other forms, resembling those which Leidy thinks may also be 
young of Triconymphe, | have represented in Figures 30-32, 35-39, and 
42-44, I can see no sufficient reason, however, for supposing them to 
be young Trichonymphe. The nucleus of these forms is situated at the 
extreme anterior end of the body. The anterior tip terminates in a 
rounded projection, but it has not the form of the knob seen in Figures 
27 and 29, and a comparison with the cap seen in adult Trichonymphe 
is more difficult in this case than in that of the forms last described. 
The cilia are of very nearly equal length over the entire body. They 
are arranged in bands running spirally around the animal, and the 
spirals run in the same direction (dexiotropic) as in the forms we have 
just been discussing. 

There are, however, so many points of difference between these two 
kinds of supposed young parasites as to render it probable that they 
are not connected with each other genetically any more than they are 
with Trichonympha. It will be observed not only that the nuclei are 
differently situated in the two forms, but also that in those shown in 
Figures 30-32 and 35-38 the interval between successive bands of cilia 
remains nearly constant throughout the entire length of the animal, quite 
unlike the condition obtaining in the other forms. 

This form is also found in a great variety of sizes (Figs. 30, 31, 35), 
the largest being considerably larger than the individuals of the other 
form represented in Figures 27 and 28. The shape of the animal is char- 
acteristic, being in the living condition long and slim. Figures 37, 39, 
and 42-44 represent living specimens. Figure 38 shows one that died in 
normal salt solution. Figures 30-32, 35, and 36 represent the shapes 
they assume when killed with corrosive sublimate. Their method of 
locomotion also serves to distinguish them from the forms which I have 
last described, for they often move, independently of ciliary activity, 
by changes in the form of the whole body (Figs. 39, 42, 44), — some- 
times with the wriggling, squirming motion of a worm, at others, swell- 
ing out in places to almost double their average diameter, and then 
slowly contracting their body-wall (Fig. 44), they produce a kind of 
peristaltic motion, which may aid in locomotion. The smaller ones 
(Fig. 43), when travelling very rapidly in a straight line, revolve around 


PORTER: TRICHONYMPHA. 59 


the longitudinal axis, their bodies changing form little or not at all. 
This rotation is undoubtedly due to the spiral arrangement of the 
cilia, but the constant motion of these organs makes it impossible to 
discover in living specimens what the arrangement of the cilia is. 

The bodies of these worm-like animalcules (Figs. 30-32, 37-39, 42-44) 
are invariably filled with great numbers of large protoplasmic granules ; 
this is not the case with any of the other parasites. 

It seems to me that the characters of this form are sufficiently definite 
and different from those of the other forms described to allow one to 
consider them the representatives of another species. 


2. Pyrsonympha vertens. 
Plate 3, Figs. 33, 34, 40, 41; Plate 4; Plate 5. 


Pyrsonympha in its mature state is much larger than Trichonympha. 
Either the parasites found in the vicinity of Cambridge attain a greater 
size than those studied by Leidy, or else this author failed to see the 
largest individuals, for the longest specimen seen by Leidy measured 
only 160 », whereas the one which is shown in longitudinal section in 
Figure 33 (Plate 3) is 275 » long, exclusive of the peduncle ; this being 
a large, but not an exceptionally large individual. 

Pyrsonympha, like Trichonympha, evidently feeds on solid food, so 
that the question again confronts us, How does this food enter the 
body? Sometimes the whole posterior portion of the animalcule is 
filled with fragments of wood fibre (Fig. 34). These fragments are 
often of considerable size. I have seen, within the posterior part of 
the body, a single rectangular piece so large that it touched both sides 
of the body, and extended anteriorly almost to the nucleus. Surely, a 
mouth large enough to swallow such a portion of food must be recog- 
nizable ; but I have been unable to discover any aperture whatsoever 
in the body-wall of the animal. 

Pyrsonympha is usually more or less club-shaped, the relative pro- 
portions of length and thickness, and the particular form, being of course 
dependent on the degree and nature of its contraction (Plate 3, Figs. 33, 
34, 40, 41; Plate 4, Figs. 45, 51, 53, 55). In the adult condition 
it appears always to be attached to the wall of the host’s intes- 
tine, a fact which Leidy, strangely enough, seems to have entirely over- 
looked. The attachment is by means of the narrower end, which is 
prolonged into a sort of homogeneous stalk or peduncle, having only a 
slender connection with the main portion of the parasite. The peduncle 


60 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


is from 1p» to 1.5 in diameter, and may attain a length of at least 
75. It is deeply embedded in the epithelial wall of the host’s intes- 
tine. J am not certain that I have been able to trace it to its end, and 
do not know if there is any specialized structure at the end; conse- 
quently I cannot state the possible maximum length. It is usually of 
almost uniform calibre throughout the most of its length. The region 
near the point of attachment, however, often shows a spindle-shaped 
enlargement (Plate 4, Fig. 45). There is invariably a spheroidal struc- 
ture that I have called the knob or tubercle (Figs. 46, 47, twb.), which 
serves as a means of connecting the peduncle with the rest of the body ; 
when an artificial separation between body and peduncle takes place, 
the tubercle may remain attached to either part, but it usually separates 
from the body (compare, however, Fig. 47). The tubercle is somewhat 
thicker, and becomes much more deeply stained, than the peduncle; it 
is homogeneous and highly refractive. 

The attached end of Pyrsonympha is that which Leidy has called the 
anterior end. Accepting this designation simply as a convenience in 
description, it may be said that the free posterior part of the parasite 
projects with a rounded end into the lumen of the host’s intestine. 
The body of the parasite, from the narrow tuberculate part to the free 
rounded end, looks like a thin sae. 

The body-wall itself is frequently excessively thin, and in places 
the body seems to be almost naked and amceboid in its nature. The 
slightest pressure of an object on its surface would cause it to enter 
the substance of the body. I believe that something of this kind is 
represented in Figure 40 (Plate 3) ; at any rate, in this case a particle 
of wood fibre was found about three fourths engulfed by the animal ; 
whether the fragment was accidentally forced into the body, or whether 
the animal was taken in the act of ingesting it for food, I am unable to 
say. The parasites, of which this was one, had been removed from the 
host for about an hour when I discovered this individual. During the 
three or four minutes that I watched this one no change took place, and 
at the end of that time it was dead. So it is still only a matter of con- 
jecture that Pyrsonympha engulfs its food by the exceedingly mobile 
posterior portion of the body. 

However that may be, the body-wall is so thin in all regions as to 
make the condition noteworthy. There is no place where its thickness 
is sufficient to allow it to be readily measured. In fully grown indi- — 
viduals the body is only sparsely covered with cilia, and portions of the 
surface seem to be entirely destitute of them. When present they are 


PORTER: TRICHONYMPHA. . 61 


scattered irregularly, apparently without relation to the contractile cords 
of the body-wall. The young, as we shall see later, are provided with 
a coat of abundant cilia unifurmly distributed. 

The body-wall is not without further differentiation, for at regular 
intervals it is marked by darker lines, which have the appearance of 
thickenings of the wall, or of separate cord-like structures applied to 
its inner surface. These are really contractile cords, which arise at the 
knob-like structure of the anterior tip of the parasite where it joins the 
peduncle. These cords are usually grouped together at the anterior 
end of the body, so that a portion of the surface is quite bare of them 
(Plate 4, Fig. 51a). They pass backward in a slightly spiral direction 
(leeotropic), leaving between one another equal spaces. As a rule, each 
cord can be traced to the posterior extremity of the animal, and thence 
back again on the opposite side of the animal to the anterior end ; but 
sometimes the cords apparently terminate in cup-like depressions in the 
body-wall, which resemble pock-marks, being outlined by coarse granu- 
lar protoplasmic rings (Fig. 51, ann.). The cords are frequently so 
superficial that in profile views of the animalcule they are seen to cause 
the surface to project in the form of ridges. In some individuals there 
is to be seen in the middle of the space between two successive cords 
another and much finer line (Plate 4, Figs. 47, 54) running parallel 
with them. This narrower line or cord is apparently embedded in a 
more or less clear homogeneous substance, while the larger cords are 
surrounded by coarse granular protoplasm. In other cases the larger 
cords occupy the clear areas, and midway between them are sharply 
marked lines of coarsely granular protoplasm, which stains more deeply 
than the rest of the protoplasm (Fig. 51). 

In living specimens these cords keep up an incessant undulatory 
motion, producing an effect which very closely resembles bands of 
vibrating cilia The body of the adult, however, is generally quite bare 
of cilia, but when it is ciliated, the cilia are, as I have said, scattered 
promiscuously over the body, irrespective of the contractile cords. 

The contents of the sac-like body are finely and rather uniformly 
granular, and not very thick or viscid, and there are appearances of 
vacuolation in them. 

The nucleus of Pyrsonympha is a large pear-shaped or sometimes 
oval body, generally situated at about one fourth or one third of the 
distance from the anterior or attached end to the posterior end, and its 
more pointed extremity is turned toward the attached end of the animal- 
cule. It always contains a nucleolus (Plate 4, Fig. 47, nil.) at its larger 

VOL. XXXI.— No. 3. : 2 


62 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


end, and frequently it is highly vacuolated at its anterior end. Figure 
52 (Plate 4) represents a cross section of a Pyrsonympha through the 
nucleus, showing its position and that of the flagellum (/g.) in relation 
to the wall of the body. The nucleus varies considerably in shape ; 
sometimes it is very much attenuated (Fig. 50), and at others almost 
spherical (Fig. 53, z/.). 

Within Pyrsonympha is a lash-like filament or flagellum (fg.), as I 
shall call it, which is by far the most remarkable structure of this para- 
site. In the living specimen it is in constant motion, great waves pass- 
ing from the attached to the opposite end of the animal, often giving it 
the appearance of a revolving polygon inside the animal. Leidy says, 
“The motion of the undulating cord and of the animal together im- 
pressed me with the idea of a snake in a bag, making its presence obvious 
in active contortions.” The effect of this flagellum is not unlike that of 
a churn, for it keeps the contents of the sac-like body, including the food, 
thoroughly stirred up. 

The flagellum is of nearly uniform thickness throughout the most of 
its length, and, as cross sections (Fig. 52) show, is oval in section ; its 
diameter varies with the state of contraction, but averages about 1.5 p 
by 2. Near its anterior end, where it joins the peduncle by means of 
the darkly staining tubercle, it sometimes becomes very much attenuated 
(Plate 4, Fig. 46). I am not certain, however, but that this may be 


due to abnormal tension induced by the increased activity of the flagel- 


lum when the parasite is put in normal salt solution. However this 
may be, the posterior portion of the flagellum tapers off very gradually, 
finally becoming very fine at its posterior end (Figs. 45, 53, 55). 

Throughout the most of its length it is quite free from the body-wall, 
and moves about with vigorous strokes in the most unexpected manner. 
It is, however, attached to the body-wall at or near the posterior end of 
the parasite, as well as at the region of the peduncle. But quite fre- 
quently it seems to break loose from this posterior place of attachment, 
— perhaps owing to a too violent whipping about caused by the stimu- 
lating effect of the salt solution, — and then it sometimes projects pos- 
teriorly in the form of a tail-like appendage. This caudal appendage is 
surrounded by a layer of protoplasm, which has considerable thickness 
at its base, but becomes reduced to a condition of great tenuity (Plate 3, 
Fig. 34) at its tip. It flaps backward and forward very violently with 
every undulation of the flagellum. 

Leidy (’77, p. 437) says of Pyrsonympha, ‘‘ Sometimes too it appears 
terminated by a caudal appendage of variable form and length, but this 


—); 


- 


PORTER: TRICHONYMPHA. 63 


has seemed to me to be a production resulting from change due to disso- 
lution.” It does not appear from anything said elsewhere, or from his 
drawings, that he connected the presence of this caudal appendage with 
the vibrating cord. 

The method employed in removing from the host the living animal- 
cules for study has proved to be important, for it is to this that I owe the 
discovery of the peduncle previously described. Leidy speaks of gently 
pressing the intestine of the host, and thus forcing out the parasites. 
Instead of doing this, I removed the intestine into normal salt solution 
and on the slide teased it into pieces with needles. In this way of course 
many fragments of the intestine were found mingled with the parasites. 
These were frequently covered with Pyrsonymphe, so closely packed 
together that they looked at first sight like large epithelial cells. Each 
of the parasites was attached to the fragment of the intestinal wall by a 
long filament (Plate 4, Fig. 45). I think that Leidy saw this filament 
or peduncle, as I have already called it, for he says (’77, p. 438), “In 
the process of dissolution of the animal, the undulating cord [flagellum] 
often appears to project to a variable extent from the narrower end of 
the body”; but he did not recognize that this projecting part was a 
means of attachment, for on forcing Pyrsonymphe from the intestine 
the most of this peduncle must invariably have been broken off. Nor- 
mally, I believe, all mature Pyrsonymphe are attached to the intestine 
by means of this peduncle. 

The few evidences of reproduction which I have observed relate to 
the conditions of the nucleus or the size of the individuals. I have 
observed nuclei in what I believe to be various stages of division 
(Plate 4, Figs. 48, 49, 55). This shows that probably reproduction by 
division takes place occasionally. Perhaps Figure 53 represents an in- 
dividual recently formed in this way ; the presence of the nucleus in the 
posterior portion of the parasite certainly points to that conclusion. 
Individuals produced by the division of a large adult Pyrsonympha 
should of course resemble the adult form in the almost total absence of 
cilia, and also in the possession of a flagellum. It therefore seems safe 
to assume that almost all unusually small Pyrsonymphe possessing these 
qualities (Plate 5, Figs. 62, 64) were recently formed by division. 

Besides young of this type, there are found swarms of immature Pyr- 
sonymphex with essentially different characteristics. These are, for ex- 
ample, profusely covered with fine, short cilia of nearly equal length 
(Plate 5, Figs. 58, 61, 63). Apparently, the flagellum is frequently 
wanting in these forms, or at least cannot be distinguished (Figs. 57, 


64 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


63). The superficial muscular cords exist, but take a longitudinal 
course as often as a spiral one (Figs. 57, 61). Sometimes they are 
distributed over only a part of the animal (Fig. 61). The nucleus of 
these small individuals resembles that of the adult, but is situated at 
the extreme anterior end. If, therefore, it were not for the presence of 
the flagellum and peduncle in some of these forms, we should be quite 
unwarranted in supposing them to be in any way related to Pyrsonympha. 
As it is, however, I think there can be no doubt of the relationship. 
The flagellum and peduncle are both absent in some forms (Figs. 
59, 60, 65) which I take to be the earliest known conditions of this 
parasite. 

I have come to the conclusion that the flagellum of Pyrsonympha is 
merely a differentiation of one of the superficial muscular cords. We 
may conceive that the cord which is destined to become a flagellum, 
after becoming larger, stronger, and more active than any of the other 
cords (Plate 5, Fig. 61), simply splits off from the inner surface of the 
body-wall, remaining fixed only at its two extremities; thus it enjoys 
free play for all its peculiar undulations. 

The peduncle is apparently a slow-growing structure. In sections of 
the intestine one often finds young parasites in closely packed masses, 
lining considerable portions of the intestinal wall (Plate 5, Fig. 56). 
Their anterior ends lie very close to the intestinal epithelium, thus 
showing that the peduncle is still very short. The young here referred - 
to (Plate 5, Fig. 56) are not more than one third or one fourth as long 
as the adult parasites shown in Figure 45 (Plate 4). The deeply 
stained body which is to be seen at the anterior tip of even the youngest 
individuals (Fig. 56) is the knob-like structure or tubercle of the adult, 
from which the peduncle arises. 

In their activity the young far exceed the adults, for besides the 
undulation of the muscular cords, the animal is constantly changing its . 
shape. Figures 59, 60, and 65 (Plate S) represent different shapes, 
taken by the same individual in the course of a few minutes. 

It is of interest in this connection to note the effect of these para- 
sites on the intestine of the host (Plate 5, Fig. 56). The epithelial 
lining is very much indented. The peduncles force their way between 
the cells, reaching sometimes almost to the underlying muscular layer. 
The cells themselves are often reduced in size, but otherwise apparently 
perfectly healthy. It is a mystery how the host can support such a vast 
number of parasites, unless they in turn are in some way of benefit 
to it. 


PORTER: TRICHONYMPHA. 65 


38. Dinenympha gracilis. 
Plate 6, Figures 66-72. 


I have found it almost impossible to draw any sharp line of distinction 
between Dynenympha and the young of Pyrsonympha; in fact I doubt 
very much whether Dinenympha should be considered as anything else 
than a very early stage of a developing Pyrsonympha. 

Dinenympha never possesses a flagellum or a peduncle, although at the 
anterior tip of the animal there is frequently a deeply stained body resem- 
bling the tubercle of Pyrsonympha. The body of Dinenympha is long 
and slim, slightly flattened on one side, and generally twisted with one or 
two dexiotropic turns. Running parallel with this fwist, but upon the 
convex side only, there are from seven to nine muscular cords, resem- 
bling those of Pyrsonympha; but they often cause the surface to project 
much farther than is usual in Pyrsonympha, giving the animal a fluted 
appearance. This is shown in Figures 69 and 70 (Plate 6), which repre- 
sent the upper and lower surfaces respectively of the same individual. 
Similar views of another individual are shown in Figures 71 and 72. 

The animal is generally almost devoid of cilia, except for a tuft at the 
posterior end (Figs. 67, 68), or occasionally a few cilia at both extrem- 
ities (Figs. 66, 69). Sometimes, however, it is thinly ciliated all over 
(Fig. 72). I think Leidy must have mistaken the undulatory motion of 
the muscular cords for the vibration of bands of cilia, for I have found 
that individuals with abundant cilia are rare. 

The nucleus is situated near the anterior extremity of the parasite ; 
it is oval and homogeneous, or finely granular, and sometimes (Fig. 66) 
shows a single darker structure like a nucleolus. The nucleus varies in 
size from 9 X 5.5 to 5.5 X 3.6. 

The motions of the living Dinenympha are exceedingly interesting, 
but Leidy has amply described them. 

Dinenympha, like its companion parasites, lives on solid food ; but it 
possesses no discoverable oral aperture. 


4. Gregarinida. 
Plate 6, Figs. 73-76. 


Leidy (81, p. 441) speaks of having only once noticed a small Gre- 
garine among the other parasites of Termes. I have, however, found 
Gregarines very common in some hosts. They are found, almost with- 


66 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


out exception, in the anterior portion of the small intestine only. Sec- 
tions through this part often reveal great numbers of cysts (Plate 6, 
Fig. 75). They belong to the Polycystidea. Figure 73 (Plate 6) shows 
a very characteristic appearance of a living specimen, and Figure 74 that 
of one filled with sporocysts. 

In sections (Fig. 76) the protomerite (pr’mer.) is seen to be com- 
pactly filled with protoplasm, the anterior half of which stains more 
deeply than the rest. The posterior half appears to be made up of 
globules somewhat flattened by mutual pressure. In living specimens 
the anterior portion of the protomerite is quite transparent, being free of 
all coarse granules (Fig. 73). 

The deutomerite_ (Fig. 76, dew’mer.) is generally only loosely filled with 
a coarse protoplasmic network. It contains the large, round nucleus, gen- 
erally situated quite centrally. Within the nucleus is always to be found 
a single large deeply stainable nucleolus. 


In conclusion, I wish to express my very deep indebtedness to Dr. 
Mark for whatever there may be of value in this paper. The work was 
taken up at his suggestion, and carried on under his very kind and care- 
ful supervision. 


CAMBRIDGE, May 12, 1896. 


‘ 
val 
—— —— 


i. ee eo 


PORTER: TRICHONYMPHA. 67 


PAPERS CITED. 


Frenzel, J. 
91. Leidyonella cordubensis nov. gen., nov. spec. ine neue Trichonym- 
phide. Arch. f. mikr. Anat., Bd. XX XVIII. pp. 301-316. 4 Fig. 


Grassi, B. 
°85. Intorno ad alcuni protozoi parassiti delle Termiti. (Nota letta 15 Gen- 
naro 1885.) Atti dell’ Accad. Gioenia Sci. Nat. in Catania, ser. 3, Tom. 
XVIII. pp. 235-240. 7 Fig. 
Kent, W. S. 
°85. Notes on the Infusorial Parasites of the Tasmanian White Ant. (Read 
Nov. 17, 1884.) Papers and Proceed. Roy. Soc. Tasmania for 1884, 
pp. 270-273. Reprinted in Ann. & Mag. Nat. Hist., ser. 5, Vol. XV. 
No. 90, June, 1885, pp. 450-453. 
Leidy, J. 
°77. On Intestinal Parasites of Termes flavipes. Proceed. Acad. Nat. Sci. 
Philadelphia, 1877, pp. 146-149. (Read Apr. 17; printed June 26.) 
Leidy, J. 
81. The Parasites of the Termites. Journ. Acad. Nat. Sci. Philadelphia, 
New (Second) Series, Vol. VIII. Part 4, (1881,) pp. 425-447. 


Lespés, C. 
°56. Recherches sur l’organisation et les meurs du Termite lucifuge. Ann. 
Sci. Nat., sér. 4, Zool., Tom. V. pp. 227-282. 


68 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


EXPLANATION OF PLATES. 


All the figures, with the exception of those of Plate 2, were outlined with the 
aid of an Abbé camera lucida. 

The high magnifications were obtained mostly by the use of a Zeiss 1.5 mm. 
apochromatic homogeneous immersion objective with No. 4 compensating ocular. 


LIST OF ABBREVIATIONS. 


Ring of protoplasmic granules. 

Cylindrical axis of the “nipple” 
in Trichonympha. 

cil. Cilia. Sim 


Spores of a fungus. 

Inner layer of “nipple” and 
“bell” of Trichonympha. 

Middle layer of same. 


ann 
az. 


spo. 
Shen 


csp. Tooth-like processes of the inner _ st.” Outer layer of same. 
surface of the outer layer of  st.ctr. Cortical layer of granular 
the “ bell.” protoplasm. 
deu’mer. Deutomerite. st, gran. The granular protoplasmic 
for. lig. Wood fibre. layer separating the “ bell ” 
Sd: Flagellum. from the posterior part 
Jis- Fissure. (body) of Trichonympha. 
mu. Muscle. st. gran.’ Granular protoplasm marking 
nl. Nucleus. the region of transition from 
nil. Nucleolus. the superficial (ectosarc) to 
pd. Peduncle. central (entosarc) portion 
pil. Cap. of the protoplasm in the 
pli. Fold. posterior half (“body ”) of 
prmer. Protomerite. Trichonympha. 
pr pl. Coarsely granular protoplasm — st.str. Striated layer of the “bell” 
of “ bell.” of Trichonympha. 
pr’pl’. Finely granular protoplasm, — tub. Knob-like enlargement of the 
making up most of the con- anterior end of the proto- 
tents of the “bell.” plasmic axis of the “nip- 
pr’pl.ax. Protoplasm situated in longitudi- ple,” Trichonympha. Also 


nal axis of the bell of Tricho- 
nympha, which is coarser and 
more deeply stained than the 
surrounding protoplasm. 


knob-like structure at the 
anterior extremity of Pyrso- 
nympha. 


i 

pi 
Shi 

rier iN 

Fh, 


PorTER. —Trichonympha. 


PLATE 1. 


All figures are of Trichonympha agilis. 


Figs. 1,2. Normal appearance of parasite when examined in diluted albumen. 
xX 520. 

Fig. 3. Appearance of anterior tip of Trichonympha viewed from in front. 

Figs. 4-6. Animals in diluted milk, showing corpuscles seized upon by the long 
rigid cilia and dragged after the animal. X 520. 


PORTER.- TRIGHONYMPHA. Praeri 


PorTeER. — Trichonympha. 


20. 
g. 21. 


ig. 22. 


. 23. 


PLATE 2. 


All figures are of Trichonympha agilis. 


Diagrammatic longitudinal section through the anterior portion of 
Trichonympha, imagined to be not thicker than one band of cilia. 

Same as Figure 7, but showing an additional layer (st.”) in the nipplelike 
anterior extremity. 

Cross section through posterior portion or “body,” showing a fold in 
the body-wall, in which cilia have been engulfed. 

Section similar to that of Figure 9, except that particles of wood fibre are 
entangled with the cilia in the engulfed portion. 

spo. Spores of Fungi eaten by the animal. 

Diagram of a portion of the surface of the anterior end, the “ nipple,” 
and part of the “ bell.” 

Diagram of side view of same region as that shown in Figure 11. 

Complete longitudinal section, showing a gathering of a clear proto- 
plasmic substance about the nucleus; probably an early stage in 
the development of an immensely thickened nuclear membrane 
such as is shown in Figure 22. 

Cross section of posterior portion, showing a fold, as in Figures 9 and 10, 
the walls of which are beginning at a to disintegrate. 

Complete longitudinal section of an individual more shortened than that 
of Figure 13. 

Cross section in the region of the line 16-16, Figure 15. 

Diagrammatic cross section in the same region as Figure 16. 

Cross section of the nipple-like anterior extremity. 

The second section in a series of longitudinal sections. This is a portion 
of the bell, the outermost layer having been cut off with the first 
section from the middle of the figure, but remaining around the 
margin. In the centre is exposed the striated layer and the roots 
of the cilia arranged in quincunx. 

Diagram of the condition shown in Figure 19. 

Cross section in the region of the line 21-21, Figure 15, showing the 
radiating structure of the wall (st. gran., Fig. 7) separating the 
anterior from the posterior half of the animal. 

Longitudinal section showing an immensely thickened nuclear mem- 
brane. 

Cross section in the region of the line 23-23, Figure 15. 


Sp 4) 


stgran 


stgran. 


st.gran! 


Ray ; 
‘ Be Ai hich ‘ 
ip eco 


Wiai' 


oe pire 


Porter. — Trichonympha. 


PLATE 3. 


Figs. 24-26. Trichonymphe showing conditions which may be preparatory to 
longitudinal division. Compare text. X 520. 

Fig. 27. A parasite supposed by Leidy to be a young stage of Trichonympha. 
x 520. 

Fig. 28. Another similar individual with a peculiar swollen cap. X 520. 

Fig. 29. An individual more highly magnified. X 1100. 

Figs. 30-32. Young heretofore considered as immature Trychonymphe, but which 
I believe to be a distinct species. X 520. 

Fig. 33. Longitudinal section of a Pyrsonympha. X 400. 

Fig. 34. A Pyrsonympha containing wood fibre, the flagellum protruding in a 
tail-like appendage from the posterior or unattached end. X 400. 

Figs. 35, 36. The same kind of parasite as is shown in Figures 30-32. 

Fig. 37. The same kind of parasite as those shown in Figures 30-82, 35, 36, but 
drawn from a living specimen. X 520. 

Fig. 38. The same as Figure 37, but just after death in normal salt solution. 
x 520. 

Fig. 39. The same kind of parasite as that shown in Figure 37. X 520. 

Fig. 40. A Pyrsonympha apparently in the act of engulfing a particle of wood 
fibre. X 400. 

Fig. 41. A Pyrsonympha with a peculiar flagellum. X 520. 

Figs. 42-44. The same kind of parasite as that shown in Figure 39. X 620. 


TRIGHONYMPHA. 


PORTER: 


TF; 
2a Boe, 


Porter. — Trichonympha. 


Fig. 45. 


Fig. 46. 


Fig. 47. 


PLATE 4. 


All figures are of Pyrsonympha vertens. 


Parasites attached by peduncles to a bit of the intestine of the host. 
Drawn from living specimens. X 400. 

Optical section of anterior portion of a Pyrsonympha, showing the rela- 
tion of the knob-like structure (tub.) to the flagellum on one side and 
the peduncle on the other. X 1100. 

Anterior portion of an individual, showing the nucleus and the superficial 
muscular cords. XX 1100. 


Figs. 48, 49. Nuclei in stages of division. 
Fig. 50. A very much attenuated nucleus. 


Fig. 51. 


Fig. 5 
Fig. 


Fig. 


Fig. 


Surface view of a Pyrsonympha. Between the spiral muscular cords are 
bands of granular protoplasm. The region at a is bare of all muscu- 
lar cords. At the extreme posterior portion are several ring-like 
markings (ann.). XX 400. 

Cross section through the animal in the region of the nucleus. X 1100. 

Optical longitudinal section of a Pyrsonympha with its nucleus in the 
posterior half of the body. X 620. 

Portion of the surface of a Pyrsonympha, greatly magnified, showing a 
finer and a coarser set of muscular cords. The cords of the finer set 
are surrounded by a clear homogeneous substance, while granular 
protoplasm surrounds those of the other set. 

Optical section of the Pyrsonympha represented in Figure 51. There are 
two nuclei flattened against each other. X 400. 


PLaTE 4 


R- TRICHONYMPHA. 


, 


B Meisel lith Basten, 


—s 


Porter. — Trichonympha. 


PLATE 5. 


All figures are of Pyrsonympha vertens. 


Fig. 56. Section of a portion of the intestine of a Termite, showing numerous 
young Pyrsonymphe, the manner in which they attach themselves to 
the host, and the effect on the cells of the intestinal wall. X 1100. 

a,a. Spaces due to shrinkage. 

Fig. 57. Young Pyrsonympha without a flagellum. X 520. 

Fig. 58. Young witha flagellum. > 520. 

Figs. 59, 60, 65. Some of the shapes taken by a single living immature individual 
in the course of a few minutes. 

Fig. 61. Young, showing that the flagellum takes the same direction as (is differ- 
entiated from ?) one of the superficial muscular cords. X 1100. 

Fig. 62. Small Pyrsonympha, perhaps recently formed by division. X 620. 

Fig. 63. Young Pyrsonympha without a flagellum. X 520. 

Fig. 64. Small individual. 

Fig. 65. Same individual as that shown in Figures 59 and 60. 


PORTER - TRIGHONYMPHA. PLate 5 


B Meisel lith Boston. 


Porter. — Trichonympha. 


PLATE 6. 


Figs. 66-68. Common forms of Dinenympha. X 1100. 


70. Surface views of the upper and lower sides of the same Dinenympha. 


LD: 


72. Similar views of another individual. > 1120. 
76. Views of a Gregarine found as an intestinal parasite in Termes in com- 


pany with Trichonymphidae. 
Normal appearance of a living Gregarine. X 520. 
A gregarine filled with sporocysts. 430. 
Longitudinal section of a portion of the small intestine of a Termite, 
showing numerous Gregarines. > 160. 
Longitudinal section of a Gregarine. X 1100. 


tas <i ey et dls ae gees eli 2 


PORTER - TRIGHONYMPHA. 


B.Metsel lith Boston 


’ 
° a 


No. 4.— Variations in the Brachial and Lumbo-Sacral Plexi of 
Necturus maculosus Rafinesque+ By F.C. WaIrrE. 


VARIATIONS in the position of the pelvic girdle in Vertebrates, espe- 
cially in Amphibia, have already been noticed by several authors ; 
Adolphi, Bourne, Case, Howes, G. H. Parker, and others. These varia- 
tions, which obviously involve an inconstancy in the number of presacral 
vertebre, are as a rule symmetrical, the entire girdle being one segment 
caudad — more rarely cephalad — to the usual position ; but infrequent 
instances are found in which the right and left constituents of the girdle 
have unsymmetrical positions. 

Closely associated with the pelvic girdle is the lumbo-sacral plexus, 
and the problem which suggests itself in this connection is to determine 
whether, with the variations known to occur in the skeletal structures 
of the girdle, there are correlated variations in the lumbo-sacral plexus. 

If, as some morphologists believe, a difference in the position of the 
sacrum is the result of increase in the number of vertebrz by splitting 
of one or more presacral vertebra, or decrease through fusion, then 
such phenomena must take place in one or the other, or both, of two 
regions, either anterior to the most posterior nerve of the brachial 
plexus, or between that point and the lumbo-sacral plexus. 

To obtain evidence upon these alternatives, 1 dissected out the 
brachial plexus, in addition to the lumbo-sacral, in all the specimens 
which I have studied. As this plexus involves less variation than does 
the lumbo-sacral, I shall discuss it first. 

In naming the spinal nerves, I have adopted the plan of calling that 
nerve which emerges between the cranium and first vertebra the first 
nerve,” succeeding nerves being consecutively numbered. 

So far as I have noticed, there is little variation among either the 
sympathetic or dorsal branches of the spinal nerves. Since only the 
ventral branches enter the plexi, I shall, for the sake of brevity, desig- 


1 Contributions from the Zo6logical Laboratory of the Museum of Comparative 
Zodlogy at Harvard College, E. L. Mark, Director, No. LXXXV. 

2 In the Anura, where there is no spinal nerve emerging anterior to the atlas, 
the so called first spinal nerve emerges between the first and second vertebre, and 
is homologous to the second nerve of Urodela. 

VOL. XXXI. — NO. 4. 


72 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


nate the ventral branch of the first spinal nerve as nerve 1, the ventral 
branch of the second as nerve U, ete. 

It has seemed necessary in comparing plexi to indicate in some way 
the relative value of the nerves entering them. I have therefore 
recorded their approximate diameters, which I shall designate as their 
strength, — a rough method, but accurate enough for the present 
purpose. 

The material which I studied consisted of thirty specimens of the 
large perennibranch salamander of the Middle United States, — Nec- 
turus maculosus Rafinesque. These specimens were obtained from the 
Great Lake Region, in the main from a single locality (near Sandusky, 
Ohio) on the south shore of Lake Erie. 

Unfortunately, I neglected to note the sex of each individual. The 
importance of this did not occur to me until the urogenital system had 
been removed, and dissection had gone so far as to prevent the deter- 
mination of the sex. The greater number (more than two thirds) were 
males, but which ones I am unable to say. 

The work represented by this paper was done at the suggestion, and 
under the direction, of Dr. G. H. Parker, to whom I am indebted for 
assistance and advice during its progress. 


THe BracHIaL PLexvs. 


The brachial plexus (Plate 1, Fig. 1) is formed from nerves I to V in- 
clusive. In no case have I found indications that nerve vi contributed 
to it. 

The greater strength of the plexus lies posterior to the pectoral 
girdle. Its major part is compacted into a large trunk, the brachial 
nerve (br.), which, after sending branches to the shoulder muscles, 
divides to supply the various muscles of the fore leg. But nerve distri- 
bution does not concern us here. It is my purpose to determine if, in 
the number and strength of the nerves which enter the brachial plexus, 
there be any as ii which may be correlated with the variations in 


the sacral region. 

If, when the sacral rib is on the 19th vertebra, — the more usual 
position, — we find the brachial plexus constant in position, and if, when 
the sacral rib is on the 20th vertebra, — a common variation, — Wwe find 
a backward displacement of the brachial plexus to the extent of one 
segment, it would be fair to infer that the seat of variation is in the 
pre-brachial region. 


WAITE: PLEXI OF NECTURUS. yf 


Nerve 1 (Fig. 1) is slender, and does not contribute to the innerva- 
tion of the appendage, but is distributed to the inner wall of the girdle. 
It often lies close to nerve 11, and may (six cases in thirty) anastomose 
with it. 

Nerve 11 is also slender, and is distributed to the same region as is I. 
It seems questionable if 1 and 1 can be properly considered parts of the 
plexus. In strength they approximate the ordinary post-brachial spinal 
nerves (VI to xv). In some cases branches of 11 run in close relation to 
the supracoragoid (su’crac.) branch of 111, and may (four cases in thirty) 
pass out through the coracoid foramen with the supracoracoid. In no 
case have I found any anastomosis between branches of 11 and 111, as im- 
plied by Hoffman (’74, p. 229) in his account of Necturus (Menobranchus); 
and from my specimens of Necturus I can assert that this is certainly 
not a constant relation, if indeed it occur at all. 

Nerve ut usually divides into three branches, of which the small 
anterior one is distributed to the thoracic wall, the middle one forms the 
supracoracoid nerve (sw’crac.), while the larger posterior branch enters 
the main trunk of the plexus. 

Nerve iv is usually the strongest trunk of the plexus. It passes 
directly to its exit posterior to the scapula, close to the margin of the 
glenoid cavity, giving off in its course a single small branch, which is 
distributed anteriorly on the thoracic wall. Before reaching the border 
of the scapula this nerve usually divides into two main branches, which 
pass side by side to the musculature of the anterior appendage. 

Nerve v is the most posterior nerve to enter the plexus, its anterior 
branch joining nerve 1v just before this reaches the scapula, while the 
small median and the posterior branches are distributed to the body wall, 
posterior to the girdle. 

There were no variations of note in the topography of the plexi in the 
thirty animals examined, except in one case of slight want of symmetry in 
the point of junction of v with rv, but this occurred in a specimen which 
was normal in its sacral structures. Some variation occurs in the rela- 
tive strengths of the nerves ; for while Iv is usnaily the strongest, it may 
be equalled by m1, as was seen in six cases, of which four were with the 

“sacrum in a normal position (19th vertebra), one with the sacrum on 

the 20th vertebra, and one with an unsymmetrical sacrum (Plate 2, 
Fig. 5). Again, nerves m1 and v are usually of about equal strength, 
but tr may be much (six to eight times) stronger than v, or more 
rarely may be weaker. 

These conditions show a tendency toward variation in the location of 


74 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


the “strength-centre”* of the plexus, such variation usually resulting 
in displacement anteriorly ; but this is not correlated with the variations 
in the position of the pelvic girdle, — indeed most such cases were found 
in specimens in which the pelvic girdle was normally placed, — nor have 
I found any correlation between the displacement of the strength-centre 
in the brachial and lumbo-sacral plexi respectively. This is not in accord 
with the conclusions of Adolphi (96, p. 118), who says for Anura “.. . 
die beiden Extremitiatenplexus, der Plexus sacralis und der Plexus brachi- 
alis, ihren Schwerpunkt in der gleichen Richtung verlegen.. .” Such 
a condition indicates that the variations in the brachial plexus are inde- 
pendent of variation in the position of the pelvic girdle ; and since there 
is no posterior displacement of the brachial plexus, nor any change of 
topography in cases where the pelvic girdle is placed on the 20th ver- 
tebra, we have evidence that there has been no interpolation of vertebre 
in the part of the column anterior to the posterior limit of the brachial 
plexus. 


Tue LumBo-SAcRAL PLEXUS. 


The variations in the lumbo-sacral plexus are most conveniently 
grouped under three heads, viz. : A, those in which the girdle is attached 
to the 19th vertebra; B, those with the girdle carried by the 20th ver- 
tebra; and C, those in which the constituents of the girdle have an 
unsymmetrical position (see the Table, page 81). 

Group A is represented by twenty specimens. The nerves here form- 
ing the plexus (Plate 1, Figs. 2, 3) are xvi to xxt inclusive, neither 
XVII nor XXII in any case entering it. The plexus is distributed by three 
trunks. The anterior is N. ileohypogastricus (éLh’ga.), which is the 
anterior branch of nerve x1x, and extends cephalad on the inner body- 
wall; its strength and relation to other nervous parts seem uniform. 
The middle trunk (erw.) is N. cruralis (obturator), which passes out 
between the ilium and pubis, to be distributed to the anterior part of the 
thigh. The posterior is N. ischiadicus (¢sch.), which is the main trunk, 
and is distributed to the posterior part of the thigh and to the leg. 

We may distinguish in the topography of the plexus two types, based 
on differences in the source of the branches which go to form N. cruralis 
and N. ischiadicus; these I shall designate as a type and f type. 


1 The term “ strength-centre ” has been adopted to designate that point in the 
plexus which is central with regard to the combined “strengths ” of the component 
nerves. We may thus conceive the whole “strength” of the plexus as concen- 
trated at this point, the “strength-centre.” 


WAITE: PLEXI OF NECTURUS. 1 


In the a type (Plate 1, Fig. 2) N. cruralis (eru.) is formed from the 
union of the posterior branch of xrx with a small anterior branch of xx, 
while N. ischiadicus (csch.) is formed by union of the remaining part 
(posterior branch) of xx with the anterior branch of xxt. 

In the f type (Plate 1, Fig, 3) N. cruralis (eru.) is the middle branch 
of xix, which may (three cases out of ten) receive a delicate branch 
from xvut; while N. ischiadicus (isch.) is formed by union of the small 
posterior branch of x1x, all of xx, and the anterior branch of xxi. 

It will be noticed that the a type (Fig. 2) presents, as compared with 
the B type, a tendency to a forward migration of the plexus as a whole, 
indicated by part of nerve xx trending forward into the next segment to 
enter N. cruralis ; on the other hand, the £ type (Fig. 3) shows a back- 
ward tendency, since nerve xIx sends a branch into the next posterior 
segment to enter N. ischiadicus, and also nerve xvilI occasionally enters 
into N. cruralis. 

The successive spinal nerves in the region of the plexus may now be 
considered individually. 

Nerve xvii1 makes only a small and inconstant contribution to the 
plexus, since in three cases only, — all in the 8 type, —it gives a deli- 
cate branch to N. cruralis. Its strength is nearly constant, and is about 
the same as that of the ordinary spinal nerves anterior to it. This 
nerve may therefore be considered as a nearly typical spinal nerve. 

Nerve xix presents much variation in its strength relative to the 
other nerves of the plexus. It may equal xx, —six cases in twenty, — 
or it may have only } the strength of that nerve. Its average strength 
as compared with nerve xx (the main nerve of the plexus) is between 
4 and 2, and this is the relation usually found. Besides the ileo- 
hypogastric branch, which is constant, this nerve possesses one (type a, 
Fig. 2) or two (type f, Fig. 3) branches. In the B type the middle 
(cruralis) and posterior branches show considerable variation in strength 
in relation to each other. 

Nerve xx is the chief nerve of the plexus. Its strength is usually 
about eight times that of an ordinary spinal nerve. In the a type, it 
gives off a branch anteriorly, which joins the posterior branch of nerve 
xIx, these together forming N. cruralis. This anterior branch is always 
much weaker than the remaining (posterior) one, which forms the major 
part of N. ischiadicus. In the B type the entire nerve becomes a part 
of N. ischiadicus. 

Nerve xxi has a uniform relation to the plexus in the two types. Its 
stronger anterior branch enters N. ischiadicus, while the weaker posterior 


6 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


branch is distributed to the wall of the pelvic cavity. The strength of 
the whole nerve before branching varies from } to 3 that of nerve xx, 
the average being between } and }; i. e. it is usually weaker than nerve 
x1x, but may (four cases in twenty) equal that nerve in strength. 

To gain evidence on the relation of the plexus to the skeleton, I have 
in each specimen determined the position of the first vertebra which 
bears a heemal arch, for this is often assumed as a fixed point in discuss- 
ing variations in the vertebral column. By reference to the Table on 
page 81, (see also Parker, ’96, p. 712, and Bumpus, ’97, p. 457,) it is seen 
that the position of the first hemal arch is variable; but, as the fol- 
lowing tabulations show, there is no correlation between these variations, 
and those of the “ strength-centre” of the plexus as represented by the 
a and £ types. 


a type, with first hemal arch on vertebra 22 = 4 cases. 


a (73 (73 [73 “ec “ce 93 — 6 “ 
B “c “ ‘cc “cc “ 99, —_ 6 “c 
B “c 6s “ “ “c 93 —_ 4 “ec 


Group B includes the individuals having the sacrum borne on the 
20th vertebra, and of these there were seven specimens. The nerves 
involved in the plexus (Plate 1, Fig. 4) are x1x to xxi inclusive. Nerve 
XVIII in no case entered the plexus. In three cases a delicate branch 
from XIX was contributed to the formation of N. cruralis. Nerve xix 
occupies the same relative position as did nerve xvii in group A, but 
shows more of a tendency to enter the plexus (three cases in seven, 
as against three cases in twenty in group A) than did the element 
in the same position in group A. Moreover, it is distinctly (a half) 
stronger than are the ordinary spinal nerves. For these reasons it 
must be considered as belonging to the plexus. 

Nerve xx corresponds in position and distribution to xIx in group 
A. In every case it branches into three parts, the anterior forming 
N. ileohypogastricus, the middle N. cruralis, and the posterior entering 
N. ischiadicus ; i. e. the B type of group A only is represented, but with 
the modification that at first sight the whole plexus is apparently one 
segment posterior to the position in the 8 type specimens of group A. 

The whole of nerve xxi in each case enters into the formation of N. 
ischiadicus, for it sends no important branches cephalad or caudad. Its 
strength averages six times that of an ordinary spinal nerve, i. e. it is a 
fourth weaker than the nerve (xx) occupying the corresponding position 


in group A. Its average strength is about 1} times that of the next — 


anterior nerve, while in group A the ratio is 13 to 1. 


———_ 


\ a 


age a 


WAITE: PLEXI OF NECTURUS. tT 


Nerve xxi has here topographically the same relations as nerve XXI 
in group A, but its strength relative to the rest of the plexus is much 
less, and in two cases (in seven) 7 fails to send a branch into the plexus. 

The position of the first hemal arch is in every case on the 23d 
vertebra. 

At first sight the whole plexus in group B seems to have moved with 
the girdle one segment caudad as contrasted with the position in group 
A, a condition which might be explained by the interpolation of a pre- 
sacral segment; but for several reasons I do not believe such an expla- 
nation to be sufficient. If a presacral segment had been interpolated, 
thus entailing a change in position to the extent of an entire segment, we 
should expect still to see variations in the position of the first hemal 
arch. In one half the cases in group A (see Table, p. 81) the first hema] 
arch was on vertebra 23, three segments posterior to the main nerve of 
the plexus. Hence in group B, if the interpolation hypothesis be true, 
the hemal arch ought to occur, at least occasionally, in the same rela- 
tive position, i.e. on the 24th vertebra, instead of maintaining as it 
does a constant position on vertebra 23, only two segments posterior to 
the main nerve (xx!) of the plexus. The specimens examined by Parker 
(96, p. 712) show the same result, i. e. a constancy of position of the 
first heemal arch on the 23d vertebra in all specimens bearing the girdle 
on the 20th vertebra. Bumpus (’97, p. 473), however, among thirty-five 
specimens having this position of the girdle finds three (9%) in which 
the first heemal arch is on vertebra 24. This shows that such a position, 
though possible, is rare. 

Again, we should not expect the tendency of the last trunk nerve 
(xvilI in group A, xIx in group B) to enter the plexus to be so much 
greater in group B (43%) than in group A (15%), nor should we expect 
to find its strength in group B distinctly exceeding that of the ordi- 
nary spinal nerves, as it does, since there is no indication of such an 
excess in group A. 

If interpolation has occurred, nerve xx11 should show about the same 
average relative strength as the element (xxi) corresponding to it in 
group A. This is not found to be so, for xxii in group B is relatively 
and absolutely much weaker than xxi in group A, and by completely 
failing in two cases (28%) to enter the plexus shows an inconstancy 
not seen in the corresponding nerve (xx1) in group A. This leads 
to the conclusion that in group B this most posterior nerve (Xxi1) 
is not an essential element of the plexus. 

Nerves xx and xxi in group B correspond, in position relative to the 


78 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


sacrum, to nerves XIX and xx in group A (cf. Fig. 4 with Figs. 2 and 3). 
In group A the main nerve (xx) has eight times the strength of an ordi- 
nary spinal nerve. In group B the main nerve (xx1) is but six times as 
strong as this unit. In group A nerve XIx has an average strength 
1 to 2 that of nerve xx; though in some cases (30%) their strengths are 
equal, in others x1x may have only { the strength of xx. In group B, on 
the other hand, in two cases only is xx weaker than xxI; in three cases 
their strength is equal, and in two cases xx is the stronger, 1. e. in 70% 
of the cases XX equals or is stronger than xxI. The average strength 
of xx as compared with xxI is as 1.1 to 1 in group B, whereas in group 
A the ratio between the corresponding elements is about 0.6 to 1. This 
shows that in group B the main nerve (xxt) tends to be weaker, and the 
one next anterior to it (xx) to be stronger than the corresponding nerves 
(xx and x1x) in group A. This, together with the weakness and incon- 
stancy of xxii in group B, and the greater strength of xIx with its in- 
creased tendency in this group to enter the plexus, is evidence that in 
group B the strength-centre of the plexus has not travelled caudad through 
an entire segment as compared with group A, but holds a position wmter- 
mediate between such an one and that of group A. 

If we adopt as an explanation of the above facts and conclusion the 
interpolation of a presacral segment, we shall have to supplement such 
an explanation by assuming a subsequent movement cephalad of the 
plexus as a whole, to account for the ascertained intermediate position 
of the strength-centre of the plexus. 

From the evidence of the hemal arch, and of the topography ( type) 
of the plexus, — which I shall immediately discuss, — it seems more 
reasonable to consider this intermediate position of the plexus the result 
of what may be called the migration caudad of the locus of the strength- 
centre. The correlation of the position of this locus with that of the 
girdle I shall discuss further on. 

There is in group B a persistence of the B type of topography. This 
at first may seem to contradict the conclusion that we have an interme- 
diate position of the locus of the strength-centre, for with an interme- 
diate position of the plexus arising from migration caudad we should 
expect traces of reversion toward the more anterior position, such as is 
indicated by the tendency cephalad in the a type. However, there are 
reasons for expecting the B type here. (1) In such posterior migra- 
tion as is shown in group B, there must also be a tendency — whatever 
has been the stimulus to cause the migration of the locus of the whole 
plexus — for a movement caudad among the parts of the plexus, since 


WAITE : PLEXI OF NECTURUS. 79 


the movement of the entire plexus is but the summation of the move- 
ments of its elements, the individual nerves. (2) Nerve xxu is so incon- 
stant and contributes so feebly to N. ischiadicus, that, in compensation 
for this deficiency, all of xxi is taken up to form N. ischiadicus, and thus 
it can contribute nothing to N. cruralis. Again, I have shown that, in 
group B, xxi is relatively weaker, and xx relatively stronger than the 
corresponding elements of the plexus in group A. Hence a contribution 
from xx to N. ischiadicus (in group B) serves to make up the deficiency 
arising from the inconstancy and stronger posterior tendency of XXII 
(cf. Fig. 4). (38) It is to be noticed that in group A (Figs. 2 and 3), 
xx always entered N. ischiadicus and that nerve only, while xx was 
(see Table) in many cases (a type) divided between N. ischiadicus and 
N. cruralis. This gives to xx the definite relation of a component part 
to N. cruralis in group A, a relation which might be expected to be 
retained after migration, i.e. in group B. But in group A xx in no 
case had any relation to N. cruralis. It is therefore less probable that 
in the new condition xx1 should form a new connection entering N. cru- 
ralis, than that it should give its entire strength to N. ischiadicus, as it 
does, with which it had the definite relation of a component part in the 
old condition. The general backward movement of the parts of the 
plexus, and the former (a type of group A) branched condition of xx, 
may account for the fact that this nerve sends a branch to N. ischi- 
adicus in this post-migration condition. 

These considerations lead me to believe that the @ type is the only 
condition to be expected in group B, if the plexus has reached its posi- 
tion there by means of migration of the locus of its strength-centre. If 
however, interpolation of a presacral segment has occurred, we should 
expect to find about the same variations in type of topography in group 
B as in group A. These do not occur, therefore the persistence of the 
B type of topography in group B; the inconstancy of nerve xxu, and 
the added activity of x1x in their contributions to the plexus; together 
with the slight variation in the position of the first hemal arch shown 
in this group ;—all combine as evidence against the theory that the 
position of the pelvic girdle in group B is the result of interpolation of a 
presacral segment. 


Group C includes three individuals (10% of the total number ex- 
amined), in which the attachment of the girdle is unsymmetrical. 

Of these one (Plate 2, Fig. 5) bore the right sacral rib on the 19th 
vertebra, the left on the 20th, while the first hamal arch was on the 


80 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


23d vertebra. The topography of the plexus is of the a type. There 
is no asymmetry in its distribution to the appendages, although the 
plexus of the left side must as a whole trend further caudad in order to 
reach the foramina of exit. On the left side the backward trend of x1x 
exposes a longer antero-posterior area of the body wall to which xvu is 
distributed than is exposed on the right side. In accordance with this, 
nerve xvill of the left side is slightly stronger than its companion of the 
right side. Also nerves xIx and xx of the right side are somewhat 
stronger than their mates of the left side; i. e. the strength-centres of 
the two sides are to a slight extent unsymmetrically placed, but this by 
no means equals the extent of an entire segment. 

To which group, A or BL, is this specimen more closely related? As 
in both these groups the first hemal arch may occur (see Table) on 
the same vertebra as in the specimen under consideration, this criterion 
is of no value in determining the affinity of the specimen. But the fact 
that the plexus of this specimen is of the a type indicates that it falls 
under group A, to which this type is restricted (see Table). A glance 
at Figure 5 shows that the main nerve of the plexus is xx, and that xx 
fails to enter the plexus, both conditions typical of group A. Hence 
I consider this specimen to represent an individual like those of group A, 
in which, however, the sacral rib of the left side is displaced one seg- 
ment caudad. ‘There is not a corresponding displacement through one 
segment of the plexus of the left side, but only a slight displacement of 
the strength-centre caudad. 

The two other unsymmetrical specimens (Plate 2, Fig. 6) have in 
each case the right sacral rib on the 18th vertebra, the left upon the 
19th. In both cases the first hemal arch is on the 22d vertebra. 
The plexus is of the 8 type, but with the modifications that in both 
cases the posterior branch of nerve xvi enters strongly into the plexus, 
and further in both cases nerve xxi does not enter the plexus, but re- 
mains a weak nerve distributed to the wall of the pelvic cavity. The 
plexus is consequently formed of nerves xvii, XIx, and xx. Nerve xv 
is stronger than usual, x1x equals the strength of xx, and, as noted, xxI 
is very weak, being no stronger than an ordinary spinal nerve, all of 
which shows a displacement of the strength-centre cephalad. I can 
detect no marked want of symmetry in the locus of the strength-centre of 
the two sides, but if there be any, indications are that it is toward a 
yet further displacement cephalad of the plexus of the right side. 

The position of the hemal arch on vertebra 22 and that of the sacral 
ribs remove these two specimens from any relationship to group B, 
and I therefore conclude that they represent individuals like those of 


ee 


a 


a 


WAITE: PLEXI OF NECTURUS. 81 


eroup A, in which the right sacral rib has been displaced cephalad 
through one complete segment. Superimposed upon this is a dis- 
placement cephalad of the strength-centre of the plexus through the 
extent of part of a segment only, as is clearly indicated by failure of 
xxr to enter the plexus, and the increased strength of xvi. These 
conditions together represent a state intermediate between that of indi- 
viduals bearing the girdle symmetrically on the 19th vertebra, and a 
hypothetical group in which the girdle would be carried symmetrically 
on the 18th vertebra, and probably with the first hemal arch on the 
21st vertebra part of the time at least. Such a variation has not been 
described in Necturus, so far as I know, but I think it likely to occur, 
although the prevailing tendency in abnormal position of the girdle is 
toward displacement caudad. Indeed, Davidoff (’84, p. 412) has found 
such symmetrical displacement cephalad in Salamandra maculosa. 

Of the 157 specimens of Necturus recorded by Parker (96), Bumpus 
(97), and myself, thirteen (8%) are found to have unsymmetrical 
sacra. Of these thirteen, nine have the sacral rib of the left side 
further cephalad, fonr that of the right side. It is thus seen that 
unsymmetrical sacra are not very rare, and further that either side may 
be in advance. Of these four specimens with the right side in advance, 
three, one recorded by Bumpus (’97, p. 466) and two by myself, invade 
the territory of the 18th segment, and these three are the only cases 
out of the 157 in which that segment is invaded. 

By way of summing up the statistics of certain of the conditions 
described, the following table of the thirty specimens which I have dis- 
sected is inserted : — 


TABLE. 
First Hemal Arch on 
Peeieaeria: || Specimens, | ¢ 2¥be-.| @ Type. 
Vertebra 22. Vertebra 23. 
19 20 10 10 10 10 
20 7 0 7 0 i 
19 (rt ) 
and 20 (Ift.) if 1 0 0 1 
18 (rt.) 
and 19 (1ft.) 2 0 2 2 0 


82 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


An interesting skeletal variation in one of the specimens of group C 
just described is the occurrence on the left side of the 19th vertebra of 
a bifurcate transverse process and a partially double sacral rib (Plate 2, 
Fig. 7). The two sacral ribs are not distinct throughout their course, 
but articulate independently with the transverse processes, and normally 
with the ilinm. This condition is parallel with the occurrence of bifur- 
cated transverse processes on the vertebre of Rana, as recorded by 
Bourne (’84, p. 87). 

There has lately come into my possession a skeleton of this species, 
the soft parts of which had already been to a considerable extent re- 
moved. This skeleton shows (Plate 2, Fig. 8) a single sacral rib on the 
left side borne on the 19th vertebra, while on the right side are two 
sacral ribs borne one each on the 19th and 20th vertebrae. These ribs 
are both well formed, but the posterior one is much the shorter, and 
from comparison with the other side, is evidently the supernumerary 
rib. Each articulates independently with the head of the ilium, and all 
the joints work easily. The transverse process on the left side of the 
20th vertebra shows no trace of a rib or articulation, and as it was well 
covered with the musculature, which had not been disturbed when I 
received it, I am certain that none existed there. The condition of the 
specimen when received was such as not to allow determination of the 
nerve relations. 


THEORETIC CONSIDERATIONS. 


The variations which have been described in the preceding pages 
involve at least two questions: (1) Does the abnormal position of the 
girdle arise by intercalation or excalation of presacral segments, by slip- 
ping of the girdle upon the column cephalad or caudad, or by some other 
means? (2) Is there any correlation between the variations of the plexus 
and those of the girdle, and if so, of what sort is it ? 

The first of these questions is far reaching, and this paper does not 
aim at an exhaustive discussion of it. It is commonly held that nerves 
are less subject to variation than either muscles or skeletal parts, and 
thus serve as a surer basis for homology ; yet this basis is in a degree 
unstable, for there are certainly considerable variations amongst nerves, 
as I have shown, for instance, in the lumbo-sacral plexus. 

The evidence for intercalation or excalation as an explanation of the 
changes in the presacral length of the column is of a diversified nature, 
and the opinions concerning such a process may be roughly grouped in 
two categories : first, that the change in number of segments is due to an 


WAITE: PLEXI OF NECTURUS. 83 


initial variation in the serial number and position of centres of metamer- 
ism, — a doctrine formulated by Bateson (92, p. 111); and, secondly, 
that such change arises from fusion of two or more adjacent somites, or 
from the splitting of one or more somites somewhere in the presacral re- 
gion. The first of these two views is the more acceptable as a morphologi- 
cal process, as it preserves the integrity of the metameres and makes the 
process a general one affecting the animal as a whole, rather than local- 
izing the activity within narrow limits, but the evidence in its favor is 
mostly a@ priori. 

Rosenberg (’76, p. 104) was one of the first to advance such a view, but 
he went further than is implied in the above statement, and held that 
this process is an actively phylogenetic one, and that in the vertebrate 
series its operation results in a constant shortening of the vertebral col- 
umn. This shortening is greatly emphasized in some orders, e.g. 
Anura, whereas such conditions as occur in Ophidia must, on the con- 
trary, be considered reversionary. 

According to his view, in the early ontogeny there is represented the 
primitive condition with a very large number of separate prosegments, 
but in course of development of the individual these are reduced to the 
number set by heredity. We may infer variation in the final number 
of segments to be the result of inaccuracy of response to the hereditary 
stimulus. As noticed by many writers upon this subject, the great 
majority of such variations are toward an increase in the number of 
segments, and according to this view such cases must be interpreted as 
atavistic. It would seem that Rosenberg’s view is insufficient, or at 
least if there be such a general tendency, it is exerted, not in a single 
unilateral series, but in a branching series of phylogenetic relationships. 

In such cases of variation between two individuals either of the same 
or of different species, no one centre of the final series of metameres in 
the normal can be directly homologous with any one centre in the vari- 
ant; i.e. the number of segments arising in the variant differs from that 
arising in the normal, and each segment occupies a new position different 
from that in the normal. 

Most of the evidence from recorded skeletal abnormalities better fits 
the second category stated. Baur (’91, p. 335) claims intercalation of 
vertebra as an actual ontogenetic process. This is based upon the evi- 
dence afforded by certain complexes of vertebrae which show incomplete 
fission on one side of the median plane, with a normal condition on the 
other side, these having been recorded in some Ophidia by Albrecht, 
Owen, and Baur himself. Other cases of vertebral “ masses ” have been 


84 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


described by Bourne (’84, p. 86), Benham (’94, p. 477), Adolphi (96, 
p- 133), and others. but such masses may, in my opinion, better be 
interpreted as ankylosis, symmetrical if the line of fusion be transverse 
to the axis of the column, or unsymmetrical if the line of fusion has been 
oblique. ‘This is borne out by the fact that near, though not necessarily 
contiguous to such an unsymmetrical mass, there is usually found a comple- 
mentary mass (see Benham, 94, pp. 477, 478) ; i. e. if by oblique fusion 
we have a mass containing one right half-vertebra and two left halves, 
we shall find near by another mass representing one left half-vertebra, 
and two right halves. The vertebre lying between these two masses 
will be morphologically oblique, their two halves in each case belonging 
to different somites, although the adjustment of growth may have 
brought these to an apparently normal position, with transverse axis at 
right angles to the main axis of the column. That the right and left 
halves of a vertebra may slide upon each other, is held by Adolphi 
(796, p. 136) to be confirmed by embryological evidence. Such unsym- 
metrical masses have not been accompanied by correlative asymmetry 
in musculature and nerves where the soft parts have been described. 

Symmetrical fusion of vertebree has been described by Howes (93, 
p- 295), Adolphi (95, p. 466, and ’96, p. 122), and others. An actual 
increase in number of vertebrze, and also an indicated increase by groov- 
ing and partial splitting of vertebre, and by bifurcation of transverse 
processes, have been noticed by Bourne (’84, p. 87), Benham (94, p. 478), 
and others. The cases of actual increase described were in Anura, and 
resulted from separation of the anterior portion of the urostyle as a 
supernumerary vertebra. Adolphi (95 and ’96) has recorded forty-three 
cases of vertebral fusion in Bufo, Pelobates, and Rana, and has deter- 
mined the nerve relations. He finds that in such cases of fusion the 
spinal nerves are not suppressed, but emerge through foramina in the 
fused mass; they are, however, liable to be weaker than normal. 

Such supposed direct evidence of intercalation and excalation is capa- 
ble of being interpreted as pathological, rather than as a disturbance 
tending primarily to alter the serial number of metameres, — especially 
since it is almost entirely confined to the skeleton, without involving 
musculature or nerves beyond the narrow limits necessitated by local 
accommodation to the distorted vertebre. No evidence of the inherit- 
ance of such abnormalities has yet been offered. Such would be valu- 
able as determining whether or not these variations show persistence. 
If so, it would indicate that they are sports; if not, the pathological 
interpretation would be strengthened. 


WAITE: PLEXI OF NECTURUS. 85 


I have shown that no intercalation or excalation is evident in the 
prebrachial region of Necturus, and since serial variation tends to be at 
one end of the series, it is more to be expected in the prebrachial region 
than in the postbrachial-presacral region. Addition or loss at the caudal 
end of the series may be disregarded, as this could not directly affect 
the number of presacral vertebree, although it might be correlated with 
an abnormal position of the sacrum, both being an indication of general 
instability in the individual. 

While the idea of variation in the number and position of centres of 
metamerism may be attractive on theoretic grounds, it has no observa- 
tional evidence to support it, and, further, is insufficient to account for 
unsymmetrical sacra and supernumerary sacral ribs. On the other 
hand, intercalation or excalation has not been shown to exist as a mor- 
phological process, the supposed evidence for it being more naturally 
interpreted as due.to pathological conditions. Since these two explana- 
tions seem improbable, we must look for some other cause of the varia- 
tions of presacral distance which we find. 

The common expression of sliding or shoving of the sacrum upon the 
column is open to criticism. It is contrary to ideas of metameric unity 
that the girdle should begin its development in one segment, and later 
in ontogeny migrate to the next. Bolk (94, p. 267) has found in Homo 
during ontogeny an actual migration cephalad on the part of the Anlage 
of the pelvic girdle ; but such migration is confined within narrow limits, 
not having extended over an entire segment, and thus the Anlage has 
not passed through a myotome. A slight migration of the musculature 
in relation to the skeleton is also known, but none of these slight 
changes are sufficient to account for sudden displacement of the girdle 
through an entire segment. Bumpus (97, p. 464) has advanced a mod- 
ification of this idea to account for the greater frequency of displace- 
ment caudad. Since the transverse processes to which the sacral ribs 
are attached lie nearer the caudad limit of the vertebra, he considers it 
“more probable that variations occurring in the course of ontogenetic 
development will fall on the side of nearer proximity,” i. e. into the 
next segment caudad, and, having once invaded the territory of this new 
segment, the ribs will be adjusted to the proper position within that 
segment near the posterior limit. The individuality of segments in the 
skeleton is very sharp, and any explanation which involves a migration 
of an organ located entirely in one segment, so that it comes to lie 
wholly in another segment, assumes a process which is not only un- 


proved, but seems to me highly improbable. Bumpus accounts for 
VOL. XXXI.— NO. 4. 2 


86 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


variation of the serial position of the sacrum by assuming that the 
appendage has a locus fixed at a point whose linear distance from the cra- 
nium is a definite and constant proportion of the entire length of the 
animal. The change in serial position of the girdle is then effected by 
compressing the presacral vertebre, making each shorter and diminish- 
ing their combined length, thus bringing a vertebra caudad to the nor- 
mal, opposite the stationary appendage locus. While such a view may 
be made to account also for asymmetrically placed sacra, it is difficult 
to see how it will adapt itself to the occurrence of supernumerary 
sacral ribs, and especially those asymmetrically placed (see Plate 2, 
Fig. 8). 

What seems most probable is that in different individuals the girdle 
may develop at primarily different distances (measured in segments) 
from the cranium. In Necturus we find a pair of sacral ribs on verte- 
bra 19 (group A), or on 20 (group B). The explanation by intercala- 
tion implies that vertebra 20 in group B is the same vertebra as 19 
in group A. That of slipping of the girdle — literally taken — implies 
that a girdle beginning to form in the 19th segment later (in ontogeny) 
is transferred to the 20th segment. Both explanations, as already 
stated, seem unsatisfactory. It is more logical to consider that the 
new position of the girdle is due to a stimulus to girdle formation 
having been applied at a new point, i. e. in a segment other than the 
normal, and hence that a sacral rib may arise in any one (or more, as 
shown in supernumerary ribs) of several points in this region. In 
Necturus these points are at least three, located in the 18th, 19th, and 
20th segments. 

Such a view explains the variation as to place of origin in different 
segments of sacra placed symmetrically, and also the condition of un- 
symmetrical sacra, such as have been described in group C,— since the 
stimulus to girdle formation is not single but paired, i. e. from the future 
appendages, and so need not necessarily be symmetrical ; and above 
all it is sufficient to explain the occurrence of supernumerary sacral 
ribs. These latter may be on one side only (Plate 2, Fig. 8, Howes, 
’86, p. 279, Lucas, ’86, p. 562, and others) ; or @ symmetrical pair of 
supernumerary ribs may appear (Case, 96, p. 232; Lucas, °86, p. 562). 

From the foregoing discussion I conclude that neither intercalation or 
excalation, nor slipping, are involved, but that the abnormal position of 
the girdle represents development of a new girdle at a new point. 

This new position is usually caudad to the normal, though in a few 
recorded cases in Urodela (Davidoff, 784, p. 412, and others) it is 


WAITE: PLEXI OF NECTURUS. 87 


cephalad. Such general displacement caudad is capable of interpreta- 
tion either as atavistic, or as indicating some force at work tending to 
lengthen the vertebral series. 

It is noticeable that unsymmetrical relations of the appendages are 
more frequently recorded in Amphibia than in Reptilia, Aves, or Mam- 
malia. This may possibly be due simply to the probably greater num- 
ber of Amphibia examined. But assuming that this is not the case, it 
is not clear why such variation should exceed in this class. I offer, 
however, the suggestion that it may be due to the position of the em- 
bryo during development. In Amphibia the position is such that the 
embryo is curled within the egg membrane Jaterally, which would tend 
to shorten the concave side, and so might induce a displacement of the 
appendage of that side cephalad or caudad. In the other groups men- 
tioned, the curling of the embryo is dorso-ventrally, and would give less 
tendency to unsymmetrical relations. 

As to the second question involved, —Is there any correlation be- 
tween the variations of the plexus and those of the girdle? — there is a 
wide difference of opinion, ranging from very close correlation, as main- 
tained by Ruge (’93, p. 393), to no correlation, as believed by von Ihering 
(78), who regards nerve and skeleton as independent in variation. 
Von Ihering holds that the plexus migrates as a whole independently of 
the girdle ; if cephalad, then a presacral pair of spinal nerves drops out, 
if caudad, a pair is inserted, and if the migration be unsymmetrical, it 
involves loss or addition of a spinal nerve on one side only. 

Such a scheme, by reason of its apparent artificiality, fails to appeal 
to me strongly, and no conditions found in Necturus warrant adoption 
of such a hypothesis. 

The plexus is a combination of spinal nerves, the nature of whose 
topography is secondary. The point which needs emphasis in this con- 
nection is that the variation of the plexus is not a variation as a whole, 
but the summation of variations of its elements (individual spinal nerves). 
The variation in strength of these elements is probably a response to the 
influence from their end organs, — the muscles, — for there appears to 
be direct correlation between the size and activity of a muscle, and the 
strength of the nerve or nerves supplying it. Whether the muscles of 
a given metamere shall develop as limb muscles, i. e. muscles of increased 
size and activity, or into less extensive trunk musculature, depends upon 
whether or not in that region an appendage —the primary determinant 
of both plexus and girdle —is to arise. If appendicular musculature 
develop, the increased activity presumably induces stronger develop- 


88 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


ment of nerves in the segments involved, the matter of topography of 
plexus being secondary to the determination of the segmental position 
of the appendage. 

That there is any direct correlation between skeletal and nervous tis- 
sues seems improbable, for there is no obvious reason why nerves should 
directly influence skeletal parts, nor is it probable that skeletal parts 
directly influence the position of nerves, since the vertebrate skeleton 
arises, both in phylogeny and in ontogeny, much later than the nervous 
system. 

The view of the relations of nerve and skeletal parts, first hinted at 
by Fiirbringer (79), and more clearly formulated by Eisler (’92), seems 
much more reasonable than the idea of direct correlation. According to 
Eisler’s conception, skeletal and nervous structures exhibit variations 
independently, but in parallel directions. The control of such paral- 
lelism rests with the musculature. The latter has very definite re- 
lations to the nerves on the one hand, and to the skeleton on the 
other. Bolk (794) has shown that in Homo the position of the nerves 
depends upon the position of the muscle segment, which is differen- 
tiated earlier than the nerve, and it is also well known that the myo- 
meres have very close relations to the vertebra, so that in any given 
segment there is an interdependence among the three systems. Either 
nerve or vertebra may vary within certain limits, without necessarily 
affecting the other, but extensive variation of either would presumably 
influence the other through intermediation of the musculature. 

From the nature of the case skeletal variations within narrow limits 
are less easily observable than nervous; but the examination of almost 
any series of vertebre shows variation not only serially, but also on the 
right and left sides of the same vertebra. Such variations have been 
recorded by Paterson (’92, pp. 523, 524) from dissections of a large 
series (265) of human adults. 

The conception of parallel variations of skeleton and nerve under 
control of the musculature is able to explain the fluctuations in posi- 
tion and strength of nerves belonging to any one segment involved 
in the plexus. It also offers an explanation of the inverse correlation 
of nerves of adjacent segments, seen in the weakening of the nerve 
of one segment, accompanied by a strengthening of that of the next 
segment; for since many muscles are innervated by two or more 
nerves from different segments, if one of these becomes weaker, the 
other becomes stronger to make up the deficiency. It should be 
remembered further that it has been shown that any one of several 


WAITE: PLEXI OF NECTURUS. 89 


myotomes in this region is able to produce sacral ribs. In the greater 
number of cases the function of rib production by the myotome is 
expressed at a constant distance (measured in metameres) from the 
cranium, resulting in what we call the normal position; but as some 
tendency arises for the girdle to be abnormally placed, such tendency 
is expressed by the appearance of a new plexus and a new girdle in a new 
position. However, as we have seen in group B, the plexus in the new 
position does not have the same relation to the girdle-bearing segment 
that it did in the old ; on the contrary, it tends to occupy a place inter- 
mediate between such a position and the old one. The girdle also may 
possibly show a parallel intermediate position ; but if so, it is more 
difficult to verify. Such an intermediate position of the plexus may be 
interpreted either as representing an atavistic tendency, or an incom- 
plete migration. I am inclined to believe im the latter, and that it 
indicates a less complete response by the plexus than by the girdle to 
the influence of the musculature which changes the locus of both girdle 
and plexus. 

The nerve relations in Necturus show that variations of girdle and 
plexus are nearly parallel, but that these are in some degree independent, 
as exhibited by the fact that the strength-centre of the plexus does not 
have just the same relation to the girdle in the variant that it had in the 
normal condition. 


February 20, 1897. 


Nors. — Since the manuscript of this paper left my hands, two papers 
bearing upon the question of intercalation have appeared. 

Ridewood (97, p. 366) considers that the point of sacral rib forma- 
tion and attachment is determined by some stimulus external to the 
column, and that the girdle does not migrate from one segment to 
another during ontogeny, —a conclusion which is along the same line 
as my own (pp. 85, 87). 

Baur (97) contends for intercalation as an ontogenetic process, but 
I find little in addition to what is contained in his earlier paper (Baur, 
*91), except that he now (p. 42) supports the idea that the pelvis, devel- 
oping in one segment, may migrate into a neighboring segment, and 
become ‘secondarily united with the vertebral column.” 


October 1, 1897. 


90 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


BIBLIOGRAPHY. 


Adolphi, H. 

°93. Ueber Variationen der Spinalnerven und der Wirbelsaule anurer Am- 

phibien. I. Morph. Jahrb., Bd. XIX. p. 313. 

Adolphi, H. 

°95. Ueber Variationen u.s. w. II. Morph. Jahrb., Bd. XXII. p. 449. 
Adolphi, H. 

796. Ueber Variationen u. s. w. III. Morph. Jahrb., Bd. XXV. p. 115. 
Bateson, W. 


°92. On Numerical Variation in Teeth, with a Discussion of the Conception 
of Homology. Proc. Zodl. Soc. London, 1892, p. 102. 


Baur, G. 
°91. On Intercalation of Vertebre. Jour. Morph., Vol. IV. p. 331. 
Baur, G. 
'97. Remarks on the Question of Intercalation of Vertebre. Zodl. Bull., 
Vol. I. p. 41. 
Benham, W. 


94. Notes on a particularly Abnormal Vertebral Column of the Bull-frog 
(Rana mugiens) ; and on certain other Variations in the Anuran Column. 
Proc. Zool. Soc. London, 1894, p. 477. 


Bolk, L. 
°94. TBeziehungen zwischen Skelet, Muskulatur, und Nerven der Extremi- 
taten. Morph. Jahrb., Bd. XXI. p. 241. 


Bourne, A. G. 
°84. Certain Abnormalities in the Common Frog (Rana temporaria). Quart. 
Jour. Micr. Sci., Vol. XXIV. p. 83. 


Bumpus, H. C. 
°97. A Contribution to the Study of Variation. (Skeletal Variations of Nec- 
turus maculatus Raf.). Jour. Morph., Vol. XII. p. 455. 


Case, E. C. 
°96. Abnormal Sacrum in an Alligator. Amer. Naturalist, Vol. XXX. 
p. 282. 
Davidoff, M. von. 
°84. Ueber die Varietiten des Plexus lumbosacralis yon Salamandra macu- 
losa. Morph. Jahrb., Bd. IX. p. 401. 


WAITE: PLEXI OF NECTURUS. 91 


Eisler, P. 
°92. Der Plexus lumbosacralis des Menschen. Abhandl. Naturf. Gesell. 
Halle, Bd. XVII. p. 279. 


Fiirbringer, M. 
°79. Zur Lehre von den Umbildungen der Nervenplexus. Morph. Jahrb., 
Bd. V. p. 324. 


Hoffmann, C. K. 
°74. Klassen und Ordnungen der Amphibia. Jz Bronn’s Klassen und Ord- 
nungen des Thier-Reiches. Bd. VI. Abth. LI. Lief. 6 u.7. Leipzig u. 

Heidelberg. 


Howes, G. B. 
86. On some Abnormalities of the Frog’s Vertebral Column. I. temporaria. 
Anat. Anz., Jahrg. I. p. 277. 


Howes, G. B. 
°93. Notes on Variation and Development of the Vertebral and Limb 
Skeleton of the Amphibia. Proc. Zool. Soc. London, 1893, p. 268. 


Ihering, H. von. 
°78. Das peripherische Nervensystem der Wirbelthiere als Grundlage fiir 
die Kenntniss der Regionbildung der Wirbelsaule. Leipzig, 1878. 


Lucas, F. A. 
*°86. The Sacrum of Menopoma. Amer. Naturalist, Vol. XX. p. 561. 


Parker, G. H. 
°96. Variations in the Vertebral Column of Necturus. Anat. Anz., Bd. XI. 
p. fii. 
Paterson, A. M. 
*92. The Human Sacrum. <Aédstr. iz Proc. Roy. Soe. London, Vol. LI. 
p. 520. 


Ridewood, W. G. 
97. On the Development of the Vertebral Column in Pipa and Xenopus. 
Anat. Anz., Bd. XIII. p. 359. 
Rosenberg, E. 
°76. Ueber die Entwicklung der Wirbelsaule und das Centrale Carpi des 
Menschen. Morph. Jahrb., Bd. I. p. 83. 
Ruge, G. I 
*93. Verschiebungen in den Endgebieten der Nerven des Plexus lumbalis 
der Primaten. Morph. Jahrb., Bd. XX. p. 305. 


92 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


DESCRIPTION OF PLATES. 


The Arabic numerals indicate the serial number of the vertebre, the Roman 
numerals the serial number of the ventral branch of the spinal nerves. 


ABBREVIATIONS. 
br. Nervus brachialis. il-h’ga. Nervus ileohy pogastricus. 
cru. Nervus cruralis isch. | Nervus ischiadicus. 
est. ser. Sacral rib. pre.t. Transverse process. 
cst. scr’. Supernumerary sacral rib. su’crac. Nervus supracoracoideus. 


il. Ilium. 


All the diagrams are drawn to natural scale of a full grown individual, and 
are viewed from ventral aspect. The size of nerves is drawn to scale roughly for 
the average condition. 


Warre. — Plexi of Necturus. 


Jara 
Fig. 2. 
Fig. 3. 
Fig. 4. 


PLATE 1. 


Diagram showing topography of brachial plexus of left side. 

Diagram of lumbo-sacral plexus of left side, as found in a type, group A. 
Diagram of lumbo-sacral plexus of left side, as found in B type, group A. 
Diagram of lumbo-sacral plexus of left side as found in group B. 


Puate. |. 
 B.Meisel lth dostn, 


}-PLEXL OF NECTURUS. . 


by 


— 


ga 0g 0g OR 


by 


c>| — 


rT 


er 


Waite. — Plexi of Necturus. 


PLATE 1. 


Diagram showing topography of brachial plexus of left side. 

Diagram of lumbo-sacral plexus of left side, as found in a type, group A. 
Diagram of lumbo-sacral plexus of left side, as found in B type, group A. 
Diagram of lumbo-sacral plexus of left side as found in group B. 


ek Dae i on 


PLATE. |. 


-PLEXI OF NECTURUS.. 


B Meisel lith Bostes. 


net 2 


Wim PG.0e i 


Fig. 
Fig. 


Fig. 
Fig. 


Waite. — Plexi of Necturus. 


~] 


PLATE 2. 


Diagram of plexus of both sides in a specimen bearing asymmetrical sa- 
cral ribs on vertebre 19 and 20. 


Diagram of plexus of both sides of a specimen bearing asymmetrical sa- 
cral ribs on vertebre 18 and 19. 


Diagram showing a bifurcate transverse process and sacral rib. 


Diagram of sacrum of a specimen with a supernumerary sacral rib on one 
side. 


BMeisel ith Best, 


u 


i 


No. 5.— Reports on the Dredging Operations off the West Coast 
of Central America to the Galapagos, to the West Coast of 
Mexico, and in the Gulf of California, in charge of ALEXANDER 
Acassiz, carried on by the U. S. Fish Commission Steamer 
“ Albatross,’ during 1891, Lieut.-Commander Z. L. TANNER, 
U. S. N., Commanding. 


[Published by permission of MarsHatt McDonavp and J. J. Brice, 
U. S. Fish Commissioners. ] 


XXII. 
The Isopoda. By H. J. Hansen. 


Tue collection contains in all fifteen species, fourteen of which, all 
marine, I have considered new to science, while one form — belong- 
ing to the Oniscidz — is terrestrial in habit, and proves to be a well 
known species. Of the fourteen marine species, eight are free-living 
forms, and one is parasitic on fishes; these nine species are easily 
referred to genera established many years ago. The remaining five 
species belong to the subfamily Bopyrine, of the very extensive family 
Epicaridea; they present several peculiarities in structure, and more- 
over they are rather interesting since no form of the Bopyrine has 
heretofore been found on truly deep-sea animals. For particulars, how- 
ever, the reader must be referred to the special description later on. 

Besides my special account a few remarks must suffice.t Since each 


1 The Director of the Entomological Department of the Zodlogical Museum in 
Copenhagen, Inspector Dr. F. Meinert, had commenced to deal with the material, 
but being engaged in other work, he transferred to me the preparation of this 
report. Only the following particulars are of interest. He had recognized the 
two species of Asellota and all the species of Cymothoidw as new to science; 
furthermore, he had furnished them with names, and on the labels briefly men- 
tioned the species already published to which each of the new forms was most 
closely allied. Some of the names and most of these hints on affinity are adopted 
in the report, which otherwise is wholly a work of my own. Yet it must finally 
be mentioned that Mr. G. Budde-Lund has determined the single species of 
Oniscide. 

VOL. XXXI.—No. 5. 


96 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


of the families is represented by only a few species, I am unable to 
attempt improvements in the classification of any of them. In a pre- 
vious paper — Isopoden, Cumaceen und Stomatopoden der Plankton- 
Expedition (Ergebnisse der Plankton-Expedition der Humboldt-Stiftung, 
Bd. II. G. c) —I have proposed a partly new arrangement of the Iso- 
poda, with observations on some of the families, and to this treatise 
the reader must be referred for several particulars. I have thought it 
useful to illustrate all the species rather fully, and to describe them 
in some detail, taking into consideration the best representations in the 
literature, yet altering and adding where it seemed advisable. 


ASELLOTA. 


Of this large tribe only two species were secured. Both belong to the 
Munnopside G. O. Sars, a family rather badly limited, and both must be 
referred to the genus Hurycope G. O. Sars. Unfortunately, the material is 
rather scanty and all the specimens are much mutilated, yet I am able to draw 
attention to a point of significance, namely, that the genus with the limits 
still adopted presents startling differences in the structure and shape of the 
mandibles of some of the species. In the two species here described the 
mandibles possess distally a cutting portion, behind this a “ lacinia mobilis”’4 
with a row of sete on each mandible and a strong ‘‘cuspis lacinie” on the 
left one, and farther backward a well developed molar process. In the small 
Norwegian species the mandibles seem to be of similar structure,? but in the 
large Ewrycope gigantea G. O. Sars they are very different. In this species 
each mandible has a very long oblique edge on the inner side, the molar process 
is very short and badly defined, no lacinia mobilis is found, ete. It may be 
added that the two pairs of jaws also present differences from those in the 
species to be described here. (The mouth organs of Hurycope gigantea were 
first described by G. O. Sars in the Norwegian North-Atlantic Expedition, 
Zool., Crustacea, Vol. I. pp. 132, 133, Plate XI. Fig. 10-14, and shortly after- 
wards by the present author in his account of the Crustacea in Dijmphna- 
Togtets zool.-bot. Udbytte, 1877, pp. 199-201, Tab. XX. Fig. 3e-3g.) Itis 
interesting to observe that great differences in the structure and armature of 


1 This and the following term are set forth and explained in my paper: Ciro- 
lanide et Fam. nonn. prop. Musei Haun. (K. Danske Vidensk. Selsk. Skrifter, 6 
Rekke, naturv.-math. Afdeling, V. pp. 239-426, Tab. I.—-X.) 

2 At my request, Prof. G. O. Sars very kindly sent me the proof-sheets containing 
the account of the Munnopsidz, in his new leading work on the Isopoda. He has 
divided the family into two families, etc., but he still maintains the genus Eurycope 
in its old and very wide extension, yet remarking that some of the species estab- 
lished by Beddard “ought perhaps more properly to be separated as types of 
nearly allied genera.” 


HANSEN: THE ISOPODA. OF 


the mouth organs are found in species which in general shape and other struc- 
tural features seem to be rather closely allied. Unfortunately, the mouth parts 
in several of the species described by Beddard in the “Challenger” Isopoda 
are entirely unknown. 


1. Eurycope pulchra, n. sp. 
Plate I. Fig. 1-1i. 


One much mutilated male and six females, three of them with the mar- 
supium well developed, the others much more than half grown or almost fully 
grown, were captured. 

Head. The dorsal surface with three acute processes, the two anterior of 
which are rather small, each lying a little behind the antennula, while the 
third odd process is rather good-sized and separated from the two others by a 
deep and rather broad furrow. On each side this furrow runs down the lateral 
surface of the head, above it bends obliquely forward, converging with the 
furrow from the other side, and finally terminating in a median impression 
between the two anterior processes. The labrum is very large and prominent, 
anteriorly rounded. 

Antennule. The basal joint of the peduncle is oblong, anteriorly cut off ; 
the most distal part of the interior side, where the second joint is articulated, 
is incised; the upper side is irregularly arched, the distal part of the under 
side longitudinally somewhat excavated. The second joint is as usual short, 
the next slender and of about the same breadth at both ends; the anterior 
inner angle somewhat produced, acute. Third joint somewhat shorter and much 
more slender than the second. The flagellum somewhat exceeding one third 
of the length of the body, with innumerable joints. 

Antenne. In no specimen are more than the four proximal short joints and 
sometimes the basal part of the very elongated fifth joint preserved. Third 
joint anteriorly on the limit between the exterior and the lower side produced 
into a very conspicuous acute process ; the exopod (squama) very small and 
quite fused with the third joint, not even set off by a transverse suture (com- 
pare the following species). 

Mandibles (Fig. 16 and 1c). Of about the same shape as in Janira and 
allied genera. The cutting portion (a) compressed, much higher (when seen 
from in front) than broad, ending with three teeth. Pars molaris (b) moder- 
ately long, somewhat compressed, so that it is broader when seen from in front 
than when seen from below as in the figure ; distally it is cut off obliquely, 
with some setz, and as usual the terminal face of the two molar processes is 
somewhat differently shaped. Lacinia mobilis (/) with numerous sete, and 
the cuspis lacinia on the left mandible strongly developed, compressed and 
much higher than broad, ending in four teeth. The palpus stout, three-jointed, 
second joint almost double as long as the first ; third joint of a peculiar aspect, 
curved, rather broad, with a conspicuous incision on the anterior margin. 


98 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


Mazillule (Fig. 1d). The lobe (A) of the first joint (1) in its distal half 
rather narrow and curved, with numerous hairs at the rounded apex, but with- 
out any spine-like seta. The oblique terminal margin of the lobe (/#) of the 
third joint, as in other species, with numerous long spines. 

Mazille (Fig. 12). The lobe (J?) of the second joint proportionally rather 
narrow, with hairs along the interior margin and on the rounded terminal mar- 
gin; the two lobes (J) of the third joint with some long and robust sete at 
the apex. 

Mazillipeds (Fig. 1f). Second joint (2) rather long, with about sixteen 
coupling hooks (h) at the inner margin; the terminal margin of its lobe (/?) 
coarsely serrated and hairy. Fourth and fifth joints, as in other species, much 
expanded, but not to such a degree as, for instance, in the following species ; 
the fourth joint conspicuously narrower than the second, and considerably larger 
than the fifth. Sixth and seventh (7) joints small and slender. The epipod (¢p.) 
with a somewhat produced acute angle at the middle of the exterior margin. 

Thorax. As usual in this genus, the thorax is divided into two parts, the 
first of which, consisting of four segments, in this species equals in length the 
second part. The first segment considerably narrower than the second, the fifth 
nearly twice as broad as the first. The four anterior segments with a trans- 
verse depression in a considerable part of the breadth. The first segment 
with a single small dorsal process. Second, third, and fourth segments each 
with a median, very high, laterally compressed, acute dorsal process, turning 
obliquely forward and rising just behind the anterior margin ; besides, the 
second and third segments with a short rounded protuberance in the median 
line a little in advance of the posterior margin. The third and the fourth seg- 
ment with the antero-lateral angle produced into an acute, almost spine-like 
process ; on the first two segments the same angle is rounded. The three pos- 
terior segments with a median, longitudinal, rather broad impression, on each 
side limited by a low keel, anteriorly produced into an acute process, which is 
long on the fifth and short on the seventh segment. The antero-lateral angle of 
the three last named segments produced into an acute process, turning forward 
and somewhat outward, the process being long on the fifth segment, shorter, 
but almost broader on the last two segments ; finally, on the lateral margin a 
little in advance of the posterior angle, a protuberant rounded process, which is 
very low on the fifth segment, somewhat larger on the two others, especially 
on the last one. The whole dorsal surface of the trunk, as of the abdomen, 
closely set with very small granulations, giving it a faintly scabrous appearance. 
While the first segment is movably jointed with the head, and the articulation 
between the four anterior segments, and especially between the fourth and the 
fifth segment, is very well developed, the three posterior segments are im- 
movably connected with one another and with the abdomen. 

Thoracic Legs. The basal joint of the four anterior pairs with the antero- 
lateral angle produced into a rather good-sized, distally almost spine-like acute 
process, and laterally with a shorter projecting process ; the basal joint of the 
three posterior pairs smooth. The first pair (Fig. 1g) scarcely of medium 


HANSEN: THE ISOPODA. 99 


length, very slender ; the fifth joint almost as long as the second, somewhat 
curved, very slender, and not expanded on its under side. Of the second, third, © 
and fourth pairs only the two proximal joints are not broken off. In the three 
last pairs the fifth joint is almost twice as long as broad (Fig. 1h), the seventh 
joint (7) as long as the fourth, and very slender (in Figure 1h all the hairs 
are omitted). 

Abdomen. As long as the four posterior thoracic segments together, narrower 
than the seventh segment, and decreasing in breadth from the anterior angle, 
which is produced into a triangular acute process, turned forward and especially 
outward. It consists of at least two visible segments — the posterior, of course, 
consisting of fused segments — fused together, the anterior of which is short ; 
besides, there is seen across the anterior part of the second segment a curved 
transverse furrow, perhaps indicating a rudiment of a second articulation. In 
the median line, just behind the furrow between the first segment and the rest 
of the abdomen, is a small tubercle, especially obvious in a lateral view as a 
rudimentary process. On the dorsal side are two deep longitudinal furrows, 
at a considerable distance from each other, and anteriorly curving outward. 
The posterior margin with three acute processes, the median one curved con- 
siderably downward and much larger than the two others, each of which is 
situated close inside the point where the dorsal furrow reaches the posterior 
margin. The oblique terminal face of the abdomen is quite similar in both 
sexes and rather peculiar (Fig. la and 17): the hind margin with the three 
processes just mentioned, the oblique lateral margin a little arcuate, while 
the infero-anterior margin is short and concave, the infero-lateral angle 
being produced into a shorter process; on the upper half of the terminal 
face are seen the two oblong-triangular anal doors (Fig. 1%, d), and just 
outside each door the uropod is attached. In the female the ventral oper- 
culum (the first pair of abdominal limbs) has an impression along with and 
somewhat inside of the lateral margin and close to the posterior margin; in 
the median line it possesses a keel, which somewhat before the middle is 
produced into a rather long, moderately compressed acute process. In the 
male the operculum (the first and second paits of abdominal limbs together) is 
represented in Figure 17, and scarcely needs a special description. (Having 
but one male specimen, I omit the description of the “appendix masculina” 
on the second limb.) 

Uropods (Fig.17%). Each consists of a moderately short and very slender 
peduncle and two 1-jointed rami, the interior of which is about as long as the 
peduncle and still more slender, the exterior one somewhat shorter. 

Size. A female specimen whose marsupium is still rudimentary (consisting 
only of small plates on the second and fourth pairs of legs) is 28 mm. long and 
11.2 mm. broad. Of the three females with the marsupium completely devel- 
oped (in two specimens filled with eggs) the largest is a little smaller than the 
specimen with rudimentary marsupium ; the smallest is 23.3 mm. long and 
9.8 mm. broad. The youngest female is 20.3 mm. long; the male is about 
24.5 mm. long and 10 mm. broad. 


100 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Habitat. Station 3361 (Lat. 6° 10’ N., Long. 83° 6’ W.), 1471 fathoms, 2 
- specimens ; Station 3413 (Lat. 2° 34’ N., Long. 92° 6’ W.), 1360 fathoms, 5 
specimens, 

Remarks. This species is closely allied to Ewrycope fragilis Bedd. (“Chal- 
lenger ” Isopoda, p. 68, Plate XI. Fig. 8-12) ; but if the drawings of Beddard 
are trustworthy in detail, my species is easily distinguished from E. fragilis by 
the processes on the dorsal surface of the head, by the shape of the lateral 
margin of the three posterior thoracic segments, by the direction of the pos- 
tero-lateral abdominal processes, ete. However, a thorough revision of many 
of the “Challenger” Isopoda, especially of the Asellota (sens. lat.), is very 
much needed. 


2. Eurycope scabra, n. sp. 
Plate I. Fig. 2-2d; Plate Il, Fig. 1. 


Only one single and ill-handled female specimen is present ; yet I hope that. 
the species can be easily recognized, especially by the aid of my figures. 

Head, thorax, and abdomen without any processes, and scarcely with sharp. 
angles; but with the exception of a transverse belt across each of the four 
anterior thoracic segments, the dorsal surface of the body is almost wholly cov- 
ered with numerous granulations, so that it becomes scabrous in a much higher 
degree than the preceding species. 

Antennule. Absent. 

Antenne. Only the four proximal joints are present. The third joint with- 
out any process, but above at the exterior side is seen a small triangular and 
rounded exopod, well set off by a suture. 

Mandibles. Only the left mandible (Fig. 2a) has been examined. The cut- 
ting portion well developed, ending in five teeth; the lacinia with about six 
sete, and the cuspis lacinie large, with teeth of very different magnitude. The 
molar process rather long and proportionally slender, seen from below (as in 
Fig. 2a) almost conical with the end cut off very obliquely ; seen from in front 
the distal part is somewhat broader, and the terminal face is vertical, with 
sharp serrulation and a few broad hairs ; but in the lower end of the face a tri- 
angular process is seen, and it is this process which in Figure 2a overlaps the 
greater part of the end. The palp is very slender; second joint but a little 
longer than the first; the third very slender. 

Mazillule (Fig. 26). The distal part of the lobe of the first joint broader 
and less curved than in the preceding species, hairy and without spine-like 
sete ; the lobe of the third joint about as in Eurycope pulchra. 

Mazille (Fig. 2c). The lobe of the second joint distally proportionally 
narrow and tapering towards the rounded apex, which is furnished with 
normal hairs. 

Mazillipeds (Fig. 2d). Second joint rather elongate, its lobe with the 
terminal margin closely serrated and with about twelve coupling-hooks at the 
inner margin. Fourth and fifth joints more expanded than in EHurycope 


HANSEN: THE ISOPODA. 101 


pulchra, the fifth almost as large as the fourth, and its inner margin partly 
serrated; sixth and seventh joints much broader than in the preceding species. 
The epipod with the exterior margin evenly curved. 

Thorax. It was badly preserved, and therefore the relative breadth of the 
segments could not be drawn with so much certainty as could be wished. The 
want of processes and the scabrous surface are mentioned above. The three 
posterior segments, without any median dorsal impression not connected immov- 
ably with each other, and somewhat shorter than the four others together ; the 
last segment seems to be movably united with the abdomen. 

Thoracic Legs. The basal joint of the four anterior pairs anteriorly or ex- 
teriorly produced into an angle or short scabrous process. The first pair (Plate 
II. Fig. 1) rather short and stout; the fifth joint conspicuously shorter than the 
second, compressed and somewhat expanded on the under side, the margin of 
which is hairy. Of the six other pairs of legs only the basal joint is preserved. 

Abdomen. It is nearly ovate and proportionally large compared with the 
thorax, but neither shape nor magnitude could be drawn with absolute cer- 
tainty, as the abdomen was roughly handled. The basal segment is very short; 
for the rest only a pair of very faint somewhat curved longitudinal impressions 
are seen on the scabrous dorsal surface. The operculum in the female without 
any keel. 

Uropods. Somewhat longer than in the preceding species, but of about the 
same shape. 

Size. The specimen described is about 25.6 mm. long, and 8.4 mm. broad. 

Habitat. Station 3413 (Lat. 2° 34’ N., Long. 92° 6’ W.), 1360 fathoms, 
1 specimen. 

Remarks. It is easily distinguished from all other large species hitherto 
known by the general shape of thorax and abdomen, and the want of processes. 


CYMOTHOIDA. 


As to the limitation and the constituent elements of this family I refer to 
the above named report on the Isopoda of the German Plankton Expedition. 
Of its six sub-families only two, namely, Hgine and Cymothoine, are repre- 
sented in the collection, the first sub-family by six, the second by one species. 
The leading work on these two sub-families is Shiédte and Meinert: Symbole 
ad Monogr. Cymothoarum, Crust. Isopod. Familia (Naturh. Tidsskr., 3 R., 
Bd. XII.-XIV., 1879-84), and further remarks on the structure of the mouth 
and the classification are found in my above named work, Cirolanide, etc. 

Tn the large genus ‘ga Leach, not rarely several species are closely allied 
to one another, and three of the four species established here differ only in small 
features from species living in the most northern part of the Atlantic (in Nor- . 
way, Greenland, etc.). In the following, some characters derived from the 
structure of the thoracic legs, and partly overlooked by earlier authors, will be 
used; besides, the shape of the posterior angles of the thoracic ‘‘ epimera,” of 


102 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


the sixth abdominal segment, and especially of the uropods, furnish us with 
more distinguishing marks than are generally recognized, but as most of these 
details are more easily apprehended from figures, I will direct the attention of 
future students to these facts, believing that proportionally rather large and 
very accurate drawings of the parts mentioned will be extremely useful. 

In specimens of gine taken on fishes, the ventral side of the thorax is 
often, nay almost generally, vaulted, and sometimes very considerably so, 
owing to the fact that the alimentary canal is greatly distended by blood 
sucked from the host ; another result of this swollen condition is that the seg- 
ments of the thorax very often become drawn out from each other. In speci- 
mens taken on the bottom of the sea by trawl or dredge, the ventral side is not 
vaulted, and the thoracic segments are not drawn out, it follows that such 
specimens are comparatively shorter in proportion to their breadth than most 
of the specimens taken on fishes, and therefore present a somewhat different 
aspect. No specimen of the ZEgine in this collection has the ventral side 
vaulted, and all seem to be taken on the bottom. 

Schiddte and Meinert divide the species of the genus Afiga into two groups. 
The first of them is thus diagnosed: “ Scapi antennarum infra plani vel con- 
cavi, invicem accommodati. Lamina frontalis plana vel concava,” and to this 
group the two first described species, A. maxima, 0. sp-, and AJ. acuminata, 
n. sp., must be referred. To the other group the two authors ascribe the fol- 
lowing characters: ‘‘Scapi antennarum teretiusculi vel compressi, invicem 
liberi. Lamina frontalis convexa vel compresse elevata,” and to this belong 
the two other species, 4. plebeva, n. sp-, and J. longicornis, 0. Sp. 


8. Aga maxima, 2. sp. 
Plate Il. Fig. 2-2¢. 


Only one specimen, a female without marsupium. 

Head. The frontal margin rather concave on each side ; the median elon- 
gation acute, reaching to about the middle of the interior margin of the first joint 
of the antennule. The frontal plate “lamina frontalis” (on the ventral side of 
the head), about as long as broad, seen as much as possible from the side con- 
siderably convex, and seen from infront with a low and rather broad sublateral 
carina, and somewhat excavated in the middle. The eyes ovate, the shortest 
distance between them only a little less than the basal joints of both antennule 
together. 

Antennule. Reaching very little beyond the end of the peduncle of the 
antenne, and a little beyond the anterior angle of the first thoracic segment. 
The peduncle very little longer than the flagellum ; its basal joint as long as 
broad, with the upper side flatly convex, and the antero-interior angle rectan- 
gular. The flagellum 17-jointed. 

Antenne. Each antenna, when bent backward, nearly attains the posterior 
margin of the second thoracic segment. The proportion between the peduncle 
and the flagellum is about that of 3 to 5; the flagellum 23-jointed. 


HANSEN: THE ISOPODA. 103 


Thorax (Fig. 2a). The posterior angle of the first segment rectangular, 
scarcely produced. For practical reasons, the “epimera” of the six following 
segments, though in reality constituting the first joint of the legs, are here 
treated as belonging to the thorax ; the epimera of the second thoracic segment 
with the posterior free angle nearly rectangular, those of the third segment 
somewhat obtuse-angular. The epimera of the four posterior segments poste- 
riorly considerably produced ; those of the fourth and fifth segments poste- 
riorly obliquely rounded ; the last two pairs with the triangular apex a little 
rounded. 

Thoracic Legs. All clumsy. In the three anterior pairs the fourth joint (the 
epimeron considered as the first joint) is shorter than the third, considerably 
incrassated, in the first pair with only one spine, in the second with six or seven 
(Fig. 2b), in the third with nine short spines at the interior margin ; the fifth 
joint only in the third pair with a spine at the antero-interior angle ; the sixth 
joint short, without keel on the inner side; the claw (consisting of the seventh 
joint fused with the real claw) short and robust. The four posterior pairs (Fig. 
2c) with numerous, comparatively short spines. 

Abdomen. The first segment partly free, a little broader than the fourth. 
The sixth segment about 1} times broader than long; the dorsal surface feebly 
convex, very slightly keeled in the median line, and between this keel and the 
base of the uropod is seen a large, but shallow depression; as the posterior 
apex unfortunately is broken off, nothing can be said about its shape, but most 
likely it was acute, and the posterior margin probably with about five spines 
on each side. 

Uropods. They reach a little beyond the end of the abdomen ; both rami are 
proportionally narrow, of the same breadth and the same length, the inner ra- 
mus therefore posteriorly surpassing the outer one. The inner ramus more than 
three times longer than broad; the interior margin from a point a little behind 
the apex of the very long and narrow process from the peduncle turning ob- 
liquely outward, thus forming a posterior margin, with five or six small spines ; 
the exterior margin somewhat convex, but at a short distance from the rounded 
tip of the branch it changes its direction, bending somewhat outward, thus 
forming a low incision. The outer ramus with the tip rounded; the distal part 
of both margins faintly serrated with a smaller number of spines. 

Color. The whole dorsal surface yellowish white, the eyes grayish. 

Size. The single specimen measures 55 mm. in length and 26 mm. in 
breadth. 

Habitat. Station 3362 (Lat. 5° 56’ N., Long. 85° 10/30” W.), 1175 fathoms, 
1 specimen, 

Remarks. The species is closely allied to 4. psora (L.), but is easily distin- 
guished by its enormous size, and the following characters : a different shape of 
the frontal plate; the eyes smaller and more distant from each other; the 
dorsal surface of the last abdominal segment slightly convex, with two large 
depressions. 


104 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


4. Aiga acuminata, n. sp. 
Plate Il. Fig. 3-3. 


Only one specimen, a female without marsupium. 

Head. The frontal margin scarcely as concave on each side as in 4. maxima, 
the median elongation not reaching the middle of the interior margin of the 
first joint of the antennule. The frontal plate conspicuously broader than 
long, seen from the side shaped as in the preceding species, seen from in front 
somewhat concave with projecting lateral margins. The eyes as in the preced- 
ing species. 

Antennule (Fig. 3). Reaching considerably beyond the peduncle of the 
antenne, to the middle of the first thoracic segment. The peduncle slightly 
shorter than the flagellum; the basal joint, seen from in front, quite as broad 
as long ; the dorsal surface somewhat convex ; the antero-interior angle a little 
produced, acute-angled. The flagellum 18-jointed. 

Antenne. When reflexed, reaching to the posterior margin of the second 
thoracic segment. The relation of the peduncle to the flagellum is about that 
of 2 to 3; the flagellum 19-20-jointed. 

Thorax (Fig. 3a). The posterior margin of the epimera of the second to the 
fifth segment and the corresponding margin of the first segment sinuate, being 
directed a little forward just inside the somewhat produced postero-lateral angle, 
which is scarcely rectangular, but a little acute-angled. The epimera of the 
sixth segment forming a transition between those of the fifth and of the seventh 
segment, the last named pair posteriorly and laterally considerably produced 
and acute. 

Thoracic Legs. They are robust, though scarcely as clumsy as in 44. maxima, 
but very similar in shape and armature. In the three anterior pairs the claw 
is somewhat longer ; the thick fourth joint in the first pair with one spine, in 
the second with five, in the third with six to eight spines. The spines on the 
four posterior pairs scarcely as numerous as in the preceding species, but some- 
what longer. 

Abdomen. The first segment almost totally covered, very conspicuously 
broader than the fourth. The last segment scarcely 14 times broader than long 
(in Fig. 3 6 it seems to be proportionally broader, owing to the circumstance 
that the figure presents the projection of the posterior segments) ; posteriorly 
it is considerably produced, acute, with about three spines on each side of the 
tip ; the dorsal surface is rather convex, median keel and sublateral depressions 
scarcely visible. 

Uropods (Fig. 3b). Much as in 42. maxima, so that only the more essential 
differences will be pointed out. The outer ramus reaching a little beyond the 
inner one; the inner ramus is more deeply incised on the exterior side, and the 
posterior margin is somewhat.longer : thus we obtain a distal part forming an 
obtuse angle with the larger proximal part. 


HANSEN: THE ISOPODA. 105 


Color. The dorsal surface is light yellowish gray with a faint purple tone 
on a part of the three anterior thoracic segments, and the last abdominal seg- 
ment yellowish white; the eyes dark grayish, almost black. 

Size. The single specimen is 31 mm. long, 16.2 mm, broad. 

Habitat. Station 3403 (Lat. 0° 58’ 30” S., Long. 89° 17’ W.), 384 fathoms, 
1 specimen. 

Remarks. The species is very closely allied to &. psora (L.), but is distin- 
guished especially by smaller eyes, longer antennule, and the last abdominal 
segment being posteriorly more produced. From 4. maxima it is distinguished 
especially by longer antennule, and by a different shape of the last abdominal 
segment and of the uropods. 


5. Aéga plebeia, n. sp. 
Plato Il. Fig. 4-4. 


Six specimens, one male and five females, three of the latter with well 
developed marsupium. 

Head. The frontal margin with the sub-median curves rather indistinct; the 
median process extends a little below the inferior edge of the antennule, its 
apex almost or quite reaching the frontal plate. The frontal plate about twice 
as broad as long and strongly compressed, forming a high tranverse keel, which, 
seen from in front, shows the shape of the half of an oval. The eyes (Fig. 4) 
are very large ; the distance between them considerably shorter than the breadth 
of the frontal process. 

Antennule. Much longer than the peduncle of the antenne (Fig. 4), and 
bent backwards, reaching almost to or even beyond the posterior angle of the 
first thoracic segment. The peduncle is somewhat shorter than the flagellum, 
and almost attains the distal end of the penultimate joint of the peduncle of 
the antenne; the first joint is about as long as broad, with the antero-interior 
angle broadly rounded; the third joint of the peduncle as long as, or a little 
longer than, the two proximal joints together. The flagellum with twenty-one 
to twenty-three joints. 

Antenne. They reach a little beyond the posterior margin of the second, or 
almost to the posterior margin of the third thoracic segment. The flagellum 
14 or 13 times longer than the peduncle, with seventeen or eighteen joints. 

Thorax (Fig. 4a). The postero-lateral angle of the first segment rectangular 
or a little acute, that of the epimera of the second and generally of the third 
segment conspicuously produced and acute; the angle of the fifth and sixth 
epimera almost or quite rectangular. The epimera of the seventh segment 
somewhat produced and acute. 

Thoracic Legs. The three anterior pairs slender and rather long: the fourth 
joint not incrassated, with concave interior margin (Fig. 45), and with a couple 
of small spines at the distal inner angle; the sixth joint rather long, with a 
strong spine on the interior margin near the end; the claw very long, and 


106 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


longer than the sixth joint. The four posterior pairs are slender; the fourth 
joint elongate, and considerably longer than the fifth. 

Abdomen. The first segment partly covered, very conspicuously broader 
than the fourth. The last segment (Fig. 4 d) 1j times broader than long; the 
tip acute, but scarcely produced ; the posterior margin with six to eight small 
serratures, with scarcely visible spines on each side of the apex; the dorsal sur- 
face slightly convex, the faint median keel and the sub-lateral impressions 
almost as in 42. maxima (see supra). 

Uropods (Fig. 4d). They reach somewhat beyond the apex of the abdomen, 
the outer ramus almost or quite attaining the end of the inner one. The inner 
ramus relatively broad, scarcely half as broad as long, of a somewhat triangular 
shape ; the posterior margin considerably shorter than the antero-interior one, 
with seven or eight rather fine serratures; the exterior margin with a break at 
some distance from the acute tip, and two or three serratures between the tip 
and the break, the rest of the margin almost straight and smooth. The outer 
ramus is conspicuously narrower than the inner, yet rather broad, the apex 
acute, not produced. 

Color. The dorsal surface yellowish white, the eyes gray, somewhat 
blackish. 

Size. The largest specimen, a female with marsupium, is 37 mm. long and 
17 mm. broad; the smallest female with marsupium is but 22 mm. long ; 
the single male is 23.4 mm. long and 10.5 mm. broad. 

Habitat. Station 3363 (Lat. 5° 43’ N., Long. 85° 50’ W.), 978 fathoms, 
4 specimens ; Station 3371 (Lat. 5° 26 20” N., Long. 86° 55’ W.), 770 fathoms, 
1 specimen; Station 3402 (Lat. 0° 57’ 30” S., Long. 89° 3’ 30” W.), 421 
fathoms, 1 specimen. 

Remarks. The species is closely allied to AZ. ventrosa M. Sars, but in the 
last named species the frontal plate is lower and of another shape, the eyes are 
more narrow, not occupying so much of the dorsal surface of the head, the 
epimera of the sixth, and especially those of the seventh segment are consider- 
ably more produced, and the outer ramus of the uropods is somewhat broader. 


6. A®ga longicornis, 0. sp. 


Plate Il. Fig. 5-56; Plate II. Fig. 1-la. 


Only one specimen, a female without marsupium. 

Head. The frontal margin with the sub-median curves rather faint; the 
median process as in the preceding species. The frontal plate forms a very 
high transverse keel, which, when the head is seen from in front, protrudes 
strongly beyond the basal parts of the antennule and the antenne, and has a 
straight inferior margin and rounded lateral angles. The eyes (Fig. 5) com- 
paratively narrow, the shortest distance between them a little shorter than 
the basal joint of both antennule and the breadth of the frontal process 
together. 


HANSEN: THE ISOPODA. 107 


Antennule. They reach considerably beyond the peduncle of the antenne, 
and almost to the postero-lateral angle of the first thoracic segment. The pe- 
duncle a little shorter than the flagellum; its basal joint about as long as broad, 
with the antero-interior angle broadly rounded, the third joint scarcely shorter 
than the two proximal joints together. The flagellum with about fifteen 
joints. 

Antenne. They are unusually long, reaching to the middle of the fifth 
thoracic segment. The flagellum more than twice as long as the peduncle, with 
twenty-two joints. 

Thorax. The postero-lateral angle of the first segment and of the epimera of 
all the other segments acute and more or less acute-angled (Fig. 5 a). 

Thoracic Legs. The three anterior pairs are slender and rather long (Plate 
III. Fig. 1) ; their fourth joint scarcely incrassated, with concave inner mar- 
gin, and in the second and third pairs with a couple of spines at the distal 
inner angle; the sixth joint rather long and without spines ; the claw rather 
long, but scarcely longer than the sixth joint. The four posterior pairs rather 
long and slender (Plate III. Fig. 1a); the fourth joint a little shorter, or at 
all events not longer, than the fifth. 

Abdomen. ‘The first segment almost covered, and very conspicuously broader 
than the fourth. The last segment (Fig. 5) about 14 times broader than 
long ; the apex acute but very little produced ; the posterior margin on each 
side of the apex with four or five comparatively coarse serratures, and a con- 
spicuous spine in each incision ; the dorsal surface very flatly convex, with a 
transverse depression near the base and the median keel not discernible. 

Uropods (Fig. 56). They reach far beyond the apex of the abdomen, the 
inner ramus scarcely attaining the end of the outer one. The inner ramus is 
relatively broad, but yet more than twice as long as broad, triangular, the 
triangle being almost isosceles, with rounded vertex, as the posterior margin 
is almost as longas the antero-interior one ; the exterior margin almost straight, 
with about five coarse serratures in the distal half, and the posterior margin is 
also serrated ; the apex is acute. The outer ramus much narrower than the 
inner one, about four times longer than broad; the apex much produced, 
acute. 

Color. The dorsal surface is yellowish white, the eyes black. 

Size. The single specimen is 14.5 mm. long and 7.5 mm. broad. 

Habitat. Station 3402 (Lat. 0° 57’ 30” S., Long. 89° 3’ 30” W.), 421 
fathoms, 1 specimen. 

Remarks. The species is easily distinguished by the following characters 
together: the long distance between the eyes, the long antenne, and the 
relative length of the rami of the uropods. 


108 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


7. Rocinela laticauda, n. sp. 


Plate III. Fig. 2-2e. 


Three specimens of very different size, one a large and in all probability adult 
male ; no female with marsupium. 

Head. The eyes of medium size, the shortest distance between them about 
as long as the last joint of the peduncle of the antenne ; the distance in the 
smallest specimen is comparatively a little shorter than in the largest one. 

Antennule, They surpass a little the middle of the last joint of the peduncle 
of the antenne (Fig. 2a); the peduncle reaching a little beyond the extero- 
anterior angle of the third joint of the peduncle of the antenne ; the flagellum 
in the small specimen with five, in the large specimens with six joints. 

Antenne. They reach a little beyond the middle of the third thoracic seg- 
ment ; the flagellum in the small specimen with fifteen, in the two other 
specimens with sixteen joints. 

Thorax. The epimera (Fig. 26) of second and third thoracic segments 
posteriorly rounded and not produced, those of the fourth segment somewhat 
produced with rounded apex, those of the three posterior segments considerably 
produced and almost acute. 

Thoracic Legs. The three anterior pairs (Fig. 2c) tolerably stout : the fourth 
joint with about four acute spines, some of them rather long ; the sixth joint 
quite as broad as the fourth, its large and broad expansion on the inner side 
with six spines. The four posterior pairs (Fig. 2d) with numerous slender 
spines. 

Abdomen (Fig. 2e). The first segment is entirely concealed under the last 
thoracic one. The abdomen increases very conspicuously in breadth from the 
second to the fourth segment. The last segment is large and broad, posteriorly 
very broadly rounded ; the dorsal surface is keeled anteriorly in the middle, 
and from the keel towards the lateral margin it is rather deeply, or, in the 
two smaller specimens, deeply and broadly depressed, the depression not 
reaching the lateral margin; the posterior margin with a number of very 
small spines. 

Uropods (Fig. 2¢). They surpass a little the last abdominal segment. The 
outer ramus reaches very little beyond the inner one, is considerably, but not 
14 times, broader than this, and is furnished with a number of spines on a 
larger part of its exterior margin. The inner ramus with spines on the terminal 
margin, and on the larger part of the outer margin. 


1 Schiddte and Meinert write (Nat. Tidsskr., 3 R., Bd. XII. p. 383) on the species 
of the genus Rocinela: “ Bene recordari debet, discrimen, quod individua speciei 
unius ejusdemque quoad figuram frontis atque sculpturam partis prioris trunci 
prebent, non sexum, sed etatem diversam notare.” This observation is a very 
valuable one, as the differences in the front sometimes lead to great confusion. 
The frontal plate seems to be very small in all species; the thoracic epimera show 
much smaller differences in the various species than in the species of da. 


HANSEN: THE ISOPODA. 109 


Color. The two smaller specimens yellowish white, with a reddish tone on 
a part of the three anterior segments, and the eyes blackish ; the large speci- 
men is more grayish, posteriorly on the dorsal surface of the last abdominal 
segment and on a part of the uropods reddish brown, the eyes black. 

Size. The largest specimen, a male, is 40.5 mm. long, and 16 mm. broad; 
the two other specimens are immature females, the smallest of them 21 mm. long. 

Habitat. Station 3418 (Lat. 16° 33’ N., Long. 99° 52’ 30” W.), 660 fathoms, 
1 specimen; Station 3425 (Lat. 21° 19’ N., Long. 106° 24’ W.), 680 fathoms, 
1 specimen; Station 3430 (Lat. 23° 16’ N., Long. 107° 31’ W.), 852 fathoms, 
1 specimen. 

Remarks. The species is closely allied to R. australis Sch. & Mein., but 
in this last species the eyes are very conspicuously larger and the distance 
between them considerably shorter than the last joint of the peduncle of the 
antenne, the abdomen does not increase in breadth from the base to the fourth 
segment, the last abdominal segment is somewhat smaller and the outer ramus 
of the uropods much broader, about 12 times broader than the inner ramus. 


8. Rocinela modesta, n. sp. 
Plate IT. Fig. 3-3c. 


Only one somewhat mutilated specimen, a female with marsupium. 

Head. The eyes are rather small, occupying only about half of the lateral 
margin of the head, and the distance between them considerably longer than 
the last joint of the peduncle of the antenne. 

Antennule (Fig. 3). Comparatively long, reaching very little beyond the 
peduncle of the antenne. The peduncle surpasses the middle of the penulti- 
mate joint of the peduncle of the antenne. The flagellum with six joints. 

Antenne. In my single specimen only the peduncles are present. 

Thorax. ‘The epimera essentially as in the preceding species, yet posteriorly 
a little more produced. 

Thoracic Legs. The first three pairs (Fig. 3 a) of medium size, rather slender: 
the fourth joint with from three to four blunt spines ; the sixth joint not as 
broad as the fourth, the expansion on the inner side rather low and short, with 
four feeble spines. The four posterior pairs (Fig. 36) nearly as in the pre- 
ceding species, but the spines are less numerous. 

Abdomen (Fig. 3c). The first segment is completely covered; the second 
quite as broad as the fourth. The last segment is smaller than in the pre- 
ceding species, decreasing considerably in breadth from before the middle 
backward ; posteriorly it is comparatively rather narrow and rounded, with 
some few fine spines on each side of the median line ; the dorsal surface is 
somewhat convex, keeled anteriorly in the median line and with a rather 
deep but not broad depression from that keel outwards almost to the lateral 
_ Imargin. 

Uropods (Fig. 3c). The inner ramus surpasses a little the abdomen and 
very little the outer ramus, which is somewhat broader than the other ; both 

VOL. XxXXI. — No. 5. 2 


110 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


rami with rather feeble spines on the major part of the exterior margin ; the 
inner ramus with some spines on the obliquely rounded terminal margin. 

Color. The dorsal surface is whitish, the eyes dark. 

Size. The single specimen, a female with marsupium, is 23.5 mm. long, and 
10.7 mm. broad. 

Habitat. Station 3384 (Lat. 7° 31’ 30” N., Long. 79° 14’ W.), 458 fathoms, 
1 specimen. 

Remarks. This species is closely allied to R. maculata Sch. & Mein., but 
it totally lacks the four large black spots ; furthermore, in the last named 
species the uropods are a little shorter and broader, and the two rami of equal 
breadth, while the outer ramus is conspicuously shorter than the inner one ; 
the three anterior pairs of legs are relatively shorter and more clumsy, ete. 


9. Irona foveolata, n. sp. 
Plate III. Fig. 4-45. 


Seven specimens, all females with marsupium, were secured. The species 
certainly must be referred to the genus Jrona Sch. & Mein., but as in my 
opinion it would be of little value to work out a long and very detailed account, 
I prefer to give a shorter description, especially pointing out the features by 
which it is distinguished from the four species described by Schiodte and Mein- 
ert in their monograph (Nat. Tidsskr., 3 R., Bd. XIV. pp. 383-395), and more 
particularly from Jrona melanosticta Sch. & Mein., to which it is rather closely 
allied. As in adult females of other species belonging to Jrona, Lironeca, etc., 
the body is unsymmetrical and somewhat variable in shape, in Some specimens 
being contorted to the right, in others to the left side ; furthermore, the last 
abdominal segment is sometimes as large as shown in the drawing (Fig. 4), 
sometimes a little larger or smaller, in the smallest specimen even conspicu- 
ously smaller. 

The body is about twice as long as broad, in the smallest specimen a little 
shorter and broader, much depressed, but the dorsal surface of the thorax and 
the median part of the five anterior abdominal segments yet more or less 
but never strongly vaulted, while the lateral part of the abdominal segments 
mentioned and the whole sixth abdominal segment are nearly or quite flat, but 
sometimes obviously contorted. 

Thorax. The epimera of the second, third, and fourth thoracic segments are 
very narrow, seen from above ; those of the fifth segment a little broader and 
posteriorly more produced. The epimera of the sixth and especially of the 
seventh segment are much broader and posteriorly much more produced than 
the others, besides on each side rising considerably above the more lateral part 
of the dorsal surface of the thorax, which is brought about by the curious fact 
that these epimera are turned outwards and somewhat upwards. 


Abdomen. All the segments are very broad. The last segment either rather - 


thin and tolerably large, or mostly, as in Figure 4, thin and very large, and in 
this instance almost membranous, so that the marginal part easily becomes 


HANSEN: THE ISOPODA. ttt 


*. folded. The dorsal surface of this last segment sometimes with tolerably dis- 
' tinct, sometimes with very faint median keel, and else almost all over finely 
and densely pock-marked by exceedingly numerous and very small depressions. 
(This structure is not clearly defined on the copperplate, as the depressions are 
far more numerous than in the figure, and the intervals form a kind of irregular 
reticular work.) 

Uropods. They have avery depressed peduncle and thin rami; the outer 
ramus is oblong-ovate, distally rounded ; the inner ramus is considerably 
longer than the outer, with sub-acute end. 

Color. In the six larger specimens the head, the thorax, the five short 
abdominal segments, and the basal part of the sixth segment, are yellowish 
with innumerable dark dots ; the epimera of the three, and especially of the 
two, posterior thoracic segments, and the lateral angles of the five anterior 
abdominal segments are white ; almost the whole last abdominal segment is 
grayish. In the smallest specimen the dorsal surface is darker, more grayish, 
with exception of the two last pairs of thoracic epimera and the angles of the 
five anterior abdominal segments, which are white. 

Size. The largest specimen is 20.5 mm. long, and 10.5 mm. broad ; the 
smallest is 14.56 mm. long, and 8.2 mm. broad. 

Halntat. Station 3355 (Lat. 7°12’ 20” N., Long. 80° 55’ W.), 182 fathoms, 
2 specimens ; Station 3389 (Lat. 7° 16° 45” N., Long. 79° 56’ 30” W.), 210 
fathoms, 4 specimens ; Station 3391 (Lat. 7° 33’ 40” N., Long. 79° 43’ 20” W.), 
153 fathoms, 1 specimen. On the labels Ido not find any mention of the 
name or names of the fishes on which the parasites must have been found. 

Remarks. The species seems to be well distinguished, especially by the 
pock-marked surface of the last abdominal segment. No males were found. 
In the marsupium of one female I found “pullus stadii primi” of Schiddte 
and Meinert; in Figure 4a a leg of the second pair, and in Figure 4 the pos- 
terior abdominal segments and the uropods of one of the specimens are shown. 
This may be sufficient, as the young one in this stage is very similar to those 
of the genus Lironeca drawn by Schiddte. Unfortunately, the “ pullus stadii 
secundi,’ always much more interesting, was not found. 


EPICARIDEA. 


As to the division of this very rich and highly interesting family into sub- 
families the reader is referred to my above mentioned treatise on the Isopoda 
of the Plankton Expedition. Of the four sub-families admitted (the very 
doubtful Microniscinz not included) only one, viz. the Bopyrinz, is represented 
in the collection. Of the five species secured both female and male — but no 
young ones —are present of the four species, while the fifth species is repre- 
sented only by a male and a small portion of a female. 

It is a rather unpleasant task to describe a few new forms of the Bopyrine. 
Most of the authors who have contributed to the knowledge of the group 


a BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


possessed very few species, and often even very few specimens, and the ani- 
mals being not very easy to examine, and still less to describe and draw, the 
result is that most of the species are imperfectly represented, and many of the 
genera badly or not at all limited. Giard and Bonnier have given full descrip- 
tions of a few species only, as their principal work on this sub-family has not 
yet been published. They have made an attempt to divide the Bopyrine into 
three groups, Phyxiens, Bopyriens, and Ioniens ; but I am unable to perceive 
the limits between the two first named groups, and even the group Ioniens is 
not very sharply defined. We must wait until a number of still unknown forms 
have been thoroughly studied and many of the already established species re- 
examined before it will be possible to divide the sub-family into natural 
groups. I must add, however, that the few descriptions just mentioned of 
the two authors have been very useful to me. In 1893, T. R. R. Stebbing, 
in his well known work, “A History of Crustacea — Recent Malacostraca,” 
gave a very good catalogue of all the twenty-one genera and almost all the 
species hitherto established. 

I must confess that I have been unable to refer more than one of my five new 
species to any of the genera hitherto established, and as they are very different 
from one another it is necessary to institute four new genera, —a result with 
which I am rather dissatisfied, not being sure that they all will prove to be 
valid. On account of the present state of things, I do not venture to lay down 
diagnoses of the new genera ; but I hope that by means of my rather numerous 
figures and tolerably full descriptions it will be easy not only to recognize my 
species, but also to place the genera properly and work out the diagnoses, when 
in the future we get a real systematic arrangement. 


10. Cryptione elongata, n. gen., n. sp. 
Plate Ill. Fig. 5-5 a; Plate IV. Fig. 1-lg. 


A fine female with its male (Fig. 1 a, m) was discovered. 


a. Female. 


The body is elongate (Fig. 1) and (the uropods not included) about twice as 
long as broad; the greatest breadth at about the middle. 

Head. It forms, when seen from above (Fig. 1), almost a regular transverse 
oval, with the anterior half projecting in advance of the antero-lateral part of 
the thorax and the frontal margin considerably and evenly curved ; the dorsal 
surface somewhat convex, with a depression a little inside of the anterior mar- 
gin. The antennule (Fig. 1, a) rather distant from each other, of medium size, 
3-jointed ; the basal joint is considerably enlarged, the terminal joint minute. 
The antenne (b) rather long, 3-jointed ; the basal joint very large, ovate, with 
the second joint proceeding from the extero-anterior part ; the second joint rela- 
tively rather long and robust (compare the following forms), the third some- 
what shorter and considerably more slender. A frontal plate is absent, and 
between the antennule, the antennz, and the labrum is found a rather large 


HANSEN: THE ISOPODA. 1s 


free space. The labrum (c) is tolerably small, a little broader than the hypo- 
pharynx, the posterior margin emarginate. The hypopharynx (A) with the 
lateral margins sub-parallel. Mandibles (d), maxillul (e), and maxille (/) 
scarcely need special mention, their general shape and position being easily seen 
in Fig. 1. In this figure the place of attachment of the maxillipeds is lettered 
with g. The left maxilliped, seen from below, is shown in Figure 1 c; the first 
joint (1) with its usual free posterior dilatation, the second joint (2) with the 
exterior dilatation (d?) which is of secondary origin — as in the females of the 
family Cymothoide —and cannot be considered as an exopod ; the palp (p) is 
longer than in the following forms, but not distinctly jointed. The peculiar 
border behind the attachment of the maxillipeds is well developed, having on 
each side two oblique, good-sized rather broad, but not long, somewhat fleshy, 
lamellar processes (Fig. 18, /). 

Thorax. On the four anterior segments the ovarian bosses are well developed, 
occupying but a little more than half the length of the sub-marginal part of 
each segment; in the other segments the bosses are wanting. The pleural 
plates (“lames pleurales” of Giard and Bonnier) of the four anterior segments 
are interesting : each of them is divided by a deep incision into two portions, 
the anterior of which is oblong, set off by a furrow and especially on the right 
side of the animal incised or emarginate exteriorly, while the posterior forms 
a shorter, rounded, not defined lobe. In the three posterior segments the pleural 
plates are larger and laterally more prominent, but neither divided nor set off. 
The ventral side of the two posterior segments is elevated and divided by nu- 
merous longitudinal ridges into low fleshy lamella; the other segments possess 
a similar, but more narrow, transverse keel divided into small protuberances. 
The legs are normal, each sitting on an eminence which often is rather promi- 
nent (its appearance on the left side of the figure conveys the impression that 
the leg has a short basal joint, which of course is not the case); the second 
joint (basipodite, Giard and Bonnier, the basal joint being fused with the 
segment) not expanded ; the claw is present, and none of the joints with keels 
or rugosities. The first left leg with its marsupial plate is exhibited in Figure 
1d, which, better than a long description, will show the differences between the 
plate in this and in the following forms; the transverse furrow on its exterior 
side is plainly seen, and on the inner side is found a transverse keel, the proxi- 
mal part of which is divided into a fewlamelle. The margins of the marsupial 
plates are more or less hairy; on the anterior margins of all plates the hairs are 
fewer and rather rudimentary, while especially the inner and posterior margin 
of the two posterior pairs of plates is densely set with rather short hairs (omitted 
in Fig. 1a). (The marsupium was empty.) 

Abdomen. The segments are distinctly separated on the dorsal side. The 
five anterior segments, all comparatively broad, on each side produced as good- 
sized free plates, which mostly are cut off in a more or less oblique direction ; 
on the left side all these pleural plates are bent obliquely upwards. The ventral 
side of these segments shows a similar but less regular division into low lamelle 
as that of the posterior thoracic segments. The pleopods of medium size, each 


114 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


with two subequal rami, the basal part of which is thicker, somewhat fleshy, 
the distal part more lamellar; some of the rami are oblong-triangular and dis- 
tally almost produced, others are distally broader and rounded ; almost the 
whole, or at least the major basal part of the ventral surface of all rami is fur- 
nished with conspicuous rounded knots, some of which plainly show that this 
structure is a rudimentary ramification ; the pleopods decrease somewhat in 
size from before backward. Each uropod (Fig. 1) is an oblong, glabrous 
lamella, which is as large as, or a little larger than, a ramus of the first 
pleopod. 

Size. From the front to the apex of the longest uropod the specimen is 
13 mm., and to the end of the last abdominal segment 11.2 mm. ; it is 6.6 mm. 


broad. 
b. Male. 


The body is very elongate, about 3} times longer than broad (Fig. le 
and 1/). 

Head. It is completely fused with the first thoracic segment. The eyes are 
very small, light grayish, and scarcely visible when the animal is seen from 
above. The frontal part bends much downward and forms a high border, which 
covers the basal part of the antennule and the antenne (Fig. 1g); the margin 
is rather slightly curved. The antennulz tolerably short, 3-jointed; the basal 
joint longer and very much thicker than the second ; the third joint very 
slender and rather short. The antenne rather long, 8-jointed; the first joint 
a little longer and about twice as broad as the second, which is about as long 
as, and much thicker than, the third and especially the fourth; the four distal 
joints exceedingly small. The mouth conical and protruding, but it was 
utterly impossible to study its elements with any certainty without a 
dissection. 

Thorax. The segments, when seen from above, with their lateral outline 
feebly rounded and the incisions between them short. Each segment with a 
median, rather high, basally very broad and distally rounded cone on the ventral 
side (Fig. 1f) ; this cone is smaller on the two first segments than on the others. 
A leg of the first pair is shown in Plate HI. Figure 5, and the corresponding 
leg of the fifth pair in Figure 5a; the general shape and the armature of the 
fifth and the sixth joint — the first joint as usual fused with the thorax and 
consequently not drawn — are easily seen. 

Abdomen. It occupies one third of the total length, and decreases posteriorly 
very little in breadth. The six segments are all well separated from each 
other. The five anterior segments with the lateral part almost triangular, 
when seen from above; each with a ventral cone as those in the thoracic seg- 
ments, and, besides, each pleopod is developed as a protuberance of considerable 
size and directed obliquely inward and a little backward. The sixth segment 
relatively broad, on each side with a large, narrow conical, obtuse process, 
probably the uropod, originating from the side and directed somewhat out- 
ward and much backward ; the posterior margin of the segment is angular. 

Size. It is 4.1 mm. long to the apex of the uropods. 


Ts 


HANSEN: THE ISOPODA. 115 


Habitat. The described pair were found in the branchial cavity of a speci- 
men of Nematocarcinus agassizii Fax., from Station 3407 (Lat. 0° 4’ S., Long. 
90° 24’ 30” W.), 885 fathoms. The swelling of the carapace is oblong, and 
not very high. 


11. Munidion princeps, n. gen., n. sp. 
Plate IV. Fig. 2-2¢; Plate V. Fig. 1-1d. 


Two females with their males were secured. 


a. Female. 


The body, when seen from above (Fig. 1), of an almost pyriform outline, 
and not quite 14 times longer than broad (the uropods not included). One 
specimen has the right margin convex, —a “right”? specimen; the other is 
a ‘‘left”’ specimen. 

Head. It is much broader than long and encircled posteriorly and on the 
major part of its sides of the first thoracic segment ; the dorsal surface is some- 
' what convex, and the frontal border tolerably broad and bent conspicuously 
upward ; the anterior margin is slightly convex. The antennule (Plate IV. 
Fig. 2) separated by a frontal plate ; they are of medium size, 13-jointed ; the 
basal joint is comparatively large, the third extremely small. The antenne 
are rather short, 3-jointed ; the basal joint is long and exceedingly broad, 
almost triangular, with the expanded inner border overlapping the outer part 
of the mandibles and the lateral angle of the labrum, the produced anterior 
angle extends to the frontal plate and the second joint is inserted on the vertex 
of the triangle ; the last named joint is short and slender, the third joint ex- 
ceedingly small. The frontal plate is broadly triangular with obtuse vertex, 
completely occupying the small space between the foot of the antennule, the 
anterior angle of the antenne, and the labrum. The labrum (c) scarcely of 
medium size, somewhat broader than the hemi-cylindrical hypopharynx. 
Hypopharynx, mandibles (d), maxillule, and maxille need no special men- 
tion. The left maxilliped is shown in Figure 2a; the most interesting 
character is that the palp has almost disappeared, as we see but a some- 
what produced angle. The border behind the maxillipeds is very well 
developed, with a number of small protuberances, and having on each side 
two oblique slender processes, of which the inner is long, the outer very 
long. 

Thorax. Ovarian bosses are developed on all segments ; they are very prom- 
inent, most of them almost semi-globular (on the drawn specimens they are acci- 
dentally — caused by pressure — more or less depressed on the right side of the 
second to the fourth segment); in the three anterior segments they are large and 
gradually decrease in size backward, the two posterior pairs almost petiolated, 
the seventh pair small (in the small specimen the two posterior pairs are even 
reduced to prominent, distally not swelled processes). The bosses do not oc- 
cupy the sub-lateral part of the segments to its whole length, only the larger 


116 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


posterior portion, yet not extending to the posterior margin. The pleural plates 
- are comparatively large, oblong, rounded, thus occupying the whole or at least 
most of the lateral margin; in the posterior segments they are broader and 
’ overlap each other considerably ; their convex ventral side with numerous 
small tubercles and oblong knots. The three posterior segments on the ventral 
side with an interrupted row of very short fleshy keels ; the other segments 
are not examined. The legs are robust; the second jum (Fig. 16 and 1d) 
on the outer side with a very high expansion, shaped as an oblique plate, which 
is about as high as long and somewhat shorter than the length of the joint, on 
both sides with irregular small protuberances ; the other joints normal. In 
Figure 1 } is shown the first left leg with the marsupial plate ; this plate shows 
on the under side a deep transverse furrow and more forward a group of low 
knots, on the upper side (Fig. 1 c) a kind of transverse keel, the marginal por- 
tion of which is divided into numerous irregular, small, thin-skinned processes. 
The basal part of the other four pairs of plates with numerous knots (Fig. 1 a). 
(The marsupium*of both specimens with eggs. 

Abdomen. The five anterior segments with very large rounded pleural ies 
the anterior of which are somewhat transverse, the posterior oblong; the lamelle 
cover the main part of one another, a large portion of the dorsal surface of the an- 
terior segments, and the whole dorsal surface of the posterior ones. In Figure 1 
the fifth pair of lamellz are lettered a’. The second to the fifth segments on 
the ventral side each with a transverse row of short fleshy processes or knots, 
or lamellar keels; the first segment with an interrupted series of low lamelle. 
The sixth segment (posteriorly behind the attachment of the uropods produced 
into an oblong, distally rounded and swelled, almost petiolated process. Each 
pair of pleopods consists of two large rami ; in the anterior pairs these are 
shorter and rather broadly triangular, backwards they gradually become elon- 
gate. The uropods biramous, the rami similar to those of the last pleopods ; 
in Figure 1a the visible distal part of the rami on the left side of the figure 
are marked with 6, on the right side with 64. The main part, or almost the 
whole, of both surfaces of the pleural lamelle and of the pleopods and uropods is 
set with very low and irregular minute keels and more rounded protuberances, 
which are most developed on the anterior pleopods. 

Size. The largest specimen — which has been taken as type for all the fig- 
ures — is 14.2 mm. long to the end of the abdomen, 17.3 mm. long to the apex 
of the longest ramus of the uropods, and 10.9 mm. broad. The other specimen 
measures 15.6 mm. to the end of the uropods. 


b. Male. 


One specimen (Plate IV. Fig. 26) is symmetrical and undoubtedly normal ; 
the other specimen (Fig. 2c) is anomalous, not symmetrical, and somewhat 
misshapen, — especially the abdomen is conspicuously different. In the fol- 
lowing the normal specimen is described, with some remarks concerning the 
other. 


HANSEN: THE ISOPODA. EV 


The body is considerably depressed and relatively broad, scarcely 2} times 
longer than broad. 

Head. Its dorsal surface almost flat, with irregular rugosities. The eyes 
are very small and dark. The frontal part bends feebly downwards ; the ante- 
rior margin is considerably curved. The antennule (Plate IV. Fig. 2d) are 
rather long, 3-jointed ; the basal joint thick, the second longer than the first 
and comparatively thick, the third rather short and slender. The antenne 
of medium length, 3-jointed ; the basal joint of medium length and almost 
twice as broad as long, with the outer margin concave ; the second joint is at- 
tached to the anterior half of the outer margin of the first joint; it is stout and 
twice as long as the basal one ; the third joint is rathershort and slender, The 
labrum of medium size, with the anterior margin very convex; its median 
part is covered by the hypopharynx, which extends forward to the middle of 
the basal joint of the antennule. The hypopharynx is long, not broad, and 
tapers somewhat towards the rounded apex. The mandibles and the maxillule 
are easily seen in the figure ; a rounded protuberance behind each maxillula 
most probably represents the maxilla ; maxillipeds I have not been able to 
discover. 

Thorax. The fifth segment is the broadest, and from that the thorax de- 
creases a little in breadth towards both ends. The segments, when seen from 
above, with the lateral outline much rounded, but the incisions between them 
are short. On the ventral side a median, very conspicuous cone on each seg- 
ment. The legs subequal in structure ; all are relatively short and very thick, 
but the fourth and fifth pairs are somewhat larger and still more clumsy than 
the first pair ; Figure 2¢ (on Plate IV.) represents the left leg of the first pair, 
and a description is scarcely needed. 

Abdomen. It does not occupy one third of the length of the body, and an- 
teriorly it is somewhat narrower than the last thoracic segment ; it is triangular 
with rounded angles, a little longer than broad, and the Jateral outline is some- 
what sinuous, which shape indicates the segmentation. Allsegments are com- 
pletely fused ; vestiges of transverse sutures are scarcely discernible on the 
dorsal, but rather distinct on the ventral side. 

Size. The specimen is 3.3 mm. long. 

The misshaped male is exhibited in Figure 2c (on Plate IV.). The outline 
of the thorax is somewhat irregular ; the abdomen is very wry, with all the 
segments well separated on the dorsal surface, and the last segment having 
about the shape of an oblique square. The result of this deformity is, in 
my opinion, very interesting. 

Habitat. The label states that the two females (with their males) were 
found in the branchial cavity of Munida refulgens Fax., from Station 3378 
(Lat. 3° 58’ 20” N., Long. 81° 36’ W.), 112 fathoms. 


118 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


12. Pseudione galacanthe, n. sp. 
Plate V. Fig. 2-2 i. 


Five adult females and the same number of males have been transmitted. 
(Compare ‘* Habitat.”) 
a. Female. 


The body about 14 times longer than broad. 

Head. It is somewhat broader than long, fused with the considerably 
curved first thoracic segment and encircled forward to the antero-lateral angle, 
while its anterior margin is slightly curved ; the frontal border is rather nar- 
row and turned somewhat upwards ; the dorsal surface is slightly convex, The 
antennule (Fig. 2, w) are in contact anteriorly, posteriorly they are separated 
by a small, triangular frontal plate (p); they are of about medium size, 3- 
jointed; the basal joint is rather large, thick, the second shorter and more 
slender, the third very small, terminating in an exceedingly short bristle. The 
antenne (b) are 4-jointed, rather short; the basal joint is very large, forming 
almost an oblique oval, yet the inner margin is almost straight, the outer very 
convex, and the second joint originates from its extero-anterior angle ; the second 
and third joints are short and slender, the fourth very small, terminating in an 
exceedingly short bristle. The frontal plate is already mentioned. The labrum 
is very broad; the hypopharynx is oblong-triangular with rounded vertex. 
Mandibles (d), maxillule (e), and maxille (/) do not present any interesting 
peculiarities. The left maxilliped is shown in Figure 2c; the palp is very 
conspicuous, with some hairs, but not jointed. The border behind the maxilli- 
peds is well developed, with numerous small, irregular protuberances, and only 
one pair of processes which are long and distally narrow. 

Thorax. The four anterior segments with ovarian bosses, which are low, 
and oceupy about two thirds of the lateral margin of each segment ; the pleural 
plates which occupy the remaining one third of the margin, are short or narrow. 
The three posterior segments without bosses, but the pleural plates occupy the 
entire margin and are developed as lamella, increasing gradually in length and 
turning more backward from the fifth to the seventh segment ; besides they are 
longer on the convex than on the other side of the animal. The legs are rather 
stout (Fig. 2d and Fig. 2); the second joint about as broad as long, owing to 
the fact that on the whole outer side it is much expanded, with the outline 
almost semicircular ; the fourth joint with a keel on the inner margin, and two 
short, knot-like keels are found on the same margin of the posterior, but disap- 
pear on the anterior pairs of legs. The first left leg, with its marsupial plate, 
is shown in Fig. 2d ; the plate has on the lower side a broad and high trans- 
verse keel, and on the upper side a structure similar to that in Munidion (see 
above). Only the last segment on the ventral side with numerous small in- 
cisions and between these low fleshy projections; this structure is found both 
at the anterior and the posterior margin of the segment. 


HANSEN: THE ISOPODA. 119 


Abdomen. It occupies less than one third of the length of the animal, and 
the segments are well separated on the dorsal surface. The pleural plates are 
very large and lamellar, partly overlapping one another, in the first segment 
somewhat longer than those of the last thoracic segment, and then gradually 
increasing in length and turning more backward from the first to the fifth seg- 
ment. The ventral side of the five anterior segments about as in the preceding 
species. Each pleopod with two triangular or ovate rami of medium size ; the 
pleopods decrease somewhat in size from before backward, and the outer ramus 
is as a rule a little smaller than the inner one. Each uropod consists of one 
ramus (Fig. 26) which is oblong-ovate and considerably smaller than the 
pleural plates of the fifth abdominal segment. 

Size. The largest specimen is 10.4 mm. long to the apex of the sixth ab- 
dominal segment, 11.8 mm. to the end of the uropods, and 7.4 mm. broad. 
The smallest specimen — with eggs in the marsupium — is only 7 mm. long to 
the end of the abdomen, and 5.8 mm. broad. 


b. Male. 


The body is very elongate (Fig. 2), between 35 and 4 times longer than 
broad. 

Head. The dorsal surface is convex, the antero-lateral margin much curved, 
and the anterior part of the head bends somewhat downward. Eyes could not 
be detected, but we find small frontal impressions, which vary very much in 
different specimens (in one specimen two pairs were found). The antennule 
(Fig. 2 9) of medium length, 3-jointed ; the basal joint thick and almost glob- 
ular, the second shorter and much narrower than the first, the third minute. 
The antennz of medium length, 5-jointed ; the first joint thick and almost glob- 
ular ; the three following joints gradually a little shorter and much narrower; 
the apical joint minute. The labrum extremely broad, crescent-shaped. The 
hypopharynx reaches to the middle of the labrum; it is rather long, of medium 
breadth, tapering somewhat towards the rounded end. Mandibles, maxillula, 
and maxille (/) normal ; the maxilliped (g) has the shape of a rather small 
oblong triangle. 

Thorax. The fifth segment is the broadest, a little broader than the seventh, 
and considerably broader than the first segment. The lateral outline of the 
segments either rounded or (Fig. 2) more straight with rounded angles ; the 
incisions between the segments narrow, triangular, and very deep. The ventral 
surface without conical protuberances. The legs more slender than in the pre- 
ceding form ; from before backward to the fifth pair they increase a little in 
length and their hand in size, and from the fifth to the last pair at least the 
hand decreases somewhat in size. In Figure 2h is shown the left leg of the 
first pair, and in Figure 27 that of the seventh pair. 

Abdomen. It occupies about two fifths of the total length of the animal, 
and decreases in breadth from before backward to the small square sixth seg- 
ment. All segments are very movable; seen from above, their lateral portion 
in the large specimen is triangular with the lateral angles more or less acute, 


120 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


in the small specimens less triangular and rounded laterally. In the anterior 
segments rudiments of pleopods are either scarcely discernible or visible as very 
low and broad rounded eminences. 

Size. The largest specimen (Fig. 2/) is 4.8 mm. long ; a smaller specimen 
‘from which the three analytical figures have been drawn, is 3.5 mm. long ; 
a small specimen is only 2.9 mm. long. 

Habitat. The label indicates that the five adult females (with their males) 
were found in the branchial cavity of Galacantha diomedee var. parvispina 
Fax., from Station 3435 (Lat. 26° 48’ 0” N., Long. 110° 45’ 20” W.), 859 
fathoms. In the Report on the Stalk-eyed Crustacea of the ‘* Albatross ’’ Ex- 
pedition of 1891, W. Faxon writes (p. 81): “Seven specimens (5 males, 2 
females) of var. parvispina house a Bopyrus in the left branchial chamber.” 


13. Parargeia ornata, n. gen., n. sp. 
Plate VI. Fig. 1-1i. 


Only one female and its male are found. 


a. Female. 


The body is much distorted and scarcely 14 times longer than broad. 

Head. It is comparatively very broad, but otherwise of the same shape as 
in Munidion (ante, p: 115). The antennule (Fig. 1b) separated by a frontal 
plate (p), of medium size, 3-jointed; the basal joint comparatively long 
and thick, the second short and narrow, the third exceedingly small. The 
antennz (6) similar in shape to those of Pseudione (see above), but larger 
and 6-jointed ; the basal joint very large, forming about an oblong oval, with 
both margins a little convex ; the second joint is attached at the antero-exterior 
angle of the first, and is tolerably short and slender, yet longer and considerably 
thicker than the third; the three distal joints are exceedingly small. The 
frontal plate (p) rather large, about three times broader than long, anteriorly 
emarginate. The labrum exceedingly large, in the middle very short, but on 
each side forming a large oblique plate (c) which overlaps the distal part of 
the mandible and the maxillula, and the lateral part of the hypopharynx. 
This organ (h) is triangular and broader towards its base than in the preceding 
forms. The mandibles (d) extend in the middle with their acute tip beyond 
the end of the hypopharynx. Maxillule (¢) and maxille (f) need no men- 
tion. The left maxilliped is shown in Figure 1c; the palp consists of a 
prominent basal part and a small terminal joint. The border behind the 
maxillipeds well developed, with two pairs of long, oblique, distally slender 
processes. 

Thorax. Ovarian bosses are found on the four anterior segments ; they are 
oblong, considerably convex, and occupy from less to more than half of the 
sub-lateral portion of each segment. By a conspicuous or even deep furrow 
they are set off from the anterior part of the pleural plates, which lie outside or 
more beneath the bosses, are much arched, and look almost like “epimera ” in 


HANSEN : THE ISOPODA. An | 


Cymothoide. The posterior portion of the lateral part of the segments men- 
tioned is more or less protruding, rounded or angular, and must be considered 
as the posterior division of the pleural plate (compare Cryptione). On the 
three posterior segments the pleural plates are deeply incised, divided into a 
larger, broader, and more produced anterior part, and a much smaller, nar- 
rower, and less produced posterior one, which is more or less obsolete on the 
last segment. At least on the posterior segments the ventral side shows the 
usual low fleshy keels. The legs are slender ; the second joint proximally on 
the outer side with a considerable rounded expansion, which is comparatively 
longer and broader on the anterior (Fig. 1d) than on the posterior (Fig. 1 ¢) 
pairs; the other joints are normal. In Figure 1d is shown the first leg with 
its unusually large marsupial plate; the transverse furrow is not deep; on 
the upper side the keel is tolerably high and much compressed, but without 
marginal processes. The marsupial plates do not quite reach each other at the 
middle ; their natural position was somewhat disturbed in the specimen, and 
therefore it was necessary to make use of construction in Figure la. 

Abdomen. The segments distinctly separated at the middle on the flat dorsal 
surface. No pleural plates. The segments fleshy on the ventral side; only 
the first segment with slight furrows. The pleopods very curious, and rather 
similar to each other; each consists of two rami; the outer ramus is a very 
long, subrectangular or distally rounded, somewhat fleshy lamella, which is 
placed at the margin of the segment ; the inner ramus is, proportionally short, 
more or less ovate, fleshy, originating at some distance from the outer ramus, 
and on the left side of the animal it conveys the impression that the basal half 
is fused with the ventral side of the segment. (I am aware that another 
interpretation of the described facts could be advanced, namely, that the outer 
ramus is a pleural plate set off by a kind of articulation, and that the inner 
ramus in reality represents the entire pleopod, but this opinion I cannot share.) 
Each uropod consists of a single lamella of about the same shape and size as 
the nearest outer ramus of a pleopod. 

Size. The specimen is 8.5 mm. long to the apex of the abdomen, 10.3 mm. 
long to the end of the uropods, and 7.2 mm. broad. 


b. Male. 


The shape of the body is interesting. It increases uniformly but rather 
slightly in breadth from the head to the last thoracic segment, and the abdomen 
is anteriorly somewhat broader than the preceding segment, triangular with ~ 
rounded angles, somewhat broader than long, the anterior margin a little con- 
cave and the lateral margins convex. The body is a little more than 2} times 
longer than the width of the abdomen. 

Head. The dorsal surface is convex, the median part of the anterior outline 
almost straight. A pair of small spots or minute depressions perhaps represent 
the eyes. When the head is seen from below (Fig. 19), it is observed that the 
frontal border arises like a broad and rather high transverse keel above the 
attachment of antennule and antenne. The antennule of medium length, 


122 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


3-jointed ; the basal joint thick, the second shorter and much narrower than the 
first, the third short and very slender. The antennz rather short, 7-jointed ; 
the first joint very thick, the second of about the same length but somewhat 
more slender, the third somewhat shorter and more slender than the second, 
the fourth rather short and very slender; the three distal joints exceedingly 
small. The mouth forms a basally broad, somewhat protruding oblique cone, 
but without a dissection I was not able to recognize several of the parts with 
any certainty ; the figure will show what I believed I saw. 

Thorax. The segments much arched on the dorsal side, the incisions between 
them of medium length, and most of them very narrow; their lateral margin 
is, when seen from the side, much more rounded than if seen from above. No 
ventral cones. The legs increase somewhat in length from before backward, 
but at the same time their hand decreases in size from the first (Fig. 1h) to the 
seventh (Fig. 17) pair, and besides alters conspicuously in shape. 

Abdomen. It occupies somewhat more than one fourth of the length of the 
animal; its outline is described above. All segments are completely fused, so 
that only some transverse, partly very indistinct furrows, but no sutures, are 
found on the dorsal surface. About half way between the median line and the 
lateral margin the dorsal surface presents a broad longitudinal depression, and 
in the median line a little behind the anterior margin a prominent knot. The 
ventral surface does not seem to be quite normal, but the following characters 
certainly are of importance: no rudiments of pleopods are to be discovered, 
but in the median line are found three protuberances: the first small, the 
second rather large, the third shaped as a short transverse keel. 

Size. The specimen is 4.1 mm. long. 

Habitat. In the branchial cavity of Sclerocrangon procax Fax., from Station 
3418 (Lat. 16° 33’ N., Long. 99° 52’ 30” W.), 660 fathoms, 1 female with a 
male. 


14. Bathygyge grandis, n. gen., n. sp. 


Plate VI. Fig. 2-2c. 
Only a male, and the posterior part of a female have been sent to me. 


a. Female. 


The rudiment consists of the posterior part of the thorax, bearing three legs 
* on one and two on the other side, and the abdomen. 

Thorax. The pleural plates are very large oval lamelli, only connected with 
the segment by somewhat less than the posterior half of their interior margin, 
and this result is due to the fact that they anteriorly are very much produced, 
highly overlapping each other, and posteriorly rather shortly produced. The 
legs are tolerably slender; the second joint not expanded ; the fifth joint elon- 
gate, in the last pair as long as the hand. 

Abdomen. It is turned to the left in a startling degree, and is proportionally 
small, — perhaps very small. The dorsal surface is soft-skinned, the segments 


HANSEN: THE ISOPODA. 123 


more or less distinctly separated. Pleural plates not developed. The pleopods 
quite soft, of medium size, decreasing conspicuously in size from before back- 
ward and attached to the lateral margin ; each pleopod consists of a short 
peduncle and two lamellar oblong rami; the outer ramus much larger than 
the inner one. The uropods biramous ; the outer ramus a little smaller than 
the outer of the fifth pleopod, the inner ramus very short, almost rudimentary. 
The pleopods are curled to such a degree that it would have been impossible 
without much construction to draw a sketch of the abdomen. 


b. Male. 


The body is a little more than three times longer than broad, and from the 
fourth thoracic segment it decreases in breadth towards both ends (Fig. 2). 

Head. The dorsal surface rather convex ; the median portion of the anterior 
margin almost straight. Noeyes. The frontal border bent slightly downwards 
(Fig. 2a). The antennule rather short, 3-jointed; the basal joint tolerably 

thick, and partly overlapped by the rostrum; the second joint slender and 
rather short, the third very small. The antenne comparatively long, 7-jointed ; 
the four proximal joints of about the same length, but decreasing much in 
breadth from the rather thick basal joint to the fourth one; the fifth joint is 
short and very slender, the two last joints exceedingly small. The mouth 
forms a rostrum which, when seen from below, is triangular, considerably de- 
pressed and directed forward, reaching almost to the frontal margin of the 
head. The hypopharynx is very large, and just outside it is seen the very 
oblong lateral part of the labrum (d), the median part of which is concealed 
by the hypopharynx; at first I believed that these oblong organs were the 
mandibles, but a closer examination gave the result mentioned, while the man- 
dibles, being needles with brown apex, were discovered within the rostrum. 
Maxillule are not observed; the maxille (f) are small semicircular lobes lying 
considerably behind the posterior edge of the labrum. The maxillipeds (g) 
are short, extremely slender, almost styliform. 

Thorax. The segments are rather convex, the incisions between them com- 
paratively broad and very deep ; the lateral margins are much curved when 
seen from the side. The legs increase considerably in length, and very much 
in thickness, from the first (Fig. 2 5) to the fifth pair (Fig. 2 ¢) which is robust, 
with the hand very large; the two posterior pairs again decrease somewhat in 
size. The terminal margin of the hand is deeply concave, thus differing con- 
siderably from the preceding forms. 

Abdomen. It occupies scarcely one fourth of the length of the animal; it is 
narrower than the last thoracic segment, shortly ovate in outline, without the 
slightest rudiment of segmentation or abdominal feet; both the ventral and 
especially the dorsal surface are very convex. , 

Size. Uncommonly large, being 7 mm. long, and 2.3 mm. broad. 

Habitat. The branchial cavity of Glyphocrangon spinulosa Fax., from Station 
3424 (Lat. 21° 15’ N., Long. 106° 23’ W.), 676 fathoms, 1 female with its 
male. 


124 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


Remarks. The species is established essentially on the very large male, the 
mouth parts of which are very different from those of other forms known to 
me. I hope that it will prove to be rather easy to recognize the form, but I 
hesitated to establish the new genus, the knowledge of the female being very 
incomplete. However, I found it impossible to refer the species to any of the 
genera hitherto published. 


ONISCID 4. 
15. Porcellio levis Latr. (1804). 


I will only refer to the account in G. Budde-Lund’s Crustacea Isopoda Ter- 
restria, 1885, which is the principal work on the Oniscide ; the author (pp. 138- 
140) describes the species, presents an enormous quantity of synonymy and 
references to earlier authors, and adds a very long list of localities for this 
almost cosmopolitan form. 

Habitat. Chatham Island, Galapagos (March 29, 1891), four specimens (de- 
termined by G. Budde-Lund). 


ZobLocicaL Museum, COPENHAGEN, 
September 16, 1897. 


a 


Fig. 1. 


Fig. la. 


Fig. 16. 
Fig. lc. 


Fig. 1d. 


Fig. le. 


Fig. 1/. 


Fig. 1g. 
Fig. 1h. 


Fig. 1i. 


Fig. 2. 


Fig. 2a. 
Fig. 2 6. 
Fig. 2c. 
Fig. 2 d. 


VOL, 


HANSEN: THE ISOPODA. 125 


EXPLANATION OF THE PLATES. 


PLATE I. 


1. Lurycope pulchra, n. sp. 


Female seen from above, X ¢. Of the antennule only the two proximal 
joints, of the antenne only the four proximal joints are drawn; the 
thoracic legs omitted, with the exception of the basal joint of the 
four anterior pairs. 

Female seen from left side, X ¢. Antennule and antennz as in the pre- 
ceding figure; the first thoracic leg is drawn, but of the six other 
pairs only the basal and the major part of the second joint are shown. 

Right mandible seen from below, X 11. 

Left mandible seen from below, KX 11; most of the palp omitted; a, cut- 
ting portion; 6, molar process ; /, lacinia mobilis; m, muscle (only the 
basal part) ; p, palp (proximal part). 

Left maxillula seen from below, X 11; 1, first joint; 1, lobe of the first 
joint; 2, second joint; 3, third joint; /?, lobe of the third joint. 

Left maxilla seen from below, X 11; 1, first joint; 2, second joint; /? 
lobe of the second joint; 3, third joint; /3, lobes of the third joint. 

Left maxilliped seen from below, X 11; 1, first joint, ep. its epipod ; 
2, second joint; A, its coupling hooks; /?, lobe of the second joint; 
7, seventh joint. 

First thoracic leg, X 3. 

Thoracic leg of fifth pair, X $; the natatory hairs omitted; 1, first joint; 
2, second joint; 7, seventh joint. This and the preceding analytical 
figures are drawn from parts of a female. 

Abdomen of a male seen from below, X 4;; a, pleopod of first pair; 
b, pleopod of second pair; c, uropod; d, anal doors. 


2. Eurycope scabra, n. sp. 


Female seen from above, X 2. The antennule completely wanting ; of 
the antennez the four proximal joints, and of the thoracic legs only the 
basal joint are seen. As to the correctness of the outline of thorax 
and abdomen, see the description. 

Left mandible seen from below, X 23. 

Left maxillula seen from below, 22. 

Left maxilla seen from below, 2. 

Left maxilliped seen from below, x 4. 

XXXI.— NO. 5. 3 


126 


Fig. 


Fig. 
Fig. 
Fig. 


Fig. 


Fig. 
Fig. 


Fig. 


Fig. 
Fig. 
Fig. 


Fig. 
Fig. 


Fig. 
Fig. 


Fig. 


Fig. 


Fig. 


20. 


AA 


3a. 


3b. 


or 
g 


or 
o 


la. 


. Left leg of fifth pair of the same specimen, seen from below, X 3 


BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


PLATE IL. 


1. Eurycope scabra, n. sp. (continued). 


First thoracic leg of female seen from the exterior side, X 6. 


2. Aga maxima, 0. sp. 


Female without marsupium, natural size; the apex of the last abdominal 
segment was wanting. 


. Right side of thorax of the same specimen showing the “epimera,” etc., 


natural size. 

Left leg of the second pair of the same specimen seen from below, 
searcely X 3. 

Left leg of the fifth pair seen from below, scarcely X 3. 


3. ga acuminata, n. sp. 


Head of female without marsupium, seen half from above and half from 
in front, X 5. 

Right side of thorax and of the two anterior abdominal segments of the 
same specimen, X 2. 

Posterior part of abdomen with the uropods of the same specimen, 
scarcely X 3. 


4, ga plebeia, n. sp. 


Head and first thoracic segment of a good-sized female without marsu- 
pium, seen half from above and half from in front, x 3. 


. Right side of thorax and of the two anterior abdominal segments of the 


same specimen, X 4. 
Left leg of second pair of the same specimen, seen from below, X 3. 


3. 


. Posterior part of the abdomen with the uropods of the same specimen, 


X 8. The hairs and spines on the uropods and on the posterior margin 
of the last abdominal segment omitted. 


5. Aga longicornis, n. sp. 


Female without marsupium, X about 3. 


. Right side of thorax and of the two anterior abdominal segments of the 


same specimen, X 3. 


. Posterior part of abdomen with the uropods of the same specimen, <4. 


PLATE III. 


1. £4 a longicornis, n. sp. (continued). 


Left leg of second pair of the female exhibited in the preceding plate, 
seen from below, X 33. 
Left leg of fifth pair of the same female, seen from below, X 33. 


Fig. 2. 


Fig. 2a. 
Fig. 26. 


Fig. 2c. 


Fig. 2d. 
Fig. 2e. 


Fig. 3. 


Fig. 3a. 
Fig. 30. 


Fig. 3c. 


Fig. 4. 


Fig. 4a. 


Fig. 40. 


Fig. 5. 


Fig. 5a. 


Fig. 1. 


Fig. la. 


Fig. 1. 


Fig. 1c. 


Fig. 1d. 


“| 


HANSEN: THE ISOPODA. 12 


2. Rocinela laticauda, n. sp. 


Male (the largest specimen), natural size. 

Head of female without marsupium, seen from below, X 5. 

Right side of thorax of the small immature female, scarcely 8. 

Left leg of second pair of the larger immature female, seen from below, 
scarcely X 4. 

Left leg of fifth pair of the same female, seen from below, scarcely x 4. 

Last thoracic segment and abdomen of the small immature female, 
scarcely X 3. 


3. Rocinela modesta, n. sp. 


Head of female with marsupium, seen from below, X 6; the flagella 
of the antennz are broken off. 

Left leg of second pair of the same female, seen from below, X 3. 

Left leg of fifth pair of the same female, seen from below, « 42. 

Last thoracic segment and abdomen of the same female, 3. 


4. TIrona foveolata, n. sp. 


Female with marsupium,  {. 
Leg of second pair of “ pullus stadii primi,” * 22. 
Posterior part of abdomen of “ pullus stadii primi,” x 22. 


5. Cryptione elongata, n. gen., n. sp. 


Leg of first pair of the male, X 111. 
Leg of fifth pair of the male, X 111. 


PLATE IV. 


1. Cryptione elongata, n. gen., n. sp. (continued). 


Female seen from above, < 43. 

Same female seen from below, X 13; m.male; mzp. maxilliped; 1’, rami 
of first pleopod on the left side (of the animal = right side of the figure) ; 
2, rami of second pleopod on the right side; 4’, rami of fourth pleopod 
on the left side; 5, rami of fifth pleopod on the right side; 5’, rami of 
fifth pleopod on the left side; 6, uropods. The marginal hairs on the 
marsupial plates are omitted. 

Head of the female seen from below and both maxillipeds omitted, X 9; 
a, antennula; 6, antenna; c, labrum; d, mandible; e, maxillula; f, max- 
illa; g, place of attachment of the maxilliped ; 1, hypopharynx; /, lobes 
or processes from the border behind the maxillipeds. 

Left maxilliped of the same female seen from below, X 42; 1, first joint 
with its posterior expansion ; 2, second joint; d?, dilatation on the outer 
side of the second joint ; p, palp. 

Left leg of first pair with its marsupial plate seen from below, X 18; 
2, second joint of the leg (the first joint being fused with the thorax). 


Fig. 


Fig. 


. 18. 


telic: 


BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Male seen from above, X 33. 
Same male seen from below, X 4. 
Head of the same male seen from below, X 36. 


2. Munidion princeps, n. gen., 0. sp. 


Anterior part of the head of the large female seen from below, scarcely 
10; c, labrum ; d, mandible ; f, maxilla. 

Left maxilliped of the large female, seen from below, scarcely X 7. 

Normal male, X #3. 

Misshaped male, X 43. 

Head of the normal male, seen from below, < 39. 

Left leg of first pair of the normal male, X 44. 


PLATE V. 


1. Munidion princeps, n. gen., u. sp. (continued). 


The large female seen from above, about X 1; a®, pleural plates of the 
fifth abdominal segment. 

Same female seen from below, about X 1£; 1, rami of first pleopod on 
the right side (of the animal, left side of the figure) ; 1’, rami of first 
pleopod on the left side; 4, rami of fourth pleopod on the right side ; 
5, rami of fifth pleopod on the right side; 5’, rami of fifth pleopod on 
the left side; 6, rami of the right uropod; 6’, rami of the left uropod. 

Left leg of first pair with its marsupial plate of the same female, seen 
from below, scarcely X 7. 

Posterior part of the marsupial plate exhibited in the preceding figure, 
and seen from above, X 3. 

Left leg of sixth pair of the same female, scarcely X 7. 


2. Pseudione galacanthe, n. sp. 


Large female, seen from above, X $; 6, uropods. 

Same female seen from below, X $; 1’, rami of first pleopod on the left 
side (of the animal); 4’, rami of fourth pleopod on the left side; 5, rami 
of fifth pleopod on the right side ; 5’, rami of fifth pleopod on the left 
side; a®, pleural plates of fifth abdominal segment. 

Anterior part of the head of female, seen from below, X 10; a, antennula ; 
b, antenna; c, labrum; d, mandible ; e, maxillula ; f, maxilla; p, frontal 
plate. 

Left maxilliped of female, seen from below, X 10. 

Left leg of first pair with its marsupial plate, seen from below, X 10. 

Left leg of sixth pair of female, X 10. 

Largest male, < 10. 

Head and a part of the first thoracic segment of a smaller male seen from 
below, X 39; f, maxilla; g, maxilliped. 

Left leg of first pair of the last named male, X 47. 

Left leg of seventh pair of the same male, X 47. 


HANSEN: THE ISOPODA. 129 


PLATE VL 


1. Parargeia ornata, n. gen., 0. sp. 


Female seen from above, about X $. 

Same female seen from below, about X 3; as to the marsupial plates see 
the description of the species. 

Anterior part of the head of the same female seen from below, X 13; 
6, antenna; c, labrum; d, mandible; e, maxillula; 7, maxilla; h, hypo- 
pharynx; p, frontal plate. 

Left maxilliped of the same female seen from below, scarcely X 10. 

Left leg of first pair with its marsupial plate seen from below, scarcely 
x10: 

Left leg of seventh pair of the same female, scarcely X 10. 

Male, X 22. 

Head of the same male seen from below, X 39. 

Left leg of first pair of the same male, X 46. 

Left leg of seventh pair of the same male, X 46. 


2. Bathygyge grandis, n. sp. 


Male, scarcely X 13. 

Head of the male seen from below, X 26; d (by error instead of c), la. 
brum; /, maxilla; g, maxilliped. 

Left leg of first pair of the male, x 19. 

Left leg of fifth pair of the male, X 19. 


PLATE VII. 


Route OF THE “ALBATROSS.” 


MW ee 
ave 4 


i tee 


e fii 
cc ye 


Lsopoda PUT. 


Albatross" Fax. 12H. 


wa 


SC. 


Lévendal 


ia 


S. 


fy 


Lsopoda Pl. ii. 


i “Albatross” Fx. 1891. 
’ 


+ J 


Lsopoda PU. I. 


Albatross" Fae. 1897. 


LT. Hansen’ del. 


Levendal ge. 


~ 


ar 


bon 


Albatross” Fa. 8H. LIsopoda Pl: IV. 


PSN 
ae 
WG : 

2—— 


4.7. Hansen del. Lévendal ge. 


Albatross” Bax. LEPL Isopoda SPE 


4 T. Hansen del. Lévendal ve. 


< 


‘Albatross ” Fa. LBA « Lsopoda LPL: VI. 


HJ. Hansen del. Lévendal se. 


a 
Ag 

* 

= 
> 


» 


PAA 


ee 


¥ 
ro 


x 


F 
pS <4 


2. fathome 


No. 6.— The Thoracic Derivatives of the Postcardinal Veins in 
Swine. By G. H. PARKER and C. H. Tozier.? 


Introduction. 


ALTHOUGH the postcardinal veins? in swine were originally studied by 
Rathke, and have since been reinvestigated by Hochstetter, our knowl- 
edge of them is admittedly fragmentary ; for Hochstetter himself regrets 
that his results in the main do little more than raise doubt as to the 
accuracy of some of the most important of Rathke’s statements, without 
giving grounds enough for full criticism. It is our purpose in this paper 
to present what seems to us a consistent account of the changes that 
these veins undergo, and to offer some critical comments on the ques- 
tions raised by Hochstetter. 

In dealing with this subject we have had recourse to the two general 
methods of serial sections and injection. The smaller embryos were 
cut into serial sections, and the courses of the veins then studied by a 
simple method of graphic reconstruction. The larger ones were injected 
with a raw starch-mass, or a celloidin-mass. In the former case the 
veins were afterwards dissected out; in the latter, corrosion preparations 
were made by dissolving away the tissues of the embryo in an artificial 


1 Contributions from the Zoological Laboratory of the Museum of Comparative 
Zoology at Harvard College, E. L. Mark, Director, No. LXX XVII. 

2 Some little confusion exists as to the terminology of the principal veins in 
lower vertebrates and the homologues of these veins in mammalian embryos. The 
principal veins from the head of a fish are usually designated by comparative 
anatomists as right and left anterior cardinal veins, and their homologues in the 
mammalian embryo are generally named by embryologists right and left jugular 
veins. For these veins, whether they be in the adult fish or in the embryonic 
mammal, we propose to use the names right and left precardinals. Ina similar 
way, the blood-vessels designated by comparative anatomists as right and left 
posterior cardinal veins, and by embryologists simply as right and left cardinal 
veins, will be called by us right and left postcardinals. These changes are in 
harmony with those by which the longer and older names, vena cava posterior 
and vena cava anterior, have been replaced by postcava and precava, and it is 
therefore hoped that they will commend themselves alike to embryologists and 
comparative anatomists. 

VOL. XXXI.— NO. 6. 


134 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


digesting fluid, thus leaving the courses of the vessels indicated by the 
celloidin. From embryos of intermediate size both reconstructions and 
injections were made, and the results obtained by the two methods 
compared. 


Observations on Swine. 


In an embryonic pig, whose greatest length as measured in a straight 
line from the crown of its head to the root of its tail, was between 
six and seven millimeters, a reconstruction of the primary veins of 
the trunk, when viewed ventrally, ap- 
peared as in Figure 1. From the ante- 
rior end of the embryo, two veins, the 
right and left precardinals (pr’erd. d. 
and pr’erd. s.), pass posteriorly to the 
region of the heart, where each is 
met by a corresponding postcardinal 
(prerd. d. and p’erd. s.). The pre- 
and postcardinals of either side unite 
to form transverse trunks, the right 
and: left Cuvierian ducts (dt. Cuv. d. 
and dt. Ouwv. s.). The ducts thus formed 
unite with each other, giving rise to the 
venous sinus (sz. vn.), through whose 
posterior wall three large veins enter 
from the liver, and in whose anterior 
wall the passage to the heart is seen. 

Of these veins the postcardinals 
(perd. d., and p’erd. s.) claim our 
special attention. At this stage they are symmetrical. Each begins 
posteriorly by a union of several small veins at the base of the hind leg 
of its own side of the body, and in a region dorsal and lateral to the 
mesonephros (ms’nph.) of the same side. From this region the vein 


PR'CRO.D.- ~~ 


FicureE 1. 


extends anteriorly over the dorsal surface of the mesonephros, penetrating — 


more or less completely the anterior end of that organ, and emerging 
from it to pass directly, as a well defined blood-vessel, to the Cuvierian 
TA 0 Se 


Fic. 1. Reconstruction on a frontal plane of the principal veins and the 


mesonephroi of an embryonic pig between six and seven millimeters long. Ventral ° 


view. X 12. dt. Cuv. d., right Cuvierian duct; dt. Cuv. s., left Cuvierian duct; 
ms’nph., mesonephros ; p’crd. d., right postcardinal ; p’erd. s., left postcardinal ; 
prerd. d., right precardinal ; prerd. s., left precardinal ; sn. vn., venous sinus. 


vie 


PARKER AND TOZIER: POSTCARDINAL VEINS IN SWINE. 135 


duct of the corresponding side. The partial penetration of the meso- 
nephros by the postcardinal might with equal propriety be described as 
a partial intrusion of the mesonephros into the cavity of the vein, for 
the loosely twisted nephridial tubules hang with such freedom into the 
venous blood-spaces that they may with justice be said to occupy more 
or less of the cavity of the vein itself. This peculiar suspension of the 
tubules in the venous blood-spaces, a 
condition which persists in the later 
stages, has been recently noticed by 
Minot (’98, p. 229). 

In an embryo whose greatest length 
(measured as before described) was be- 
tween twelve and thirteen millimeters 
the veins just considered present the 
condition shown in Figure 2. Except- 
ing for differences of size, and slight 
changes in outline, the precardinals 
are essentially the same as in the pre- 
ceding stage. In the region of the ve- 
nous sinus the hepatic opening, which 
is now single, and the anterior opening, 
which leads into the heart, have shifted 

somewhat toward the right side of the 
_ body, and the root of a coronary vein 
(un. cor.) has been formed. MsnPH 

The postcardinals are very much 
altered. With the growth of the me- 
sonephroi, they have become entirely 
interrupted in the middle part of their 
course ; their posterior portions, how- 
ever, persist near the base of the hind Ficure 2. 
legs, finding an outlet toward the heart 
through the newly formed postcava, and their anterior portions now 
begin at the anterior ends of the mesonephroi, and each extends to the 
Cuvierian duct of its own side. The anterior portion of each postcar- 


---|---PR'CRDS 


----f-0T Cuv 


-/--P'cros 


Fic. 2. Reconstruction on a frontal plane of the principai veins and parts of 
the mesonephroi of an embryonic pig between twelve and thirteen millimeters long. 
Ventral view. X12. dt. Cuv., Cuvierian duct; ms’nph., mesonephros; p’crd. s., 
left posteardinal ; pr’erd.s., left precardinal ; sn. vn., venous sinus ; v7. acc., accessory 
vein ; vn. cor., coronary vein. 


136 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


dinal receives blood not only from the anterior end of the mesonephros 
but from the region between the two mesonephroi. This is accomplished, 
however, not by the postcardinal proper, but by a new outgrowth that 
takes its origin from the postcardinal near the anterior end of the me- 
sonephros. The walls of these new vessels, which may be called the 
accessory veins (wz. ace.), are extremely delicate; the exact places of 
union between them and the postcardinals is indicated not only by the 
general topography, but also by the rather abrupt change in the thick- 
ness of the walls where the accessory vessel meets the postcardinal. 
Moreover, the cavities of the accessory vessels are not freely open 
throughout their whole extent, as those of the postcardinals are, but 
are here and there partly interrupted by flattened trabecule. In fact, 
posteriorly these trabeculz become so numerous that the cavities of the 
vessels are finally merged in the interspaces thus formed. The acces- 
sory vessels at this stage may be traced posteriorly to a point about 
midway the length of the mesonephros. 

In slightly smaller embryos the accessory veins are much shorter, but 
even in these they always open freely into the postcardinals, and we 
therefore believe them to be outgrowths of the postcar- 
dinal vessels. The place of union between the accessory 
vessels and the postcardinals in the specimens studied 
was at the level of the tenth rib, and the accessory ves- 
sels could usually be traced posteriorly some distance 
beyond the last or fourteenth rib. 

In an embryo whose greatest length was about forty- 
eight millimeters, the postcardinal and the accessory con- 
stituents of each vein could no longer be distinguished, 
for they had fallen so well into line with each other that 
they were represented by a perfectly continuous vein 
(Figure 3). That on the left side, which may now be 
called the hemiazygos vein (vn. hm’az.), retained its 
earlier connections and extended from the left Cuvierian 
duct along the left side of the vertebral column to a point 
some distance posterior to the last rib. That on the 
right side, the azygos vein (vn. az.), had lost its anterior connection with 
the right Cuvierian duct, but otherwise extended over a tract corre- 
sponding in the main to that of the hemiazygos. The blood collected 


FIGURE 8. 


Fic. 3. Reconstruction of the azygos and hemiazygos veins from an embryonic 
pig about forty-eight millimeters long. Ventral view. X i2. vn. az., azygos vein; 
vn. hm’az., hemiazygos vein. 


PARKER AND TOZIER: POSTCARDINAL VEINS IN SWINE. 137 


by the azygos vein was transferred by transverse connecting vessels to 
the hemiazygos, by which it was carried to the heart. In the specimen 
from which the reconstruction shown in Figure 3 was made, three such 
transverse connections were found. In a specimen fifty-five millimeters 
long, studied by injection, five such vessels occurred, and these were so 
placed that their ends were opposite the mouths of the newly forming 
intercostal veins. 

The most striking peculiarity of the stage illustrated by Figure 3 is its 
lack of symmetry. In the earlier conditions described these veins have 
been bilaterally symmetrical ; but with the loss of connection between 
the azygos and the right Cuvierian duct this symmetry disappears, and 
a connection with the heart is retained only through the left side. In 
this respect the pig and probably all ruminants differ from other mam- 
mals, in which as a rule the azygos, not the hemiazygos, retains its 
original connection with the heart. 

The further changes that the azygos and hemiazygos undergo may 
be seen in pigs ranging in length from seven to twenty centimeters. 
The chief features of these changes consist in the further reduction of 
the azygos, together with the retention of the transverse connecting 
vessels, by which the right intercostals are brought to connect directly 
with the hemiazygos. Depending upon the way in which the azygos is 
reduced, three types can be distinguished. These are illustrated in 
Figure 4. 

In the first type (Fig. 4, A) the hemiazygos reaches from the heart 
posteriorly to the eleventh intercostal space, receiving in its course the 
intercostal veins on the right from the sixth to the eleventh, and on the 
left from the fifth to the eleventh. Posterior to the eleventh intercostals 
two longitndinal veins appear, which are of about equal size, and extend 
posteriorly two segments farther, receiving the twelfth and thirteenth 
intercostals. Of these the left one (vn. hm’az.) obviously represents the 
posterior continuation of the hemiazygos, the right one (vn. az.) the last 
remnant of the azygos, which in the region of the eleventh intercostal 
still retains its transverse connection with the hemiazygos. 

In the second type (Fig. 4, B) the hemiazygos (vn. hm’az.) extends 
as the predominant vessel from the heart to the fourteenth intercostal 
space. The azygos is entirely suppressed, except for a small part run- 
ning from the twelfth to the thirteeth intercostal and possessing at its 
two ends transverse connections with the hemiazygos. 

In the third and last type (Fig. 4, C) the hemiazygos is a well devel- 
oped trunk from the heart to the ninth intercostal, beyond which the 


138 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


blood is conducted in the main through what is obviously the persistent 
azygos (vn. az.), though remnants of the hemiazygos occur between the 
ninth and tenth right intercostals, as well as between the eleventh and 
twelfth. 
Thus in the three types considered the anterior part of the system is 
always formed exclusively from the hemiazygos. The posterior part may 
i 


--VWNAZ 


— VN HAZ, 


Figure 4. 


be derived from the equally persistent azygos and hemiazygos (type one) 
or from a preponderant hemiazygos (type two) or finally from a pre- 
ponderant azygos (type three). 


Fic. 4. The hemiazygos and connected veins, showing three structural types. 
Raw starch injections. Ventral views. vn. az., azygos vein; vn. hm’az., hemi- 
azygos vein; 6, 10, 13, bases of the sixth, tenth, and thirteenth intercostal veins 
respectively. 

A. Type one, with equally persistent azygos and hemiazygos. From an em- 
bryonic pig about seven centimeters long. X 3. 

B. Type two, with preponderant hemiazygos. From an embryonic pig about 
eleven centimeters long. X 2. 

C. Type three, with preponderant azygos. From an embryonic pig about 
twenty centimeters long. Natural size. 


PARKER AND TOZIER: POSTCARDINAL VEINS IN SWINE. 139 


It will be recalled from the earlier part of this description that the 
two components (postcardinal and accessory vein) which make up the 
azygos and the hemiazygos were united at about the level of the tenth 
rib. The hemiazygos from the region of the heart to the tenth rib is 
therefore to be regarded as the persistent anterior portion of the left 
postcardinal. As the corresponding part of the azygos has aborted, the 
right postcardinal of this region is entirely absent. Consequently the 
variable portion of this system — most of which lies posterior to the tenth, 
or at least to the ninth rib — represents the parts derived from the 
accessory veins. 

Although the main stem of the hemiazygos from the heart to the 
region of the tenth rib has been stated to be derived exclusively from 
the left postcardinal, it is possible that occasionally a portion of its 
posterior extent may come from a fusion of both right and left post- 
cardinals ; for in one instance we found between the levels of the ninth 
and tenth intercostals (compare Fig. 4, C) an “island” formation which 
was so narrow that the right and left components may be said to have 
almost completely united. While the rareness of such cases makes it 
improbable that a process of fusion is at all usual, the possibility of its 
occurrence cannot be ignored, and, where fusion does occur, the incor- 
poration of a part of the right postcardinal into what becomes the main 
stem of the hemiazygos is at least a possibility. Aside from this, how- 
ever, the right postcardinal certainly plays no part in the ultimate for- 
mation of the system of veins under consideration. 


Historico-critical Remarks. 


The postcardinals of swine were first described by Rathke (’30, p. 64), 
whose account, though mainly taken from the sheep, applies, according 
to this author, almost equally well to the pig. The same account was 
subsequently somewhat amplified and published by Rathke (32, p. 82) 
in a second paper. In both these papers the postcardinals are called 
posterior ven cave (hintere Hohlvenen), for Rathke believed at this 
time that the right postcardinal persisted throughout its whole extent 
as the adult postcava. He further believed that the thoracic portion of 
the left postcardinal became the hemiazygos. This interpretation agreed 
well with the fact that, as Rathke (’30, p. 67) pointed out, adult sheep 
and pigs have no azygos veins, structures that might be supposed to 
represent the right postcardinals. Stark, as we gather from the histor- 
ical account given by Hochstetter (’93, p. 611), subsequently showed 


140 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


that in embryonic sheep, in addition to a postcava, an azygos vein also 
occurred. Rathke seems to have become cognizant of this fact, for in 
his third paper on this subject he abandoned his earlier views on the 
fate of the postcardinals, and, without stating reasons for the change, 
adopted views more nearly in accordance with the new observations 
made by Stark. 

According to Rathke’s (’38, pp. 3, 4, and pp. 10, 11) later view, the 
anterior thoracic portion of the left postcardinal is involved in the for- 
mation of the hemiazygos, and the right postcardinal in a similar way 
enters into the formation, not of the postcava, but of the embryonic 
azygos. This view agrees in the main with the results obtained by recent 
investigators (Hochstetter, 93, and Zumstein, ’96, 97) on other mam- 
mals, and is abundantly confirmed by our own observations on swine. 

The facts thus far stated are only a partial exposition of Rathke’s later 
opinion. Only the anterior portions of the azygos and hemiazygos 
are formed, according to Rathke’s later view, from the postcardinals, 
the posterior parts being developed from a system of longitudinal 
anastomosing trunks between the successive intercostal veins. These 
anastomosing trunks receive the blood from the intercostals, pass- 
ing it forward towards the heart, and thus form a longitudinal ves- 
sel, which gradually replaces a part of the original postcardinal. The 
extent to which this replacement occurs may be indicated as follows. 
The part of the hemiazygos extending from near the heart to the sixth 
intercostal vein represents a persistent part of the postcardinal, and the 
remaining part from the sixth intercostal posteriorly to the last one is a 
new formation from the longitudinal anastomosing vessel ; the part of the 
embryonic azygos from its connection with the heart posteriorly to the 
eighth or tenth intercostal represents the right postcardinal, the remain- 
ing posterior portion having been derived from the longitudinal anas- 
tomosing vessel of that side. After the right intercostals establish 
transverse connections with the hemiazygos, the azygos disappears, thus 
leaving the hemiazygos as a return trunk for the blood from the right 
as well as from the left intercostals. 

That the posterior portions of the azygos and of the hemiazygos in 
many mammals are new formations added to the remnants of the post- 
cardinals is now, we believe, generally admitted, and, as we ourselves 
have seen, is certainly true for swine ; but that these new formations, 
the accessory veins, develop from anastomosing branches between 
the intercostal veins, as stated by Rathke, is not, we believe, in accord- 
ance with fact. Of the several embryos examined by us at the stage in 


PARKER AND TOZIER: POSTCARDINAL VEINS IN SWINE. 141 


which the accessory veins were developing, no trace of intercostals or 
intercostal anastomoses could be discovered, but the accessory veins 
grew at their posterior ends by a process that seemed like the formation 
of an opening through the tissues independent of any pre-existing blood- 
cavities. We therefore believe that Rathke was in error in ascribing to 
the non-cardinal parts of the azygos and hemiazygos veins in swine 
an origin from intercostal anastomoses. 

In justice to Rathke, however, it should be mentioned that longitudi- 
nal anastomosing vessels between the intercostals do occur in embryonic 
pigs. We have never identified these in early stages, but their presence 
can be easily demonstrated in older embryos by means of celloidin in- 
jections that have been converted 
into corrosion preparations. In a 
pig about six centimeters long, the 
veins between the ninth and eley- 
enth intercostals, when thus prepared 
and viewed from the dorsal side, are 
represented in Figure 5. From the 
longitudinal hemiazygos (vn. hm’az.) 
pass off on the left the three inter- 
costals 9, 10, and 11; on the right, 
intercostal 11 and intercostal 9, to 
which intercostal 10 is attached by a 
longitudinal trunk (wn. az.), which 
probably represents a part of the 
once complete azygos. 

Each of the six intercostal veins wae 
mentioned gives off a short dorsal 
vessel, which opens into a zigzag longitudinal trunk (wn. lg.) of the 
corresponding side of the body. Thus the intercostals of a given side 
are put into communication with one another by a longitudinal anasto- 
mosing trunk. The right and left anastomosing trunks are moreover 
connected transversely, at regular and frequent intervals, in regions 
where their inwardly directed angles approach each other, thus pro- 
ducing a series of more or less hexagonal “islands” bounded by blood- 


Fie. 5. The hemiazygos and connected veins from the region of the ninth to 
the eleventh intercostals in a pig about six centimeters long. Celloidin injection 
freed by artificial digestion. Dorsal view. X 10. vn.az., azygos vein; vn. hm’az., 
hemiazygos vein ; vn. /g., longitudinal anastomosing vein; 9, 10, 11, respectively 
ninth, tenth, and eleventh intercostal veins of the left side. 


142 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


spaces. This system of longitudinal anastomosing vessels may be traced 
through at least the whole length of the thorax, and lies only a little 
dorsal to the region occupied by the hemiazygos, being thus in the 
neighborhood of the forming vertebral column. It seems to us highly 
probable that these anastomosing vessels were the ones seen by Rathke, 
and supposed by him to enter partially into the formation of the azygos 
and hemiazygos veins, a conclusion which, though in our opinion errone- 
ous, is nevertheless not surprising when one considers the methods of 
investigation employed in his time. What the origin of these vessels 
may have been and what their subsequent fate may be have not been de- 
termined by us, but that they contribute nothing to the formation of 
the azygos or of the hemiazygos we feel perfectly assured. 

The development of the azygos and hemiazygos veins in swine con- 
forms, then, in general to that found by Hochstetter (’93) and by Zum- 
stein (’96, 97) in other mammals. 

In one respect, however, there is in this connection still ground for 
difference of opinion among recent students; this has to do with the 
proportions in which the postcardinal and accessory components enter 
into the formation of the azygos and hemiazygos. According to Zum- 
stein (96, p. 601), in the human being the azygos and the hemiazygos, 
if there be one, are formed from the postcardinals exclusively, whereas 
in the Guinea pig (Zumstein, “97, p. 188) both vessels are almost entirely 
formed from the accessory veins. In the rabbit, and probably also in 
the cat, according to Hochstetter (93, pp. 589 and 595), the plane of 
separation between the two components lies in the eighth thoracic seg- 
ment, and this agrees very nearly with our observations on swine, where 
it lies near the tenth pair of ribs. These differences, which are clearly 
not fundamental, are more likely due to peculiarities in the develop- 
ment of the respective species than to errors of observation, though such 
topographical determinations are by no means simple. 


Conclusions. 


1. Small embryonic pigs possess well developed right and left post- 
cardinals (posterior cardinal veins), which extend from the bases of the 
corresponding posterior extremities anteriorly over the dorsal surfaces 
of the mesonephroi to the Cuvierian ducts. 

2. The thoracic portion of each postcardinal persists from the heart 
to the region of the tenth pair of ribs, beyond which a new vessel, the 
accessory vein, is developed to a point some distance posterior to the 
last pair of ribs. 


PARKER AND TOZIER: POSTCARDINAL VEINS IN SWINE. 143 


3. The united postcardinal and accessory veins of the right side give 
rise to the azygos vein ; those of the left side, to the hemiazygos. 

4, The azygos and hemiazygos veins receive the intercostal veins of 
their respective sides and become mutually connected by several trans- 
verse veins. 

5. The cardinal portion of the azygos vein usually degenerates com- 
pletely, and the right intercostal veins formerly connected with it then 
find an outlet through the corresponding part of the hemiazygos, which 
persists in the adult pig. 

6. The accessory parts of the azygos and hemiazygos veins may re- 
main connected with the cardinal part of the hemiazygos, and by their 
variations give rise to three structural types: first, one in which both 
accessory parts are equally developed ; secondly, one in which the hemi- 
azygos accessory predominates; and thirdly, one in which the azygos 
accessory predominates. 


144 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


PAPERS CITED. 


Hochstetter, F. 
93. Beitrage zur Entwicklungsgeschichte des Venensystems der Amnioten. 
III Sauger. Morphologisches Jahrbuch, Bd. XX. pp- 543-648, Taf. 
XXI.-XXITI. 


Hochstetter, F. 
°94. Entwickelung des Venensystems der Wirhbeltiere. Ergebnisse der Anat. 
und Entwg., Bd. III. pp. 460-489. 


Minot, C. S. 
°98. The Veins of the Wolffian Body. Science, N. S., Vol. VII. No. 164, 
p- 229. 


Rathke, H. 
°30. Ueber die friiheste Form und die Entwickelung des Venensystemes 
und der Lungen beim Schafe. Arch. Anat. u. Physiol., Jahrg. 1830, 
pp. 63-73, Tab. I. 


Rathke, H. 
°32.  Abhandlungen zur Bildungs- und Entwickelungs-Geschichte des Men- 
schen und der Thiere. Erster Theil. F.C. W. Vogel, Leipzig. 114 pp., 
Tab. I.-VII. 


Rathke, H. 
‘38. Dritter Bericht iiber das naturwissenschaftliche Seminar bei der Uni- 
versitat zu Kénigsberg. Nebst einer Abhandlung iiber den Bau und die 
Entwickelung des Venensystems der Wirbelthiere. Konigsberg. 23 pp. 


Zumstein, J. 
‘96. Zur Anatomie und Entwickelung des Venensystems des Menschen. 
Anat. Hefte. Erste Abt., Arbeiten. Bd. VI. pp: 571-608, Taf. XXVI.- 
XXXVII. 


Zumstein, J. 
97. Zur Entwickelung des Venensystems bei dem Meerschweinchen. 
Anat. Hefte. Erste Abt., Arbeiten. Bd. VIIL. pp. 165-190, Taf. 
XXI-XXX. 


No. 7.—The Segmentation of the Nervous System in Squalus 


acanthias. 


brate Head. 


A Contribution to the Morphology of the Verte- 
By H. V. NEAL. 


CONTENTS. 
Page Page 
Introduction . ; . 148 b. Relation of Encephalomeres 
Criteria of Segmentation : - 148 to Somites . . 187 
Summary of Results of former Inves- c. Somatic Value of the Pre-otie 
tigations on the Pe gee ae of Mesoderm Segments 187 
the Encephalon . 150 d. Summary 206 
I. Locy’s ‘‘ Neural Segments”? or VI. The Relation of Neuromeres to 
“ Metameres ” . 154 Nerves . 5c 207 
a. Material . - . 154 a. Historical Review . - 208 
b. Method of Study = . 155 b. Nerve Relations in the 
c. Description of auuiee “* Neu- Trunk of S. acanthias. . 210 
ral Segments ” . 156 c. Nerve Relations in the 
d. Continuity of the “ & Seg- Cephalic Region . - 211 
ments’’. . 159 d. Development of the 
e. Interpretation of the Evi- 1. Oculomotorius . 220 
* dence . 2 Ao 2. Abducens . 230 
f. Limit of f @enhalic Plate Je Gye 3. Trochlearis . 235 
II. The ‘‘ Hindbrain Neuromeres”’ in VII. Segmental Value of Hindbrain 
S. acanthias : 166 Neuromeres . 240 
a. Bacon of the Term “Nen- a. Non-phylogenetic interes 
romere ’ . 166 tation 240 
b. Development of Hindbrain b. Phylogenetic Tnterpretsane 243 
Neuromeres 167 c. Interpretation of Hindbrain 
c. Summary. . - 172 Neuromeres in S. acan- 
III. The Neuromeres in the Trunk He: thias . . 245 
gion . » + . « - - 178) VIII. Primitive Relations nt ‘Gephalic 
a. Development of Myelo- Segments I.-VII. . 254 
HENS OG - . . 178 a. Encephalomere VII. . 254 
b. Summary 176 b. Encephalomere VI. . 255 
IV. The Neuromeres anterior a he c. Encephalomere V. . 255 
Hindbrain . : 5 lees d. Encephalomere IV. . 256 
a. Essential Criteria of Rea: e. Encephalomere ITI. . 258 
meres a yt f. Encephalomere II. . 258 
b. Development of the ee g. EncephalomereI.. . . 259 
brain and Midbrain . 5 are h. Comparison with the See 
ce. Summary . 185 mentation of Amphioxus . 260 
VY. The Relation of iocemeros ‘6 i. General Conclusions . 268 
Somites . . - 186 IX. Summary 275 
a. Relation of Myclonercs to Bibliography aa ty 275 
Somites - 186 | Description of Plates . 293 


1 Contributions from the Zodlogical Laboratory of the Museum of Comparative 
Zoology at Harvard College, under the direction of E. L. Mark, No. LXXXIX. 


VOL. XXXI. — NO. 7. 


148 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Introduction. 
CRITERIA OF SEGMENTATION. 


Morpuotocists have long sought to compare in Vertebrates a head 
Segment with a trunk segment. They have assumed that in the ances- 
tors of Vertebrates head and trunk were differentiated from each other, 
and that similar segments once extended throughout the entire length 
of the body. Direct evidence in favor of this assumption is now fur- 
nished, it is held, by Amphioxus. Because of the many difficulties in- 
volved, the problem has become a favorite one, and since the early 
attempts made by the poet Goethe and anatomists of the “ Transcen- 
dental” school, many men have contributed evidence and theory in the 
hope of its solution. Since Goethe and Oken maintained the bony 
cranium to be composed of fused vertebre comparable with those in the 
vertebral column, the problem has passed through several phases. First, 
Huxley (’58), upon broad comparative anatomical evidence, proved that 
nothing like a vertebra is to be found in the cranium of either high 
or low Vertebrates, and he concluded as a result of his researches that 
morphologists, in attempting to find a primitive metamerism in a struc- 
ture which is so late in its phylogenetic appearance as the bony cranium, 
were approaching the problem in the wrong direction. In thus dis- 
proving the “vertebral theory” of the Vertebrate cranium, however, 
“ war die Frage doch noch nicht aus der Welt geschafft,” as Gegenbaur 
wrote in his famous “Kritik.” By Gegenbaur (’72) the question was trans- 
formed into a problem of the phylogenesis of the entire head. By using 
as criteria the visceral arches and the nerves which innervate them, he 
attempted to determine the number of primitive segments in the head 
of those low Vertebrates, the Selachii, which in his opinion most re- 
sembled the hypothetical Vertebrate ancestors. 

With the gradual acceptance of the “ fundamental law of biogenesis,” 
that the development of an individual is an epitome of the develop- 
ment of the race, the evidence offered in the solution of the problem 
of the morphology of the Vertebrate head has become more and more 
embryological. 

After Balfour’s (’78) discovery that the primary body cavity of Sela- 
chian embryos extends unbroken into the head region, and the further 
discovery of Marshall (’81) that in these embryos the body cavity of the 
head undergoes an independent segmentation into mesodermal cavities, 
Selachian embryos became the chief objects of research. It was finally 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 149 


left to van Wijhe (’82) to demonstrate in Selachian embryos an uninter- 
rupted continuity, and a direct morphological comparability, of head 
and trunk ‘ Mesodermsegmente,” and thus, in the opinion of many 
morphologists, the existence of an “acraniote” stage in the development 
of craniote embryos. Since the “mesodermal segments” cr somites 
were regarded as the best evidence of the primitive segmentation, it 
was at first believed that the problem of the morphology of the Ver- 
tebrate head, as regards both number and nature of segments, had at 
last been solved by van Wijhe. His conclusion was that nine seg- 
ments, four of which were pre-otic and five post-otic, enter into the 
formation of the Vertebrate head, or at least the Selachian head. 

Yet one who studies the literature of the decade and a half that has 
elapsed since van Wijhe wrote his famous paper must conclude, from 
the great divergence of opinion which still prevails among the most 
competent investigators as regards both nature and number of head 
segments, that the problem is “ noch nicht aus der Welt geschafft.” 
According to Froriep, Kastschenko, and Rabl, the segments of the pre- 
otic and post-otic regions are of a fundamentally different kind. Fur- 
thermore, while Rabl (92) finds not over three segments in the entire 
pre-otic region, Dohrn (90) finds in the same region twelve to fifteen 
segments, serially homologous with trunk segments. These, indeed, 
represent extremes of opinion, for the majority of morphologists agree 
with Gegenbaur and van Wijhe that pre-otic segments are few but com- 
parable with trunk segments. The chief causes of the present dis- 
agreement of morphologists are two. In the eager search for evidence 
of segments investigators have often failed (1) to control their results, 
based upon the study of a single organ system, by a comparison of the 
actual conditions which obtain in other organ systems in the same 
organism ; and (2) to control conclusions based upon a single organ- 
ism by appeal to the facts and conclusions of comparative anatomy and 
embryology. As the result of the healthful scepticism of such accurate 
observers as Froriep, Kastschenko, and Rabl, the necessity for such con- 
trol now seems too obvious to need repetition here. 

While morphologists (excepting Gegenbaur) in attempting to elu- 
cidate the problem of cephalic segmentation have based their con- 
clusions chiefly on the study of the mesodermal segments, — since these 
have seemed to afford the best criteria of segmentation, — yet other 
embryonic structures have also been studied, viz. the segments of the 
central nervous system, or “neuromeres,” the nerves, the epibranchial 
organs, the blood-vessels, and the visceral arches. 


150 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


It is well known that in embryos of all classes of Vertebrates the 
central nervous system shows a segmentation which consists in a series 
of constrictions and dilatations extending throughout the length of the 
neural tube, giving to it a beaded appearance. In the trunk the nerves 
have a definite relation to the segments of the spinal cord, the “ myelo- 
meres,” as I shall call them, adopting the term introduced by McClure 
(’89), and it is believed that the cranial nerves also have definite re- 
lations to the segments of the brain, the “ encephalomeres,” although 
their relations are less clear. Even if we believe with Ahlborn (84*) 
and Froriep (’94) that the nervous system is segmented in adaptation 
to associated segmental structures, it is not @ priori improbable that the 
number of primitive segments in the Vertebrate may be shown by the 
number of neural segments, for in some Invertebrate embryos segmental 
cephalic ganglia appear even when most other traces of mesodermal 
segments and related sense organs have (it is believed) disappeared. 

In view of the present discrepancy between the results based upon the 
study of neuromerism and those based upon the study of mesomerism, it 
devolves upon one who attempts to elucidate the question of cephalic 
segmentation in Vertebrates by using the segments of the central ner- 
vous system as criteria, to show the comparability of encephalomeres 
with myelomeres, not only structurally, but also in relation to ner- 
vous outgrowths, and to those divisions of the mesoderm on which 
the segmentation of the motor nerves ultimately depends. The inter- 
dependence of motor nerve and muscle has seemed so evident that 
morphologists have not hesitated to make the number of cranial nerves 
conform with the number of somites previously determined by them. 
Yet the majority of investigators of the segments of the encephalon have 
failed to take into consideration the relation of these to the segments of 
the mesoderm, and consequently we find in the literature upon neuro- 
merism a diversity of opinion such as we have learned to expect in 
results based upon insufficient knowledge. ‘ 


SuMMARY OF RESULTS OF FORMER INVESTIGATIONS ON THE 
SEGMENTATION OF THE ENCEPHALON. 


The results of former investigators concerning the number of enceph- 
alomeres and their nerve relations may be summarized in the form of 
two tables. Table I. shows the number of segments as determined by 
previous investigators, as well as their relation to the primary vesicles 
of the brain. The total number of segments has been given in the 
cases where it has been stated by the observer. The observations of 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 


Taste I.— ENCEPHALIC SEGMENTS. 


BS | eMCB: || (HB: (cee. 


151 


742 | Bischoff . Dog. 6 
: (OO |eDursy | ar Oxia 32 6 
a "77__| Mihalkovics Rabbit Rte 5-6 
5 86 «| Kupffer . Sheep, Mouse, Man 5 
= 789 ~+| Prenant . Swiners =i sie mo) Ne 5 
= 791 | Zimmermann . Rabbit 2 3 8 13 
92,928 | Froriep . Mole 2 3 7 12 
795 | Broman . Man. 7 
75D) | Remak .-~. Chick . 5-6 
"77 ~+| Mihalkovics as ; 5-6 
a 785 «=| Rabl . s 6-7 
S 87 | Béraneck s 2 1 6 9 
ra) 789 | Platt . . ss 1 1 6 8 
790 ~| McClure ; s 2 2 6 10 
791 +| Zimmermann . s 2 3 8 18 
784 | Beraneck Lizard . 5 
a ’87—=« | Orr i 2 1 6 9 
S| ’85,’89 | Hoffmann’ . Sh ce 7 
& | 785,’89 | Hoffmann Tropidonotus a 
a 90 | McClure. Lizard . . . 2 |.-2 6 10 
an 91 ‘+| Clarke Alligator . 5 
792 ~+| Herrick . Snake . 5-7 
9 "15 =| Goette Bombinator . 2 
% | 786,793 | Kupffer . Salamandra . 5 3 2 
< 790 =| McClure. Amblystoma 2 2 5 9 
a 791 | Waters . sf : 3 2 5 10 
| 792,’928 | Froriep . Salamandra . + 
S| 7°92, ’92* | Froriep . - “batryngueg & 5 
< 792 +| Wiedersheim . Salamandrina 2 2 2 a 
"75 | Dohrn Teleostei . 8-9 
784,’85 | Kupffer . ae sf: 3 5 
’°87-—«| Scott . Petromyzon . 3 5 
a Sonal Platte = Salmon 5 
= 791 | Waters . ee Codi. : 3 2 6 11 
L 791 | Zimmermann . Acanthias. 2 3 8 3 
= 793. -| Kupffer . Acipenser. 5 3 5 13 
94 | Kupffer . Ammocetes . 3 
94 | Locy . Squalus 3 2 6 11 
795 | Locy . 6 3 2 | 6 (8) | 14-15 


investigators upon neuromerism have been seldom, if ever, so far ex- 
tended as to determine the number of segments finally included in the 
head. The reason for this has been that the hindbrain neuromeres dis- 
appear before their relations to the posterior limit of the cranium can be 
determined. Table EI. gives the nerve relations in different Vertebrates, 


BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


KE oe DA Al * UB uewoig | 9, 
See a SR eA ITA ISP IS AN. aN eth en snjenbg A00T | &6, 
See SST |) a : Nod aN daiio1y | Ze, 
Me eT 3 IA | A axvug WI | Bb 
Xe sal IA » A | AI Til I * sBIyjUBOYV UUBUIdSMUWTZ | 16; 
S| Me heh ,) A | AI Ill I YOO UUBUOMUUIZ | 16, 
WRT ae: > A | AI Ill I ; * HqqByy uuBWOMUTZ | 16, 
XI ; * TEN. Ae aatae vhf * vuoyss|quy SIVA | 16, 
x fr » IA! A RT 1 re |i pe se ed) 1010) s19IVA | 16: 
EL ” ” IN | | OID 49U1 I 68. 
XI ” A "+ + MOWTRS Wd | 68 
Ne 95 A AY TIL I * BmMojss[quy ‘ aNIOOW | 6% 
X rf A IN = st I ‘+ paeZry * ANID | 68 
Xx + A AL Yi I YO * aANIOOW | 68 
{x- F A AI plvZvy uuvuyoy | 68. 
¥ 5 TE a ae VG hae at pav2ry ATO) BE 
fn et Ae eke | TEE II | 1 WUD yoouviog | 18, 
E IIA ! BI190¥ auBld 
Iey ILA A W00V'T yoounlog | FR 
} ¢ Z i it g z Tt ‘SNOISIAIQ] TVINUNNAG 40 ‘ON 
SY +, _—_—_—~_ 


‘SNOILVIAY AAUYAN GNV SLNAWOAS OLTVHdHONG — ‘Il ATIVE 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 153 


so far as they have been studied. In both tables it has been impossible 
to exclude much that is theoretical, and in view of this fact general 
conclusions are obviously dangerous. One important result, which 
should be borne in mind during the discussion of the evidence presented 
in this paper, is established, viz. the constancy, in all classes of Verte- 
brates, of five “hindbrain neuromeres ” (“ Falten” or “vrais replis”), 
and of their nerve relations. When siz have been counted, usually the 
Anlage of the cerebellum has been included with them, and when seven 
(see Hoffmann, 90), another fold behind the true fifth neuromere has 
been counted. There is consensus of opinion that from the third 
“hindbrain neuromere” (designated in Table II. as 5, and as V in my 
figures) the acustico-facialis nerve takes its origin. In counting hind- 
brain neuromeres, then, this may safely be used asa check. In regard 
to the presence of true neuromeres, comparable with those of the hind- 
brain, in the region of the encephalon anterior to the hindbrain, much 
is theoretical, and, as I believe, uncritical. Morphologists have natu- 
rally been more or less prejudiced in favor of the view that a serially 
homologous segmentation extends throughout head and trunk. This 
preconception has led to the search for resemblances at the risk of dis- 
regarding differences which obviously exist, and as a result structures in 
the encephalon which are morphologically incomparable with the myelo- 
meres have been homologized with them. Moreover, this has been done 
in utter disregard of their relations to the segments of the mesoderm. 

The study of neural segments and their relations to nerves and somites 
in embryos of Squalus acanthias has given me some facts bearing 
on the problem of cephalic segmentation, which are, so far as I know, 
new. The conclusion yhich I have reached is as follows. Jn S. acan- 
thias there exists in early stages a continuous primitive segmentation of" the 
nervous system serially homologous throughout head and trunk, — the 
“neuromeric”’ segmentation. In later stages there appears in the en- 
cephalon a secondary (in time) segmentation resulting in the so called 
vesicles, which are not serially homologous with the segments of the 
myelon, but give rise to an anterior cephalic tract, which is a region sw 
generis. 

In the following discussion I propose (1) to trace the development of 
neuromeres; (2) to compare the structure of the segments of the enceph- 
alon with those of the myelon; and (3) to note the relation of the 
neuromeres to the sensor and motor nerves, to the mesodermal somites, 
and to the visceral arches. I shall begin with the description of the 
first appearance of neural segmentation in the embryo. 


154 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


I. Locy’s ‘‘Neural Segments” or ‘‘Metameres.” 


a. MATERIAL. 


Much of my material was collected with a view to the study of the 
“neural segments” or “ metameres” described by Locy (94 and 795). 
In a preliminary paper, which appeared with numerous illustrations in the 
“Anatomischer Anzeiger,” 1894, Locy affirmed the discovery of “neural 
segments’ in embryos of Squalus acanthias* at stages preceding the 
formation of the medullary folds and “ before the mesoblast has, to any 
extent, become divided into somites.” He therefore believed that these 
“epiblastic segments must be independent of any formative influence of 
the segments of the mesoblast.” This discovery is interesting, and, if 
confirmed, one of most fundamental importance. I have therefore col- 
lected a large number of Squalus embryos in early stages of develop- 
ment, in order to confirm, if possible, Locy’s results. 

S. acanthias is abundant along the coast of Massachusetts in early 
summer, and the embryos are very easily obtained. My collecting was 
done at Rockport, Massachusetts, during the months of July and August, 
1894, 1895, and 1896, and the number of specimens obtained exceeds 
twenty-five hundred. Locy has well insisted on the necessity of abun- 
dant material in closely connected stages of development. 

The killing agents which I have used were (1) Davidoff’s corrosive 
sublimate-acetic ; (2) Kleinenberg’s picro-sulphuric (undiluted); and (3) 
a mixture of Kleinenberg’s picro-sulphuric (1 vol.), with $% chromic 
acid (3 vol.), especially recommended by Locy. 

In this material were more than two hundred and fifty embryos 
corresponding to Balfour’s stages C, D, and E. The specimen which 
shows Locy’s “ neural segments ” best was killed in Kleinenberg’s picro- 
sulphuric mixture (Plate 1, Figs. 1 and 2). I cannot recommend the 
mixture composed of picro-sulphuric and chromic acids, since specimens 
killed in it were not well preserved histologically. Davidoff’s corrosive 
sublimate-acetic seems to me the best for general purposes of all the 
killing agents I have used, and consequently most of my material has 
been so killed. For the special study of the development of the nerves 
and the fibre courses in the wall of the brain, I have used material killed 
with vom Rath’s fluid, followed by pyroligneous acid. This method I 
regard as most valuable, since with it nerve fibres are differentiated by 


1 Squalus acanthias (Linneus, 1748), synonymous with Acanthias vulgaris 
(Risso, 1826). 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 155 


the precipitation of osmium in the very earliest stages of development, 
and it has given me results which I have been able to obtain in no other 
way. For staining sections Kleinenberg’s hematoxylin has been used 
chiefly, while Heidenhain’s iron hematoxylin and Grenacher’s alcoholic 
borax carmine have both given excellent results. 


6. MetHop oF Stupy. 


In studying Locy’s “ neural segments” in Squalus embryos, reflected 
light was used, and in consequence low powers of the microscope were neces- 
sary. Ihave used constantly a small Zeiss stand in which the upper half 
of the stage and the superstructure revolve on the lower half of the stage, 
and his objective A and ocular I. My method of procedure has been, 
first, to make with the aid of a camera lucida an outline of the embryo 
cleared in clove oil and viewed as a transparent object. The irregularities 
of the edge of the neural plate may thus be represented accurately, 
and may serve as landmarks in the subsequent study of the specimen as 
an opaque object. After the outline drawing has been made, the speci- 
men is transferred to a watch glass filled with alcohol. Now the im- 
portant question is illumination. In order to bring out the delicate 
structures along the edges of the neural plate, oblique illumination should 
be used, since it brings into strong contrast the shadows and the high 
lights. The embryos should be rotated, so that light may be obtained 
successively from all directions and thus the chance of deception by 
false lights avoided. As the embryo is studied chiefly from the ventral 
side (for reasons given by Locy), careful manipulation with brush and 
needle is necessary in order to remove the yolk, which would otherwise 
obscure the edges of the neural folds. In studying these surface con- 
ditions, I have found a very faint hematoxylin stain and a black back- 
ground to be of advantage. 

In representing the specimen under observation, I have not had 
recourse to photography, but have made as faithful a representation 
as possible with pencil, seeking to preserve the relative values of light 
and shade. Since it is possible by careful illumination to increase the 
contrast of light and shade to a considerable amount, it is well to study 
the same embryo with different kinds of illumination. In this way it is 
possible to determine more satisfactorily what is permanent and what is 
not. The study of the segments is by no means easy, and the labor 
is considerable because it is necessary to study so many individuals. 
It is evident from a comparison of Locy’s photographic representations, 
given in his final paper (’95), with his drawings, that the latter are, 


156 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


probably for the sake of clearness, semi-diagrammatic in character. While 
his photographic reproductions show absolutely no segmentation in the 
early stages, his drawings on the contrary show in these same stages 
“segments” as clearly marked as those of later stages. Photography 
is obviously unsatisfactory as a means of reproducing these delicate 
structures, 

Before taking up the consideration of the evidence which I have obtained 
from my studies, it is well to give a brief review of Locy’s results. In 
his final paper (795) he qualified his statement that the segmentation is 
solely epiblastic, since he discovered in sections that it may be found 
in both mesoderm and ectoderm. He therefore concludes that the 
Segments seen in surface study are the remnants of a primitive metam- 
erism of the Vertebrate body. The more important points in Locy’s 
description may be briefly summarized as follows. The evidence of seg- 
mentation appears first in the non-axial part of the embryo, i. e. along 
the thickened blastodermic rim. The segments later extend along the 
lateral margin of the neural plate from the anterior unsegmented tip of 
the embryo backward into the non-axial part. The segments are most 
clearly seen in “ marginal bands along the neural plate,” though “in 
the trunk region the lines of division may be traced inward toward the 
median furrow. This is probably due to the appearance of the meso- 
dermic somites in that region.” The “marginal bands,” he thinks, 
“represent the dorsal nerve cord.”! “These segments, once estab- 
lished in this very early stage, may be traced onward in an unbroken 
continuity until they become the neuromeres of other observers, and 
sustain definite relations to the spinal and cranial nerves.” In the con- 
clusion of his preliminary paper Locy writes, “No one is likely to ques- 
tion but what the segmented condition I have described represents a 
survival ” (i.e. of an ancestral segmentation 7). My own observations 
on embryos of S, acanthias lead me to question in large part the accu- 
racy of Locy’s observations, as well as his interpretations. 


c. DeEscrIPpTION oF Locy’s “ NEuRAL SEGMENTS.” 


I shall now give an account of the conditions, as I have found them, 
in the head region of a shark embryo with 6 to 61 somites.? This 


* In his final paper Locy speaks of the “neural folds or ridges” as “ divided 
throughout their length into a series of segments.” 

* Icount the somites beginning with van Wijhe’s 7th somite (somite 7 of my 
figures), the myotome of which becomes the first segment of the lateral trunk 
musculature (van Wijhe). 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 57 


stage has been selected to begin with, because it gives the strongest 
evidence that I have seen of a segmented condition in the neural plate. 
I shall describe first two embryos which represent fairly well the con- 
ditions I have found at this stage. These two embryos are represented 
in Figures 1, 2, and 3, Plate L. The neural or “ medullary” plate is 
seen to be a spatula-like expansion of the anterior end of the embryo, 
raised somewhat above the blastodermic area. Figure 1 (Plate L) repre- 
sents an embryo viewed from the dorsal side ; the neural plate, it will be 
observed, is not perfectly flat, for its edges bend slightly ventrad and a 
shallow depression extends along its median portion. The chorda, lying 
in the axis just beneath the neural plate, causes a slight elevation of the 
floor of the groove in the median line. Anteriorly the chorda passes into 
the common tissue which later becomes differentiated into entoderm, 
mesoderm, and chorda. The anterior more expanded portion of the 
neural plate has been called the “cephalic plate.” At the posterior 
portion of this cephalic plate its lateral wing-like expansions undergo 
their greatest bending ventrad. The posterior or trunk portion of the 
neural plate extends back into the tail folds and along the embryonic 
rim. I do not wish to seem to imply by this statement that the tail 
folds and the embryonic rim become included in the neural tube, because, 
although in general I believe in the concrescence theory, I do not find in 
the continuity stated above any evidence of addition to the posterior 
part of the neural plate by a concrescence of the tail folds and the embry- 
onic rim. 

In the dorsal view of the embryo shown in Figure 1, Plate 1, little or 
no evidence is afforded in either cephalic or trunk regions in support of 
Locy’s contention that the edges of the neural plate are segmented. We 
see only that the edges of the plate are slightly and zrregularly lobed, 
and not in the true sense segmented. For the lobes on the opposite mar- 
gins of the plate do not correspond in number or position, neither do they 
show any definite relation to the mesodermal somites, as seen in the cleared 
specimen. 

Figure 2, Plate 1, shows the same embryo viewed from the ventral 
side, and gives the strongest evidence I have seen of Locy’s interpreta- 
tion of the condition of the neural plate. The “segments” appear much 
more marked in embryos of this stage when viewed from the ventral 
side, for reasons already stated by Locy, who has well insisted upon the 
importance of ventral views. There are several reasons, however, for re- 
garding the structures which appear along the edges of the neural plate 
as not true segments. These so called segments, even in the cephalic 


158 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


region, are not equally distinct, it being very difficult, if not impossible, 
to determine the boundaries of some of them. They also differ consider- 
ably in length and apparently without any regularity, a condition not 
easily reconciled with the interpretation of them as true segments. It 
would certainly be impossible even in this specimen to point out with 
certainty corresponding segments on opposite sides of the cephalic plate. 
In the trunk region of the same specimen no correspondence between 
somites and ‘‘neural segments” is seen. However, a faint lobing of 
the inner margin of the tail fold is seen on the right side of the embryo. 
Locy’s (95, p. 519, Fig. 29) description of a stage as close to this as 
any figured by him is as follows: “They [the segments] appear like.a 
row of beads running along the ventrally recurved margin, and extend 
with great distinctness the entire length of the embryo. Those in the 
trunk region are continuous with those of the head, and pass into the 
latter without any transition forms. There is, however, some individual 
variation in size of the neuromeres, and they are not absolutely sym- 
metrical on the right and left sides, but the significant thing is, [that] there 
is uniformly the same number on each side in a given region, such as the 
hindbrain, or the brain region as a whole. . . . There seems now to be 
a natural landmark separating the ‘cephalic plate’ from the rest of the 
embryo ; this is an abrupt downward bending in the medullary folds, 
which, as I have determined, lies just in front of the future origin of 
the vagus nerve. There are eleven metameres* in the lateral margins 
of the cephalic plate, including the ones embraced in this fold.” The 
accuracy of this conclusion I shall discuss in treating of the question of 
the limit of the cephalic plate (p. 162). I wish here only to call atten- 
tion to the fact that none of the reproductions of Locy’s photographs, 
with two possible exceptions (his Figs. 2 and 23), show a segmentation 
of the neural folds in either the trunk or the embryonic rim. 

If now we turn to Figure 3 (Plate 1), we find an embryo of about 
the same stage as that shown in Figures 1 and 2; at least, it has the 
same number of somites (6 to 63). The conditions are these. The “seg- 
ments” at the margin of the neural plate differ markedly in distinct- 
ness, and are irregular in size. In the region of the cephalic plate — 
the posterior boundary of which is marked by the arrow — the number 
of segments on the right and left sides is not the same. I was not 
able to assert this with so much confidence in regard to the embryo of 
Figures 1 and 2, since in that embryo the limits of the cephalic plate 
were less clearly defined. If the segments of the two sides of the neu- 


1 Italics mine. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 159 


ral plate in Figure 2 do not admit of a satisfactory comparison, neither 
is it possible, even with a prejudice in favor of finding uniform condi- 
tions, to state exactly which segments of Figure 3 correspond to those 
of Figure 2. 

An examination of many embryos (more than fifty) in this stage of 
development —at which, in agreement with Locy, I have found that 
the segments are more clearly marked than at any other stage — has 
served only to establish the opinion that there is no constancy in their 
number in different individuals, nor agreement in number or position 
upon the two sides of the plate of a single individual. After an ex- 
amination of a large number of embryos at this and closely related 
stages, I have been compelled to abandon my first opinion, which was 
based chiefly on the study of the embryos of Figures 1 and 2, and was 
favorable to Locy’s contention. In no case that I have seen do the 
segments appear symmetrical, and in no case have I been able to de- 
termine a definite relation with the somites. 


d. CoNTINUITY OF THE “ SEGMENTS.” 


My observations have of course not been confined to this most favor- 
able stage. While the evidence given above, based on the study of 
embryos at a stage when the segments are most plainly seen, appears 
to my mind satisfactory proof that true segments do not exist at this 
time, the study of embryos in both earlier and later stages shows that 
even these segments are only transitory structures. This evidence, 
though in a sense negative, is not without weight in the treatment of 
the question. It constitutes, it is true, neither proof nor disproof of the 
genuineness of the segments. It is, however, what we should expect, 
if we find the segments unlike in number and size on the two sides 
of the same embryo and in different embryos of the same stage. A 
want of continuity in successive stages is not, however, what we should 
expect if we were dealing with true segments. These structures would 
certainly have much less morphological value than is assigned to them 
by Locy, were they simply transitory and without definite relation to 
organs which appear in later stages. Locy believes that he has traced 
them “ up to the time when they form neuromeres,” but he by no means 
makes it clear how structures which appear “like beads” along the edges 
of the neural plate become transformed into ventral structures such as, 
according to his own account, the “‘neuromeres” are. “In the trunk 
region,” he says (’95, p. 516), “ the lines of division may be traced inwards 
toward the median furrow. This is probably due to the appearance of 


160 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


the mesodermic somites in that region.” In the head region, where 
somites do not similarly press upon the neural plate, it still remains for 
Locy to show how structures morphologically dorsal, as his “neural 
segments” are, become converted into structures morphologically 
ventral as well as dorsal, as are the “hindbrain neuromeres,” for 
example. 

My own evidence of their continuity in time is, as I have said, nega- 
tive. Figure 4, Plate 2, represents an early stage with three or four 
somites. One sees the “marginal bands” of which Locy has spoken, 
but only the faintest traces of segments are visible. On one side— 
the right —they are exceedingly irregular. At this stage the lateral 
edges of the neural plate are not flexed ventrally, and such segments as 
are to be seen at all show best from the dorsal side. A quite regular 
segmentation is seen on the left side of the cephalic plate, yet the seg- 
ments are by no means all of the same size or distinctness, nor do they 
equal in size the mesodermal segments. In the trunk region the lobes 
of the edge of the neural plate show no definite relation to the meso- 
dermal somites, the boundary between two somites coinciding in some 
cases with the depression between lobes, in others with the apices or 
with other parts of the lobes. I wish to call especial attention to the 
fact that here, as in the embryos shown on Plate 1, the segments are con- 
fined to the marginal bands, and therefore do not extend into the median 
plate. Here, again, there is a considerable discrepancy between Locy’s 
observations and my own. 

I have found it impossible to trace definite segments into the later 
stages, for in these stages, before the closure of the neural tube, in the 
majority of specimens little or no evidence of segments along the cephalic 
plate can be seen. 

Two embryos in later stages of development are seen in Figures 5 and 
6, Plate 2. There is practically no evidence of segmentation or lobing 
of the edge of the medullary folds. The segments which Locy has 
numbered 1, 2, and 3 are visible in many specimens, in some very dis- 
tinctly, as shown in his photographs; but behind them there is an 
irregularly sinuous or entirely smooth edge, as shown in my Figures 5 
and 6, and in Locy’s photographic reproductions. These three anterior 
segments, according to Locy, shift their position. Since, however, 1 
do not find them constant in appearance and position, I have not been 
able to regard them as of morphological importance. It is worthy of 
note that they appear in the region of the neuropore, and that possibly 
they may be partly accounted for as the result of the difficulty of fusion 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 161 


of the neural folds at this point (the angulus terminalis). Their late 
appearance is possibly also to be correlated with the late appearance of 
the anterior portion of the neural crest. 

The evidence which I have given leads me to conclude that the so 
called neural segments cannot be traced into the “neuromeres” of 
later stages. Of the accuracy with which Locy has traced them I shall 
have more to say, when I speak of the limits of the cephalic plate. 


é. INTERPRETATION OF THE EVIDENCE. 


Locy interprets the “neural segments,” as has already been stated, 
as “survivals of a primitively segmented condition of the body.” In 
search for evidence to support this phylogenetic interpretation, he has 
studied the early stages of the Torpedo, Amphibia is, and the chick. 

Torpedo embryos (p. 531) are found to be “not so favorable for the 
study of the segments as Acanthias,” yet “the number [of folds] in a 
given region in Torpedo corresponds to that in Acanthias.” In the three 
Amphibian forms which Locy has studied (Amblystoma, Diemyctylus, and 
Rana) “there are about ten pairs of segments in the broadly expanded 
neural folds of the head.” In the chick, “‘there are eleven segments 
in front of the first formed protovertebrze.” Locy has also found (p. 539) 
that in the chick “ the walls of the primitive groove are also divided into 
segments that are similar to those that appear in the neural folds.” } 

Owing to the evidence stated above, I am unable to regard the seg- 
ments in S. acanthias as of phylogenetic value. Are they then arti- 
facts, as suggested by Eycleshymer?? I do not think so. Several of 
the best fixing agents have been used by Locy and myself, and he has 
in addition observed these structures in living embryos. It is known, 
however, that different fixing agents cause differences in internal and 
external conditions, as the result of swelling or contraction. They may 
have served to intensify the distinctness of Locy’s segments, yet it is 
hardly probable that they are the sole cause of them. 

I believe that the segments are the results of unequal growth 
along the margin of the neural plate. It is obviously not necessary to 

1 Italics my own. 

2 Eycleshymer’s (95, p. 394) observations on Amblystoma do not agree with 
those of Locy. Eycleshymer states that “certain markings which might be in- 
terpreted as neuromeres are often observed in the neural folds, yet their arrange- 
ment is decidedly irregular, and one is led to believe that they indicate nothing 
more than artifacts caused by the killing agents.” I have carefully examined 
Amblystoma embryos, at a stage when the neural folds are widely open, and my 


observations agree with those of Eycleshymer. 
VOL. XXXI. — NO. 7. 2 


162 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


regard such irregularities of the edge of a rapidly expanding plate of 
tissue as of morphological importance. It is very significant that the 
segments show most prominently in the cephalic-plate region just before 
the edges of the plate begin to rise dorsally, for it is likewise at this 
stage that I find the first evidence of the disassociation of cells along 
the edges of the neural crest. Such a disassociation of cells, or even a 
rapid proliferation of cells, — which certainly does occur in this region, 
— would lead to such phenomena as those reproduced in Figures 1 and 
2, Plate 1. An examination of cross sections of the cephalic plate 
(Plate 7, Figs. 55 and 56) before the edges have fused dorsally to form 
a closed tube shows that the neural crest is already differentiated from 
the tissue which will form the walls of the neural tube; it is differen- 
tiated as a region of rapid cell proliferation and of less compactly 
arranged nuclei. If the centres of cell proliferation were fixed, then 
we should have a segmented neural ridge, as affirmed by Beard (88). 

My interpretation differs from Locy’s, since he finds the “neural 
ridges” segmented regularly, and considers the segments as sur- 
vivals of an ancestral segmentation; whereas I find the edges of the 
neural plate irregularly and somewhat transitorily segmented, the 
irregularity and inconstancy of the segments precluding, in my opinion, 
a phylogenetic interpretation. Locy’s results from surface studies seem 
to me to be a confirmation of those reached by Beard (88), who, in 
studying the development of the peripheral nervous system in Selachii, 
found from the examination of sections that the neural crest is differen- 
tiated before somites appear, and that it is from the beginning segmented. 
Beard’s conclusions have, however, never been confirmed, and have in- 
deed been regarded by Dohrn (’90, p. 55) as quite untenable. 

To demonstrate that Locy has not accurately traced the “ neural seg- 
ments ” onward in unbroken continuity until they become the “ neuro- 
meres of other observers,’ I propose to discuss the relation of the 
neuromeres to the posterior limit of the cephalic plate. 


f. Lit or “Crpnaric PLate.” 


Locy (’95, p. 543) has stated that in early stages of the embryo, before 
the neural plate has formed a closed tube, head and trunk may be distin- 
guished. “It is possible,” he says, “‘in very young stages to draw a line 
indicating where the expanded part of the cephalic plate joins the non- 
expanded part of the embryo. . . . This is, in Squalus acanthias, just in 
front of the point where, subsequently, the vagus nerve begins. . . . In 
this animal, we may identify that part of the head which lies in front of 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 163 


the vagus nerve by counting the first eleven neural segments. It will 
be merely a question of agreeing upon the number of primitive segments 
belonging to the vagus, to enable us to locate with definiteness the 
hindermost limit of the head. Besides being of use in other ways, this 
would enable us to say, even in the earliest stages, what is head meso- 
blast and what is trunk mesoblast.” + 

I cannot see that Locy’s determination of the limits of the cephalic 
plate helps us at all in the determination of the boundary of head and 
trunk. This boundary, as he states, has still to be determined. To fix 
the limits of head-mesoderm by a direct study of the mesoderm itself is 
quite as easy as to determine its boundary by the still hypothetical pos- 
terior boundary of the vagus region. According to Locy, the posterior 
limit of the cephalic plate separates neither what is pre-otic from what 
is post-otic, nor head from trunk. 

My own observations on this point differ fundamentally from those of 
Locy, since according to my determination the line which separates the 
expanded cephalic plate from the region posterior to it marks the pos- 
terior boundary of the auditory invagination. This is of value, in so far 
as it enables us to distinguish those two regions— which on other 
grounds have always been held to be distinct —in stages earlier than 
was formerly possible. The posterior boundary of the cephalic plate is 
a clearly marked point at a stage before the neural folds begin to be 
raised dorsally, and it is situated just behind the region of greatest 
ventral flexure of the cephalic plate (marked by an arrow in Fig. 3, 
Plate 1). This point may be traced into later stages, until the neural 
plate is transformed into a closed tube, when it is seen that it corre- 
sponds exactly with the hinder boundary of the hindbrain neuromere 
numbered VI in my figures (Locy’s 10th “neural segment”) ; opposite 
this neuromere, as has been stated by many observers, lies in early 
stages the centre of the auditory invagination. The thickened auditory 
epithelium extends anterior and posterior to this neuromere ; but it is 
opposite this neuromere that the first invagination to form an enclosed 
capsule takes place (see Plate 3, Figs. 15 and 16). In later stages the 
ear capsule shifts backward, so that its centre comes to lie opposite the 
hindbrain neuromere numbered VII in my figures, which, as may be in- 
ferred from the statement above, lies in—or rather is afterwards differ- 
entiated from — the region behind the cephalic plate. I have been able 
to determine with certainty that the posterior limit of the cephalic plate 


1 Locy finds that in later stages segments are added to the occipital region from 
the region of the trunk (see Tables I. and IL.). 


164 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


is a fixed one from a stage with seven somites, until the plate no longer 
exists as such. That it corresponds with the posterior boundary of neu- 
romere VI of my figures, I am able to state with equal positiveness. 
Not having found, as Locy has done, eleven segments in the cephalic 
plate, by counting which one could determine the limits of the plate, I 
have been obliged to resort to other means. My method of determina- 
tion has been as follows. As a fixed point in all the,stages examined, I 
have taken the mesodermal somite marked 7 in Plates L to 4. This, as 
I determine, is the most anterior somite which becomes innervated by a 
ventral spinal root ; it therefore corresponds, I believe, with van Wijhe’s 
7th somite.! Anterior to this is formed a somite (van Wijhe’s 6th), which 
in early stages possesses embryonic muscle fibres, but never becomes 
innervated by a motor root. Rabl (’92) said he could affirm with confi- 
dence that the somite (Urwirbel) which van Wijhe holds for the 6th or 
7th head segment in an embryo with 48 somites is identical with that 
which he counts as the first trunk segment in an embryo of 76 
somites. This mistake [?] of van Wijhe’s, the accuracy of whose 
work in general is so well known, has led me to take especial pains to 
verify the identity of somite 7 in the stages most carefully examined, 
viz. from the stage with 6 to 7 somites, until after the neural tube is closed. 
Its identity has been determined as follows. I have carefully measured 
the distance from the constriction between van Wijhe’s 2d and 3d 
somites-—the mesodermic constriction which appears above the hyo- 
mandibular cleft —to the partial constriction anterior to van Wijhe’s 
6th somite. This distance measured in over two hundred embryos by 
means of camera-projection images, I have found to be practically con- 
stant, since it increases only very slightly as the embryo increases in 
length. Having thus determined the identity of this somite in suc- 
cessive stages, I have had a safe starting point for the determination of 
the posterior limit of the cephalic plate. I have measured the distance 
from the posterior cleft of van Wijhe’s 7th somite, in the manner de- 
scribed above, to the posterior boundary of the widely expanded cephalic 
plate, and I have found this distance also to be constant. I chose to 
measure from the posterior boundary of van Wijhe’s 7th somite, because 
by the measurement of this rather than a less distance the chances of 
error were diminished. The reader can verify the constancy of this dis- 
tance by measuring the Figures (3 to 10) on Plates 1, 2, and 3, which 
were drawn with the aid of a camera, and are magnified forty-three 
diameters. This distance is almost precisely the same as the distance from 


1 Van Wijhe’s le occipital Somit. Rabl’s 3¢ distale Urwirbel. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 165 


the posterior boundary of somite 7 to the posterior boundary of neuromere 
VI, after the closure of the neural tube (see Figs. 7 and 10, Plate 3, and 
Fig. 6, Plate 2), and, as previously stated, the posterior boundary of the 
auditory invagination at first coincides with the posterior boundary of 
encephalomere VI. Again, and in direct confirmation of the evidence 
stated above, the posterior boundary of encephalomere VI is the pos- 
terior boundary of a greatly enlarged portion of the neural tube (Figs. 
7-10, Plate-3), as one would naturally expect, if it coincides with the 
posterior boundary of the previously widely expanded cephalic plate. 
With this fact in mind it is interesting to compare the conditions I have 
found with Locy’s results. I believe he would not contest the assertion 
that my encephalomere VI is identical with his neuromere 10 (Zimmer- 
mann’s encephalomere 11), because its relation to the ear vesicle at the 
time this is formed makes its identification a simple matter. Locy (’95, 
p- 522) says of the auditory vesicle: ‘ When first established its centre 
occupies the space of the segment marked 10. Sometimes, in its earliest 
stages, the circular area spreads over the space of the three segments 
marked 9, 10, and 11, but I should say from my observations that, more 
frequently, it is not so widely expanded. It always settles down in 
Squalus acanthias to occupy the position first indicated, and subsequently 
it is shifted backwards.” This accords with my identification of his seg- 
ment 10 with my encephalomere VI, and this conclusion is corroborated 
by his statement that ‘‘the segment marked 8 is seated above a depressed 
region in which the first visceral cleft appears,” for that is precisely the 
position of the encephalomere IV of my figures. On page 528, however, 
he says, “ When the ear vesicle first arises it makes its appearance 
opposite the ninth neuromere”’ (!). Again, in his Figures 6 and 9, Plate 
XXIX., “neural segments,” which are described (p. 528) as 8 and 9, but 
which I believe to be segments 9 and 10 (as a comparison with my 
Fig. 46, Plate 7, shows), are numbered 7 and 8(!). Here, then, are 
three conflicts. Despite the elusive nature of Locy’s “neural segments,” 
I am disposed to regard his neural segment 10 (opposite which, as he 
has twice stated, the auditory invagination occurs) as identical in posi- 
tion with encephalomere VI of my figures. If this be true, there is no 
room on the cephalic plate for his neural segment marked 11, since, 
according to my determination, encephalomere VII is differentiated from 
the region of the neural tube which lies behind the broad cephalic 
plate, and does not become clearly marked off from the spinal cord 
region before a considerably later stage (stage H of Balfour). There- 
fore, if Locy’s neural segment 11 is identical in position with my en- 


166 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


cephalomere VII, I see no escape from the conclusion that he has not 
“traced neural segments accurately up to the time they form neuro- 
meres.” It is hardly conceivable that he will bring forward in this 
instance the explanation previously offered in a similar case of mistaken 
identity, that somehow, between the stage with an open neural plate and 
a closed tube, segment 10 (neuromere VI) has tnstdiously come to as- 
sume the position previously occupied by his segment 11, and that seg- 
ment 11 has been crowded backward. And it is likewise improbable 
that he would follow this explanation with another, —as he did in the 
case mentioned, — that encephalomere VI of my figures represents the 
“combined vesicle” of his segments marked 10 and 11.? 

I now turn to the study of what I regard as the true primitive 
segmentation of the nervous system,—the so called neuromeric seg- 
mentation. 


Il. The ‘‘Hindbrain Neuromeres” in S. acanthias. 


a. DEFINITION OF THE TERM “‘ NEUROMERE.” 


In the preceding description the term “ neural segment,” or simply 
“segment,” has been used as a non-committal term for structures of 
such different morphological value as those described by Locy under that 
name and the regular foldings of the neural tube. Locy (’95) has used 
the term “metamere” as synonymous apparently with his term ‘neural 
segment.” Since, however, the term “ metamere” is applicable by usage 
only to the successive similar parts of the body as a whole, it cannot be 
applied wisely to the successive parts of a single organ system, such as 
the nervous system. 

Ahlborn (’84*) was the first to use the term “neuromere,” and he 
applied it to all the successive similar segments of the central nervous 
system. Béraneck (’84) applied the term “replis medullaires” and 
Kupffer (’86) the term “ Medullarfalten ” to the regular foldings seen in 
the brain region of Vertebrate embryos, those of the hindbrain being 
given by Béraneck the special appellation of “ vrais replis.” Since the 


1 In a paper which comes to hand just as this goes to press, Locy (’97) states 
that he finds two sets of vesicles in the brain of chick embryos. Of these the 
first set, numbering seven in all, called by Locy “ optic vesicles,” are very ephemeral 
in existence, and have nothing whatsoever to do with the second set, called by him 
“brain vesicles.” In Acanthias (Squalus) also he finds at least nine pairs of “ optic 
vesicles,” likewise very transitory. The exact relation of these to the “meta 
meres ” or “ neuromeres ” he does not state. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 167 


English equivalent of Kupffer’s ‘ Medullarfalten” and of Béraneck’s 
“replis medullaires ” (medullary folds) is used with an entirely different 
meaning from that intended by these writers, Orr (’87, p. 335) employed 
the term “ neuromere ” for the folds due to symmetrical constrictions seen 
in the hindbrain and the thalamencephalon, and distinctly stated that 
in Lizard embryos no neuromeres are found behind the vagus nerve. 
This limitation of Ahlborn’s term has not, however, been accepted by 
later investigators. McClure (’89 and ’90) again extended the term 
neuromere so as to include all the constrictions and dilatations of the 
neural tube, and classified neuromeres into: (1) myelomeres, due to con- 
strictions of the myelon; (2) encephalomeres, resulting from constric- 
tions of the encephalon. The latter term had, however, been previously 
used by Wilder (’89) for the large encephalic vesicles. Zimmermann 
(91) adopted the term encephalomere, although he did not attempt to 
compare “ Encephalomeren ” with “ Myelomeren,” and Froriep (94) used 
the term for theoretically homodynamous segments of the neural tube in 
the region of the head. He stated that the encephalomeres may corre- 
spond with neuromeres, but that this correspondence is not self-evident. 

I shall adopt the nomenclature proposed by McClure (’89 and ’90). 
In my account of the segmentation of the brain I shall begin with the 
conspicuous constrictions and enlargements of the hindbrain, which have 
uniformly been regarded by morphologists as typical neuromeres or 
encephalomeres. Orr’s (’87) criteria for neuromeres, based on the study 
of the hindbrain of Lizard embryos, are as follows: (1) “ Each neuro- 
mere is separated from its neighbors by an external dorso-ventral con- 
striction, and opposite this an internal sharp dorso-ventral ridge, so that 
each neuromere (i. e. one lateral half of each) appears as a small are of 
a circle.” (2) ‘The constrictions are exactly opposite on each side of 
the brain.” (3) ‘The elongated cells are placed radially to the inner 
curved surface of the neuromere.” (4) “The nuclei are generally 
nearer the outer surface, and approach the inner surface only toward the 
apex of the ridge.” (5) “On the line between the apex of the internal 
ridge and the pit of the external depression, the cells of adjoining neuro- 
meres are crowded together, though the cells of one neuromere do not 
extend into another neuromere.” Later investigations have served only 
to confirm this clear analysis of the structure of a neuromere. 


6. DEVELOPMENT OF HINDBRAIN NEUROMERES. 


Previous investigators have assumed that the hindbrain neuromeres 
possess the same characteristics at their first appearance that they do in 


168 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


later stages, whereas it will be shown in Squalus that this is not the 
case. 

The want of abundant material of early stages felt by investigators in 
most cases is not a hindrance in the case of Squalus, for the early stages 
are as easily obtained as the later ones. In the study of the develop- 
ment of neuromeres, I have made use, first, of specimens very lightly 
stained in hematoxylin and mounted 7m toto in balsam, and secondly 
of the usual cross, frontal, and sagittal sections. The series of embryos 
represented in Plate 3 is chiefly of value in showing the neuromeres in 
successive stages, and the relations of the masses of cells composing 
the neural crest, or ganglionic Anlagen (colored blue in the figures), as 
seen in cleared specimens. The neural tube is represented as seen in 
optical section, while the other structures of the right half of the embryo 
are projected upon the median plane. 

The earliest evidence of hindbrain neuromeres which I have found is 
seen in embryos of 14 or 15 somites in which the cephalic plate has not 
closed in the hindbrain region. In most embryos with that number of 
somites the plate is already closed, but in cases where it has not, neuro- 
meres IV, V, and VI are seen as thickenings of the lateral walls of the 
hindbrain before its closure. Usually closure takes place, as in the 
chick, first in the region of the so called trigeminus Anlage, and later in 
the region of neuromere V, the most anterior portion of the cephalic 
plate remaining open as the neuropore until considerably later stages. 
Figure 7, Plate 3, shows that in embryos of 14 to 16 somites (in the 
specimen figured, after the closure of the cephalic plate) four expansions 
of the neural tube in the hindbrain region are differentiated (neuromeres 
III, IV, V, VI). The hinder boundary of neuromere VI marks the for- 
mer posterior boundary of the cephalic plate. The figures show (and this 
is a point of considerable importance in considering the morphological 
value of neuromeres) that each neuromere corresponds to the region of 
a dorsal as well as a ventral expansion of the neural tube, and that the 
neuromeres are separated from one another by both dorsal and ventral 
constrictions, which are to be seen both in sagittal sections and in 
cleared specimens. 

Frontal sections at this stage give additional evidence concerning the 
structure of hindbrain neuromeres. A frontal section just below the 
axis of the neural tube is shown in Figure 22, Plate 5. The section 
shows that the cephalic plate is still open in the region of the forebrain. 
The dorsal portion of the mesoderm in the region of van Wijhe’s 2d and 
3d head somites (2 and 3) is cut on the right side only, the sections not 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 169 


being exactly frontal on account of the torsion of the embryo. The lat- 
eral walls of the neural tube are seen in the figure to be thickened in 
that region which lies just posterior to the constriction opposite van 
Wijhe’s 3d somite. A comparison of many frontal and sagittal sections 
leaves no doubt that this thickening lies in the region of neuromere IV. 
That expansion of the neural tube which lies between the 2d and 3d 
somites, and which is separated by an external constriction from neuro- 
mere IV behind and from the midbrain vesicle (encephalomere II of my 
figures) in front, is the most anterior of the primary expansions or 
encephalomeres of the hindbrain. It has been called by Zimmermann 
(91) “‘Hinterhirn.” This corresponds to the third expansion of the 
neural tube in the chick (Fig. 44, Plate 7), as may be determined by its 
relation to the acustico-facialis Anlage and the auditory invagination. 
Failure correctly to identify this vesicle in the chick led Miss Platt (’89) 
to call the second vesicle, viz. the primary midbrain, the hindbrain. 

At a later stage, when 17 to 18 somites are differentiated, a well 
marked local thickening in the posterior half of encephalomere III 
appears.’ A frontal section of an embryo at this stage, showing neuro- 
mere IV as a local thickening posterior to neuromere III, is seen in 
Figure 23, Plate 5. Encephalomere III is separated by a constriction 
from encephalomere II. At this stage, then, only four of the hindbrain 
neuromeres (III, IV, V, and VI) are differentiated, and the conditions 
remain the same when one more somite is formed. 

In a similar frontal section of an embryo with 19 somites, such as is 
represented in Figure 24, four symmetrical thickenings of the lateral 
walls of the hindbrain (III-VI) appear. Opposite neuromere V lie 
the cells of the Anlage of the acustico-facialis nerve (blue), and opposite 
neuromere VI the thickened auditory epithelium. Neuromere VII is 
not present at this stage, and it does not begin to be differentiated until 
after one or two more mesodermal somites are formed, when a faintly 
marked dorsal and ventral dilatation appears in the region of the neu- 
ral tube just behind neuromere VI (Fig. 9, Plate 3). The lateral walls 
of this neuromere never become so markedly thickened as the walls of 
the other neuromeres, nor does the neuromere show a constriction at its 
posterior border before the embryo reaches the condition of Balfour’s 
stage H, and then only a faintly discernible one. A cross section 


1 Such a secondary subdivision of encephalomere III. (‘‘ Hinterhirn”’) occurs in 
the chick as in S. acanthias. I regard the primary vesicle as of different morpho- 
logical value from that of its subdivisions, for reasons which will be made more 
apparent when the relations of the vesicles are studied. 


170 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


through neuromere IV, which serves to show how greatly thickened 
the lateral wall is at this stage, is shown in Figure 32, Plate 5. The 
dorsal wall of this neuromere is considerably thicker than that of the 
neuromeres anterior and posterior to it, possibly because few cells are 
proliferated from this neuromere to form the ganglionic Anlage or neu- 
ral crest. 

I pass now to a description of the hindbrain neuromeres (encephalo- 
meres) at a stage with 28 or 30 somites (Balfour’s stage H). Since at 
this stage the neuromeres are clearly differentiated, and the thinning and 
expansion of the roof of the hindbrain have progressed very little, this 
is a most favorable stage for the study of the structural and histological 
peculiarities of the hindbrain neuromeres Figure 13, Plate 3, repre- 
sents a cleared specimen at this stage, and Figure 25, Plate 5, a frontal 
section of the same. Opposite neuromere III (Fig. 25) lies part of the 
trigeminus Anlage ; opposite neuromere V lie the cells of the acustico- 
facialis Anlage ; and opposite neuromere VI lies the thickened auditory 
epithelium, which is just beginning to invaginate. The acustico-facialis 
Anlage always remains in relation with neuromere V, so that this serves 
as an excellent starting point in counting the neuromeres. In order to 
get a clear conception of the structure of the neuromeres, cross, frontal, 
and sagittal sections are necessary. The series represented in Figures 
36-38, Plate 6, are frontal sections taken at different levels (a, B, ; 
Fig. 40, Plate 6) in the medullary tube. Only the right wall of the 
medullary tube in the region of neuromeres IV and V is shown in de- 
tail. The first section (Fig. 36) is dorsal, in the region of the “ Deck- 
platte.” In this section it is seen that what Orr (87) has said for the 
Lizard (see page 167) is true for Squalus. The section reproduced 
in Figure 37, more ventral than Figure 36, shows that the conditions 
which obtain in the region of the lateral zones are somewhat different 
from those of the dorsal zone. Since no sharp internal ridge exists, each 
lateral half of a neuromere does not appear in section as an arc of a 
circle, but as a thickening of the wall of the medullary tube. The cells 
and nuclei are fewer in number and more crowded in the region of con- 
striction between neuromeres. Although there is no inner concavity at 
this level, the cells and nuclei (Fig. 37) show a radial arrangement sim- 
ilar to that shown in Figure 36. The ventral section (Fig. 38) differs 
in no essential respect from the dorsal one. I have chosen these two 
neuromeres (IV and YV) for description, since they with neuromere VI 


1 The head somites, likewise, appear at this stage most clearly differentiated. 
It is, in fact, the “acranial stage ” of the embryo. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. na 


show the characteristics stated above in the most marked way. Only 
a faint external constriction, without internal constriction or ridge, sep- 
arates neuromere VII from the region of the spinal cord. 

At a stage with fifty somites (Balfour’s stage K) the structure of the 
neuromeres is slightly but not materially changed. In Figure 17, Plate 
3, is represented an embryo of this stage, viewed as a transparent ob- 
ject. Figures 26-29, Plate 5, show four frontal sections of such an em- 
bryo, Figure 26 being the most dorsal, and Figure 29 the most ventral 
of the series. Figure 26 shows that the most dorsal portion of the 
Deckplatte has become very thin, being only one layer of cells thick. 
The constrictions and dilatations are only faintly shown, the nuclear 
arrangement being the same in the region of the constriction as in the 
region of dilatation. Figure 27, more ventral than Figure 26, though 
still in the region of the Deckplatte, shows the conditions, both nu- 
clear and cellular, to be almost precisely the same as in Figure 36, 
Plate 6. The internal ridges, or cusps, are sharp, and the cells in the 
region between the internal ridge and external constriction are closely 
crowded together. It is to be noted that the separation of the lateral 
walls of the hindbrain is least marked in the region of neuromere VI, 
opposite which the ear capsule lies (compare Fig. 17, Piate 3). Figure 
28 seems to show that the neural walls have become considerably 
thickened in the region of the lateral zones. There is no doubt that 
the lateral zones are absolutely and relatively thicker than at the stage 
last described, while the neuromeres have increased in length. It is to be 
observed that this thickening is accompanied by a change in the outline 
of the lumen of the tube, vertical grooves appearing in the place of the 
vertical ridges of the more dorsal sections, In the most ventral of the 
sections, Figure 29, the internal ridges appear again, though the con- 
cavity of the inner surface of each neuromere in the antero-posterior 
direction is only faintly indicated. 

During stage K, as the result of the great expansion and thinning of 
the Deckplatte in the region of the medulla oblongata, the neuromeres 
come to affect only the lateral zones. Locy (95, pp. 524 and 525) 
notes changes in the appearance of the “‘ neural segments” at this stage, 
the explanation of which he does not state with precision. His opinion 
seems to be, however, that a union of part of each of the original seg- 
ments with the segment lying just in front of it, accounts for this con- 
dition. An examination of the series of Figures 7 to 21 of my Plates 
3 and 4, and of the frontal sections of Plate 5, shows that no such 
fusion of neuromeres takes place, The constrictions and ridges between 


ive BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


neuromeres never shift their position, the only change being a gradual 
assumption, by each of the local thickenings, of an inner concavity in 
the region of the lateral zones. 

Frontal sections of an embryo 15 mm. long show that dorsally all 
traces of the neuromeres are lost. A frontal section in the region of 
the lateral zones from an embryo of this stage is represented in Figure 
30, Plate 5. A great separation of the lateral walls of the medulla is 
seen to have taken place in the region of neuromeres III, IV, and V. 
At this stage only do the neuromeres possess the characteristics de- 
scribed by Orr for the Lizard (see page 167). While the external con- 
strictions are only faintly shown, owing to the increase of the “ white 
substance’ on the sides of the medulla, the internal ridges and con- 
cavities are well marked. From this stage onward the neuromeres 
begin to disappear. In embryos of 40 mm. to 50 mm., neuromere VI, 
in relation with the facialis nerve, is the most clearly marked of the 
neuromeres. 

Before passing to an examination of the evidence of neuromeres in the 
trunk region, I wish to emphasize the fact that the hindbrain neuro- 
meres cannot be regarded as structures dependent upon the pressure of 
mesodermal somites. Being local thickenings of the lateral wall of the 
neural tube they are obviously inexplicable on such « simple mechanical 
basis. They are structural differentiations of the tube in regions where 
the mesoderm has not yet extended, — that is, in the dorsal and lateral 
portions of the tube, the mesoderm of the head being still ventral in re- 
lation to the neural tube. 


c. SUMMARY. 


In the preceding study of the hindbrain neuromeres in 8. acanthias, 
I have supplemented Orr’s criteria (applicable to later stages) by a de- 
scription of the structure of the neuromeres in Squalus in earlier stages 
of development, i. e. in embryos of 14-50 somites. The characteristics 
possessed by hindbrain neuromeres in these earlier stages may be sum- 
marized as follows. Each neuromere is separated from its neighbor by 
an external constriction, which passes entirely around the neural tube. 
There is dorsally and ventrally an internal ridge corresponding to this 
external constriction ; but the ridge vanishes in the region of the lateral 
zones, being replaced by an internal depression or groove. The nuclei 
of the lateral wall are, however, still arranged (Fig. 37) in a manner 
which approximates that of the region of the internal ridges, notwith- 
standing that the thickening of the lateral wall of the neuromere has 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 173 


actually obliterated all surface evidence of such a condition. Each hind- 
brain neuromere, therefore, consists of a lateral thickening and a dorsal 
and ventral dilatation of the wall of the neural tube. The constrictions 
are exactly opposite on the two sides of the brain. The elongated cells 
are placed radially to an imaginary point situated in the middle of the 
thickening of the wall opposite. The nuclei are generally nearer the 
outer than the inner surface, and approach the latter only in the region 
of the constriction between the neuromeres. In this region the cells 
are more crowded, but the cells of one neuromere do not extend into . 
the adjacent neuromeres. 

The hindbrain neuromeres, being structural differentiations of the 
walls of the neural tube, are not to be explained as the result of a 
simple mechanical process. The essential similarity of these serial 
groupings of nerve cells to the metameric ganglia of Annelids will, I 
believe, impress others as well as myself. A reconstruction of the neu- 
romeres as they appear in this typical condition is shown in Figure 40, 
Plate 6. 


III. The Neuromeres in the Trunk Region. 


a. DEVELOPMENT OF MYELOMERES. 


It might seem that a more natural sequence in the study of neuro- 
meres than the one here followed would be to pass from the simpler 
conditions which obtain in the trunk to the more complicated ones in 
the head region. Instead of this, I follow the historical sequence, hav- 
ing begun with the “ Krauselungen,” or foldings, first seen by observers 
in the region of the hindbrain, and now pass to the study of the con- 
ditions in the spinal cord. That ‘hindbrain neuromeres” could be 
compared with segments of the spinal cord was an afterthought on 
the part of embryologists, evidently born of the conception that the 
head has a segmentation comparable with that in the trunk. 

While the neural plate in the trunk region is still widely open, its 
dorsal surface exhibits cross furrows, which are proved by longitudinal 
sections to correspond with the interspaces, or clefts, between the meso- 
dermic somites. The number of the cross furrows exactly equals that or 
the interspaces, increasing in number as the constrictions between the 
somites do. They do not, however, extend to the edges of the neural 
plate, but are restricted to the region where the plate rests upon the 
somites. In these cross furrows we have the first indications of those 
structures which were called by McClure (’89) ‘myelomeres,” and were 


174 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


compared by him with the ‘“‘neuromeres” of the medulla? Such sym- 
metrical cross furrows on the widely expanded neural plate of embryos 
of Salamandra atra were described by Kupffer (86), and considered by 
him as remnants of a primitive segmentation. Since Kupffer believed 
that at this stage there was no trace of mesodermic somites, he regarded 
the segments as “ primary,” i. e. not formed secondarily in adaptation to 
the mesodermal segmentation. Both Froriep (’92) and Wiedersheim (’92) 
have, however, declared that mesodermal somites are present at the stage 
described by Kupffer, and that the segments could be explained as the 
passive result of the pressure of these somites. Locy (95) finds in the 
trunk region of embryos of S. acanthias with a widely expanded cephalic 
plate that the lines of division between his “ neural segments” may be 
traced inward toward the median furrow, probably as the result of the 
appearance of somites in that region. As stated on page 160, I have 
failed to find this exact correspondence between the neural segments 
of Locy and the somites. 

When the neural plate has closed to form the neural tube, the regions 
of elevation between the furrows become constrictions, which however 
affect only the ventral half of the tube, i. e. that portion against which 
the somites lie (see Fig. 41, Plate 6). Neither frontal sections nor 
cleared specimens give evidence of constrictions in the dorsal half of the 
tube. The constrictions in the ventral half of the tube are most clearly 
marked in the early stages, when the mesodermal somites are most rounded 
in form, and they disappear as this rounded form disappears.” 

Figure 39, Plate 6, represents a frontal section in the ventral half of 
the myelon of an embryo with 28-30 somites (Balfour’s stage H). The 
right half of the neural tube and of the mesoderm is shown. It is seen 
that the wall of the neural tube shows a rounded constriction opposite 
the somite, while opposite the cleft between two somites, and conform- 
ing with it, an outer ridge and an inner rather sharp groove are seen. 
This section affords evidence more favorable to the contention that 
‘‘neuromeres” exist in the spinal cord than that seen at any other stage 
of development, or in any other plane of sectioning. In dorsal sections 
of the same series the constrictions disappear, as do the somites also. 


1 Marshall (’78) had previously stated that in the chick “the cord is slightly 
constricted opposite the centres of the protovertebre, and slightly dilated opposite 
the intervals between successive protovertebre.” 

2 Miss Platt (’89) has said with regard to the chick, “ Here [in the trunk region] 
as in the medulla, the segmentation is more manifest in the ventral region than in 


the dorsal.” 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 175 


The structure of the myelomeres in embryos of 40-50 somites is 
represented in Figures 42 and 43, Plate 6. As in Figure 39, only the 
right half of the embryo is shown. The only evidence of the structural 
peculiarities of neuromeres at this stage consists in an external constric- 
tion opposite the myotome and the spinal ganglion (Fig. 43). In 
sections dorsal or ventral to the one shown in Figure 43, even this con- 
striction becomes lost (see Fig. 42, which is more ventral, occupying the 
region of the ventral roots). All traces of an internal dilatation and 
constriction, and of the concomitant radial arrangement of cells, have 
disappeared. In the head, on the contrary, the “neuromeres ” still pre- 
setve all the characteristics seen in the earlier stages. 

An examination of the structure of the myelomeres shows that the 
conditions are easily explicable on the mechanical grounds stated. There 
are no serial thickenings of the wall of the neural tube, as in the hind- 
brain, and the radial arrangement of cells and nuclei shown in the frontal 
sections (Fig. 39) presents no difficulty ; for the cells composing the 
epithelium of the neural tube always have their long axes perpendicular 
to the surface of the tube, so that, if the tube becomes constricted oppo- 
site each somite, the cells will necessarily show a radial arrangement in 
frontal sections. In view of this fact, it is difficult to understand how 
investigators should have thought that the existence of a radial arrange- 
ment of cells and nuclei was evidence sufficient to establish the morpho- 
logical value of myelomeres, and their serial homology with hindbrain 
neuromeres. McClure (’90), for example, says, “‘ The lateral walls of the 
spinal cord are divided into neuromeres which, while less conspicuous, 
have all the cellular characteristics seen in the typical neuromeres of the 
hindbrain, and in fact are a continuation of the latter.” That all of the 
cellular characteristics seen in the typical neuromeres of the hindbrain 
are also found in the myelomeres is demonstrably untrue for Squalus, as 
may be seen by comparing the sections shown in Figures 38 and 39, Plate 
G6, both from the ventral half of the neural tube of the same embryo, one 
in the head and the other in the trunk. The cellular arrangements are 
decidedly unlike. Inthe head (Fig. 38) the cells and nuclei are crowded 
in the region of constriction between neuromeres, while in the trunk, 
if the cells are crowded at all, it is in the region of dilatation of the 
myelomere. 

It has seemed a strong argument for the serial homology of myelo- 
meres and hindbrain neuromeres that the former continue into the latter 
gradually and in an unbroken series. For example, McClure (’90) 
stated that “the constrictions of the myelon (in Lizard embryos) gradu- 


176 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


ally pass or merge into those of the encephalon, thereby forming a con- 
tinuous series of constrictions throughout the entire length of the neuron, 
which increase in size anteriorly.” Also, in demonstrations of this con- 
tinuity, Miss Platt (’89) stated (for the chick) that “ the difference (in 
size) between the fifth neuromere [last neuromere of the medulla] and 
the next posterior fold is not as great as the difference between the sec- 
ond and third neuromeres.” (Compare Fig. 44, Plate 7.) Locy (’94 
and °95) says of his neural segments that “those in the trunk region are 
continuous with those of the head, and pass into the latter without any 
transition forms.” Zimmermann (’91), on the other hand, does not find 
the spinal cord in S. acanthias to be segmented. 

While I am able to confirm the evidence of continuity of encephalo- 
meres and myelomeres as stated by previous investigators, I am unwilling 
on this ground alone to regard these structures as of the same morpho- 
logical value. Moreover, it has been shown that the hindbrain neuro- 
meres and the myelomeres differ both in structure and in development. 


6. Summary. 


The evidence presented by the constrictions of the myelon warrants 
the inference that the existence of the myelomeres is dependent upon 
the presence of the somites, an explanation by no means possible for the 
hindbrain neuromeres. The constrictions of the myelon appear only 
after the somites are formed, and increase in number with the addition of 
new somites. They are opposite the somites, and are confined to that 
portion of the neural tube against which the somites lie, i. e. the ventral 
portion. They present no histological or structural conditions which are 
not easily reconcilable with the hypothesis of their mechanical formation. 
In those Vertebrates in which the somites extend farther dorsally with 
reference to the neural tube, the constrictions of the myelon also have a 
greater dorsal extent. As soon as the somites lose their rounded form 
and no longer lie close to the neural tube, the constrictions of the tube 
disappear. As a whole, the evidence in the spinal region of Squalus 
fully confirms the explanation given by Minot (92), viz. that the ap- 
pearance of the myelomeric constrictions “seems to depend upon the 
development of the primitive segments of the mesothelium. When the 
segments are fully formed, and before their inner wall has changed into 
mesenchymal tissue, they press against the medullary tube and oppose 
its enlargement; at least one sees that the tube becomes slightly con- 
stricted between each pair of segments and slightly enlarged opposite 
each intersegmental space.” Structurally, therefore, myelomeres and 


wees. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. AUT 


encephalomeres differ. While a mechanical explanation is possible for 
- the one, such is not possible for the other. They are, it is true, con- 
tinuous serial dilatations of the neural tube. The proof, however, that 
they are of equal morphological value, that is to say, serially homologous, 
rests, I believe, in the demonstration of a similar metameric relation to 
organs known to be segmental. The myelomeres correspond meta- 
merically with the somites, as has been stated. Do the encephalomeres 
likewise correspond with somites? Upon the answer to this question 
obviously depends largely the decision as to their metameric value. Be- 
fore stating the evidence bearing upon this question it is necessary to see 
if there is any evidence of neuromeres anterior to the hindbrain. 


IV. The Neuromeres anterior to the Hindbrain. 


a. ESSENTIAL CRITERIA OF NEUROMERES. 


I believe that those who find neuromeres in the brain region anterior 
to the hindbrain have assumed the presence of a homodynamous seg- 
mentation of the entire encephalon. Yet it must be admitted that even 
if a serially homologous segmentation extends from the spinal cord 
into the medulla oblongata, it by no means follows that such segmenta- 
tion also extends into the anterior brain region. Compare with the 
analogous case of the skull. Because the occipital.region is segmen- 
tal, i.e. composed of fused vertebree, it does not follow that the pre-otic 
region is. It is well, at least, to study the conditions in the anterior 
brain region with the mind as unprejudiced by any theory as possible. 
What criteria, then, warrant the conclusion that any given division of 
the neural tube is a neuromere? Certainly, no one criterion would 
be held to be sufficient. The best criteria are such as associate the 
supposed neuromeres metamerically with other structures known to 
be segmental, e.g. the mesodermic somites or the segmental nerves. 
But where such direct evidence is wanting, to say that a radial arrange- 
ment of cells and nuclei is evidence of a neuromere, and thus indirectly 
evidence of a metamere, is obviously dangerous, since the radial arrange- 
ment of the nuclei appears whenever the neural tube is constricted from 
any cause whatever. 

If, however, we have rudimentary somites in the head, may we not 
also have rudimentary neuromeres? McClure (’89) finds between the 
- midbrain and the optic vesicle of the Lizard a structure which resembles 
a portion of a neuromere, —a “half-neuromere.” He accepts the evi- 

VOL, XXXI.— NO. 7. 3 


178 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


dence of neuromeres in the primary forebrain also, although the arrange- 
ment of nuclei does not always conform to the typical condition. . 
Waters (91, p. 143) says: “In this area [that of the posterior com- 
missure| the Cod brain shows little or no segmentation, but from the 
fact that it nearly corresponds in extent to neuromere II, and that its 
existence is quite evident in Amblystoma, it seems probable that this 
space is occupied by the third and last of the forebrain neuromeres.” 
In other words, though none of the characteristics of a neuromere are 
present, it is a priori probable that a neuromere exists here ! 

Orr, Beraneck, and Miss Platt have regarded the midbrain vesicle as 
a single enlarged neuromere. It has an external constriction separating 
it from its neighbours, a corresponding internal ridge, an inner concavity, 
an outer convexity, a radial arrangement of cells and nuclei, and in 
addition is primary in time of appearance. On the other hand, Waters 
(92) says that it is an error to confound the neuromeric segmentation 
with the so called vesicular segmentation, since he finds in the midbrain 
region “two well marked convolutions of the brain wall,” and the 
characteristic radial arrangement of nuclei. Kupffer (’93*) believes 
that, since, with Froriep (92°) and Zimmermann (91), he finds evidence 
of three encephalomeres in the midbrain,? this confirmation gives a cer- 
tainty to their results. 

Surely the divergence in the results of other investigators has not 
proved that Orr, Béraneck, and Miss Platt were wrong in considering the 
primary midbrain as a single neuromere, especially since the midbrain 
and forebrain form parts of a continuous series of primary enlargements 
of the encephalon. The majority of investigators (Orr, Béraneck, 
McClure, Froriep, and Zimmermann) find that the forebrain consists of 
two neuromeres, without however giving a satisfactory explanation of 
its marked divergence, in the matter of secondary division, from the 
typical hindbrain dilatations. If we count dorsal expansions, as is done 
by Waters and others, we may find evidence of at least three neuromeres, 
which correspond, says Kupffer (93°), with his Grosshirn, Nebenhirn, 
and Schalthirn. Furthermore, if dorsal diverticula be regarded as evi- 
dence of neuromeres, we must agree with Kupffer that it is impossible 
to disregard the epiphyses and plexus formations.* On this basis 


1 Waters says (p. 465) that he thinks McClure is mistaken in assigning to the 
midbrain region, on purely speculative grounds, a third neuromere. 

2 Kupffer found these three secondary subdivisions of the midbrain in Cyclo- 
stomes, Zimmermann in Selachii, and Froriep in Mammalia. 

8 See Kupffer (’98%, p. 549). 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 179 


Kupffer finds at least five encephalomeres in the primary forebrain. 
This conclusion seems strengthened by the conclusion of Burckhardt (’93), 
that the median zones of the neural tube retain throughout the Verte- 
brate series the primitive segmentation best, and therefore are the best 
for comparison. 

My conclusions from a study of the evidence presented by those who 
have assumed a segmental value for the secondary subdivisions of the 
forebrain and midbrain vesicles are, (1) that morphologically different 
structures have been described by them as ‘‘neuromeres” or “ encepha- 
lomeres,” and (2) that the divergence in their results does not seem to 
justify this assumption. 

I now turn to an examination of the development of forebrain and 
midbrain regions in S. acanthias, in order to determine whether or not 
it is probable that structures morphologically comparable with hindbrain 
neuromeres exist in these regions. Since hindbrain neuromeres involve 
all three zones —dorsal, ventral, and lateral—of the walls of the 
encephalon, the value of forebrain and midbrain segments as morphologi- 
cal equivalents of them will clearly depend on their similarly involving 
those zones. If they do not, it is incumbent upon one who holds to 
their equivalency to demonstrate how modification has probably obscured 
or obliterated the primitive conditions. Evidence in such a highly 
specialized region can be at best only probable. Here, however, as 
always, the demonstration of morphological comparability must be 
“controlled” by the demonstration of similar relationships to other 
organ systems. 


6. DEVELOPMENT OF THE FOREBRAIN AND MIDBRAIN. 


At a stage with 19 or 20 somites the conditions in the anterior brain 
region are very simple. The primary forebrain and midbrain are simple 
vesicles or enlargements of the neural tube. A parasagittal section cut 
through the right wall of the neural tube is represented in Figure 45, 
Plate 7. Six vesicles are counted, all of them being included in the 
region of the cephalic plate. The anterior vesicle shown is the wall of 
the forebrain in the region of the optic vesicles. Behind lies the mid- 
brain, separated by a slight constriction from that region of the hind- 
brain to which Zimmermann (91) has given the name “ Hinterhirn.’’* 
Hindbrain neuromeres IV, V, and VI are clearly defined. 

A frontal section of an embryo of the same stage, so cut as to coincide 


1 The English term hindbrain has been applied to the region separated by 
the Germans into “Hinterhirn” and “ Nachhirn.” 


180 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


with the axis of the midbrain vesicle, is shown in Figure 49, Plate 7. 
The vesicle of the primary forebrain (I) appears as an almost circular 
enlargement of the anterior portion of the neural tube. Behind this, 
and separated from it by a constriction, lies the narrower and somewhat 
more elongated midbrain vesicle (II). Posteriorly a small portion of the 
‘“‘ Hinterhirn”’ vesicle also appears in the section. 

Sagittal sections of embryos at this stage are seen in Figures 8 and 9, 
Plate 3. Faint dorsal constrictions separate forebrain, midbrain, and 
“ Hinterhirn” (III), the separation between midbrain and “ Hinterhirn ” 
being very slight. A deep depression in the floor of the forebrain marks 
the position of the infundibulum, which is bounded posteriorly by a faint 
constriction, the first indication of the tuberculum posterius (Kupffer). 
Another constriction of the ventral wall of the neural tube is seen behind 
the tuberculum posterius in the region of the midbrain, — the plica en- 
cephali ventralis. In later stages the region of this constriction becomes 
the point of greatest fiexure of the neural tube. The constrictions 
marking off the brain vesicles appear as rather broad depressions, not 
sharply defined as are the constrictions between neuromeres. The brain 
vesicles are also seen to be considerably larger than the hindbrain 
neuromeres, the difference in size constantly increasing from this stage 
onwards. Except for a local thickening of the lateral zones, the two 
anterior brain vesicles are structurally quite comparable with the hind- 
brain neuromeres. They similarly involve all three zones of the neural 
tube. 

An examination of embryos at a stage with 28 to 30 somites, i.e. 
early in Balfour’s stage H, shows that slight changes have occurred. A 
parasagittal section of such an embryo is shown in Figure 46, Plate 7. 
The anterior vesicle, the forebrain, is so cut that one sees its lumen. 
Behind this, and separated from it by a constriction which extends to 
the ventral portion of the tube, lies the midbrain, which dorsally is a 
single expansion passing almost without constriction into the hindbrain. 
The depth of the constriction is much less than it appears to be in this 
figure, because the section passes to one side of the median plane. In 
the ventral half of the midbrain there is a constriction, which more 
median sections of this stage (Fig. 13, Plate 3) show to correspond with 
the region of sharpest flexure of the neural tube (plica encephali ven- 
tralis). This constriction does not extend, however, to the dorsal portion 
of the neural tube, and therefore is not equivalent to a constriction 
which separates neuromeres. By it the midbrain is separated ven- 
trally into two lateral expansions on each side of the head, — one 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 181 


anterior, the other posterior, — while dorsally it remains a single dilata- 
tion. The anterior of the two expansions narrows as it extends ventrally, 
and terminates at a point in the ventral wall near, but anterior to, the 
tuberculum posterius. The posterior of the two midbrain expansions is 
bounded behind by the faint lateral constriction between midbrain and 
hindbrain vesicles. 

The conditions shown in a sagittal section at this stage are not essen- 
tially different from those presented at the stage previously described 
(Fig. 13, Plate 3). The forebrain, midbrain, and “ Hinterhirn” vesi- 
cles are separated by very faint dorsal constrictions. In the constric- 
tion between forebrain and midbrain vesicles appears later Miss Platt’s 
‘¢thalamic nerve.” Ventrally two constrictions are seen, one corre- 
sponding with the tuberculum posterius, and the other, more posterior, 
with the point of greatest flexure of the neural tube. Two frontal sec- 
tions of an embryo at this stage are shown in Plate 7, Figures 48 and 
50. Figure 48 represents the more dorsal of the two, and shows only 
the expansion of forebrain and midbrain vesicles separated by the pri- 
mary constrictions spoken of above. A small portion of the “ Hinter- 
hirn” is shown. The section shown in Figure 50 is more ventral, being 
in a plane about midway between the dorsal and ventral sides of the 
neural tube. An arrow is drawn at the constriction separating forebrain 
and midbrain vesicles. This constriction corresponds with the one seen 
in the more dorsal section, also indicated by an arrow. Behind this, in 
the region of the midbrain, another constriction appears, one which was 
not seen in the dorsal section. This may be traced in more ventral 
sections into the constriction previously described as occupying the floor 
of the midbrain at a point corresponding with the point of greatest flex- 
ure of the neural tube. In my opinion everything in front of the arrow 
belongs to the primary forebrain, the lateral walls of which are expanded 
to form the optic vesicles. Behind these two vesicles are seen “ two well 
marked constrictions and two convolutions” of the neural wall which 
show radially arranged nuclei. It is found in later stages that the pos- 
terior of the two constrictions corresponds in position with the posterior 
commissure, and therefore that what lies anterior to this constriction must 
be considered as part of the thalamencephalon. It is seen, therefore, 
that the constriction between primary forebrain and midbrain vesicles 
does not correspond with the posterior commissure, which in later stages 
forms by common consent the anterior boundary of the midbrain (see 


1 Probably the two “neuromeres” of the thalamencephalon described by Orr 
(87). 


182 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Plate 7, Fig. 47, coms. p.). This constriction corresponds, instead, with 
a point just behind the epiphysis, and is separated from the posterior 
commissure by that portion of the brain which Kupffer has named 
Schalthirn, or diencephalon (the Schaltstiick of Burckhardt). Neither 
of the so called neuromeres (Orr) is in relation with a nerve, motor, or 
sensor, and neither possesses a dorsal expansion of its own. 

A parasagittal section of the next older stage represented is seen in 
Figure 47, Plate 7 (compare Fig. 19, Plate 4); it is of an embryo with 
65 somites (Balfour’s stage K), and the changes in the anterior brain 
region are seen to be considerable. In the dorsal portion of the region 
called primary forebrain, i. e. the region anterior to the constriction in 
which the “thalamic nerve” (¢hl., Fig. 18, Plate &) lies, two expansions 
now appear. These are median, unpaired, and separated from each 
other by a constriction which extends toward, but does not reach, the 
optic stalk. The anterior expansion is the prosencephalon (Grosshirn, 
epencephalon of Kupffer), which involves, as determined by His (88°), 
the “ Deckplatte” and both “ Fliigelplatten.” The second, which at this 
stage is a simple expansion, later becomes differentiated into “ Zirbel- 
polster ” (Kupffer’s parencephalon, Nebenhirn) and the epiphysis. The 
latter, according to His, is derived from the “ Deckplatte” only. The 
primary constriction between forebrain and midbrain is marked in Fig- 
ure 47 by the dorso-ventral line, behind the second expansion. The 
midbrain now shows three lateral expansions. The anterior is bounded 
in front by the primary constriction between forebrain and midbrain, and 
behind, as in the previous stage, by the ventral (and now lateral) con- 
striction which extends dorsad toward the posterior commissure from a 
point just in front of the chief root of the oculomotor nerve. The sec- 
ond dilatation has as its posterior boundary a ventral constriction which 
I do not consider of morphological importance, because it simply cor- 
responds with a point of flexure of the ventral wall of the tube, never 
extends to a dorsal position, and has no corresponding inner ridge. The 
constriction exists, however, at this stage, and forms the posterior boun- 
dary of a neural segment related to the oculomotor nerve. Behind this 
lies a third expansion, faintly marked anteriorly and also posteriorly, 
where it merges into the isthmus. In later stages the trochlear nerve 
arises from the region of the posterior constriction of this expansion ; it 
is the chiasma of fibres of this nerve which defines the posterior con- 
striction of the midbrain vesicle. In this stage, as in the preceding, the 
midbrain vesicle remains dorsally a simple expansion, the constrictions 
affecting only its lateral and ventral walls. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 183 


Two frontal sections of an embryo at this stage are seen in Figures 51 
and 52, Plate '7. Anteriorly in the more dorsal section (Fig. 51) is seen 
the expansion of the prosencephalon. Behind this lies an expansion 
which might be considered as a neuromere, if a radial arrangement of 
nuclei and a constriction of the brain wall were alone considered suffi- 
cient criteria for such a structure. Since, however, it is simply a dorsal 
expansion, which is unrelated to nerves, and soon becomes differentiated 
into adult organs, I am unable to regard it asa neuromere. From it are 
differentiated “ Zirbelpolster ” (parencephalon, Nebenhirn, or Zwischen- 
hirnblase) and epiphysis. Posterior to the constriction marked by 
the arrow, which corresponds with the point so marked in Figure 50, 
is situated a long expanded portion of the encephalon which passes with- 
out constriction into the midbrain vesicle. In the more ventral section 
(Fig. 52), however, there is seen in this region a constriction which may 
be traced ventrally to that point from which the anterior root of the 
oculomotor arises. Two neuromere-like expansions, separated by the 
constriction between primary forebrain and midbrain, are seen in this 
stage as in the previous stage described. 

Passing now to a much later stage (21-22 mm.), we find (Plate 4, 
Fig. 21) that the posterior commissure has come to lie much nearer the 
base of the stalk of the epiphysis, and thus that the portion of the dor- 
sal wall which is called by Kupffer diencephalon has become much re- 
duced in the region of the midbrain vesicle. Thus it has come about 
that frontal sections in a plane midway between the dorsal and the ven- 
tral walls of the neural tube (Fig. 53, Plate 7) show only a single neuro- 
mere-like expansion. In more dorsal as well as more ventral sections 
this undergoes constriction, so that it is by no means a simple neuro- 
meric enlargement. A median sagittal section, such as that shown in 
Figure 21 (Plate 4), is the most satisfactory for the study of segmenta- 
tion at this stage. The primary forebrain is now differentiated into the 
successive dorsal dilatations epencephalon, paraphysis (parencephalon), 
and epiphysis. Dorsally the midbrain still continues to be a simple 
expansion, while ventrally traces of the three segments still remain, 
the anterior one having become much reduced in length. 

With the exception of Locy, Zimmermann (’91) is the only investigator 
who has studied the “neuromeres” in Selachii. For the purpose of 
comparison, it is well to state his results here. He finds at first edght 
“primaére Abschnitte” in the encephalon, the first three of which 
exceed in size the last five. The first three are the Vorderhirn, Mittel- 
hirn, and Hinterhirn, each of which he regards as a complex of en- 


184 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


cephalomeres, since they later subdivide into segments which dorsally 
are equally long and broad. The Vorderhirn divides into two encepha- 
lomeres, the Mittelhirn into three, and the Hinterhirn into three. Thus, 
since the posterior five “‘primare Abschnitte” do not further subdivide 
there are in all thirteen “encephalomeres.” As a result of cephalic 
flexure some of the encephalomeres become wedge-shaped, but all are 
clearly separated from one another by constrictions. Zimmermann’s 
paper was a preliminary one without figures, and it has not as yet been 
followed by a final paper. 

It is seen that Zimmermann’s account, based on the study of S. acan- 
thias embryos, differs somewhat from my own. At the closure of the 
neural tube I find six vesicles or expansions of the encephalon. The first 
three correspond with those called by Zimmermann Vorderhirn, Mittel- 
hirn, and Hinterhirn; the last three are hindbrain neuromeres IV, V, 
and VI. Since Zimmermann’s 7th and 8th “ primaire Abschnitte ” are 
not differentiated at this stage, Iam unable to accept his conclusion that 
there are at first eight primary ‘‘ encephalomeres ” or “Abschnitte.” The 
primary forebrain subdivides into the two dorsal expansions which Zim- 
mermann calls “‘Secundire Vorderhirn” and “ Zwischenhirn.” But, if 
these are “encephalomeres,” I am unable to see how later differentia- 
tions, such as the prosencephalon (epencephalon), paraphysis (paren- 
cephalon), and epiphysis can be excluded from the same category. 
May we not have tertiary as well secondary “encephalomeres”? Iam 
unable to accept Zimmermann’s single criterion of size as sufficient to 
enable us to make a distinction between those segments which are prim- 
itive, i. e. remnants of ancestral structures, and those which are the early 
beginning of adult organs. A most serious objection to regarding such 
structures as Zimmermann’s “‘ Secundire Vorderhirn ” and “ Zwischen- 
hirn ” morphologically comparable with neuromeres or myelomeres has 
been stated by Herrick (’92), and consists in the difficulty of homol- 
ogizing dorsal expansions with ventral ones. 

The primary midbrain, as stated by Zimmermann, subdivides into 
three segments, the most anterior of which lies in front of the posterior 
commissure and in front of the place of origin of the oculomotor nerve. 
In all stages the midbrain is seen in median sagittal sections to present 
a simple dorsal expansion, its constrictions affecting its ventral and 
lateral walls only. 

The third vesicle, Zimmermann’s Hinterhirn, which he says subdivides 
into three “ Encephalomeren,” I find to become differentiated into the 
cerebellum Anlage and a posterior enlargement or thickening, but nothing 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 185 


more. The only evidence which I find of Zimmermann’s anterior “ Hin- 
terhirn Encephalomer” consists of a flexure of the median ventral wall 
appearing in late stages in the anterior portion of the Hinterhirn. Since 
no dorsal or lateral constriction corresponds with this, and since there- 
fore it cannot be regarded as a vesiculation of the neural tube, I do not 
consider it as of morphological importance, but explicable simply as a 
passive result of the flexure of the neural tube. 

Locy (95, p. 542) finds five “neural segments” in the forebrain and 
midbrain, — three in the former and two in the latter. He clearly figures 
aud mentions in the description of plates, however, the three secondary 
midbrain expansions described by Zimmermann and myself. 


c. SUMMARY. 


An examination of the literature bearing on the question of neuro- 
meres in the region anterior to the hindbrain had led me to the conclusion 
that structures of different morphological value had been described as neu- 
romeres, and the examination of the secondary subdivisions of the fore- 
brain and midbrain of embryos of S. acanthias has served to strengthen 
this opinion. These subdivisions have been shown to differ from the 
typical neuromeres in shape, in structure, and in rglation to the dorsal 
and ventral zones of the neural tube. The attempt to establish a serial 
homology on the basis of such structures alone seems to me quite mis- 
leading ; not less so, indeed, when we attach hypothetical nerves (dorsal, 
lateral, and ventral roots) to them. 

Moreover, the late appearance of the so called neuromeres of the ante- 
rior brain region, together with the fact that they are secondary subdivisions 
of primary vesicles, and thus differ from the hindbrain and spinal expan- 
sions, seems a serious objection to the contention that they afford satisfac- 
tory evidence of a primitive metamerism. Zimmermann (’91) attempted 
no explanation of this difficulty, saying merely that the differentiation of 
the anterior encephalomeres 7s retarded for reasons unknown to him. 
Waters (’92) alone offers an explanation. To him it seemed “not unrea- 
sonable to conjecture that these constrictions, being essentially primitive 
and in a state of degeneration, have gradually been more and more 
crowded out by the specializing brain development, and hence appear at 
a much later period in the ontogeny than would be expected.” What 
right, we are tempted to ask, has one to assume the primitive nature of 
“forebrain neuromeres,” in view of the facts that they are late differen- 
tiations, and that some of them are the fundaments of adult organs, and 
in this respect differ both from the typical hindbrain neuromeres and 


186 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


from the expansions of the myelon? Every fact which we possess seems 
to me to argue against their primitive nature. In my opinion the 
assumption of Herrick (’92), that, “if neuromeres once existed in the 
forebrain, they would be visible only at an early stage, and would be 
obscured by altered conditions,” is the more reasonable of the two 
assumptions. On the basis of structure and of relation to other seg- 
mentally arranged organs, however, I conclude that the primary vesicles, 
the forebrain and midbrain, give evidence — as do the primary expansions 
of the hindbrain — of the primitive segmentation of the Vertebrate head. 
I now turn to an examination of these relations, first, to those of nevro- 
meres and somites, since they are the most important. 


V. The Relation of Neuromeres to Somites. 


a. RELATION OF MYELOMERES TO SOMITES. 


Since the myelomeres, as has been stated, show a definite (numerical) 
correspondence with the trunk somites, the expansions of the spinal cord 
alternating with the somites, it is evident that proof of the serial homol- 
ogy of myelomeres and encephalomeres will rest very largely on the dem- 
onstration of a similar correspondence of the latter with head somites, if 
there be such. Yet, so far as I know, Miss Platt is the only investigator 
who has affirmed that there is such correspondence for the head region. 
She writes (’91, p. 82) as follows: “The line of somites [in Squalus] 
alternating with the neuromeres is continued into the head as far for- 
wards as the alimentary pocket which is to form the second visceral cleft. 
Here complete divisions of the mesoderm cease, but serial depressions 
in its dorsal wall indicate incomplete divisions into three parts above 
the hyoid arch (van Wijhe found two somites here) and two parts above 
the mandibular arch [van Wijhe found one somite here]. Like the 
somites of the trunk, the divisions thus marked off alternate with 
the neuromeres, lying opposite successive constrictions of the brain. 
The anterior division of the mandibular cavity corresponds to the con- 
striction that separates the midbrain from the hindbrain, or to that from 
which the trochlear nerve arises.”” The same investigator likewise says 
in regard to Necturus (94, pp. 960, 961): “ Hinter der Hyomandibular- 
spalte wechseln die primitiven Neural- und Mesoderm-Segmente regel- 
miissig mit einander ab. Die mesentodermale Segmentation ist dieselbe, 
die von v. Wijhe den Selachiern zugeschrieben wird.” * 


1 It is seen that Miss Platt finds the segmentation, both neuromeric and meso- 
meric, different in Squalus and Necturus. While in embryos of the former she 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 187 


6. RELATION OF ENCEPHALOMERES TO SOMITES. 


With Hoffmann (’94 and ’96) I am able to confirm the presence of 
van Wijhe’s head somites in Squalus (Acanthias) and also Platt’s “ ante- 
rior’? somite. Valuable as this repeated confirmation appears to me, 
I regard as equally important the fact that anterior to the sixth (van 
Wijhe’s) somite a mesodermal segment corresponds to each of the primary 
encephalic vesicles (encephalomeres I to VII). A topographic alterna- 
tion, however, such as that affirmed by Miss Platt for the hindbrain 
region of Squalus and Necturus, I do not find. In the early stages of 
development van Wijhe’s sixth somite lies opposite the posterior constric- 
tion of encephalomere VII, but this relation is soon lost. However, the 
numerical correspondence seems important, and I believe that it will be 
shown by a study of nerve relations that the correspondence is not with- 
out morphological significance. 


ec. SOMATIC VALUE OF THE PRE-oTIC MESODERM SEGMENTS. 


Although it has been stated that the purpose of this paper is to dis- 
cuss the nature of the neuromeric segmentation and the relations of 
neuromeres to other segmental structures, it seems to me not incon- 
sistent with this purpose to inquire into the credentials of those meso- 
dermal segments in the Selachian head which van Wijhe in his famous 
paper considered of somatic value. The confirmation of their presence 
in Squalus given by Hoffmann (94 and 796) and myself (’96), while 
strengthening the belief in their permanency, which has been greatly 
shaken by the discovery of more numerous segments in other Selachii 
(Torpedo), by no means demonstrates their somatic value. The dis- 


finds in the hindbrain region two more somites than were seen by van Wijhe (’82) 
and a numerical correspondence of these with the neuromeres, in the latter, on the 
contrary, she finds neuromeres corresponding with a somatic segmentation which 
is the same as that found by van Wijhe. She finds, therefore, it may be inferred, 
two less hindbrain neuromeres in Necturus than in Squalus. In embryos of Ambly- 
stoma I find, in agreement with McClure (’90), no neuromere corresponding with 
encephalomere IV of Squalus, i. e. there is one less neuromere in the Urodele than 
in the Selachian. Now, since I find a numerical correspondence of van Wijhe’s 
somites with hindbrain neuromeres (encephalomeres III-VII) in the Selachian, it 
is clear that they could not likewise correspond in the Urodele. However, I have 
been unable to find evidence of pre-otic somites in Amblystoma, and therefore am 
unable to affirm or deny a correspondence of neuromeric and mesomeric segmenta- 
tion in this form. 

2 It is a matter of great interest that the latest investigation upon Torpedo 
(Sewertzoff, 98) shows that the mesodermic segmentation in Torpedo and Pristiurus 
is the same. Thus van Wijhe’s results receive repeated confirmation. 


188 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


crepancy in the results of investigators of the mesomeric, as well as of the 
neuromeric, segmentation most certainly justifies Rabl’s ’89) complaint 
of the hasty way in which investigators have given mesodermal segments 
somatic value. In no question of morphology to-day is conservative 
judgment more needed. Before stating my own evidence I will briefly 
summarize the arguments advanced by previous investigators for and 
against the somatic value of the mesodermal segments of the head. 

(1) In addition to the evidence first stated by Marshall (’81), that the 
dorsal mesoderm of the head of Selachian embryos undergoes a segmenta- 
tion independent of the segmentation of the visceral arches, van Wijhe 
(’82, p. 4) uses the following arguments for the somatic value of his 
somites : (2) Dass die Lange der Somite sich im ganzen Korper gleich 
verhilt. (3) Dass die obere Grenzlinie der Rumpfsomite ununterbrochen 
in diejenige der Kopfsomite tibergeht. (4) Dass die untere Grenze der 
Somite sowohl im Kopfe als im Rumpfe nur wenig unter der oberen 
Grenze des Darmes liegt.” The latter proof has been amplified by. 
Killian (91) from the evidence that the head somites are dorsal in 
relation to chorda, dorsal aorta, and epibranchial (medio-lateral) line. 
(5) Hoffmann (’94) and Miss Platt (97) have confirmed van Wijhe’s 
statement that the development of the somites begins in the neck region 
and proceeds continuously both posteriorly and anteriorly. Furthermore 
(6) the same constituent parts, viz. myotome and sclerotome, may be 
distinguished in the head as well as in the trunk somites (van Wijhe, ’82, 
Killian, 791). To this Miss Platt (91) adds (7) the evidence that, as 
in the case of the somatic musculature of the trunk, the muscles derived 
from the “anterior,” the first, the second, and the third somites (rudi- 
mentary in the case of the anterior and somewhat modified in the case of 
the first somite) first appear in the median wall of these somites. Finally 
(8) there is a correspondence of the neuromeres and mesodermic segments 
throughout the entire length of the neural tube (Neal, ’96). 

The following are the arguments advanced in opposition to the somatic 
value of the mesodermal segments of the head. 

(1) The divisions of the mesoderm of the head are due to the me- 
chanical influence of the neighboring parts, chiefly that of the visceral 
pouches (Kastschenko, ’88). 

(2) The divisions are irregular in size (Kastschenko, ’88, Rabl, ’89). 

(3) In van Wijhe’s third proof there is “nicht die Spur eines Beweises 
fur die Richtigkeit seiner Ansicht ” (Rabl, ’89, p. 234). 

(4) The Ist somite is an exception to van Wijhe’s fourth argument 
(Rabl, ’89) ; moreover, the constrictions are never complete in the case 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 189 


of the somites 2 to 5 (Katschenko, ’88, Rabl, ’89), so that it is impos- 
sible to state the position of their lower boundary with reference to the 
dorsal wall of the alimentary canal (Rabl, ’89). 

(5) The development of the “head cavities” is discontinuous with 
that of the trunk somites (Rabl, ’89, Kupffer, 93). While the develop- 
ment of the pre-otic segments takes place later than that of the trunk 
somites, the differentiation of mesenchyma takes place much earlier 
in the head than in the trunk. This conflicts with the law, that in the 
Anlagen of serially homologous organs the older the Anlage the earlier 
the histological differentiation (Rabl, ’89). 

(6) There never appears in the case of the pre-otic segments a differ- 
entiation into myotome and sclerotome (Rabl, ’89, p. 235). 

(7) While the musculature of the trunk and occipital somites arises 
exclusively from the median wall of the somite, the musculature of the 
pre-otic segments has its origin in greater part from the lateral, and in 
smaller part from the posterior wall of the so called somites. Moreover, 
while only a distinct and sharply defined portion of the trunk somites 
proliferates mesenchyma, the entire median wall of the pre-otic segments 
participates in the formation of mesenchyma (Rabl, ’89). 

(8) The topographic relations of the dorsal nerves in later stages are 
different in head and trunk. In the head the nerves grow laterad to the 
somites, while in the trunk they grow mediad to them (Rabl, ’89). 

Special arguments, in addition to the general ones stated above, con- 
cerning the nature of the anterior, the Ist, and the 2d mesoderm 
segments have been made, because of their marked peculiarities in de- 
velopment, structure, and relations, and of their important bearing upon 
the question of the morphology of the eye muscles. It will therefore be 
necessary to state these also. 

Two chief opinions concerning the nature of the anterior (Platt’s) and 
the Ist and 2d (van Wijhe’s) mesoderm segments are now held: (1) that 
they are serially homologous with trunk somites (van Wijhe, Platt, 
Hoffmann, Neal, Fiirbringer) ; (2) that they are abortive visceral pouches 
(Kupffer, ’88, Froriep, 792, Sewertzoff, 95). The discussion, therefore, 
turns upon the question whether these structures represent diverticula 
(dorsal) of the mesoderm, or lateral diverticula from the alimentary canal. 

Miss Platt (’91, 791") argues for the somatic value of the anterior 
somite (cavity) as follows: —(1) In position, independence, and time of 
origin this cavity resembles the following ones. (2) Many cells from its 


1 Balfour (’81) holds that both median and lateral walls of the trunk somites 
form the lateral trunk musculature. 


190 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


median wall migrate into the centre of the cavity, and cells bounding 
the inner wall above and below assume the elongated contour of muscle 
cells. 

Hoffmann (94, p. 649) also, while not able to state with definiteness 
that the anterior cavity is a dorsal or lateral diverticulum from the 
alimentary canal, i.e. whether it represent a mesoderm segment or a 
visceral pouch, considers it probable that it represents the former, since 
it is very similar to the succeeding head cavities of van Wijhe. Hoff- 
mann mentions the migration of cells into the cavity of the somite, but 
does not specify from which wall they are proliferated. He also states 
that from the walls of the somite “entstehen keine Muskelfasern ” (796, 
p- 256). Against these views of Miss Platt and Hoffmann no special 
arguments have as yet been raised. 

The first somite of van Wijhe possesses the peculiarity of a median 
stalk connecting the somites of the opposite sides of the body.’ The re- 
lations of this stalk to the dorsal wall of the alimentary canal, to chorda, 
and to dorsal aorta have been used as the chief criteria in contending 
for its dorsal or its ventral nature. The evidences that the first somite 
represents somatic (dorsal) mesoderm are as follows: (1) Its cells are 
proliferated from the dorsal wall of the alimentary canal (Platt, ’91*).? 


1 Such a median connection, however, also appears in the early stages of develop- 
ment of the “anterior cavities.” The connecting stalk of the “anterior cavities,” 
however, as stated by Hoffmann, never possesses a lumen, as does the median con- 
necting stalk of the premandibular cavities. 

2 Miss Platt, in her earlier paper (’91, p. 81), states that the mesoderm of the 
premandibular cavity is formed, at least in part, by a proliferation of cells from 
the mandibular cavity, while in her later paper (7914, p. 256) she writes, “ The 
most anterior mesoderm of the head does not take its origin from the mesodermic 
plates, but from the dorsal wall of the alimentary canal. The mesodermic plates end 
with the mandibular cavities.” The lumen of the connecting stalk, according to Miss 
Platt, is, as stated by Marshall (’81), formed secondarily by the fusion of a median 
with the two lateral cavities. This evidence is interesting, since it bears on the 
question whether the cavity of the connecting stalk is to be regarded as a part of 
the archenteron, and would seem to answer this question in the negative. Killian 
(91, p. 102), however, finds the connecting stalk a “ Sklerotomkommissur,” and 
thus, it is to be inferred, the lumen of the stalk, which according to his account is 
formed secondarily, not a part of the archenteron. He states (p. 102): “ Erwahnt 
sei noch, dass zwischen den beiden ersten Mandibularsomiten vor dem vorderen 
Chordaende und iiber dem Aortensinus ein Mesodermzellenhaufen liegt, der die 
Sklerotomanteile beides Somiten in Verbindung setzt (Sklerotomkommissur). 
Was nun der Oralzone angeht, so entsteht sie dadurch, dass die vordersten Zipfel 
des urspriinglich schwalbenschwanzformig endenden dorsalen Mesoderms die vor- 
dere Darmkuppe (vordere Ektodermtasche von Petromyzon nach Kupffer) iiber- 
und umwachsen, und so einen medianen Zellkomplex bilden, aus dessen hinterer 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 191 


(2) Its connecting stalk is axial in position (van Wijhe, ’82); dorsal to 
carotis (Dohrn, ’88). (3) This section of the head cavity is so similar 
to the remaining sections, that it must be considered as serially homolo- 
gous with them (Balfour, ’81). Oppel (90, p. 623), however, states 
(for Anguis) that “nur der histologische Bau und die Art der ersten 
Entstehung dieser voriibergehend im Mesoderm auftretenden Somiten 
gestattet, an der Deutung festzuhalten dass es sich hier in der That um 
Somiten handelt.” 

That the 1st cavity is ventral would seem to follow from the evidence 
that (1) it arises as an entodermic diverticulum from the prechordal 
portion of the alimentary canal (Seessel’sche Tasche), whose cavity is at 
first continuous with that of the alimentary canal (Kastschenko, ’88, 
Kupffer, 88, *90, ’93). ‘Streng genommen,” says Froriep (’92°, 
p- 589), ‘‘ konnte es tibrigens immer noch eher ventral, als dorsal genannt 
werden, wenigstens was sein Lumen und seine untere Wand anlangt. 
Denn das Aequivalent der Chorda, welche als Achsenfaden dorsale und 
ventrale Gebilde scheidet, ist selbstverstindlich nur in der oberen Wand 
des medianen Verbindungsstiickes zu suchen.”? (2) The method of 
separation of the premandibular head cavity from the entoderm, as well 
as the presence of a median connecting stalk, serves to distinguish this 
from the following mesoderm segments (Kupffer, 93). (3) According 
to Kupffer (94) the connecting stalk of the premandibular cavity is 


Hilfte fiir jede Seite ein Somit entsteht (van Wijhe’s erster), wihrend die vordere 
Hilfte zu Grunde geht.” It is readily seen that this evidence tells decidedly 
against the view that the connecting stalk is ventral, and against the view of 
Kupffer and Froriep, that its lumen is a part of the archenteron. 

Furthermore, Goette (’90) has given evidences concerning the method of forma- 
tion of the anterior mesoderm in Ammocetes which stands in direct contradiction 
with that stated by Kupffer, and, if true, takes away the chief support of the 
theory of the visceral-pouch nature of the anterior mesoderm in that animal. 
Goette writes: “‘ Unbedingt muss ich aber die angeblichen ‘Coelomdivertikel’ 
des Urdarms im Kopf und Vorderumpf fiir tauschende Bilder erklaren, was sich 
am Besten versteht, sobald man die Mesodermbildung durch die ganzen Schnitt- 
serien yon vorn nach hinten verfolgt und dabei die vollkommene Uebereinstimmung 
derselben in allen Regionen antrifft. Ein Blick auf die Abbildungen lehrt, dass von 
einer verschiedenen Auffassung derselben nicht die Rede sein kann: giebt es im 
Rumpf keine Coelomdivertikel, so fehlen sie auch im Kopf. Auch die beiden 
‘praoralen Kopfhohlen’ sind weiter nichts als das erste Mesomerenpaar, welches 
allerdings wenn man seine erste Anlage in unmittelbarer Fortsetzung der folgenden 
Mesomeren iibersah, spater eine Ausstiilpung des Urdarms vortaéuschen kann, wie 
ich es weiter oben auseinandersetzte.” Goette’s figures, especially Figures 42, 43, 
and 44, strongly support his statements. 

1 Willey (’94, p. 175) accepts Kupffer’s and Froriep’s conclusions. 


192 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


ventral to the dorsal aorta. The vessel which Dohrn (’88) called the 
carotis, and which he stated lay ventral to the connecting stalk of the 
first cavity, if comparable at all, is comparable only to the carotis ven- 
tralis of Amniota.? 

The chief arguments concerning the nature of the 2d (mandibular) 
cavity have already been given in connection with the general question 
of the pre-otic mesodermal segments, and it is therefore not necessary to 
repeat them here. The evidence of a continuous lumen between this 
cavity and the alimentary canal stated by Miss Platt (’91*) has been 
interpreted by her as favoring the view that the cavity is formed as an 
outgrowth from the dorsal wall of the alimentary canal, similar to the 
mesodermal pouches in Amphioxus. Kupffer (’94), however, regards it 
as evidence in favor of his view, that these cavities are abortive visceral 
pouches. It is necessary, finally, to recapitulate a point in evidence 
which has only an indirect bearing on the question of the somatic value 
of the Ist, 2d, and 3d cavities, but which concerns vitally the morphol- 
ogy of the eye muscles (derived in Selachii from these cavities). It 
has been stated by Hatschek (92) and Kupffer (92~96) for Ammo- 
cceetes (Petromyzon Planeri). Their results tend to show that the eye 
muscles of that low Vertebrate are, with the possible exception of the 
muse. rectus posterior (externus), derived from splanchnic and not from 
somatic mesoderm. According to Hatschek (’92), the musc. obliquus 
superior appears as a differentiated portion of the muscles of the velum, 
which correspond with the musc. adductores mandibule. His evidence 
(pp. 149, 150) is as follows: “‘ Vom vorderen inneren Rande dieses Mus- 
kels [velar muscle] dringt nimlich ein Muskelfaserbiindel dorsal in das 
Bindegewebe ein und zieht seitlich am Trabekel vorbei zwischen dem 
ersten und zweiten Trigeminusganglion hindurch bis in die Nahe des 
Auges, wo es im Bindegewebe zugespitzt endet. Von da beginnt — 
wie ein zweiter Muskelbauch — mit seinem zugespitzten hinteren Ende 
der muse. obliquus superior und zieht, wieder anschwellend, in gleicher 
Richtung weiter zum Auge. Die histologische Uebereinstimmung beider 


1 Kupffer’s statement applies to that cavity in Ammocctes which has been 
homologized, in my opinion correctly, by most investigators (Balfour, Dohrn, 
Shipley ’87, Kupffer) with the premandibular cavity of Selachii. 

2 In my opinion those writers who have quoted Balfour (’81) and Marshall (’81) 
as holding that the connecting stalk of the premandibular (1st) cavity is ventral 
have misunderstood them. They both spoke of the two lateral parts of this cavity 
as prolonged ventralwards to meet below the base of the forebrain. They give no 
proof that the stalk is morphologically ventral, and in my opinion speak of it as 
ventral only with reference to the wall of the brain. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 193 


Muskelteile ist eine vollkommene. Dieses Verhiltnis ihnelt in hohem 
Grade jenem, welches van Wijhe als ein embryonales von den Selachiern 
abgebildet hat. Seine Deutung ist aber darin zu korrigieren, dass der m. 
obliqu. sup. nicht dem parachordalen Muskelblatte, sondern den Seiten- 
platten zugehért. Die tibrigen Augenmuskeln, die in Form eines Kegel- 
mantels an’der medialen ventralen Seite des Augapfels sich finden, 
bilden in Bezug auf ihre Lage und histologische Beschaffenheit eine 
dritte Gruppe, deren Ableitung nicht ganz sicher erscheint. Sie sind 
wahrscheinlich von den Konstriktoren des Visceral-apparates abzuleiten. 
Keinesfalls kénnen sie nach ihre Lage, Verlaufsrichtung und Struktur 
zu den Seitenrumpfmuskeln in Beziehung gebracht werden.” 

Kupffer’s (94) results are essentially a confirmation of those of Hat- 
schek. Finding that the premandibular cavity entirely disappears, and 
that its cells contribute in no part to the formation of the eye mus- 
cles, Kupffer is led to doubt the conclusions of those investigators who 
derive the muscles innervated by the oculomotorius from the epithelium 
of this cavity. According to Kupffer all the eye muscles (with the possible 
exception of the muse. rectus posterior) are derived from two visceral 
arches, the “trabecular” and the mandibular. This evidence, as well 
as that given by Hatschek, obviously stands in direct contradiction to 
the somite theory. I am, however, after my study of the literature, 
inclined to be optimistic concerning the ultimate settlement of the ques- 
tion as to the somatic value of the pre-otic mesodermal segments, for 
the differences of opinion are not due to equivocal evidence, but to 
directly contradictory and equally positive statements. We have chiefly 
to determine who has stated the facts correctly in order to determine 
whether we shall accept the opinion of van Wijhe, or that of Kast- 
schenko, Rabl, and Froriep. The evidence obtained by me, which leads 
me unhesitatingly to accept the view of the first, that the head somites 
are serially homologous with trunk somites, is as follows. I find the pre- 
otic mesodermal segments as described by van Wijhe (’82) most clearly 
defined by mesodermal constrictions or clefts in embryos of Squalus 
with 28 or 30 somites (Plate 3, Fig. 13, Plate 6, Fig. 40, Plate 7, 
Fig. 46). They are so distinctly marked that they may be seen in 
whole specimens properly cleared, as well as in sections. Moreover, 
they are found to be the same on both sides of the embryo.? 


1 Van Wijhe’s post-otic mesoderm segments have indisputable somatic value, 
and need not be brought into discussion. 
? An examination of some finely preserved embryos of Torpedo ocellata, kindly 
given me by my friend, Professor A. N. Sewertzoff, leads me to agree with Sedg- 
VOL, XXXI.— NO. 7. 4 


194 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


The contention that the constrictions between van Wijhe’s somites are 
incomplete does not appear to me to militate greatly against the view 
that they have morphological value, inasmuch as their permanency has 
been repeatedly attested (van Wijhe, Hoffmann, Neal, and Sewertzoff). 
Nor does Rabl apparently consider this argument as of great weight, 
since he regards van Wijhe’s 5th (1st post-otic) somite —though the 
constrictions which are found in front and behind are incomplete —as 
a true somite. The reduction in the myotomic portion of the dorsal 
mesoderm accounts in great part for the incompleteness of the constric- 
tions. I believe that one who follows the development of the pre-otic 
and sub-otic mesoderm in Ammoccetes, and observes the ontogenetic 
dissolution of the compact dorsal mesoderm into loose mesenchyma, 
which follows the great enlargement of the nerve ganglia and of the 
otic capsule, is in a position to understand the reduction of the dorsal 
mesoderm in this region in Vertebrates higher in the phylogenetic scale 
than Ammoceetes.? 


wick (92), that this is not true of the mesoderm segments discovered by Dohrn 
(790, 902) in that form. Dohrn apparently did not endeavor to ascertain whether 
they were symmetrical or not. Iam unable to determine, even in carefully made 
reconstructions of well oriented frontal sections of embryos at the same stage of 
development as that described and figured by Dohrn (’90*), whether or not there 
is a correspondence of the mesodermal segments on the two sides of the head an- 
terior to the one which, in my opinion, corresponds with the 15th segment of Dolirn. 
While my own negative conclusions cannot be regarded as in any sense disproof of 
the segmental value of Dohrn’s somites, it is my opinion that the evidence of their 
variability shown by the conflicting results of Killian (’91) tends to throw consider- 
able doubt upon it. Since Killian (’91, p. 103) finds that of the anterior of these 
segments one is to be regarded as the sclerotome portion of a somite, while others 
are simply vesicular enlargements of the mesoderm of the mandibular arch, it is 
to be inferred that Dohrn subjected the head somites of Torpedo to little critical 
examination. To regard as evidence of somites all vacuolar spaces in the dorsal 
(and lateral!) mesoderm which appear between the somatopleure and splanchno- 
pleure at the time these layers separate, seems to be too uncritical. Similar phe- 
nomena appear in the mesoderm of Squalus in those early stages of development, 
when the celom is in the process of formation, viz. in stages when the neural plate 
is widely expanded and the embryo possesses 4 or 5 somites. Recent studies by 
Sewertzoff (’98) render still more doubtful the results of Dohrn and Killian. 

1 A mechanical explanation of the constrictions between the head somites of 
van Wijhe, such as that offered, but without evidence, by Kastschenko (’88), 
seems hardly worthy of consideration. That such constrictions as those, for ex- 
ample, between somites 3 and 4, and 4 and 5, cannot result from the so called 
mechanical influence of visceral clefts, follows from the evidence already stated by 
Hoffmann (794 and ’96) that in Squalus, the constrictions lie dorsal to the visceral 
arches. I cannot, however, agree with Hoffmann that we may conclude from this 
evidence that the visceral arches are intersomitic in position, as are the ribs in the 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 195 


The evidences of irregularity in size and discontinuity in development 
and differentiation are not, in my opinion, the more serious of the objec- 
tions raised. Such differences may indeed be explained as ccenogenetic. 
Rabl himself has given the evidence (’89) that the first rudimentary 
visceral cleft is differentiated later than the second. Moreover, it is 
well known that the first rudimentary myotome in Amphioxus develops 
later than the following. Differences in time of development and of dif- 
ferentiation are to be expected when a comparison is made between the 
Anlagen of serial organs, some of which become highly differentiated 
(e. g. the eye muscle somites, Ist, 2d, and 3d), while the others (e. g. the 
anterior, the 4th, and the 5th somites) are becoming rudimentary. It 
is interesting to find that the last intersomitic constrictions to be formed 
are those between the anterior and the lst cavity, and between somites 
4 and 5, that is, the constrictions separating the most rudimentary 
somites. The separation of the anterior somite from the premandibular 
is first complete in an embryo with 26 or 27 somites, while the con- 
striction between somites 4 and 5 appears first in an embryo with 28 
somites. Consequently van Wijhe’s statement, that the segmentation 
of the dorsal mesoderm begins in the neck region and proceeds con- 
tinuously anteriorly and posteriorly, is true only in part. But it also 
follows that the discontinuity in the development of the more anterior 
constrictions may be explained as in great measure due to degeneration. 
The retardation in development due to degeneration, already apparent 
in the lst somite of Amphioxus, makes itself manifest in the somites of 
the more highly specialized Squalus as far posteriorly as the 7th somite of 
van Wijhe (equivalent to the 8th somite of Amphioxus 2), which I believe 
to be the first somite differentiated, as well as the first to develop a per- 
manent myotome.t The correlation between degeneration and retarded 
development serves to explain, for the occipital somites at least, why 
the development of the somites in the Craniota begins in the neck 


trunk. Such purely topographic relations in the Selachian cannot be regarded as 
weighty evidence in the settlement of this question, in comparison with the evi- 
dence stated for Amphioxus (van Wijhe, 93, Hatschek, 92), Bdellostoma (Price, ’963), 
and Amphibia (Houssay, ’91, Platt, 94), which has led these investigators to regard 
the visceral clefts as intersomitic in position. In view of the great probability of a 
shoving forward of the visceral clefts with reference to the somites in Squalus, I am 
unable to accept Hoffmann’s conclusion on the basis of the evidence he presents. 

1 On account of the considerable variation in the length of embryos in early 
stages of development, I am unable to state positively that the seventh somite is 
the first to develop. It may be the eighth somite which does so, as stated by Hoff- 
mann (’94, 96). The seventh somite shows some signs of degeneration, having a 
small myotome and losing its ventral nerve during development. 


196 BULLETIN MUSEUM OF COMPARATIVE ZOOLOGY. 


region. So far as I know, hitherto no explanation of this phenomenon 
has been suggested. 

That which I have regarded as the more serious of the objections 
made by Rabl (’89 and ’92), viz. that the pre-otic segments are not 
morphologically comparable with trunk somites, inasmuch as they do 
not show a differentiation into myotome and sclerotome, may be met by a 
denial of the statement, so far as it applies to the 3d somite of van 
Wijhe.t I have followed the development of this somite through closely 
connected stages of development, until it becomes converted into the muse. 
rectus posterior and assumes relation with the eye, in order to determine 
whether in its development it exhib- 
its those marked differences which, 
as stated by Rabl, serve to distin- 
a ~ thn. guish pre-otic and post-otic meso- 
dermal segments. The evidence 
which I have obtained may be sum- 
marized as follows: Cross sections 
of embryos in early stages of devel- 
| my'coel. opment leave no doubt that the 3d 
somite, as its topographical relations 
to chorda, dorsal aorta, epibranchial 
line, and dorsal wall of alimentary 
canal show, is composed of only dor- 
(gx----6rs.vsc-4. sal mesoderm. Figure A represents 
a cross section in the region of this 
somite from an embryo with 28 
somites (compare Plate 3, Fig. 13). It is seen that a well marked 
cavity (myoccel), surrounded by a single layer of epithelial cells, may 
be distinguished.? 


FicgureE A. 


1 Both van Wijhe (’82) and Killian (’91) have affirmed a differentiation of head 
segments into myotome and sclerotome. 

2 That the epithelial walls of the cavity (Fig. A) are not continuous with the 
two layers of the lateral plates is due to the obliteration of these two layers caused 
by the great development of the first visceral pouch. 


Fig. A. Cross section of a Squalus embryo in the region of van Wijhe’s 3d 
somite and encephalomere IV. X 240. The dorsal nature of this mesodermal seg- 
ment is attested by its relations to dorsal aorta and wall of alimentary tract. At this 
stage (Acranienstadium) the region of proliferation of mesenchyma is seen to be a 
definite one, and to correspond in its relations with the sclerotome of trunk somites. 

ao. d., dorsal aorta; Ors. usc. 1, first visceral pouch; cd., chorda dorsalis ; ec’drm., 
ectoderm ; en’drm., entoderm; my’cal., myocel, enlarged ventrally to form a sclero- 
tome vesicle; ¢b. n., neural tube. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 197 


The cavity of the somite is enlarged ventrally opposite that portion of 
its median surface where a rapid proliferation and migration of cells 
appears to take place. I see no reason why the more dorsal and lateral 
portion of the mesoderm should not be homologized with the myotome 
portion, and the ventral median region with the sclerotome portion of 
trunk somites. I am unable to detect any essential difference between 
the phenomena presented in this section and those presented in sec- 
tions made in the region of van Wijhe’s 5th and 6th somites, to which 
Rabl grants “‘ Biirgerrecht” as true somites. The greater dorsal extent 
of the latter cannot be regarded as an essential difference. Here, as 
there, we find a well marked and definite region of cell proliferation. 
As development goes on, the cavity of the 3d somite increases in volume, 
and at the same time the somite grows forward, chiefly by the elongation 
of its anterior end, median to the Gasserian ganglion. In confirmation 
of the statement of Miss Platt (91), I find that the first muscle cells 
are differentiated in the median wall of that portion of the somite which 
at this stage lies posterior 
to the Gasserian ganglion. 
The great extension of the 
anterior portion seems to 
retard its histological dif- 
ferentiation. But in this 
portion also, when muscle 
cells appear, they are found 
in the median wall. Rabl 
(89, p. 236) says: “ Wah- 
rend ferner die Muskulatur 
der Urwirbel ausschliesslich 
aus der medialen Wand ent- 
steht, nimmt sie im Vorderkopf zum gréssten Theil aus der lateralen und 
zum kleineren Theil aus der hinteren Wand der sogenannten Somite den 
Ursprung.” A cross section of the myotome of the 3d somite at a late 
stage of development appears to me to refute this statement (Figure B). 
It is clear from an examination of the phenomena presented in such a 


Fic. B. Portion of a cross section through the middle of the myotome of van 
Wijhe’s 3d somite, in a late stage of development (20 mm.). Elongated muscle 
cells are already differentiated in the median wall (muscle plate), while the lateral 
wall (cutis plate) retains its epithelial character. X 50. 

a, cell migrating from the median wall of the myotome into the myocel; 
gn. Gas., Gasserian ganglion; /a. ct., cutis plate; Ja. mu., muscle plate; my’cal., 
myoceel. 


198 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


section, that the greater part of the cells proliferated into the cavity of 
the myotome, the cells of which are at this stage already converted into 
elongated muscle cells, arise from its median wall. While the outer 
wall still maintains its primitive epithelial character, the inner wall has 
become many cells in thickness and some of these cells appear in the 
act of migrating into the now greatly diminished lumen of the cavity. 
Later, however, the cells of the outer wall also are converted into muscle 
cells, and thus both walls of the cavity participate in the formation of the 
muse. rectus posterior. We have therefore in the 3d somite of van Wijhe 
a pre-otic segment of the dorsal mesoderm, which becomes differentiated 
into myotome and sclerotome, and whose musculature is derived in greater 
part from its median wall. Furthermore, as is well known, its musculature 
is innervated by a nerve (abducens) which all the later morphologists, with, 
so fur as I know, one exception (Kupffer, 94, 795), regard as a ventral 
nerve comparable with spinal ventral nerves. Finding this to be the case 
with at least one pre-otic mesoderm segment, we are in a better position 
than we otherwise should be to understand the more modified, or at least 
more divergent, conditions presented by the remaining pre-otic segments, 
viz. the anterior, the lst, the 2d, and the 4th. That in these segments 
marked peculiarities appear is certain. In the 4th somite we have a 
segment of the dorsal mesoderm divided by constrictions from the 3d 
and 5th somites at a time when it presents essentially the same evidences 
of differentiation into myotome and sclerotome which appear in the 3d 
and 5th somites. That no muscle cells are formed in its inner wall, and 
that it soon breaks up into loose mesenchyma, are phenomena which are 
to be expected in a somite destined to become rudimentary. That it is 
more rudimentary than the 5th somite is due to the development of the 
otic capsule, under which it lies. The 5th somite — in whose inner wall 
elongated cells appear, without however developing into muscle fibres 
(as stated by Sedgwick, ’92) —thus forms a natural transition to the con- 
ditions presented by the 4th. Ifthe 3d and 5th are to rank as somites, 
it is in my opinion impossible to deny that the 4th, which lies between 
them, is serially homologous with them, even though it should lack some 
of the characteristics of a typical trunk somite. 

Passing forward in the embryo to the 2d (mandibular) somite, it 
seems to me indisputable that this is the anterior continuation of the 
dorsal mesoderm. In very early stages it grows ventrally to form the 
mesoderm (mesothelium) of the mandibular arch, a process which, accord- 
ing to Kupffer (’88), occurs in Petromyzon also. However, only the 
dorsal part of the “mandibular cavity,” which later becomes separated 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 199 


from the ventral to form the muse. obliquus superior, can by virtue of its 
topographic relations to chorda and aorta be regarded as the somatic 
portion of this mesoderm segment (Plate 7, Fig. 56). Its ventral portion, 
which later becomes differentiated into the musc. adductor mandibule, 
is therefore splanchnic. While the indications of the differentiation of 
the 2d somite into myotome and sclerotome are less clearly expressed 
than in the case of the 3d and 4th, I have no reason to question the 
correctness of Killian’s (91) interpretation that such appear. The 
great enlargement of the cavity of the somite is the chief factor in 
modifying its form and the relations of its constituent parts. While 
Miss Platt (91) finds the musculature to arise first in the median wall 
of the somite, that is to say, the dorsal part of the so called “ mandibular 
cavity,” Hoffmann (’96) states that the muse. obliquus superior arises in 
its upper and lateral walls. In my opinion their conclusions are not so 
divergent as they might at first sight seem to be, for I believe that the 
portion of the somite which Hoffmann calls dorsal is morphologically 
median ; in other words, that it is the portion which in early stages lies 
against the wall of the neural tube (Plate 7, Fig. 56). I agree with 
Hoffmann that the muse. obliquus superior arises in the dorsal and lateral 
walls of the second (van Wijhe’s) somite, but with the qualification that 
the dorsal wall is morphologically median.? 

The first (premandibular) somite shows in its development even greater 
peculiarities than those of the mandibular; yet it appears to me to pos- 
sess somatic value as unquestionably as the latter does. The first and 
most important question to answer is whether this segment represents 
dorsal mesoderm or a diverticulum from the alimentary canal, and for 
this purpose the relations of the connecting stalk furnish us with the 
decisive evidence. In a median sagittal section of an embryo with 
14 or 15 somites, such as that shown in Figure C, the tissue which is later 
differentiated as the connecting stalk of the first somite appears as a 
mass of cells between the base of the brain, in that region which lies 
just posterior to the pit of the infundibulum, and the dorsal wall of the 
alimentary canal. Posteriorly this mass of cells is continued into the 
chorda and its relations are seen to be such that, if the chorda is dorsal, 
so must the mass of cells be also. The lumen of the alimentary canal 
may be traced to a point directly ventral to the pit of the infundibulum, 
where it ends as the so called “ Seessel’sche Tasche ” (Kupffer’s “ pra- 


1 Miss Platt’s (’91*) evidence of the continuity of the cavity of the alimentary 
canal and that of the mandibular cavity, as well as her evidence of two segments 
in the latter, appears to me illusory. 


200 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


orale Darm”), while its walls become continuous anteriorly with that 
mass of tissue which later differentiates into the “‘ anterior cavities.” 
Furthermore, a cross section of a corresponding stage of development in 
a plane immediately posterior to the infundibulum (i. e. along the line 
a 8 of Figure C) gives equally convincing evidence (shown in Figure D) 
that the mass of cells (1) lies dorsal to the wall of the alimentary canal, 
with which, however, they are in close connection in this somewhat earlier 
stage (11-12 somites). There exists not the faintest shadow of evidence 
that the mass of cells which forms in its lateral part the premandibular 
cavities and in its median part their connecting stalk, represents entoder- 


Peeneear” 


FiGureE C. 


mal diverticula. During development, as the result of the ventral growth 
of the infundibulum, the pre-oral (Seessel’s) pouch becomes obliterated and 
the mass of cells surrounding the “anterior cavity” is cut off from those 
posterior to the infundibulum (26-27 somites). By this change in rela- 
tions the Anlagen of the connecting stalk and of the premandibular cavity 
take a position apparently anterior to the alimentary canal and in close 


Fic. C. Median sagittal section of a Squalus embryo with 14 or 15 somites. 
The neural folds have not as yet met in the mid-dorsal line. X77. The meso- 
derm of the connecting stalk of van Wijhe’s first somite is seen as a thickened 
mass of cells lying between the base of the brain and the dorsal wall of the alimen- 
tary tract. 

1, mesoderm which later becomes differentiated as the connecting stalk of the first 
head cavity. a, mesoderm of the “ anterior cavity ” (Platt); ar’ent., pre-oral pouch 
of the archenteron; ent., dorsal wall of alimentary canal; 7’/d., infundibulum ; 2b. n., 
ventral wall of the neural tube; a 8, projection of plane of the section shown in 


Figure D. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 201 


connection with the ectoderm immediately posterior to the infundibulum. 
Still later (40 somites) cavities appear both in the median connecting 
stalk and in the lateral meso- 
derm, and these by their fusion 
form the continuous cavity in 
the manner already described 
by Miss Platt (’91*). It follows '% ~---4 
therefore that the premandib- 7” ~~. 
ular cavities comprise dorsal 
and only dorsal mesoderm,* 


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OL 


1 Hoffmann (’94, p. 648), how- 
ever, finds evidence of a splanchnic 
portion of the premandibular so- 
mite in a “ Zellstrang, welcher 
dem Mandibularbogen parallel ver- 
lauft und der Vorderfliiche deises Bogens unmittelbar aufliegt” (his Fig. 4 x, 
p. 648). He adds, “Ein Lumen dieses Bogens habe ich im diesem Strange nie 
gesehen,” and he uses this evidence to support his conclusion that the mandibular 
arch is double. JI can confirm Hoffmann’s statement as to the presence of this 
“Zellstrang ” in the anterior portion of the mandibular arch; but there is another 
cord, not mentioned by Hoffmann, which is in every respect similar to this one and 
extends parallel and close to the posterior wall of the arch. I hold Hoffmann’s in- 
terpretation, however, to be incorrect, since, according to my determination, the cells 
of these strands are in large part if not entirely ectodermal in origin, i. e. derivatives 
of the neural crest. The cells of the Anlage of the Trigeminus may be followed in 
closely connected stages as they migrate ventrad until they enter the mandibular 
arch, where they come to surround the mesothelium as a ring of loose cells between 
the mesothelium and the superficial ectoderm. This evidence confirms the pre- 
vious results of Kastschenko, Platt, and Goronowitsch (’93). While the fate of 
these cells is not clear to me, Miss Platt (’94 and ’97) finds that in Necturus they 
contribute in large part to the formation of the cartilage of the mandibular arch. 
Considering the similarity in the origin of the anterior and posterior cell strands, 
as seen in parasagittal sections through the mandibular arch, it becomes note- 
worthy that Hoffmann (’94) in his preliminary paper failed entirely to reproduce in 
his figures the posterior, while in his later paper (96, Taf. III. Fig. 22), he figures 
two cell strands as histologically quite different from each other. This appears to 
me a notable illustration of the prejudicial influence of a theory. Although I 


Figure D. 


Fic. D. Cross section of a Squalus embryo with 11 or 12 somites in a plane cor- 
responding with that of the line a 8 of Fig. C. X 50. The section shows clearly 
the dorsal position of the connecting stalk of van Wijhe’s first somite (I) in rela- 
tion to the pre-oral pouch (ar’ent.). 

I, mesoderm of the connecting stalk of van Wijhe’s first somite; av’ent., arch- 
enteron = pre-oral pouch; cl. crs. n., neural-crest cells; ec’drm., ectoderm; ent., 
entoderm; 2’fb., infundibulum; ¢b. n., neural tube. 


202 BULLETIN: MUSEUM OF G)MPARATIVE ZOOLOGY. 


and it may also be inferred that the median portion of the connect- 
ing stalk is morphologically the undifferentiated anterior portion of 
the chorda, while the more lateral portions of the connecting stalk 
may be regarded, as they have been by Killian (91, p. 102), as 
representing the sclerotome of the somite. Furthermore, the inference 
drawn by Froriep (’92*), on the ground of evidence presented by 
Kastschenko (’88) and Kupffer (’88, 90, 94), that the lumen of the 
connecting stalk must be ventral and morphologically a part of the 
procwlom, receives no support. If Kupffer’s statement that the pre- 
mandibular cavities of Ammoccetes are formed as diverticula from the 
alimentary canal is correct, their development in Ammoccetes must 
differ essentially from that in Squalus. Goette (’90), however, flatly 
contradicts Kupffer’s statements. My own observations on Ammoccetes 
lead me unhesitatingly to accept the evidence presented by Goette.' 
Besides, the criteria furnished by the study of the early stages of devel- 
opment of the premandibular cavity in Squalus seem to me more satis- 
factory, because more decisive, than the evidence used by Kupffer (93°, 
p. 522) to demonstrate the ventral nature of the connecting stalk of 
the premandibular cavities in Ammoccetes, viz. the relation to a blood- 
vessel which is only hypothetically the complete homologue of the dorsal 
aorta. I find this blood-vessel in embryos of Ammoccetes of somewhat 
advanced stages of development (4 mm.) extending above the connecting 
stalk of the premandibular cavities, as the apparent anterior continuation 
of the dorsal aorta, as stated by Kupffer. But there is also ventral to 
the connecting stalk a similar blood-vessel, which unites with the dorsal 
vessel both anterior and posterior to the connecting stalk. It is conse- 
quently difficult for me to comprehend why the more dorsal vessel rather 
than the more ventral one is to be regarded as the anterior continuation of 
the dorsal aorta. Kupffer gives no reasons, simply stating that the ven- 
tral vessel can be homologized, if at all, with the carotis ventralis of Mam- 
malia. Now, if we are to apply rigidly such a criterion as Kupffer’s to 


am unable to accept Hoffmann’s conclusion on the basis of the evidence he pre- 
sents, I believe there are good grounds for holding that a visceral arch, which 
once existed between the mandibular and the hyoid (first and second visceral) 
arches, has disappeared in phylogeny. The evidence in favor of this view will be 
summarized later. 

1 That Kupffer has not in his studies come to a right understanding of the 
development of the anterior head mesoderm seems to me certain from a comparison 
of my sections with those figured by him (’90, Figg. 31 und 82, Taf. 28). The 
cells which he calls ganglionic are in my opinion the anterior mesoderm. This 
appears to me to be Kupffer’s fundamental error. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 203 


determine what is dorsal and what is ventral, it would follow from the 
evidence already stated by Platt (91) that the anterior portion of the 
dorsal aorta in Squalus embryos comes to lie in part dorsal to the chorda, 
and therefore that this organ, commonly known as chorda dorsalis, could 
more correctly be named chorda ventralis. Kupffer’s argument thus 
leads to a reductio ad absurdum. 

According to Hoffmann (’96) the muscles innervated by the oculo- 
motorius have their origin from the posterior part (“ Fortsatz”) of the 
premandibular cavity. Because of the complicated development and 
the secondary subdivisions of this cavity, it is difficult to be certain ; 
yet it seems to me that, as in the case of the second and third cavities, 
the epithelium of both median and lateral walls participates in the pro- 
duction of the muscles formed from this cavity, viz. musc. obliquus 
inferior, and recti inferior, superior, and anterior. 

Before passing to a consideration of the nature of the “anterior 
cavities,” I wish to discuss, in connection with the preceding study of 
the morphology of the eye-muscle somites in Squalus, the evidence of 
the development of the eye muscles of Petromyzon which has been given 
by Hatschek (92) and Kupffer (’94), and to determine in how far this 
brings us to an understanding of the morphology of the eye muscles in 
Vertebrates in general. The repeated confirmation of Marshall’s conclu- 
sion that the eye muscles in Selachii and Reptilia are derived from the 
epithelium of the first, second, and third cavities—van Wijhe (’82), 
Dohrn (’85), Orr (87), Kastschenko (’88), Miss Platt (’91), Oppel (’92), 
Hoffmann (’96), and myself — seems sufficient to remove any doubt (so 
far as those groups of animals are concerned) which Kupffer (94) may 
have sought to throw upon that conclusion. In Amphibia, Birds, and 
Mammals, as is well known, the eye muscles are differentiated from the 
connective-tissue capsule surrounding the eye. Although the source of 
these cells is not known with certainty, there is no reason to doubt that, 
as in Selachii and Reptilia, they have their origin from the dorsal meso- 
derm. In direct contradiction to these facts, which hold true for higher 
Vertebrates, stand the conclusions of Hatschek and Kupffer, that in Cy- 
clostomes the eye muscles are splanchnic in their origin, i. e. derived from 
the mesoderm of the visceral arches. Let us examine the evidence given 
by them, in order to determine in how far it seems to warrant their con- 
clusions. Hatschek’s briefly summarized evidence has been stated on 
pages 192, 193, and needs no repetition. 

In sections of a 5 cm. Ammoceetes I find the relationship of the 
median posterior musculature of the eye capsule to the velar muscle, 


204 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


which is the probable homologue of the muse. adductor mandibule of 
Selachii, to be those stated by Hatschek. Whether in this muscle 
group we have to do with the musc. obliquus superior, I am not able to 
state, since its innervation still remains uncertain to me. I know, how- 
ever, that it is not innervated by the oculomotorius. Its fibres, more- 
over, are not continuous with those of the velar muscle at this stage of 
development, if indeed they are at any stage. Hatschek’s chief evi- 
dence that this muscle is derived from the velar muscle apparently 
consists in their histological resemblance, which he states is complete. 
At the stage studied by me this is certainly untrue. For I find that 
while the velar muscle is composed of large fibres, at least 7 in diam- 
eter, the fibres of the muscle in question are in their widest part not 
over 3u in diameter, and also that, while the fibres of the former show 
well marked longitudinal and cross striations, those of the latter show 
these very faintly. Moreover, the nuclei of the former are for the most 
part round or oval, while those of the latter are exceedingly elongated. 
It is of course possible that Hatschek bases his statements on the exam- 
ination of the histological conditions in embryos of a different stage of 
development. But even if we grant that the musc. obliquus superior in 
Cyclostomes is, as in the Selachii, derived from the dorsal part of the 
musculature of the mandibular arch, this evidence no more warrants the 
conclusion that the muscle is splanchnic in origin in the former group 
than in the latter. Of its dorsal origin and somatic nature in the latter 
group, proof has been given above. 

Even more theoretical than his conclusions concerning the origin of 
the muse. obliquus superior appears Hatschek’s inference that the eye 
muscles innervated by the oculomotorius are derived from the con- 
strictors of the visceral arches, a conclusion which he draws apparently 
by the method of exclusion. It does not seem to have occurred to him 
that these muscles may have had their origin from the connective-tissue 
capsule of the eye, the cells of which are in my opinion derived from the 
dorsal mesoderm in this region, which in early stages becomes disinte- 
grated and surrounds the eye vesicle. Kupffer (94) thinks that the 
more difficult part of the task of tracing the development of the eye 
musculature in Ammoccetes is accomplished when he has followed the 
growth of muscle cells from the so called “ Trabekular” and the mandib- 
ular arches until they come into close relation with the eye capsule in 


1 That Hatschek (’92) incorrectly identified the musc. rectus posterior, has been 
shown by M. Fiirbringer (’97) from the study of its innervation, a matter to which 
Hatschek seems to have paid no attention. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 205 


a6mm.embryo. In consideration of the facts that he does not even 
know that these muscle cells become differentiated into the eye muscles, 
and that he has not determined their innervation, the doubt which he 
seeks to throw upon the results which differ from his own appears quite 
unwarranted. Furthermore, I find that the anterior and posterior velar 
muscle strands described by Kupffer are in essentially the same relations 
to the eye capsule in stages of 6—9 mm. as in those of 5 cm., and that 
these strands show no relation —except that relation of the posterior 
(mandibular) muscle strand described by Hatschek (’92)—to the eye 
muscles, which are already clearly differentiated in the latter stage. I 
must therefore conclude that Kupffer has not seen the early stages of 
the development of the eye muscles of Ammoccetes. I regard the 
determination of their origin in this animal as an embryological task 
yet to be accomplished, —a task in which the well known difficulty of 
obtaining material in stages between 9 mm. and 30 mm. will be encoun- 
tered. For it is in these stages, in my opinion, that the eye muscles 
are differentiated. 

I turn now to the development of the “anterior cavity,” which has 
been so thoroughly studied by Miss Platt (91, ’91*) and by Hoffmann 
(796) that I need say but little, and that of a general nature. It 
seems very clear, since the “anterior” mesoderm segment develops 
from a perfectly solid mass of cells anterior and lateral to the infundi- 
bulum of the brain, that the statement of their formation as lateral 
diverticula of the alimentary canal is purely hypothetical. It seems also 
warrantable to infer that the connecting stalk which unites the lateral 
halves of the segments in early stages of development, the cells of which 
according to Hoffmann (’96) entirely disappear, represents in part the 
anterior continuation of the alimentary canal. But it is impossible 
to state, because of want of such criteria as chorda and dorsal aorta, 
whether we have here to do with dorsal mesoderm. Without proof to 
the contrary, and with the evidence that these cavities assume a histo- 
logical appearance similar to that of the following ones, I conclude with 
Platt and Hoffmann that the “anterior” mesoderm segment, which 
appears, so far as is known, in only two Selachii (Squalus and Galeus), 
is serially homologous with those behind it. I am able to confirm the 
evidence given by these two observers, that mesenchyma cells migrate 
into the lumen of the cavity in the later stages of its development, and 
to confirm the former, that such cells first migrate from the median wall 
(Figure E), in which also some cells assume an elongated spindle form, 
possibly indicating rudimentary muscle cells. Such histological evi- 


206 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


dence would seem to tell in favor of the view that this mesoderm 
segment, like the following ones, is to be regarded as of somatic 
value.? 


Figure E. 


d. SUMMARY. 


Neuromeres and somites show an exact numerical correspondence 
_ throughout the length of the embryo. ‘The serial alternation of myelo- 
meres and somites evinces the metamerism of the former, while the 
exact numerical correspondence of the encephalomeres and head so- 
mites appears equally convincing evidence of the metameric value of 
the encephalomeres. The head somites in Squalus are homologous with 
those described by van Wijhe (82) for Scyllium and Pristiurus, and 
there is yet another anterior to these, viz. the “anterior” somite first 


1 As the most anterior of the cavities of the Selachian embryo, it would seem 
more probable that the anterior cavities described by Miss Platt should be ho- 
mologized with the “head cavities” (vordere Entodermtasche) of Amphioxus, 
than that the next following, the premandibular, should be. 


Fic. E. A cross section through the “anterior cavity ” (frontal section of 
the embryo) in an embryo with 78 somites. X< 240. To show the proliferation of 
cells into the myocel from the median wall of the cavity. 

* 1, 2, first and second head cavities; a, “anterior cavity” (Platt); ec’drm., 
ectoderm; vs. opt., optic vesicle. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS.: 207 


seen by van Wijhe in Galeus. The somatic value of the post-otic head 
somites is indisputable. The pre-otic somites, five in all, are also in my 
opinion homodynamous with trunk somites. They are segments of the 
dorsal mesoderm (with the possible exception of the “ anterior”), which, 
as exemplified in the third somite (van Wijhe’s), become differentiated 
into myotome and sclerotome. While the “anterior” and the fourth 
somites become rudimentary and develop no muscle fibres, the eye 
muscles are differentiated from the median and lateral walls of the first, 
second, and third. The eye muscles of Selachii are therefore somatic 
in their origin, not splanchnic,’ as has been held by Hatschek (’92) 
and Kupffer (94). It will furthermore be shown that the nerves 
which supply them are serially homologous with ventral spinal nerves. 
It is to the consideration of the nerve relations that I now pass. 


AVA Ee The Relation of Neuromeres to Nerves. 


Ahlborn (’84*) said: “ Es bleibt auch im Auge zu behalten, dass die 
gesammte Neuromerie secundirer Natur ist: sie ist nur eine Wieder- 
hohlung aller vor ihr entstandenen Metamerien des Korpers. Line 
primiire Metamerie, wie sie z. B. im dorsalen Mesoderm vorliegt, ist 
weder im centralen, noch im periferischen Nerven-system vorhanden,’ 
und wenn im Rumpfe die Neuromerie mit der primiren Mesomerie 
iibereinstimmt, so reicht diese Eigenschaft im Allgemeinen nur so weit, 
als die Nerven sich innerhalb des primir segmentirten Mesoderms be- 
finden, und sie hért auf, wenn die Nerven in solche Organe eintreten, 
die ausserhalb der Mesomeren liegen, oder die in einer anderen nicht 
segmentalen Metamerie entwickelt sind.” 

In the trunk, the arrangement both of myelomeres and nerves is 
clearly metameric, being correlated with the segmentation of the meso- 
derm. Related to each mesodermal somite is a ventral nerve (motor 
root), which arises from segmentally arranged groups of ganglionic cells 
in the anterior (ventral) horn of the spinal cord, that is, from each myel- 
omere. Into the posterior (dorsal) horn pass the fibres of the dorsal 
nerve, which have their peripheral distribution in the skin of that seg- 
ment (rami cutanei) and in the intestine (sensor and motor sympathetic 
fibres). 

In a study of the simple, and it has been assumed primitive relations 
in the trunk, it is important to consider not only the peripheral distribu- 


1 With the exception of the musc. rectus posterior (Hatschek). 
2 Compare Froriep (’94). 


208 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


tion of nerve fibres, but also their distribution in the central nervous 
system. Gaskell (’89) has rightly insisted that the position of the cell 
groups which are in connection with the nerve fibres, is the true crite- 
rion of what forms a nervous metamere, rather than the position of the 
exits of the nerve fibres. The shifting of nerve roots is too well known 
to need discussion here. In regard to sensor nerves Miss Platt (96) 
says: “ Both development and comparative anatomy tend to show 
that it is a matter of little moment whether these fibres [of the lateral- 
line nerves] enter the brain by one nerve root or another.” I find as a 
result of my own studies that the ganglionic cells of cranial nerves enter 
into fibrillar relation with the neural tube at points quite widely sepa- 
rated from the encephalomere from which the cells were proliferated, 
and also that in embryos of different Vertebrates the relations of the 
fibres of the same nerves to the encephalomeres are variable, not only 
in the case of ganglionic roots but of medullary roots also, as those of 
the trigeminus, abducens, and glossopharyngeus. In the swine and the 
chick the abducens arises from encephalomere VI, whereas in 8S. acan- 
thias it is in relation with encephalomere VII. Also in swine and chick 
the root of the glossopharyngeus is in relation with encephalomere VII, 
whereas in S. acanthias it passes from the neural tube posterior to this 
neuromere. It is obvious, then, that we must take into consideration, 
particularly in the case of cranial nerves, both the location of the 
“Kerne ” of the medullary roots, and the points or regions of prolifera- 
tion of the ganglionic cells of ganglionic roots, in order to determine 
their primitive relationships. 


a. HistoricAL REVIEW. 


An examination of the literature bearing on the question of the re- 
lation of nerves to neuromeres is rendered difficult by the fact that 
many investigators have failed to distinguish between medullary and 
ganglionic nerve relations, and thus have not made clear what they 
meant by the statement that a nerve “develops” from, or has its 
“origin” from, the expansion or constriction of a neuromere. The 
figures of McClure (90) and of Waters (92), for example, show a pro- 
liferation of the ganglionic Anlagen of nerves from the neuromeres, but 
not the relationship of the neuromeres to nerve fibres. While it seems 
very probable that the proliferation of ganglionic Anlagen has a bearing 
on the primitive relationship of the dorsal nerves (sensor portion), our 
best criteria of the segmental value of encephalomeres, as well as of 
myelomeres, is their relation to medullary nerves, —i. e. ventral nerves 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 209 


and the motor components of the dorsal nerves. Furthermore, we 
must determine the primitive relations of medullary nerves, not by the 
place of exit of their fibres (i.e. by their roots), for we know these to be 
variable, but by the position of their “ Kerne” in the walls of the neural 
tube.! 

There is considerable difference of opinion as to whether nerves 
(“roots”) arise primarily from the expanded portion of the encephalo- 
mere (or myelomere), or from the constrictions between these segments. 
As early as 1878 Marshall said, ‘ My investigations tend very strongly 
to prove that all the nerves arise primitively from the widest parts of 
the dilated vesicles, whether of brain or cord, and never from the inter- 
vening constrictions.” Later, McClure (’89), who is in agreement with 
Marshall as well as with Orr, Béraneck, and Waters, said, “ The dorsal 
roots of spinal nerves take their origin from the apex of their respective 
myelomeres in exactly the same manner as the nerves of the medulla 
do from their respective encephalomeres.” Minot (’92) criticises McClure 
for overlooking the fact that the “ neuromeres can have no genetic rela- 
tion to the ganglionic nerves.” The ground of Minot’s statement does not 
seem to me to be so self-evident as not to be in need of explanation. 

In disagreement with McClure, Miss Platt (’89) claimed that “the 
concavity in both medulla and spinal cord is the source from which the 
nerve originates,” and her conclusion, which Minot accepts, is that 
the origin from the expanded portion of the neuromere is secondary. 
In view of this difference of opinion it is of interest that Balfour (’85) 
stated that in Selachian embryos the dorsal and ventral roots of spinal 
nerves alternate with each other, the dorsal roots being intersegmental 
(intersomitic) and the ventral roots segmental (somitic) in position. 
Miss Platt did not, however, in her statement of nerve relations make 
a distinction between dorsal and ventral nerves. 


1 The most serious obstacle to the use of this criterion is the difficulty of apply- 
ing it in those early stages of development when metameric relationships appear 
least modified. Martin (’90 and ’91, p. 230) has noted an ontogenetic ventral 
shifting of motor “ Kerne” in the cat. 

2 It is to be regretted that McClure gave no figures of the nerve relations of 
myelomeres. Minot apparently assumes that the neuromeres are constituted 
solely in adaptation to a motor segmentation, and therefore that the neuromeres 
are segmental localizations of ganglionic cells (i.e. motor “ Kerne’’) in the wall of 
the neural tube, just as are the segmental ganglia of Annelida. It seems to me 
therefore that McClure might have met Minot’s criticism by reminding him that 
neurologists have recognized in the medulla groupings of ganglion cells which are 
in relation with sensory fibres, i. e. sensory “ Kerne ” or “ Endkerne” (see Edin- 
ger, 96, p. 366), and may well contribute to the metameric enlargements. 

VOL. XXXI.— NO. 7. 5 


210 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


6. NERVE RELATIONS IN THE TRUNK OF S. ACANTHIAS. 


An examination of sections in the trunk region of embryos of S. acan- 
thias leaves no doubt whatever that the chief proliferation of ganglionic 
cells occurs in the regions of constriction between myelomeres, i. e. 
opposite the somites, and that the ventral roots also arise opposite the 
somites. Motor roots appear long before the sensor roots, as was first 
stated by Sagemehl (’82). Dohrn has affirmed that they arise as early 
as Balfour’s stage H. I find them in embryos of S. acanthias in which 
34 somites are differentiated, stage H. From the very first, i.e. at 
this early stage, they are in relation with the ventral portion of the 
neural tube at a point directly opposite the middle of the somite. That 
the relation with the tube is opposite the middle of the somite is most 
easily demonstrated in frontal sections (see Plate 6, Fig. 42, which 
represents a frontal section of an embryo with 50 somites); but that 
their relation is with the ventral wall of the tube, is most clearly seen 
in cross sections (Plate 6, Fig. 41, rz. v.). In frontal sections more 
dorsally situated than those which show the ventral roots, the spinal 
ganglia are likewise seen to lie opposite the middle of the somites? 
(Plate 6, Fig. 43). In later stages, however, the spinal ganglia lie 
opposite the anterior portion of the somites, i. e. intersomitic in position, 
as a result, probably, of the shifting of the somites. Since by this time 
the constrictions between myelomeres have disappeared, zt ts quite im- 
possible to state that dorsal roots arise either from the constrictions or from 
the dilatations of the myelomeres. 

McClure (’90, p. 42) has said that in the forms studied by him “ the 
dorsal branches of the spinal nerves pass from the external surface of the 
myelomeres to the space between two somites, which is opposite their 
point of origin, and fuse with the epiblastic thickenings to form the 
spinal ganglia.” Such a statement, if true, is certainly of great impor- 
tance in settling the question of the morphology of cranial nerves. For 
it is now generally stated by morphologists that the chief distinction be- 
tween spinal and cranial nerves consists in the fact that the ganglia of 
cranial nerves receive cellular material during development from the 
ectoderm of the lateral surface of the head, whereas the spinal ganglia 
do not. So far as I know, McClure’s statement remains unconfirmed, 


1 Similar relations of dorsal ganglia and ventral roots have been shown by Mar- 
shall (’78, Plate III. Figs. 27 and 28) for birds; by Hoffmann (90, Taf. CLV. Fig. 7) 
for reptiles; by Dohrn (’91, Taf. V. Figg. 16 und 17) for Selachii; and by Sewert- 
zoff (’95, Taf. V. Fig. 16) for Amphibia. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. At 


and it is certainly not true for Squalus, and not true, so far as I am 
able to determine, for Amblystoma. In Petromyzon, however, as has 
been previously stated by Scott (’87) and Shipley (87), the spinal gan- 
glia lie opposite the constrictions between the somites (in later stages 
opposite the myosepta).*_ Thus, inasmuch as the dorsal nerves of Ammo- 
coetes are tntersomitic and never unite with the ventral nerves which 
are somitic in position, and inasmuch as the dorsal ganglia show close con- 
nection with the ectoderm in early stages of development and lose this 
connection during development, the spinal nerves of this animal form 
a natural transition from the nerves of Amphioxus to those of Squalus 
and higher Vertebrates. For in Amphioxus ventral nerves are somitic 
in position, dorsal nerves intersomitic, and the connection of the ganglia 
of the latter with the skin is retained throughout life.2_ Two chief causes 
seem to have brought about the change in the relations of the dorsal 
spinal nerves in the Vertebrate series. The first cause appears to have 
been the great dorsal and anterior extension of the trunk myotomes, and 
the second cause the posterior extension of the ramus cutaneus dorsalis 
vagi (ramus lateralis vagi), which takes the place of the rami cutanei of 
the spinal nerves. The physiological reason for the extension of the 
vagus is to be found in the advantage obtained from the centralization 
of sensory impulses in the brain. With van Wijhe (’92), Hatschek (93), 
and M. Firbringer (97), I accept the theory of Prochaska, Simmering, 
and Gegenbaur that cranial and spinal nerves are homodynamic, and the 
view of Hatschek (’92) that dorsal and ventral nerves primitively alter- 
nated with each other.* Of these, the former were mixed in function and 
the latter motor, as in Amphioxus. 


ec. NERVE RELATIONS IN THE CEPHALIC REGION OF S. ACANTHIAS. 


In the head, where the nerve relations are much more complicated, it 
will be necessary to trace the development of the nerves in different 
stages. The series represented in Figures 7 to 21 (Plates 3 and 4) is 
intended to show the changes which the neural crest (colored in blue) 
undergoes, and likewise to show the development of the brain vesicles 


1 Because of this relation to the myomeric constrictions in Ammoceetes and the 
relation of the ganglia to the expansions of the spinal cord (myelomeres) deducible 
from it, it is obvious that not very great morphological value can be given to the 
fact that in Squalus the ganglia lie opposite the constrictions of the spinal cords. 

2 JT hold with Hatschek (’92) and M. Fiirbringer (’97) that in Amphioxus the 
homologues of the dorsal ganglia of Craniota are found in the cell groups at the 
place where the dorsal nerves meet the skin. 

8 See also Ransom and Thompson (’86). 


Zi2 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


up to the time when a fibrillar connection of the nerves with the neural 
tube is effected and the chief peripheral branches are differentiated.? 

Minot (’92) and Mitrophanow (’93) have stated that the neural crest 
in Selachii is not differentiated before the closure of the neural tube, and 
Rabl (’89) found that in Pristiurus embryos the “ Trigeminus Anlage ” 
first appears at a stage with 18 somites. On the other hand, Beard 
(88) and Dohrn (90) have shown that in some Selachii,? as well as in 
Sauropsida, the neural crest is differentiated in the region of the head 
before the closure of the neural tube. 

As has been previously stated, my observations confirm those of Beard 
and Dohrn, since I find that at an early stage, when the cephalic plate 
is still widely open, the fundament of the trigeminus is clearly differen- 
tiated from that portion of the neural plate which is destined to form 
the neural tube. The disassociation of the neural-crest cells in this 
region and their resultant loss of compact arrangement have taken place 
to a considerable extent before the neural folds meet in the mid-dorsal 
line. Usually the neural folds first close in the trunk region behind the 
cephalic plate, and later in the region of the midbrain, i. e. in the region 
of the “Trigeminus Anlage.” The closure of the cephalic plate occurs 
last in the forebrain, where the “ neuropore ” persists for a considerable 
period. 

At a stage with 15 or 16 somites (Plate 3, Fig. 7), when the cephalic 
plate is closed except in the region of the forebrain, the neural crest is 
clearly differentiated in that region of the brain which extends from the 
constriction between forebrain and midbrain to the anterior constric- 
tion of hindbrain neuromere (encephalomere) IV, i. e. in the region 
of the so called cephalic flexure. In the region of encephalomere IV 
a few cells with protoplasmic processes occur in the space between the 
neural tube and the overlying ectoderm. These may indicate that at 
one time this encephalomere was a region of cell proliferation and thus 
possessed a neural crest; but since the cells soon disappear, and since 
no new ones take their place, this encephalomere may be said to be a 
region of the neural tube which now (in S. acanthias) possesses no neu- 
ral crest. That portion of the neural crest which arises anterior to this 
neuromere has been variously called ‘‘ Trigeminus Anlage,” “ germe du 


1 A study of the histogenesis of nerve has been made only in the case of the 
eye-muscle nerves, whose morphology still remains a matter of much dispute. 

2 I am surprised by Hoffmann’s (’94) statement that in S. acanthias the tri- 
geminus Anlage first appears in an embryo with 17 somites, that is, after the closure 
of the neural tube. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 213 


Trijumeau ” (Mitrophanow, ’93), and “ erste periaxiale Strang ” (Gorono- 
witsch, 93). Its cells at this stage (15 or 16 somites) have already 
migrated half way down the side of the neural tube (Fig. 7). In the 
region of encephalomere V the disassociation of the cells of the neural 
crest has begun, and the dorsal part of the encephalomere in consequence 
appears enlarged. A ventral migration of its cells, however, does not 
take place until a later stage. 

In an embryo of 18 or 19 somites (Plate 3, Fig. 8) two regions of cell 
proliferation, separated sharply by encephalomere IV, are seen, Mi- 
trophanow (’93) has stated that at the beginning the facialis is not 
wholly separated from either the trigeminus or the vagus group. I find 
on the contrary, as already stated, that no neural crest is found in the 
region of encephalomere IV, and that consequently the “ Trigeminus 
Anlage” is separated by the space of this encephalomere from the pos- 
terior portion of the neural crest. Apparently as a consequence of cell 
proliferation and migration, the dorsal wall of encephalomere III is very 
thin at this stage, while that of encephalomere IV is considerably thicker 
and its cells are more compactly arranged. The cells of the neural ridge 
which form the “ Trigemiuus Anlage ” now extend ventrally as far as van 
Wijhe’s second somite. The second region of cell migration is at this 
stage sharply confined to encephalomere V. LBehind this a disassocia- 
tion of neural-crest cells has begun in the region of encephalomere VI, 
but no migration has taken place. From an examination of later stages, 
the cells proliferated from the region of encephalomere V are easily 
proved to pass ventrally into the hyoid arch, and to form the gangli- 
onic Anlage of the acustico-facialis. From a study of mitotic cells and 
from the grouping of cells one is led to believe that the greatest cell 
proliferation takes place in the posterior part of this neuromere. 

It is to be noticed that the advancing ventral end of the ganglionic 
Anlage extends toward the cleft between van Wijhe’s third and fourth 
somite. Also that cell processes from each of these somites now extend 
toward the ganglionic Anlage. 

When the embryo possesses 19 or 20 somites (Plate 3, Fig. 9) the 
“Trigeminus Anlage” shows a differentiation into an anterior smaller 
portion, which passes in front of the midbrain vesicle toward the optic 
evagination, and a posterior larger portion, which extends ventrally into 
the mandibular arch, just beneath the superficial ectoderm and external 
to the second somite. I am inclined to believe that this division of the 
Anlage is partly due to the enlargement of the vesicle of the midbrain, 
since frontal sections show that the lateral wall of the midbrain lies very 


214 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


close to the ectoderm. It is evident that the neural-crest cells migrate 
around the most expanded portion of the vesicle, so that they come to 
lie in the regions of constriction anterior and posterior to the dilated 
vesicle. They migrate, as it were, into the spaces where there is room 
for them. The cells of these two portions are in continuity dorsally, as 
in the previous stage. As a result of the expansion of the dorsal wall of 
the neural tube in the region of encephalomere III, the cells of the 
neural crest are laterally displaced in this region, so that they appear in 
optical sagittal section (Fig. 9) to have taken a more ventral position. 
Mitrophanow (’93) has given the name “le groupe nerveux anterieur” 
to the anterior smaller portion of the trigeminus Anlage, and states 
that “dans la plupart des cas, ce groupe est peu séparé” (i.e. from the 
“oroupe du nerf trijumeau”). Coggi (95) finds that in Torpedo this 
anterior portion of the trigeminus Anlage arises as a paired structure, 
the lateral halves of which secondarily unite in the mid-dorsal line ; 
Coggi, however, agrees with Mitrophanow that this anterior part of the 
trigeminus is at first distinct from the posterior larger portion. In 
S. acanthias, however, I find that both anterior and posterior parts 
form at first a continuous neural ridge, which lies dorsal to the midbrain 
vesicle. Only in later stages does the anterior portion become separated 
as the so called thalamic nerve. At the stage with 19 or 20 somites the 
cells proliferated from encephalomere V extend somewhat farther ven- 
trad toward the hyoid arch than in the preceding stage, and at the same 
time a proliferation of cells from the mesoderm extends dorsad to meet 
them. The mesodermal cells migrate from both sides of the constriction 
between van Wijhe’s second and third somites, and from them extends a 
cellular process toward the ganglionic Anlage.’ 

The conditions remain practically unchanged in a stage with 21 or 22 
somites (Plate 3, Fig. 10). The anterior and posterior portions of the 
trigeminus Anlage now extend into the region ventral to the midbrain 
vesicle, and are about to unite with each other. The cells in the region 
of encephalomere III have undergone a still greater lateral displace- 
ment, from which one may infer that cells are no longer proliferated 
from the neural crest of this encephalomere. It is seen that the cells of 
the acustico-facialis are now united with the cellular process from the 

1 I have been unable to determine that these mesodermic cells participate in the 
formation of the Anlage of the nerve. It appears to me, however, that such a re- 
sponse on the part of the somites to the development of a nerve Anlage is a fact 
which cannot be ignored in dealing with the question of nerve development. See 


also similar evidence in the description of the development of the trochlearis and 
oculomotorius. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 215 


mesoderm.! This process may be traced dorsally to a point outside of 
the ganglionic Anlage, i. e. between it and the superficial ectoderm. 
The future course of the nerve is along the line of the process. Between 
this and the next succeeding stage, which is represented in Figure 11, 
the trigeminus Anlage undergoes a considerable change. The anterior 
(thalamic) and posterior (trigeminal) portions having fused ventrally 
below the lateral midbrain swelling, now extend ventrad as a continuous 
sheet with two ventral processes, one reaching into the mandibular arch 
and the other to a point below the eye vesicle. The anterior (thalamic) 
portion has assumed a more compact appearance, and extends from the 
region of the constriction between forebrain and midbrain, both ventrad, 
to a point above and behind the eye vesicle, — where, as already stated, 
it meets the anterior prolongation of the trigeminus portion, — and an- 
teriad to a point in front (dorsad) of the eye vesicle. The acustico- 
facialis Anlage now extends into the hyoid arch, dts position being clearly 
inter-somitic. Posteriorly, in the region of encephalomere VI, and to a 
considerable extent behind this, the cells of the neural crest have begun 
their ventral migration. At this time, then, a continuous neural ridge 
or crest extends from the anterior boundary of encephalomere V back- 
ward into the region of the spinal cord. In cleared specimens and in 
parasagittal sections the neural crest cells seem discontinuous in the 
region of constrictions between encephalomeres IV, V, and VI. Both 
Rabl (’92) and Hoffmann (’94) have held that the pre-auditory portion 
of the neural crest is discontinuous with the post-auditory portion, and 
Rabl considers this another proof that the pre-auditory region is one 
“sui generis.” On the other hand, Dohrn (90) and Mitrophanow (93) 
have stated, like the present author, that they find the crest continuous 
in the two regions. 

A well marked proliferation of cells seems to take place in the region 
of encephalomere VI. These cells may be traced continuously into later 
stages, until they enter the first branchial arch and form the Anlage of 
the glossopharyngeus. Since previous investigators, with the exception 
of Herrick and Broman (see Table IT., p. 152), have stated that the glosso- 
pharyngeus is related to hindbrain nenromere VII,” it seems well to call 
attention to the fact that the cells of the ganglionic Anlage of this nerve 


1 Ts this mesodermal process the median branch of Kupffer’s typical segmental 
nerve? Its relation to the mesoderm leads me to believe that this is the case. It 
soon disappears, as stated by Kupffer (91). 

2 Miss Platt (’89) stated that the glossopharyngeus is connected with the pos- 
terior constriction of encephalomere VI. 


216 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


are proliferated from the region of encephalomere VI, the greatest pro- 
liferation occurring, however, as in the case of encephalomere V, in the 
posterior part of the eucephalomere. No previous observer has stated 
that the cells of the ganglionic Anlage of the ninth nerve are proliferated 
from encephalomere VI. However, that previous observers have seen 
the proliferation of cells from this encephalomere is possibly shown by 
the fact that both Shipley (’87) and Kupffer (’94) have found in Petro- 
myzon, between the Anlagen of the 7th and 9th nerves, a “weak 
primitive acusticus, which soon vanishes.” Hoffmann (’94) stated that 
in Acanthias embryos with 32 to 35 somites, a new outgrowth appears 
between the facialis and the glossopharyngeus, which to all appearance 
is a rudimentary and early aborting segmental nerve. Although Hoff- 
mann published no figures, I infer from his description that this out- 
growth, or rudimentary nerve, is that portion of the neural ridge which 
is proliferated from the region of encephalomere VI. I am at least able 
to say positively that no other outgrowth of cells takes place just pos- 
terior to the Anlage of the acustico-facialis. In the phenomena pre- 
sented by this outgrowth Hoffmann finds the chief support for his 
contention that the Anlagen of cranial nerves arise as paired segmental 
outpocketings of the neural tube, corresponding to, or comparable with, 
the outgrowth of the eye vesicles. He figures diagrammatically the out- 
growth of the neural crest in the region of the glossopharyngeus Anlage 
as an outpocketing of the dorsal wall of the neural tube possessing a 
lumen continuous with that of the tube. At no time do I find evidence 
of a lumen between the neural-crest cells, although in later stages the 
nuclei in the VII and IX ganglionic Anlagen tend to take a peripheral 
position. 

At a stage with 26 or 27 somites (Plate 3, Fig. 12) the thalamic por- 
tion of the trigeminus Anlage is no longer continnous dorsally with the 
posterior portion of the Anlage, the cells of which come to lie in the 
region of constriction between midbrain and hindbrain. The thalamic 
portion extends from the constriction between primary forebrain and 
midbrain toward the eye vesicle, just behind which it unites with a line 
of cells, ectodermal in origin, which extends along the dorsal border of 
the eye close to the superficial ectoderm. Some of the cells of the 
trigeminus Anlage now extend into the mandibular arch, and have there 
come to surround the mandibular mesoderm. 

A displacement of the cells of the Anlage of the acustico-facialis and of 
the glossopharyngeus has begun at this stage. This is clearly to be ac- 
counted for by the invagination of the auditory epithelium, which is now 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 217 


beginning opposite encephalomere VI. In parasagittal sections the 
Anlage of the glossopharyngeus appears clearly distinct from that of the 
vagus, while in the median plane they are seen to be continuous portions 
of the neural crest. 

When the embryo has 28 to 30 somites (Fig. 13) the conditions, so 
far as the trigeminus is concerned, are practically unchanged. Neural- 
crest cells still persist in the regions of constriction between the primary 
brain vesicles. Thus, three strands of neural-crest cells are seen to lie 
in the region of constriction between the brain vesicles, just as they do 
in the trunk between the myelomeres. The ganglionic Anlage of the 
acustico-facialis, which had fused with the thickened auditory epithelium 
in the early stages of its development, now, as the nerve Anlage recedes 
from the ectoderm, retains this connection, forming thus the Anlage of 
the acusticus, The acusticus therefore in its development and relations 
resembles a ramus dorsalis of a cranial nerve. 

The cells of the glossopharyngeus have been further displaced. In all 
the specimens of this stage which I have examined, two distal portions 
of the nerve Anlage may be distinguished. The fate of the posterior of 
these is unknown to me. The cells of the anterior portion pass ventrally 
into the third visceral arch, and are related to the constriction between 
van Wijhe’s somites 4 and 5. In precisely the same way the Anlage 
of the seventh nerve occupies the cleft between the third and fourth 
somites. The advancing ganglionic Anlagen pass close to the superficial 
ectoderm in the plane of the constrictions between the somites. Simi- 
larly the Urvagus Anlage meets the mesoderm at the posterior cleft of 
the fifth somite. This fact seems to me to be of some importance in 
considering the question whether the branchial nerves are somitic or in- 
tersomitic in position, and to warrant the conclusion that the cranial 
nerves resemble the dorsal nerves of Amphioxus in being intersomitic, 
as well as in other respects. At a stage with 33 or 34 somites (Plate 3, 
Fig. 14) the trigeminus Anlage retains connection with the mid-dorsal 
line of the neural tube in only two restricted regions, anteriorly by the 
‘thalamic ”’ portion, and posteriorly (in the region of the constriction be- 
tween midbrain and hindbrain) by a strand of cells to which Miss Platt 
has given the name “ primary trochlearis.” Posteriorly the cells of the 
trigeminus Anlage are grouped into a somewhat thickened mass opposite 
the posterior part of encephalomere III, the first indication of the differ- 
entiation of the Gasserian ganglion. The Anlagen of the acustico-facialis 
and the glossopharyngeus have become farther separated by the invagina- 
tion of the auditory epithelium, the displacement affecting the cells of 


/ 


218 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


the glossopharyngeus so much that they now lie opposite encephalomere 
VII. The two nerve Anlagen, however, usually remain connected with 
each other dorsally by a thin cellular strand. This strand is wanting 
in some cases, or may be present on one side of the embryo only. 
Dohrn (’90) has also stated that the separation of the seventh and ninth 
nerves is due to the crowding caused by the ear capsule, and he held 
that the connecting strand of cells was evidence of the original conti- 
nuity of the neural crest on the dorsal side of the ear. Behind the 
glossopharyngeus the neural crest extends in unbroken continuity into 
the trunk, but only its anterior portion, which forms the ganglionic 
Anlage of the Urvagus, extends ventrally between the mesoderm of the 
side plates and the superficial ectoderm into the region of the pharynx. 

In embryos with 38 or 39 somites (Plate 3, Fig. 15) the thalamic 
portion still extends as a compact cellular cord from the region of con- 
striction between forebrain and midbrain to a point above the eye, where 
it unites with the line of ectodermal cells which in later stages forms the 
ophthalmicus profundus trigemini. This nerve, because of its relations 
with the trigeminus, “ primary trochlearis,” and “thalamic” nerves, is 
regarded by Marshall (’82) and Miss Platt (91) as a commissural nerve 
connecting the three nerves mentioned. It has also been regarded as an 
independent nerve (van Wijhe, ’82, M. Fiirbringer, ’97), and as a ramus 
dorsalis either of the trigeminus or the oculomotorius. The acustico- 
facialis Anlage, opposite encephalomere V, is still in continuity with that 
of the glossopharyngeus by means of a cellular cord dorsal to the audi- 
tory invagination, while the cells of the glossopharyngeus and vagus 
Anlagen no longer appear to be continuous dorsally, as they were in the 
previous stage. 

At a stage of development when the embryo possesses 42 to 44 somites 
(Plate 3, Fig. 16), and when two visceral clefts are formed, both the 
thalamic and trochlear portions of the trigeminus Anlage are much re- 
duced. In an embryo with 48 somites the thalamic portion consists of 
a strand or cord of cells which extends dorsally from the ophthalmicus 
profundus, at a point just above the eyestalk, toward the region of con- 
striction between primary forebrain and midbrain, where the two cel- 
lular strands coming from opposite sides of the head unite above the 
wall of the brain. Because of this union, Coggi (’95) has considered 
this portion of the trigeminus Anlage as a connective “nerve,” uniting 
the lateral halves of the ophthalmicus profundus. Its position in Tor- 
pedo, according to Coggi, is anterior to the thalamencephalon. If 
Coggi is correct, its position in Torpedo is clearly different from that 


oe 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 219 


in S. acanthias. Coggi’s account differs, however, from that of Dohrn 
(’90*), who found its relations in Torpedo to be similar to those described 
by Miss Platt (91) for S. acanthias. The relations of the acustico- 
facialis and glossopharyngeus remain unchanged. For a long time 
cellular strands persist, showing the primitive relation of these nerves 
to the constrictions between the encephalomeres IV, V, and VI, re- 
spectively. 

Some important changes in the relations of the neural-crest cells 
appear in the next (48-somite) stage, and are shown in Plate 3, Figure 
17. For the first time, we find “ fibrillar” connections of the trigeminus 
Anlage with the neural tube. Protoplasmic or fibrillar processes ex- 
tend from the cells which lie opposite the constriction between enceph- 
alomeres III and IV toward both encephalomeres. It has been stated 
by some investigators (Miss Platt, 91, Locy, ’95), that this nerve has its 
origin from the constriction between the neuromeres. Two main roots 
are differentiated later, an anterior, in relation with encephalomere III 
(the ‘‘portio minor”), and a posterior, in relation with encephalomere 
IV (the “portio major”). The uearness of the ganglion cells to the 
brain wall renders it impossible for me to determine in which direction, 
whether toward the brain or toward the ganglion, the fibres are first 
developed. The two chief roots of the trigeminus have been described 
for other Vertebrates. 

The thalamic and trochlearis portions of the trigeminus Anlage are 
now much reduced in size, each retaining connection with the rest of 
the nerve fundament by means of an attenuated protoplasmic fibre. 
The acustico-facialis Anlage has assumed fibrillar connection with enceph- 
alomere V, with which it remains connected until the encephalomere 
disappears. Marshall and Spencer (’81, p. 481, ’86, p. 100) have stated 
that in Scyllium “there is an important difference between the fifth and 
seventh nerves, inasmuch as in the former the primary root is lost 
and the secondary alone retained, whilst in the latter both primary and 
secondary roots are retained up to stage N, and indeed . . . through- 
out life. The difference between the roots of the fifth and seventh 
uerves just noticed does not occur in the chick.” They also state that 
in early stages in Scyllium embryos the fifth nerve arises from the brain 
by three distinct roots, but that in later stages only two roots are found. 
Their distinction between primary and secondary nerve “roots” is 
obviously unnecessary, since the only true “roots” are the so called 
secondary ones. Before these are established we have to do with 
neural-crest cells, some of which have been shown to be non-nervous 


220 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


in function, and to contribute to the mesenchyma of the head. The 
ear capsule now lies with only its anterior portion opposite encephalo- 
mere VI. Behind the ear capsule and opposite the posterior portion 
of encephalomere VII lie the cells of the glossopharyngeus, as yet with- 
out fibrillar connections with the neural tube. Behind the glossopha- 
ryngeus and now separated from it lie the cells of the vagus, extending 
ventrally as a broad sheet between the mesoderm and ectoderm into 
the region of the pharynx, where the Anlage becomes segmented by the 
formation of the visceral clefts. The trochlear and thalamic portions 
of the trigeminus soon disappear without assuming fibrillar relation 
with the neural tube. 

At a stage with 52 somites, when the embryo is about 8 mm. in length, 
the thalamic portion remains as a group of cells lying in the constriction 
between the forebrain and midbrain vesicles (Plate 4, Fig. 18), but with- 
out connection with the ophthalmicus profundus. It very soon disappears 
entirely, and I think probably contributes to the loose mesenchyma of 
this region. In precisely the same way the disassociation of cells of 
the trochlear portion takes place, scattered clumps of cells indicating 
its previous extent. The Gasserian ganglion and the ganglion of the 
ramus ophthalmicus profundus (mesocephalic ganglion) are both clearly 
differentiated. Three branches of the fifth nerve may now be distin- 
guished, viz. the two sensor branches, r. ophth. profundus and r. maxil- 
laris (inframaxillaris ? Dohrn), and the mixed mandibular branch. Nerve 
relations to the neural tube remain the same as in the previous stage. 


d. DEVELOPMENT OF THE 
1. OcULOMOTORIUS. 


By the time the embryo has reached the length of 8 mm. (52 so- 
mites), the oculomotorius has however appeared as a fibrillar process 
from the base of the midbrain (encephalomere II, Figures F to H), arising 
as processes from neuroblast cells in the ventral horn of this encephalo- 
mere. Since this nerve throws light on the morphology of the pre- 
mandibular somite, whose musculature it innervates, its development is 
of great interest and has been studied by many investigators ; viz. Mar- 
shall (81), Rabl (89), Dohrn (’91), Platt (91), Mitrophanow (’93), 
and Sedgwick (’94). Neither Marshall (’81) nor Rabl (89) saw the 

1 Kastschenko (’88), Goronowitsch (’92), Miss Platt (’95). z 

2 This is, I believe, the nerve which in Ceratodus van Wijhe (’82) named ramus 


maxillaris superior, which in Amphibia Strong (’95) called accessory branch of 
the fifth, and Miss Platt (96) r. buccalis profundus V. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. Pink: 


early stages of its development, and their conclusions are therefore 
purely theoretical. Both agree in considering the nerve a derivative of 
neural-crest cells. Rabl (’89, p. 221) thinks he has some right to bring 
this portion of the neural crest into genetic connection with these nerves, 
since the course of the third and fourth nerves in later stages corre- 
sponds with a portion of the trigeminus Anlage, which I infer from his 
description to be the “trochlear” portion. He adds, “ Ferner darf ich 


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aus einer Reihe von Beobachtungen, die ich nicht bloss an Selachiern, 
sondern auch an Vogeln und Saugethieren angestellt habe, schliessen, 
dass die Oculomotoriuswurzel, die nach dem gesagten Anfangs ebenso 
wie die des Trochlearis aus der dorsalen Kante des Mittelhirns austreten 
musste, aus dieser Lage allmihlich durch die Ausbildung der Pedun- 
culusbahnen verdrangt und an die ventrale Seite verschoben wird.” 


Fie. F. Left face of a parasagittal section through the left half of an embryo 
with 52 somites, showing the relations of the oculomotorius to encephalomere II 
at this stage. x 50. J, IJ, IJ, first, second, and third encephalomeres; 1, 2, 
van Wijhe’s first and second head cavities; a, ventral fibre tract; oc-mot., oculo- 
motorius; vn. crd., anterior cardinal vein. 


229 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


More theoretical and farther from the truth Rabl could scarcely be, yet 
several investigators have in essential respects confirmed his theory, 
that the oculomotorius is a derivative of the neural crest. Dohrn’s 
(91) observations, however, differ fundamentally from those of his 
predecessors. He sees the beginnings of the oculomotorius in em- 
bryos intermediate between Balfour’s stages ] and K. At first cells in 
the base of the brain assume a more transparent appearance, and later 
migrate into the “ Randschleier,” where they send out processes which 
unite in a network just outside the base of the brain to form the stem of 
the nerve. Immediately at the beginning of the outflow of the plasma 

POS CSCS SHOT SOCR RIG GO 

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Ficure G. 


cells are seen half in and half ont of the wall of the tube, and later, 
but before the oculomotorius has any connection with the mesocephalic 
ganglion, large deeply staining nuclei are seen in the protoplasmic net- 
work which forms the root of the nerve. Dohrn does not lay great 
stress on the fact that these nuclei are larger than those of the surround- 
ing mesenchyma cells, but from the fact that similar nuclei lie nearer 
the medullary wall, from which they appear to emerge in increasing 
numbers during the course of development, he holds the opinion to be 
permissible, that the nuclei of the early network are emerged medullary 
elements, and not mesoderm cells which press close to the medullary 


Fic.G. A portion of the same section as that shown in Figure F.  X 240. 
The fibrillar nature of the oculomotorius is clearly shown. az-cyl., axis cylinder 
process; c/. n’b/., neuroblast cell ; oc-mot., oculomotorius. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 233 


wall. Dohrn considers it as the punctum saliens of the evidence given 
by him, that ganglion cells and ganglia which may be traced to the 
adult are to be found in the course of the oculomotorius before this 
comes into connection with the mesocephalic ganglion, and concludes 
that such ganglion cells can have had no other source than the ventral 
horn of the midbrain. He thus takes the view of Balfour, Marshall, 
Kupffer, and others, that this ventral nerve is formed as a chain of 
medullary cells, in opposition to the views of His (89), Kolliker (92), 
von Lenhossék (’92), and others, that ventral nerves are formed from 
processes of “neuroblast” cells in the ventral horn ‘of the medullary 
tube. 


Figure H. 


Miss Plattg(’91) comes to fundamentally different conclusions from 
those of Dohrn (’91). She finds that the oculomotorius appears first as 
a single cell proliferated from the mesocephalic (ciliary) ganglion toward 
the base of the midbrain, with which it at first has no connection. Ob- 
servations on Squalus, Raja, Pristiurus, and Torpedo convince her that 
the oculomotorius develops after the type of a sensor nerve [?] by a pro- 
liferation of ganglion cells toward the brain wall. _Mitrophanow’s (93) 


Fie. H. Left face of a parasagittal section through the right half of the same 
embryo as that represented in Figures F and G, showing the oculomotorius in an 
early stage of development (52 somites). XX 447. The relation of the nerve fibre 
with an axis-cylinder process from the neuroblast cell z seems clear. az-cyl., axis- 
cylinder process; oc-mot., oculomotorius ; x, neuroblast cell. 


224 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


observations confirm those of Miss Platt. According to Sedgwick (94) 
the third nerve is formed directly from the neural crest as are the dorsal 
cranial nerves {?], but arises as a differentiation of the reticulum formed 
by the breaking up of the neural crest, and first makes its appearance as 
a projection of nuclei from the mesoceplalic ganglion. His observations 
thus do not essentially differ from those of Miss Platt, their conclusions 
differing chiefly by reason of difference in theoretical views as to the 
mode of nerve development. My own evidence differs quite fundamen- 
tally from that given by previous investigators, since I find that the 
nerve develops after the manner described for spinal ventral nerves in 
Selachii and other Vertebrates, as an axis-cylinder process from “ neuro- 
blast” cells in the ventral horn of the midbrain. At the earliest stage 
in which I have been able to detect the oculomotorius the extent of its 
development and its relationships are such as are shown in Figures F 
to H, which represent sagittal sections of a Squalus embryo with 
52 somites (approximately 8 mm. long). At this stage the thalamen- 
cephalon is just becoming differentiated from the primary forebrain 
(encephalomere I). The identification of the fibrillar process as the 
oculomotorius is made easy by a comparison of its point of attachment, of 
the direction of its long axis, and of its histological appearance with those 
of an embryo with 54 somites, where the oculomotorius is already con- 
nected with the mesocephalic ganglion. Under higher powers of the 
microscope the nerve appears as a deeply staining, highly refractive 
process, clearly distinguishable by these characteristics from the granu- 
lar and faintly staining processes of the mesenchyma cells at the base 
of the midbrain. Owing to a shrinkage, which however appears in very 
few of the specimens killed by the fixing agent used (vom Rath’s fluid) 
and always most markedly in the region ventral to the midbrain, the 
mesenchyma cells and the roots of attachment of the nervé have broken 
away from the base of the brain. Since, however, similar deeply stain- 
ing processes are seen to extend from cells in the ventral horn of the 
medullary tube towards the points where the roots may be supposed to 
have once united with the wall of the brain, the inference seems war- 
ranted that the nerve is made up of the processes of these cells. The 
latter show the characteristics described by His (’89) for the neuro- 
blasts of the spinal cord, viz. a highly chromatic nucleus surrounded by 
a thin, very deeply staining protoplasmic ring, which is prolonged into 
the axis-cylinder process. The precipitation of osmium serves to render 
the processes quite opaque and easily traceable among the remaining, as 
yet undifferentiated, cells of the medullary wall, and to make it possible 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 225 


to determine that other processes, instead of leaving the medullary wall, 
extend posteriorly in the wall and parallel with it to form the ventral 
fibre tract. The nerve process (Figure G) shows a differentiation in its 
distal portion into two deeply staining fibrils surrounded by more faintly 
staining plasma, the two fibrils dividing distally into three, which enter 
the fine processes with which the nerve ends. The nerve process on the 
other side of the same embryo (Figure H) does not, however, show this 
same evidence of histological differentiation. Here the nerve appears as 
a highly refractive fibril, and, while having a greater extent than that of 
its mate of the opposite side, is composed, except at its root, of a single 
undivided fibril. The connection of this fibril with the axis-cylinder 
process from a neuroblast cell in the ventral horn seems indisputable, 
since this passes directly through the limiting membrane at the base of 
the brain wall, and projects into the shrinkage space directly opposite 
the chief root of the nerve, as is shown at az-cyl. I have no evidence 
to offer, such as that stated by His (’88, ’89), for Mammals and other 
Vertebrates, of a migration of the neuroblasts from the “ inner layer” 
of medullary cells, nor do I find any evidence of migration of cells from 
the neural tube, as stated by Dohrn (91). I find at this stage neither 
nuclei connected with the roots of the nerve outside the neural tube, 
nor such as are half in and half out of the tube. 
* The connection of the oculomotorius with cells of the mesocephalic 
ganglion is attained very quickly, and in embryos of 54 or 55 somites has 
already taken place. At this stage of development, as seen in embryos 
fixed with the corrosive sublimate-acetic mixture (Davidoff’s fluid), the 
nerve appears (Plate 8, Fig. 58) as a cellular strand, which extends from 
the inner side of the mesocephalic ganglion toward the ventral wall of 
the midbrain, with which the nerve unites by at least two main roots. 
To detect the proximal roots as well as the relations of these with 
medullary cells, sagittal sections are much more favorable than frontal, 
since the nerve roots are situated one behind the other. The fact that 
the nerve is several cells in thickness near the ganglion, while its calibre 
diminishes as it passes toward the brain wall, would naturally, if one 
Were unacquainted with the conditions shown in the embryo of 52 
somites, lead to the inference that the growth of this nerve takes place 
from the ganglion toward the brain (vide Miss Platt, ’91, Mitrophanow, 
1 Also, for the reason already stated by His (’88", p. 344) for spinal ventral 
nerves, that ‘‘die Wurzelbiindel treten in grisseren Abstinden aus dem Riicken- 
mark hervor. Jedes Biindel bezieht seine Fasern aus einem entsprechend breiten 


Bezirk des Riickenmarks. Die Sammlung derselben erfolgt zum Theil noch inner- 


halb des Markes, zum Theil erst in der Leibeswand.” 
VOL. XXXI. — NO. 7. 6 


226 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


°93, Sedgwick, 794). It is interesting to compare the phenomena thus 
observed in specimens prepared by the Davidoff method with those pre- 
pared by the vom Rath method, since the latter clearly differentiates 
the nerve fibrils, and gives the clue as to the meaning of the cells 
proliferated from the mesocephalic ganglion. _ Figure I is drawn from a 
sagittal section of an embryo with 55 somites killed by the vom Rath 
method, and fortunately so oriented as to show the oculomotorius in its 
course from the inner side of the mesocephalic gan- 
glion to a point very near the brain wall. The 
nerve itself is composed of three deeply impreg- 
nated fibrils, which near the brain wall are closely 
united to one another, while peripherally they be- 
come separated. Two lightly ‘staining cells with 
granular protoplasm lie closely adherent to the 
nerve, and with low powers are indistinguishable 
from it. Others appear in the process of migra- 
tion from the mesocephalic ganglion to assume 
similar relation. Whether these cells become ele- 
ments of the oculomotorius ganglion, which would 
thus conform in its mode of development to the 
type of a sympathetic ganglion,’ or whether they 
form the nuclei of Schwann’s sheath, I am not at 
present in a position to state, since I have not 
been able to trace their fate. It is of course pos- 
sible that they contribute to both ganglion and 
sheath. Whether cells from the mesenchyma in 
this region contribute to both of these ends, 
seems to me a question of not great morphologi- 
Fieure IL. cal importance, since in my opinion these cells are 

in great measure, if not entirely, derivatives from 

the neural crest, and thus ectodermal, not mesodermal, in origin. From 
the evidence thus stated it is seen that the oculomotorius must be 


1 Many investigators (Riidinger, Arnold, Gegenbaur, Schwalbe, Hoffmann, Onodi, 
van Wijhe, Dohrn, Beard, Ewart) have, on histological and embryological grounds, 
agreed that this ganglion belongs to the sympathetic system. 


Fic. I. Sagittal section of a Squalus embryo with 55 somites, showing the 
oculomotorius in its course from the mesocephalic ganglion toward the brain. 
> 477. The fibrillar nerve and the peripheral nuclei may easily be distinguished. 
cl. ms-ce., migratory cell from the mesocephalic ganglion; oc-mot., fibres of the 
oculomotorius. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 207. 


regarded as from the earliest stages of development a fibrillar nerve 
formed by axis-cylinder processes“of medullary cells, and that it is no 
more to be regarded as a cellular process or cellular nerve in its earlier 
than in its later stages. The unfavorableness for purposes of nerve 
study of material killed, with the fixing agents commonly used, has 
been the chief cause which has kept us so long from the true under- 
standing of the method of the development of the oculomotorius in 
Selachii. I was at first disposed to consider as of some morphological 
importance the fact that in stages of development before the appearance 
of the oculomotorius a process extends from the mesocephalic ganglion 
to the premandibular somite (Plate 8, Fig. 61). Its earlier appearance 
precluding the view that this process has connection with the oculomo- 
torius, I concluded that it furnishes us with evidence of a primitive rela- 
tion of the ramus opthalmicus profundus with this somite (Plate 8, Fig. 
61). The observations of J. Miiller in 1840, P. Fiirbringer (’75), Price 
(96), and Max Fiirbringer (’97), have established that this nerve possesses 
motor fibres in the Myxinoids, confirming van Wijhe’s view of its segmental 
value. Iam, however, not inclined to lay stress on the fact mentioned 
above as confirmatory of this view, since in later stages (65 somites) I 
also find a similar process, apparently in connection with the “anterior 
cavity” (Plate 4, Fig. 19). 


At a stage with 65 somites (10 mm.) the relations of the trigeminus 
are unchanged (compare Plate 4, Fig. 19). The r. ophthalmicus pro- 
fundus trigemini is well differentiated, and shows a marked fibrillar struc- 
ture, especially clear in embryos killed with vom Rath’s fluid. The nuclei 
seen along the truuk of the nerve are distinctly peripheral in relation to 
the nerve fibres. The facialis nerve (VIL) now possesses four branches, 
viz. the sensor acusticus branch, connected with the median and ventral 
side of the otic capsule ; the mixed hyoid nerve, innervating the muscles 
and skin of the 2d visceral (hyoid) arch; the r. ophthalmicus superfi- 
cialis VII (ophthalmic branch of the 2d trigeminal root of older anato- 
mists), whose sensor fibres develop in close connection with the skin 
along what in the head corresponds with the dorso-lateral line of the 
trunk ; and the r. buecalis VII (incorrectly called supramaxillaris V by 
Wiedersheim), developing along the medio-lateral line of the head. 


1 Allis (97, p. 742) also describes in Amia calva a small and apparently degen- 
erating nerve in connection with the ganglion of the profundus. He however, on 
grounds of the topographical relation of the eye-muscle nerves (III and IV), regards 
this nerve as homologous with the ophthalmicus profundus trigemini. 


228 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


The glossopharyngeus is now in fibrillar connection with the lateral 
walls of the neural tube at a point behind encephalomere VII. The 
fibres from the ganglion cells of the vagus enter the neural tube at a 
point somewhat behind the point of origin of the glossopharyngeus. 
The cells of the two nerve Anlagen, however, still appear continuous. 
Posteriorly, and at the same level as the origin of the roots of nerves IX 
and X, the neural-crest cells appear as a commissure (coms. d.) con- 
necting the vagus Anlage with the ganglia of the dorsal spinal nerves 
Ventrally the vagus divides into four mixed (post trematic) branches, 
each of which innervates the skin and musculature of a visceral arch, 
and posteriorly is continued beneath the skin as the ramus lateralis 
vagi along the medio-lateral line.* 


At this stage, I find the first evidence of the olfactory nerve (I) in 
the form of connecting strands or fibres between the anterior lateral wall 
of the forebrain (prosencephalon) and the thickened lateral epithelium 
of the olfactory plate. The connection between the median portion of 
the “ Riechplatte” and the brain wall (neuropore) has disappeared at a 
somewhat earlier period (8-9 mm.). According to Marshall (’78) and 
Beard (’85) the olfactory nerve develops, as do the other dorsal cranial 
nerves, from cells of the neural crest, and is therefore regarded by them as 
a nerve morphologically comparable with the dorsal cranial nerves. The 
evidence given by van Wijhe (’86*) and Hoffmann (96), however, serves 
in the opinion of these investigators to render this view improbable. 
Van Wijhe (86%, p. 680) states that ‘‘das Riechorgan und der Nerv ent- 
stehen beide aus dem vorderen Neuroporus. Der Olfactorius entwick- 
elt sich nicht aus der Nervenleiste, denn er tritt in einer Periode auf, 
wann dieselbe im Kopfe schon langst geschwunden ist; auch ist er von 
Anfang an mit der Haut in Verbindung und unterscheidet sich durch 
diese zwei Merkmale von allen iibrigen dorsalen Nervenwurzeln. Der 
Riechnerv entsteht also erst nach dem Acranienstadium und in Ueberein- 
stimmung damit ist seine Abwesenheit beim Amphioxus.” 

Confirmatory of this view is the evidence given by Hoffmann (96, 
p- 272) that “der Riechnerv fehlt [in Squalus] aber bis zu diesem Ent- 
wicklungsstadium [10-12 mm.] noch vollstandig und erst bei Embryo- 
nen, welche eine Lange von 133-14 mm. erreicht haben, beginnt er sich 
anzulegen. Bis zu dieser Periode liegt die Riechgrube der Medullar- 

1 Squalus possesses no dorso-lateral line nerve corresponding with that of 
Cyclostomata, Dipnoi, and Ganoidei. I also find no evidence in Squalus such as 


that found by Miss Platt (’94) in Necturus, to show that there once existed a ventro- 
lateral line in Vertebrates. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 229 


wand immer noch unmittelbar an, dies ist auch jetzt noch grodssteutheils 
der Fall, aber mit ihrem medialen Rand fangt sie jetzt an sich von der 
Gehirnwand zuriickzuziehen, bleibt aber mit ihr durch einen kurzen, 
dicken Zellstrang kontinuirlich verbunden. Dieser Zellstrang bildet die 
Anlage des Nervus olfactorius, aber es ist nicht mdglich zu sagen, wel- 
chen Antheil die Epidermis und welchen das Gehirn an der Anlage der 
Riechnerven nimmt, denn er entsteht aus dem letzten Rest des kontinu- 
irlichen Zusammenhanges von Epidermis und Medullarwand, welcher 
von Anfang an bestanden hat.” 

His (’89*) had previously found in the human embryo that the first 
step in the formation of the olfactory nerve was the migration of mesen- 
chymatous cells between the olfactory plate and wall of the brain. 
Later the olfactory ganglion is formed by the migration of cells from the 
lateral walls of the olfactory epithelium. Finally, the olfactory nerve 
results from the assumption by these cells of a bipolar form and the 
elongation of the poles both centripetally and centrifugally to form 
fibrillar connection with brain and olfactory pits. 

My own observations concerning the development of the olfactorius are 
as yet incomplete, and I am not able to add much to the evidence which 
has been given. In agreement with Hoffmann (’96) I find that, as the 
olfactory plate and the brain wall separate, they retain connection with 
each other by faintly staining fibrils in the region of the future olfactory 
pits. Whether these fibrils enter into the formation of the definitive 
olfactorius I am not able to state, and the observations of Hoffmann 
appear to me insufficient to establish this fact. My results and those 
of Hoffmann do not agree; for he finds in embryos of 16 mm., and still 
more clearly in embryos of 18-20 mm., that mesenchymatous tissue 
“schiebt sich von allen Seiten zwischen Medullarwand und basale Nasen- 
grubewand ein, und in demselben Grade als beide sich entfernen, nimmt 
natiirlich der Riechnery an Linge zu.” I infer this mesenchymatous tissue 
to be the same as that which Hoffmann previously states to be derived 
from the “anterior head cavities.” My observations, however, lead me 
to agree with Marshall (’78), that the cells which appear between the nasal 
pit and the brain wall, as these separate, are neural-crest cells. Van 
Wijhe may he technically correct in stating that the neural crest has 
disappeared in the region of the forebrain at the time when the olfactory 
nerve is established ; but it is certainly not true that the neural-crest 
cells in the region of the forebrain have done so at this stage. They 
persist in the region of the forebrain which lies opposite and anterior to 
the optic vesicle, aud in my opinion are the cells which migrate between 


230 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


olfactory pit and brain wall as these separate from each other. I regret 
that I am not yet in position to describe the later differentiation of 
these cells, but it appears to me not improbable that they enter into 
the Anlage of the olfactorius. The evidence given by many histologists, 
from Schulze to Retzius, establishes the ganglionic character of the 
olfactorius ; while the evidence presented by His (’89*) appears confirma- 
tory of the view of Beard, that the olfactory plate is to be regarded as 
the anterior of the sense organs of the lateral line, since from it are 
derived, at least in part, the ganglionic cells of the olfactorius. There- 
fore, if neural-crest cells also entered into the Anlage of this nerve, it 
must be regarded as homodynamous with the sensor component of a 
dorsal segmental nerve. 


2. ABDUCENS. 


Another of the eye-muscle nerves, viz. the abducens, is now (65 so- 
mites, 10 mm.) differentiated. (Compare Figs. 20 and 21 with Fig. 19.) 
The latter nerve has arisen as an outgrowth from neuroblast cells in the 
ventral horn of encephalomere VII, and its roots retain connection with 
this encephalomere until the latter disappears. Zimmermann (’91) stated 
incorrectly that its connection in Squalus is with the neural segment 
which corresponds with my encephalomere VI. Dohrn (’90*) describes. 
the nerve as having its origin from the neural tube opposite the otic 
capsule, and between nerves VII and IX. Its position in different 
Vertebrates seems inconstant. Some investigators (Orr, ’87, Waters, ’92, 
and Herrick, ’92) have stated that in the forms studied by them it arises 
from the hindbrain neuromere corresponding with encephalomere IV of 
my figures. In the chick and swine I have found that its roots are 
in connection with encephalomere VI, whereas in Necturus its fibres 
may be traced from the muse. rectus posterior to a point behind the ear, 
and thus have, as I believe, their origin from a segment of the hindbrain 
corresponding with encephalomere VII. At least, in this form, as in 
Squalus, it appears as a post-otic nerve. Dohrn (’91) gave a careful 
account of the early stages of its development in embryos of various 
Selachii. He states that the nerve first appears at a stage corresponding 
with Balfour’s stage L, arising by two roots which unite at a short dis- 
tance from their point of exit from the ventral wall of the neural tube. 
In Mustelus the roots are more numerous than in the other forms exam- 
ined, there being as many as six on each side of the brain. The roots 
are directed backward, as in the case of spinal nerves, but later form a 
network from which arises the stem of the nerve; this runs forward, 


92 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. Bol 


parallel to the neural tube, toward van Wijhe’s third somite. In later 
stages the nerve divides into two branches, one passing along the inner 
side of the somite, and the other along the outer side. At first the nerve 
root, which appears as a plasma outflow from the neural tube, is of the 
thickness of one, or at most two, medullary cells. Later the roots 
increase in thickness, apparently by the continued outgrowth of plasma 
from the neural tube, as well as by the migration of cells from the 
ventral horn of the medulla. The larger size and different staining 
qualities of the medullary cells enabled him to distinguish them from 
the mesodermal cells in their vicinity. Such (medullary) cells are often 
found with a part of the nucleus within and a part without the medul- 
lary wall. This outflow (migration) of medullary cells takes place also 
in later stages after the white substance has become quite thick on the 
side wall of the neural tube.’ 

My observations upon the development of the abducens differ from 
those of Dohrn, as in the case of the oculomotorius, inasmuch as I find 
the nerve to arise from axis cylinder processes of neuroblasts in the 
ventral horn of the medulla, and therefore to resemble in its mode of 
development that of a ventral spinal nerve, as stated by His (’89). At 
the earliest stage which I have been able to detect the abducens, it 
possesses but a single root, formed by the processes of several neuro- 
blasts, as is represented in Figure J. The union of these takes place 
just outside the medullary wall, yet peripherally the nerve appears 
as a single process with deeply staining axis and a more lightly stained 
sheath. I find neither at this stage nor in later stages any convincing 
evidence of a migration of the neuroblast cells from the wall of the 
neural tube. In later stages of development sections show that the 
nuclei seen along the course of the nerve are distinctly peripheral in re- 
lation to its fibres. Even the phenomena presented in sections of 
embryos fixed with corrosive-sublimate acetic, such as are represented in 
Figures 62-65 (Plate 9), warrant in my judgment only the inference 
that the nuclei of the nerve are peripheral, as held by Miss Platt (91). 
The darker appearance of the nuclei lying upon the nerve results more 
from the opaqueness of the nerve than from any peculiar staining proper- 
ties of the nuclei. During development the number of roots in the 
nerve increases from one to three or four, the number being variable 
even upon the two sides of the same embryo. The method of develop- 


1 Since Marshall (’81), van Wijhe (’82), and Miss Platt (91) never saw the early 
stages of development of the abducens, it is unnecessary to restate their results in 
this connection. 


232 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


ment of the secondary roots is the same as that described for the pri- 
mary one, viz. as processes from neuroblast cells in the ventral horn. 
By following the fibres of the roots in the wall of the brain, it is 
easily ascertained that the motor “nucleus” of the abducens is a 
very elongated one, as is known to be the case in higher Vertebrates 
(see Edinger, ’96). 

The study of the development of the abducens is simpler than that of 
the oculomotorius, since the nerve never comes into relation with a 
ganglion, and thus resembles the primitive ventral nerves of Amphioxus 
more than do the ventral spinal nerves. The gradual extension of its 


: cl.n’ bl. 


xe a) KE es 5 = wer . 
: Sy ahd.) eee ae 


FIGURE J. 


fibres through the mesenchymatous tissue at the base of the medulla 
may therefore be easily followed. It is a matter of some morphological 
importance, in my opinion, that not all the nerve fibrils extend anteriorly 
toward the third somite (van Wijhe’s), but that in later stages of develop- 
ment, e.g. in embryos with 78-80 somites (Plate 4, Fig. 20), a nerve 
fibril is seen to pass from the posterior root of the nerve in a posterior 
direction toward the myotome of the sixth somite, which has at this stage, 
however, only a few rudimentary muscle fibres. Miss Platt (91) like- 
wise has mentioned the fact that this nerve also distributes fibres to 
mesoderm posterior to the third somite (muse. rectus posterior). In the 
abducens, therefore, we have to do with a post-otic ventral nerve, 


Fic. J. Parasagittal section of a Squalus embryo with 60 somites, showing the 
abducens as a fibril formed by the processes of at least four neuroblast cells. 
x 447. abd., abducens; cl. n’bl., neuroblast cell. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 233 


which develops in precisely the same way as do ventral (medullary) 
spinal nerves, possesses a much elongated motor nucleus in the ven- 
tral horn of the medulla, and innervates pre-otic (possibly also in the 
embryo post-otic) musculature (musc. rectus posterior). These facts 
seem significant in dealing with the question of the primitive metameric 
relations of this nerve. 


At a stage when the embryo has a length of 17 mm. (78-80 somites) 
the ramus opthalmicus superficialis V (Plate 4, Fig. 20; compare Fig. 21) 
appears as a fibrillar nerve with peripheral nuclei extending from the 
Gasserian ganglion just dorsal to the point of exit of the fibres of the 
r. ophth. profundus V, and passing anteriorly close to the ectoderm 
below the r. ophthalmicus superficialis VII. The relations of these two 
ophthalmic nerves are therefore such that they have usually been re- 
garded as of the same morphological value, i. e. as rami cutanei dorsales 
of nerves V and VII respectively. Yet an interesting relation of proto- 
plasmic processes from the r. ophth. sup. V with the myotome of the 
second somite, such as is represented in Plate 8, Figure 60, has been 
called to my attention by Miss Platt. Since at this stage of develop- 
ment the fibres of the trochlearis have not appeared, the inference would 
seem warranted that motor impulses may have primitively passed to this 
myotome (musc. obliquus superior) through the fibres of the r. ophth. 
sup. V. Such a supposition, however, is greatly diminished in force, 
and in my opinion rendered untenable, by the fact that in embryos of 
19 mm. — therefore before the fibres of the trochlearis are in connection 
with the m. obliquus superior —the r. ophth. sup. V shows no longer 
connection with this muscle (Figure K). The fibres of the anterior root 
(portio minor) of the trigeminus nerve may now be traced from their 
origin through the Gasserian ganglion into the mandibular arch, where 
they give off fibres both to the muscles of the arch and to the skin of its 
anterior and lateral surface. The fibres appear in large part motor. 
Since this is the only motor branch of the V, it would follow that the 
posterior root (portio major) includes chiefly, if not entirely, sensor 
fibres. It would moreover follow that encephalomere III is chiefly, if 
not wholly, connected with motor fibres, which may be traced forward to 
a considerable distance in it to the neuroblasts in the lateral horn, with 
which they are in connection, while encephalomere IV has chiefly sensory 
fibres in connection with it. Mitrophanow’s (’93, p. 178) evidence is, 
however, considerably at variance with that just stated. He finds that 
in an embryo Squalus of 18 mm. “la racine du nerf trijumeau est large 


234 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


et se devise en deux parties (Pl. XIV. Fig. 8) dont les fibres sortent du 
cerveau en formant différents coins et se croisent ensuite au dedans du 
ganglion Gasseri; de cette maniére le ramus ophthalmicus profundus les 
recoit de la partie postérieure ; le ramus maxillaris, de la postérieure et 
de l’antérieure ; le ramus mandibularis, principalement de la postérieure.” 
Mitrophanow’s results are seen to differ markedly from mine as to the 
relationship of the fibres of the ramus mandibularis. My results, how- 
ever, agree with those of His (’88°, p. 365, Tab. IT, Fig. 3) for the human 


Vv rx.ma}.. 


trch. 
: V.rx.min. gn fac. 


{ ENS 
: On.MS-Ce. 
m2.ob.su ? : 


41. Vi Lopt.su. V opt.su. ‘V opt p'fnd 


Wnu.ttSU.+0. 


fy 


Figure K. 


embryo. The clear relationship of the motor fibres of the trigeminus 
with encephalomere III and the visceral part of van Wijhe’s second cavity 
appears to me a matter of considerable morphological importance, and 
seems to establish the metameric relations of these elements. 


Fic. K. Parasagittal section through the left side of a Squalus embryo of 
19-20 mm. X 40. 

II, IIT, IV, second, third, and fourth encephalomeres; V. md., r. mandibularis 
trig.; V. mz., r. maxillaris trig.; V. opt. p’fnd., r. ophth. profundus trig. ; V. opt. su., 
r. ophth. superficialis trig.; V. rz. maj. and V. rz. min., major and minor roots of the 
trigeminus; VJ. ac., r. acustico-facialis ; VII. buc., r. buccalis facialis ; VII. hoi., 
r. hyoideus facialis ; VII. opt. su., r. ophth. superficialis facialis ; gn. fac., ganglion of 
the facialis nerve ; gn. ms-ce., meso-cephalic ganglion still retaining connection with 
the ectoderm by the process x; mu. ob. su.,m. obliquus superior; mu. rt. su. + a. 
muse. recti superior and anterior (1st cavity); trch., trochlearis. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 235 


3. TROCHLEARIS. 


In an embryo about 21-22 mm. in length (Plate 4, Fig. 21) the 
trochlearis, the last cranial nerve differentiated, appears, as stated by 
Kastschenko (’88, p. 465), in the form of ‘parallel gehende kernlose 
und, dem Anschein wenigstens nach, vollstandig structurlose Faden, 
welche in ihrer ganzen Ausdehnung vom Gehirndach bis zum ent- 
sprechenden Muskel verfolgt werden k6nnen.” The great variety of 
opinions concerning the morphology of this eye-muscle nerve make in- 
teresting the facts of its development. Hoffmann (’89, p. 338), who 
was the first to study its development, states that in Lacerta one finds, 
as the Anlage of the trochlearis, “einen ziemlich grossen, zelligen Aus- 
wuchs” between midbrain and hindbrain. At certain stages the trochle- 
aris possesses “ein sehr deutliches und zwar ziemlich grosses Ganglion, 
welches aber friihzeitig wieder vollstindig abortirt.”1 In later stages 
of development the trochlear emerges as “ diinner, feinfaseriger Nerven- - 
stamm von der oben erwaéhnten Stelle aus dem Gehirn und wird in 
seinem weiteren Verlauf bald so schméchtig, dass er nur aus einzelnen, 
sehr diinnen Fasern besteht.”’ In other reptiles, in birds, and in car- 
tilaginous fishes, Hoffmann was unable to find evidence of this gan- 
glion of the trochlearis. In 1890 and 1891 Dohrn announced that, in 
early stages of the development of the trochlearis, erratic ganglia, which 
were evidently products of the neural crest, are found in Selachian 
embryos in connection with this nerve. Whether these ganglia send 
fibres into the trochlearis stem, he was not able to determine. In later 
stages anastomosing fibres appear to connect the trochlearis with the 
r. ophth. sup. V and VII. Moreover, Froriep (91) thinks he is able to 
establish in Torpedo the genetic connection of a pear-shaped ganglion 
with the trochlearis. From his studies upon Torpedo embryos, he is 
also forced to conclude that the trochlearis arises im situ through the 
“ Umwandlung oder Auslauferbildung der Ganglienzellen.” According 
to Miss Platt (91, p. 95), the trochlearis in Acanthias first appears as a 
small fibrous nerve growing from the constriction between midbrain and 
hindbrain. This may be followed a short distance into the mesoderm, 
but, becoming extremely attenuated, is soon lost. ‘Soon after the 
appearance of this small nerve, which is the root of the permanent 
trochlearis,” cells are proliferated to meet it from the ganglion cells that 

1 Confirmed by Oppel, ’90. 


2 Miss Platt makes, in my opinion, an unnecessary distinction between a “ pri- 
mary ” and a secondary, or “ permanent” trochlearis. The “ primary trochlearis ” 


236 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


lie above the superior oblique muscle. Thus the permanent trochlearis 
arises from two sources, from the brain and from ganglion-cells.” Fi- 
nally, Kupffer (’91) stated that he had found a nerve in Ammoceetes, 
which he thought to be the trochlearis (for reasons not clear to me), 
directly connected with the second epibranchial ganglion. Were this 
opinion correct, the trochlearis would be the serial homologue of a 
branchial (dorsal), not of a spinal dorsal nerve. 

From this summary of previous embryological evidence bearing on 
the question of the morphology of the trochlear nerve, it is clear that 
little support is given to the view, based on the later histological struc- 
ture and relations, that it is morphologically a ventral segmental nerve. 
Only Kastschenko (’88) finds the nerve in early stages fibrillar in 
structure. The following evidence, however, leads me to conclude that 
its mode of development is the same as that of the oculomotorius and 
abducens, and that therefore it must be regarded, like these, as a ven- 
tral (medullary) nerve. I first find the trochlearis in sections of embryos 
of 19-20 mm. as a fibrillar nerve bundle extending from the dorsal con- 
striction between encephalomeres II and III. Two roots are already 
present at this stage, but neither in these, nor in the nerve bundle as 
far as its fibres may be traced in the mesenchymatous tissue at the sides 
of the brain, are nuclei to be found. While proximally the nerve fibres 
are united in a compact bundle, they distally separate so as to form a 
loose brush of structureless fibres, which are lost in the mesenchyma at 
a considerable distance dorsal to the muse. obliquus superior (Figure K). 
While I am able to offer no direct evidence in favor of the view that the 
fibres of the trochlearis, as above described, are processes from neuro- 
blast cells in the ventral horn of encephalomere III, I hold that they 
are such, since their well known later histological relations support this 
conclusion. Dorso-ventral fibres in this region of the neural tube may 
indeed be traced in embryos of this stage, but their connection with the 
fibres of the trochlearis is not clear to me. The dorsal chiasma of fibres 
is present. Of a ganglion, or of any grouping of cells which might 


is that portion of the trigeminus Anlage which I have for convenience called its 
trochlear portion, which persists for some time in the constriction between midbrain 
and hindbrain vesicles. Since the proof of its morphological value has not been 
given, and since the “ permanent” trochlearis is not developed from the “ primary ” 
trochlearis, as Miss Platt herself states (p. 96), the use of the latter term appears 
to me apt to mislead. 

1 The explanation for this dorsal chiasma may be sought in some physiological 
advantage in codrdination gained, but it may also be seen that in case the dorsal 
exit of fibres were of physiological advantage, it would be easy for the fibres to 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. PW 


receive the name of ganglion, there is no evidence at this or later 
stages. The last traces of scattered groups of neural-crest cells found 
in some (not all) embryos of earlier stages (17 mm.) have been 
lost. 

In embryos of 21 mm. some of the distal fibrils of the nerve appear 
to have united with migratory cells from the r. ophth. sup. trigeminus, 
a process in my opinion comparable with that which takes place in the 
growth of the oculomotorius. At least, in embryos of 21 or 22 mm. the 
distal portion shows nuclei in relation with the nerve fibres, whereas 
proximally no nuclei are seen. In still later stages the nerve has a dis- 
tinctly cellular appearance throughout its length. The nuclei are, how- 
ever, seen in thin sections to be peripheral in relation to the nerve fibres, 
as in the oculomotorius (Figure 1). The evidence of anastomosis of the 
fibres of the trochlearis and the r. ophth. sup. trigemini I consider 
very doubtful. During development the loose brush of fibres at the 
distal termination of the trochlearis becomes united into a compact nerve 
stem. It has, therefore, seemed to me that the primary widely spread 
brush of nerve fibrils may be explained on the ground of advantage 
gained in seeking the terminal organ, the musc. obliqu. superior. 

The phenomena observed by me during the development of the trochle- 
aris are seen to correspond very closely with those observed by Miss Platt 
(91). To her, as to me, the trochlearis first appears asa fibrillar process 
from the dorsal wall of the brain. But while she interprets the evidence of 
cellular growth toward the advancing end of the nerve as of morphologi- 
cal or phylogenetic significance, I am unwilling to give it such interpre- 
tation, since I find that these nuclei have nothing to do with the nerve 
proper. In my opinion, it is probable that they become converted into the 
nuclei of Schwann’s sheath, an opinion which seems confirmed by their 
peripheral position in relation to the nerve fibres. When the only sec- 
tions I possessed were of embryos killed with corrosive-sublimate acetic, 
and stained with carmine or hematoxylin, the evidence seemed to me 
confirmatory of the view of Froriep (91), viz. that the trochlearis is 
differentiated from mesenchymatous cells ¢n sdtu. But better methods 
of preparation have taught me to distrust that evidence, and the results 
appeared to me too distinctly contradictory to the later histological 


cross each other in growth, since the direction of their growth would thereby be 
unchanged. I assume that it is easier for a nerve fibre to grow ina direct line 
than to bend back and reverse the direction of its growth. The possibility even of 
a primary connection of muscle and nerve appears to me to be excluded in the 
case of the musc. obliq. sup. and the trochlearis. 


238 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


structure of the trochlearis to be worthy of trust, or even of serious 
consideration. 


Before closing my account of the development of the cranial nerves 
and their chief branches in Squalus, I wish to call attention to a phenom- 
enon seen in still later stages of development, already noted by me ina 
former paper (’97, p. 455). It appears to me a matter of considerable 
morphological importance that the ganglion of the dorsal nerve of van 
Wijhe’s eighth somite (fourth post-otic)—the ventral root of which 
forms at this stage the first of the five hypoglossus roots — unites in 
late stages of development with the ganglion cells near the root of the 
vagus. Kupffer (90) was the first to make evident the morphological 
importance of the clearly marked distinction between dorsal and lateral 
(epibranchial) ganglia in embryos of Cyclostomata. While in the em- 
bryos of Selachii there is not such a clearly marked distinction, there 
nevertheless exist at the roots of the vagus groupings of ganglion cells, 
or at least of neural-crest cells (quite distinct from the lateral, epi- 
branchial ganglia of this nerve, the ganglion nodosum), which in my 
opinion are to be regarded as homologous with the dorsal ganglion of 
the vagus of Ammocetes.1. The evidence of the union of dorsal seg- 
mental ganglia in the vagus is as follows. During development the 
continuous neural crest in the occipital and trunk regions of Squalus 
becomes differentiated into clearly marked ganglia, lying opposite the 
myotomes and connected by a cellular “dorsal commissure ” (Balfour, 
’81), as far forward in the embryo as van Wijhe’s seventh somite. Oppo- 
site the sixth and seventh somites no distinct ganglia appear ; but instead 
a wide sheet of cells, lying in close juxtaposition to the extended roots of 
the nerve, is observable. While in early stages the ganglion of the eighth 
somite is separated by a considerable interval from the roots of the vagus, 
in later stages it approaches these, and in embryos of 30 mm. is seen to 
be in union with them as a well marked ganglionic appendage. In later 
stages, its fusion appears complete. The ganglion cells do not degen- 
erate, but send axis-cylinder processes both centripetally and centrifu- 
gally, the latter forming the posterior of the roots of the vagus nerve. 
The ganglion of the second hypoglossus root (ninth somite) does not, 
however, so fuse with the vagus, but is seen in embryos of 50 mm. as a 
group of cells without nerve relations, so far as I am abie to determine, 
enclosed in the cartilage of the cranium. It apparently disappears in 

1 These are probably the homologues of the intracranial ganglia of Ganoids 
(see Allis, ’97, p. 747). 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 239 


later stages, as does its ventral root, the hypoglossus of the adult having, 
according to Gegenbaur (’72) and M. Fiirbringer (97), only two ventral 
roots without dorsal ganglia. Since the reduction of dorsal and ventral 
roots takes place from the anterior towards the posterior, these two ven- 
tral roots of the adult hypoglossus are in all probability the posterior of 
the five roots of the embryo.’ A similar process of fusion of dorsal ganglia 
with the dorsal ganglion of the vagus takes place in Petromyzon ; but in 
that animal the fusion of the ganglion — viz. that of the “ spinalartige 
Vagusanhang,” which for reasons already stated by me (’97, pp. 454, 
455) I regard as the exact homologue of the dorsal ganglion of van 
Wijhe’s eighth somite in Squalus ?— appears by a comparison of the 
results of Wiedersheim (’80), Schneider (’80), Ahlborn (’84*), Hatschek 
(92), Kupffer (96), and M. Firbringer (97) to be a variable one. 
This union of dorsal spinal ganglia with the ganglion of the vagus, taken 
in connection with the fact previously stated by me (’97, p. 453), that 
the dorsal ganglia of the glossopharyngeus and vagus lie primitively ® 
median to the dorsal portion of post-otic somites, is a further link in 
_the chain of evidence which shows that no fundamental distinction be- 
tween spinal and cranial nerves exists. From the foregoing account it 
will be seen that, as in the case of spinal nerves, we are able, using as 
criteria the central and peripheral relationships of the motor fibres, to 
divide cranial motor nerves (roots) into two classes, viz.: (1) dorsal 
(splanchnic) roots, having their nucleus in the lateral horn of the neural 
tube and their peripheral distribution in the musculature (ventral) of 
the visceral arches; and (2) ventral (somatic) roots, which have their 
nucleus in the ventral horn of the neural tube and their peripheral 
distribution in the musculature (dorsal) of the somites (somatic muscu- 


1 Hexanchus and Heptanchus both have jive hypoglossus roots in the adult 
(M. Fiirbringer, ’97). 

2 Homologized, however, by Ahlborn (’84*) with van Wijhe’s tenth somite, and 
by Hatschek (’92) with van Wijhe’s seventh somite, 

8 Goronowitsch (’92) first observed in the chick that the topographic relation of 
the vagus to the head somites is the same as that of the spinal nerves to the trunk 
somites. Sewertzoff (95, p. 92) also states that “ Die Beziehung der Kopfmyotome 
zu den Kopfnerven, z. B. zum N. vagus ist dieselbe, wie diejenige zwischen den 
Rumpfmyotomen und Riickenmarknerven, d. h. sie liegen nach aussen von Nerv 
(Cyclostomata, Ganoidei chondrostei, Urodela, Reptilia, Aves).” This is stated, 
however, by Kupffer (’94, 96) not to be a primitive relation of the post-otic myo- 
tomes in Petromyzon. My own observations and conclusious, however, differ from 
those of Kupffer (see Neal, ’97, p. 453). Miss Platt’s (’97) observations on Nec- 
turus, and her conclusions likewise, confirm the conclusions of Goronowitsch and 
Sewertzoff. 


240 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


lature). The motor fibres of nerves V, VII, IX, and X belong to the 
former, and nerves III, IV, and VI to the latter class. 

While it is possible, as has been stated, to establish a numerical cor- 
respondence of encephalomeres and somites, the nerve relations are not 
so clear. We find, for example, that encephalomeres II, III, and VII 
are connected by ventral (motor) nerves with somites (van Wijhe’s) 
1, 2, and 3. Such evidence of a want of segmental correspondence 
would seem at first sight to render untenable the assumption that 
encephalomeres have the same segmental value as myelomeres. We 
have already seen that these two classes of neuromeres have structurally 
little in common. Moreover, a want of correspondence of encephalomeres 
and visceral arches is shown by the fact that the dorsal motor fibres 
which are connected with encephalomeres III and V innervate two suc- 
cessive visceral arches. In view of this discrepancy in the segmental 
relations of encephalomeres and nerves, can we regard the former of 
segmental value? Do they afford evidence in support of the assumption 
that a Vertebrate head segment is comparable, i. e. homologous, with a 
trunk segment? Before expressing my own opinion in regard to the 
answer to this question I will briefly review the interpretations given 
by previous investigators. Two antithetic views concerning the neu- 
romeres have been given, viz. (1) that they are not of segmental or 
phylogenetic value, and (2) that they are of phylogenetic value. 


VII. Segmental Value of Hindbrain Neuromeres. 


a. NoN-PHYLOGENETIC INTERPRETATION. 


In 1877 Mihalkovics, speaking of the foldings in the medulla of birds 
and mammals, expressed the opinion that the want of correlation be- 
tween these structures and the nerves and visceral arches seems to favor 
the view that they are of mechanical origin, i. e. formed by the bending 
and shoving of the neural tube as it rapidly grows in a confined space. 
This view seems strengthened by the consideration that the ventral wall 
of the neural tube of chick embryos is, in early stages, markedly folded 
into segments, irregular in size and inconstant in appearance, and that 
these folds in the head region are visibly exaggerated by certain fixing 
agents which result in shrinking the embryo. Balfour, who with Foster 
(’74) had been the first to express the opinion that these structures 
were of phylogenetic significance, afterwards (81) said that it is uncer- 
tain whether they have any morphological significance. In 1892 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 241 


Froriep, in speaking of neuromeres and their nerve relations, said that 
these relations are in no way of such a kind that both nerves and neu- 
romeres appear to be constituent parts of a simple organ system. The 
nerves, especially the trigeminus and facialis, are not so constant in 
their relation to the folds as would be expected, if the latter were 
primary segments of the nervous system. It looks much more as if the 
presence and position of the nerves determines the position of the folds, 
and as if the appearance of the folds is itself a passive, mechanical one, 
necessitated by quick growth in length in a confined space. On the 
basis of his research, he was therefore much inclined to consider these 
late appearing and transitory segments of the brain as morphologically 
unimportant phenomena, 

Broman (’95) has given a somewhat extended description of the fold- 
ings in the hindbrain of a human embryo about three weeks old. Although 
he nowhere formulates his conclusions as to the significance of the folds, 
it is evident that he does not regard them as of phylogenetic significance, 
for he says that the correspondence which earlier investigators have 
found in their relation to nerves seem to him of little help. In his 
opinion the results differ too much to allow one to think that a general 
rule prevails in the Vertebrate series as regards the number and rela- 
tions of the foldings. The foldings, he says, are intensified in the re- 
gions of the greatest flexure of the neural tube, and in these regions the 
radial arrangements of cells in the foldings is also more marked. This, 
together with the fact that the foldings are confined to the ventral half 
of the medulla, harmonizes well with a mechanical explanation of their 
origin. Upon the evidence that rounded cells (which he thinks are the 
neuroblasts) with round nuclei may be distinguished in the centre of the 
most strongly developed parts of the brain foldings, Broman (’95, p. 189) 
forms an hypothesis concerning the origin of the separation of lateral and 
ventral roots. He says: ‘‘ Wenn wir noch einmal alle die oben von mir 
als Neuromeren bestimmten Falten durchmustern, finden wir, dass nur 
das als Abducensneuromer bezeichnete die ventrale Wand des Hirnrohres 
ausbaucht. Alle iibrigen sind entweder ganz und gar davon abgedrinst, 
oder auf dem Wege es zu werden. Dies kann natiirlicher Weise ein 
blosser Zufall bei diesem Embryo sein.” 

Since Broman’s paper is, with the exception of Locy’s, the most recent 
one on the question of neuromeres, I will discuss his evidence and con- 
clusions at some length. It is unfortunate for the purpose of discussion 
that he has failed to identify correctly the cerebellum Anlage. What 
he calls Cerebellumanlage is the posterior of the two secondary sub- 

VOL. XXXI.— NO. 7. 7 


242 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


divisions of encephalomere III, as may easily be determined by a com- 
parison of his figures with frontal sections of swine embryos. He 
says (p. 188), ‘‘ Die ventrale Ausbuchtung der zweiten Falte kann man 
also mit vollkommene Sicherheit fiir die beginnende Ponsanlage, und 
ihren oberen Theil fiir die Cerebellumanlage halten.” But, as may be 
determined by the relations of the neuromeres to the ear capsule and to 
the ganglia of the acustico-facialis and the trigeminus in his figures 
(Taf. X. Figs. 4, 5), the true Anlage of the cerebellum lies anterior to 
this and is his “erste Falte,” which on theoretical grounds he considers 
related to the trochlearis nerve. As a result of this mistake it happens 
that the neuromere which he calls ‘“‘ Abducensneuromer” (VI), and to 
which on purely theoretical grounds he assigns the sixth nerve, is in 
reality encephalomere V, which is connected with the acustico-facialis. 
With this neuromere the abducens never is connected in any Vertebrate yet 
studied. In the swine, as I am able to affirm from my own observations, 
the abducens arises from the ventral portion of hindbrain neuromere VI, 
which in the early stages of all Vertebrates lies opposite the ear capsule. 
In Necturus, the chick, and S. acanthias, its origin is ventral and poste- 
rior to the origin of the acustico-facialis. In support of this theory of 
the mechanical origin of the “ Falten,’”’ Broman finds that, as a result, 
as he thinks, of the flexure of the neural tube, those neuromeres which 
correspond with encephalomeres IV and VI of my figures are wedge- 
shaped, and that their ventral edges do not reach the ventral wall of the 
neural tube. Moreover, none of his neuromeres extend to the “ Deck- 
platte.” But a study of swine embryos leads me to conclude that this 
is not characteristic of all mammalian embryos, and indeed that it may 
be “ein blosser Zufall” in the case of Broman’s human embryo. In 
young swine embryos (killed 19 days after coitus) none of the neuromeres 
are wedge-shaped ventrally or dorsally ; moreover, the constrictions be- 
tween them extend into the Deckplatte. The posterior constriction 
of encephalomere V extends aeross the Deckplatte until a somewhat 
later stage, and in this constriction a mass of neural-crest cells persists 
in a way precisely similar to that in which neural-crest cells in S. acan- 
thias persist in the regions of constriction between the primary brain 
vesicles (encephalomeres).* 

From an examination of the evidence presented by those who have 
held that the neuromeres are purely the result of mechanical influences, 

1 In connection with this fact, it is to be noted that the walls of the medulla in 


this region are little distended laterally, which may be ascribed to the influence of 
the ear capsule. (See Plate 5, Fig. 30.) 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 243 


it is evident that the chief support for the hypothesis consists, first, in an 
apparent want of a definite relation of the nerves to the neuromeres in 
the different Vertebrate groups, — i. e. an apparent inconstancy in the 
structures themselves, — and, secondly, in the fact that the hypothesis 
seems to explain the structural conditions presented. 

I turn now to a consideration of the arguments supporting the view 
that the neuromeres are of morphological (phylogenetic) importance. 


6. PHYLOGENETIC INTERPRETATION. 


A phylogenetic interpretation of the foldings of the medulla was first 
given in 1874 by Foster and Balfour. The following year Dohrn 
accepted this explanation. Béraneck (’84) showed that in the Lizard 
the hindbrain folds (‘‘replis”) were definitely related to certain nerves. 
Having later (87) confirmed his observations by studies of chick em- 
bryos, he concluded that the foldings are the last indisputable remnants 
of the primitive segmentation of the head. It is notable that he reached 
this conclusion notwithstanding the fact that, in his opinion, the seg- 
ments of the spinal cord do not have the same characters as those found 
in the foldings of the hindbrain. Subsequent investigators, however, 
have sought to compare encephalomeres with myelomeres. In 1885 
Rabl found in chick embryos a regular folding of the side walls of the 
myelencephalon, the segments of which showed the same characteristics 
as the foldings in the region of the spinal cord. During the same year 
Kupffer (’86), in studies on different Vertebrate embryos, found that the 
foldings extended into the midbrain region. Because of the relatively 
late appearance of the folds, — “after the closure of the neural tube, 
after the formation of three brain vesicles, and long after the segmenta- 
tion of the mesoderm,” — Kupffer thought that there was much against 
the interpretation of these folds as remnants of a primary general metam- 
erism of the neural tube, but his later observations — previously cited 
in another connection (p. 174) on an embryo of Salamandra atra at a 
stage before the closure of the neural plate —led him to believe that in 
this particular case there is a primary segmentation. 

The fact that Kupffer here found eight cross furrows in the brain 
region, representing as many “ancestral segments,” appears to have 
strongly influenced his subsequent interpretations of the morphology of 
the forebrain in different Vertebrates, for in his later studies he has sought 
to find evidence of these eight primary ‘“ encephalomeres” in the fore- 
brain and midbrain, even “after the closure of the neural tube, and the 


244 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


formation of the three brain vesicles, and long after the segmentation of 
the mesoderm”! 

McClure (’89, p. 435, and ’90, p. 37) concluded, from studies on em- 
bryos of Amblystoma, Anolis, and chick, ‘that the symmetrical con- 
strictions or folds found in the lateral walls of the embryonic brain are 
remains of the primitive segmentation of the neural tube, in part atavis- | 
tic, extending [from the spinal cord region] into the primary forebrain.” 
The serial homology of the segments of the myelon and the encephalon 
seemed to him certain, since he found both the structural characteristics 
and the nerve relations to be the same in the two regions. ‘“ The dorsal 
roots of spinal nerves take their origin from the apex of their respective 
myelomeres in exactly the same manner as the nerves of the medulla 
do from their respective encephalomeres ” (’89, p. 437). 

In the same year Miss Platt (’89) also advocated the view that there 
is a serial homology between the encephalomeres and the segments of 
the spinal cord. While she agrees with Orr and Béraneck in regard to 
the number and appearance of the neuromeres and the ultimate rela- 
tions of the nerves, she finds that the cranial nerves develop from the 
constrictions between neuromeres, precisely as the spinal nerves do. In 
answer to objections to the attempted homology between cranial and 
spinal segments, she says that in both head and trunk the segmentation 
is transitory, and that in both regions it is more manifest in the ventral 
portion of the neural tube. 

The conclusions of Waters (91) are largely confirmatory of those of 
McClure, viz. that there is a similar segmentation in brain and spinal 
cord, with similar sensor nerve relations in both these regions. 

Zimmermann (91), as a result of his studies on rabbit, chick, and 
Squalus embryos, thinks he is able to confirm Kupffer’s discovery of 
eight primary cephalic segments or “encephalomeres,” although his 
eight “primére Abschnitte” include forebrain, midbrain, and hindbrain 
regions, while Kupffer’s theoretical conclusion was that his eight “ pri- 
mire Medullarfalten” do not include the hindbrain.’ Although Zim- 
mermann states that the spinal cord does not appear segmented, he 
finds in later stages thirteen homodynamous “ encephalomeres,” and has 
given a table of these with their nerve relations. He supposes three 
roots, a dorsal, a lateral, and a ventral one, to be related to each en- 
cephalomere, but his table gives chiefly the impression of numerous gaps 
to be filled with hypothetical nerve roots. 

Herrick (’92) states that he finds the segmentation of the medulla 


1 At first Kupffer thought they did not include the forebrain! 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 245 


and spinal cord of snake embryos not explicable on mechanical grounds. 
“The neuromeres of the medulla cannot be ascribed to the mechanical 
influence of the Anlagen of the nerves, for those segments which have 
no nerves develop equally with the others” (cf. Froriep, 791). He 
considers however “‘the neuromeres of the forebrain” region wholly 
illusory from a morphological standpoint, since they involve only dorsal 
structures. 


c. INTERPRETATION OF HINDBRAIN NEUROMERES IN SQUALUS ACANTHIAS. 


I believe that the evidence which I have obtained from a study of the 
development of hindbrain neuromeres in S. acanthias excludes the 
possibility of a simple mechanical explanation of them. In their earlier 
stages they were seen to be local thickenings of the lateral walls, a 
phenomenon intelligible only on the ground of unequal growth, and not 
in the least explicable as the result of the passive bending or shoving 
of a tube already formed.1’ Since the somites do not extend into the 
region of the dorsal part of the encephalomeres, the possibility that the 
neural tube in this region is constricted by them is excluded. They are, 
then, in both structure and mode of development, clearly not to be ex- 
plained in the same way as the myelomeres. Again, that they are not 
due to the effect of the Anlagen of the nerves, as supposed by Froriep, 
is shown by the fact that encephalomere IV develops equally with the 
others, although there is no nerve in relation with it until a compara- 
tively late stage. Since the fibrillar connection of nerves with neuro- 
meres is established almost at the same time that the inner surface of 
the hindbrain neuromeres becomes concave, it might be thought that this 
change is due to the mechanical effect of nerve fibres. That such is not 
the case seems clear, however, because no nerve fibres come into relation 
with the outer convexity of encephalomere VI. The hindbrain neuro- 
meres, from their early appearance onwards until they disappear, are 
local differentations of the walls of the medulla, and as such are not, I 
believe, to be satisfactorily explained on simple mechanical grounds. On 
the other hand, I hold that they do possess certain characteristics which 
admit of a mechanical explanation. This seems to be supported by evi- 
dence from two sources. In the first place, a fixing agent which causes 
a contraction of the tissnes of the embryo intensifies the constrictions 
between the neuromeres. By this means the radial arrangement of cells 

1 This is true also in swine and chick embryos; but I do not find in Ambly- 


stoma as good evidence that the neuromefes are local thickenings of the neural 
wall. 


246 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


and nuclei is still more sharply emphasized. I think we may safely 
assume that this effect is the same as that resulting from a shoving of 
the neural tube due to rapid growth in a confined space. Figure 28 
(Plate 5) shows a frontal section of a preparation of a shark embryo, 
nearly 10 mm. Jong. The specimen was fixed in the mixture of picro- 
sulphuric and chromic acids, and then transferred directly to 50 per cent 
alcohol. Inadequate fixation and immediate transference to a fluid of 
very different osmotic power resulted in a strong contraction of the em- 
bryo, particularly emphasized in the wall of the neural tube. (In the 
figure the constrictions appear exaggerated, since only the regions of 
the nuclei are shaded.) Moreover, a comparison of embryos of differ- 
ent Vertebrates gives evidence, as it seems to me, that the bending of 
the neural tube results in the intensification of the characteristics of 
neuromeres. I have studied in frontal section embryos of Petromyzon, 
Gadus, Amblystoma, S. acanthias, chick, and swine. The radial arrange- 
ment of cells is more pronounced in those forms which have a stronger 
flexure, and in which, therefore, we may safely assume that there is a 
greater shoving of the neural tube, due to rapid growth in a confined 
space. These characteristics are considerably more pronounced in Sau- 
ropsida than in S. acanthias, in which the flexure of the neural tube is, 
however, considerable. This explanation tends to remove the doubt as to 
the phylogenetic value of such structures as the neuromeres which nat- 
urally arises when these are shown to be structures slightly if at all 
visible in the lowest Vertebrates (Amphioxus and Cyclostomes), while well 
marked in the highest. I believe that the presence of yolk makes the 
conditions in both Petromyzon and Amblystoma less primitive than in 
Squalus, chick, and swine. 

In Gadus and Amblystoma the radial arrangement of cells and nuclei 
is even less pronounced than in S. acanthias, and this seems to be corre- 
lated with the fact that the flexure of the neural tube in the former is 
less marked than in the latter. It must be admitted, however, that the 
presence of much yolk in the cells of the neural tube of Amblystoma 
(Plate 5, Fig. 35), in which no sign of encephalomere IV is present, may 
be concerned in producing the different condition of this form, in which 
the outpocketing of the neural tube takes place in the region of the pro- 
liferations of the ganglionic Anlagen only. Broman (’95, p. 186) has 
given proof, satisfactory as it seems to me, that the nuclear and cellular 
characteristics of the neuromeres of the buman embryo may be explained 
partly on mechanical grounds. Embryologists are agreed that the flex- 
ures of the neural tube may be accounted for by the rapid growth of the 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 247 


tube in a confined space. Such growth would clearly result in a shoving 
of the neural tube, and also in a flexure in weaker portions, as in the 
regions between local thickenings, like the hindbrain neuromeres. The 
crowding of the cells in the regions of constriction between neuromeres 
may be accounted for in the same way. I therefore conclude that some 
of the structural characteristics of neuromeres may be intensified by the 
bending or shoving of the neural tube during its growth. 

The conditions presented in Amblystoma (Plate 5, Fig. 35) led me to 
believe at one time that the neuromeres might be related to the prolifera- 
tion of the cells of the ganglionic Anlage. In this animal the neural 
tube is evaginated in the regions of the proliferation of cells for the 
ganglionic Anlagen of nerves V, VII, IX, and X, while in the region 
where no neural-crest cells are proliferated — the region corresponding 
to the position of encephalomere IV (in other forms) — no neuromere 
appears.’ In 8. acanthias we have seen (page 215) that from two of the 
hindbrain neuromeres, viz. V and VI, are proliferated the cells of two 
distinct nerve Anlagen. But since no nerve Anlage is proliferated from 
encephalomere IV, although this is as well marked as other encephalo- 
meres, I was compelled to abandon the hypothesis, to which the study 
of Amblystoma had inclined me. The fact that particular nerve Anlagen 
are proliferated from particular encephalomeres may, however, be a clue to 
the primitive mutual relationships of these nerves and of the encephalomeres 
to each other. The fact that the local thickenings are confined to that 
region of the neural tube from which the great nerves of the head — V, 
VI, VII, IX, and X —arise, must also give us some clue as to their signifi- 
cance. Such local thickenings are seen neither in the region anterior, nor 
in that posterior to the medulla, but they are not limited by the ear cap- 
sule posteriorly, and the anterior boundary of them does not coincide with 
the anterior boundary of the primary hindbrain vesicle. It is to their 
nerve relations, then, that an investigator must first turn his attention. 
We have seen that in the development of the neural crest some of the 
cells of the trigeminus are proliferated from encephalomere III; that few 
cells are proliferated from encephalomere IV; that from encephalomere V 
come the cells of the acustico-facialis, from encephalomere VI the cells 
of the glossopharyngeus, and from encephalomere VII the cells of the 
Urvagus. The clearly marked relations of the Anlagen of the two suc- 


1 The migration of cells from certain regions of the neural tube would certainly 
weaken these regions, and the tube would in consequence, if subjected to a longi- 
tudinal pressure, or to distention by growth, tend to bend or distend most readily 

‘in such places. 


248 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


cessive nerves, the facialis and the glossopharyngeus, to two successive 
encephalomeres (V and VI), seems to me to be a very important fact.1 
The cells of the glossopharyngeus are crowded back by the ear capsule, 
but the fact that in their ventral course they are directed anteriorly into 
the third visceral arch goes to prove that their posterior position is not 
their primitive one. Almost as clear is the relation of the cells prolifer- 
ated from encephalomere VII to the second branchial (4th visceral) arch. 
From these facts I was led to think that the primitive relations of the 
hindbrain neuromeres were with the visceral arches. The fact that the 
hindbrain neuromeres are local thickenings of the lateral walls of the me- 
dulla also leads to the opinion that they are segmental groupings of the 
“*Kerne ”? of the nerves of the visceral arches. With this hypothesis in 
mind, I have examined the evidence in S. acanthias, as well as in other 
forms, in order to see if the facts support it. The more primitive rela- 
tions would be expected to occur in S. acanthias. From encephalomere III 
are proliferated neural-crest cells which enter the mandibular arch ; later 
this encephalomere becomes related with the motor root which innervates 
the muscles of this (mandibular) arch. Its relations, then, are clearly 
with the first visceral arch, and we may therefore assume that its local 
thickening contains, at least in part, the “nucleus” of the trigeminus. 
The evidence obtained from the study of the relations of encephalo- 
mere IV seems at first sight strongly against the hypothesis. Few cells 
are proliferated from this neuromere. Late in its development the fibres 
of the sensor root of the trigeminus connect with its convexity. It 
forms a marked exception in its nerve relations to the other hindbrain 
neuromeres. Were it not that other facts are found which serve to bring 
this apparent exception into harmony with the hypothesis, the adverse 
evidence it presents would seem an insurmountable obstacle to the 
acceptance of my view. Neural-crest cells which pass into the second 
visceral (the hyoid) arch are proliferated from hindbrain neuromere V, 
and the motor fibres in relation with this neuromere innervate the mus- 
cles of this arch. From hindbrain neuromere VI are proliferated the 
neural-crest cells which pass into the third visceral (1st branchial) arch, 
and the motor fibres of the glossopharyngeus, of which these cells form 
the ganglionic Anlage, innervate its musculature. The place of origin 
of the fibres of the glossopharyngeus is crowded backward, evidently by 


1 Hoffmann (’94) has spoken of the paired segmental outpocketings of the 
neural tube of this region. 

? That is, they may be localizations of the motor “Kerne” and of the sensor 
“Endkerne” of the nerves primitively related to them. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 249 


the growth of the ear capsule. The place of origin of the roots of this 
nerve are variable. In swine and chick, for example, they have their 
origin from encephalomere VII, while in S. acanthias they arise behind 
this encephalomere. This is correlated with the fact that the ear cap- 
sule in S. acanthias is crowded backward into the region opposite enceph- 
alomere VII, whereas in the swine and chick the ear capsule continues to 
lie opposite encephalomere VI until long after the nerve assumes fibrillar 
connections with the neural tube. We may thus explain the variation 
in the position of the roots of this nerve, and still believe from the evi- 
dence that their primitive relations were with encephalomere VI. 

Again, the cells proliferated from encephalomere VII are those which 
pass into the fourth visceral (2d branchial) arch, and form the Anlage 
of the Urvagus, whose motor fibres innervate the musculature of that 
arch. The Urvagus assumes fibrillar connections with the neural tube 
at a point behind the origin of the glossopharyngeus, and the cause of 
this change of relation may safely be assumed to be the same as in the 
case of that nerve. We have good evidence, then, that the primitive 
relationships of four of the hindbrain neuromeres were with the first four 
visceral arches. This relationship consists chiefly, but not wholly, in the 
fact that from these four neuromeres are proliferated cells which enter 
these arches and there form, in part at least,! the ganglionic Anlagen of 
the nerves related with them. The origin of these cells from the neural 
crest would naturally lead us to infer that in dealing with them as nerve 
Anlagen we are not dealing with motor nerves. We are, however, 
really dealing with the Anlagen of nerves which later become mixed. 

ut in later stages, when the nerve roots are established, the roots of 
only two of the nerves in question, viz. V and VII, have their exit from 
the encephalomeres from which their ganglionic Anlagen arose. Have 
we a right, then, to assume that the exits of the roots of the other two 
nerves, IX and X (Urvagus), have been pushed back from the position 
which may be assumed, on the evidence of the relations of their gan- 
glionic Anlagen, to have been the primitive one? I believe that we 
have, because, as we have seen from the examination of the relations of 
the roots of these two nerves, these roots lie as close to the point of 
origin of their ganglionic Anlagen as the ear capsule will permit. Ina 


1 Part of the neural-crest cells surrounds the mesoderm of the visceral arches, 
and very probably gives rise to some of the connective tissue of the arches. (See 
Plate 6, Fig. 40, cl. ers. n.) Whether or not they later form the cartilages of the 
arches, as they are said to do in Necturus (Platt, 94, 97), is a question which 
requires more careful and prolonged study than I have been able to give. 


250 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY... 


form like S. acanthias, where the ear capsule shifts backward, the exit of 
the root of the glossopharyngeus lies behind encephalomere VII, whereas 
in such forms as the chick and swine, where the ear capsule does not 
similarly shift backward, the exit of its root is from the expansion of 
encephalomere VII. Inall Vertebrates, the roots of the glossopharyn- 
geus and the Urvagus lie close to each other, but in S. acanthias, where 
there is a greater amount of posterior displacement than in any other 
Vertebrate that I have studied, these roots are more crowded together 
than in other forms. These facts seem to me to warrant the conclusion 
that the roots of the glossopharyngeus and the Urvagus primitively made 
their exit from those encephalomeres which give rise to their ganglionic 
Anlagen. And we may likewise assume that the local thickenings of 
these encephalomeres have their significance in this primitive relation, 
i. e. they contained the “ Kerne” of these roots. I am able to find no 
facts which render this assumption untenable. 

On the other hand, encephalomere IV never has nervous connection 
with a visceral arch. From it few neural-crest cells are proliferated, 
and in consequence it never forms the ganglionic Anlage of amerve, 
nor does it ever in ontogeny have a motor nerve in connection with it. 
Since the other four encephalomeres are related to visceral arches, I 
incline to think that this encephalomere was once related to a visceral 
arch of its own. Otherwise, so far as I can see, its existence is in- 
explicable. In this condition, then, I find additional evidence of a 
lost visceral arch, which van Wijhe (82), Miss Platt (’91°), and Hoff- 
mann (’94) believe once existed in the region of this neuromere. These 
investigators have found a want of exact correspondence between the 
somites and the visceral arches in the region of the spiracular cleft. 
Van Wijhe was led to believe that, the Ayoid (2d visceral) arch is 
double, —i. e. represents two arches, the fusion of which has resulted in 
the obliteration of the visceral cleft between them, — while Miss Platt 
and Hoffmann have held that the mandibular arch is double, and that 
an anterior gill cleft has disappeared. The disappearance of a visceral 
cleft is rendered plausible, if we assume that such a loss would greatly 
strengthen the mandibular arch when it came to function as a lower 
jaw. The evidence from a study of mesomerism and neuromerism there- 
fore seems mutually confirmatory. , 

Tf encephalomere IV was related to a lost visceral arch, it follows that 
the lost arch must have been situated posterior to the mandibular (1st 
visceral) arch, for the musculature of this arch is innervated from en- 
cephalomere III. It also follows, because of the relation of the nerve 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 251 


of encephalomere V (facialis) to the present spiracular cleft, that this 
was once the second visceral cleft instead of the first (disregarding for 
the present the possibility that the mouth represents a pair of gill clefts), 
as it now is. It seems entirely possible that the outpocketing of the 
present first visceral (hyo-mandibular) cleft was originally a double one," 
and that the fusion of these two outpocketings resulted in the loss of the 
visceral arch which once separated them, and therefore in the loss of the 
nerye primitively related to that arch. Moreover, between the second 
head somite of van Wijhe, which extends into the mandibular arch, and 
the fourth somite, which is widely connected with the mesoderm of the 
hyoid arch, there lies the third head somite, in correlation with which 
there is no intermediate visceral arch. This somite (the 3d) lies opposite 
the posterior constriction of neuromere IV, and speaks plainly for the 
previous existence of a lost head segment, for which neuromere IV may 
once have furnished the nerve centre. Did such an arch exist, each of 
van Wijhe’s somites from the second to the sixth, and each of the en- 
cephalomeres from III to VII would correspond with a visceral arch. 

I give a brief summary of the line of reasoning which leads me to 
believe that the significance of the hindbrain neuromeres lies in their 
primitive relationship to the visceral arches. In the young embryos 
of S. acanthias two facts, both so far as I know new, present them- 
selves. In the first place, the hindbrain neuromeres, five in number, 
are found to be successive similar thickenings of the lateral zones of 
the medulla. Secondly, from four of them, viz. III, V, VI, and VII, 
are proliferated the ganglionic cells of the four cranial nerves which in- 
nervate the first four visceral arches, viz. the trigeminus, the facialis, 
the glossopharyngeus, and the Urvagus. A clue to the significance of 
the local thickenings of the neural wall in the tract of the encephalo- 
meres is given in the fact that from those two encephalomeres which 
(in other Vertebrates as well as in S. acanthias) most closely retain 
these primitive nerve relationships, viz. III and V, emerge the fibres 
which innervate the visceral arches (primitively) related to them. The 
thickenings are the first expression of the “Kerne” (nuclei) of the 
nervous centres related to the visceral arches, and possibly also, primi- 
tively, of those related to the somites. 


1 Kupffer (’93) finds in Acipenser embryos an entodermal outpocketing or pouch, 
which soon disappears, just anterior to the hyomandibular pouch. The position of 
this pouch would identify it with the cleft whose former existence seems probable 
on the evidence given above. Houssay (’91) also recognizes in Amblystoma a vis- 
ceral cleft between the oral and the hyomandibular. 


252 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


A study of neural segments anterior and posterior to the medulla has 
led me to the conclusion that the local thickening is a more essential 
characteristic of a hindbrain neuromere than the commonly accepted 
criteria, viz. the radial arrangement of cells in the neuromere, and the 
crowding of them in the regions of constriction between neuromeres, 
both of which may be the result of mechanical influences. 

The shifting of the point of exit of the roots primitively related to 
encephalomeres VI and VII may easily be explained as the result of the 
crowding caused by the ear capsule. Since four hindbrain neuromeres 
are clearly related to four visceral arches, we should expect the remain- 
ing one, encephalomere IV, to have been primitively related to a vis- 
ceral arch. That such an arch has been present in the region of this 
neuromere during phylogeny, has been made probable by the studies of 
van Wijhe (82), Miss Platt (91), and Hoffmann (’94). The evidences 
from the study of neuromerism and mesomerism are mutually confirma- 
tory, and to the effect that a visceral arch has been lost in the region of 
encephalomere IV and van Wijhe’s third somite. Having established an 
exact numerical correspondence between encephalomeres and somites 
(head cavities), and a probable primitive correspondence of hindbrain 
encephalomeres with visceral arches, I conclude that in the head region 
there existed primitively a correspondence between neuromerism, mesom- 
rism, and branchiomerism. Since this correspondence is not to-day 
exact in Squalus or in any other known Vertebrate, it seems necessary 
to discuss somewhat in detail the constituent parts of the anterior or 
more highly modified metameres, and to inquire what may be inferred as 
to their previous conditions. The table on the opposite page, although 
in part theoretical, will help to make the discussion clearer. 

I have in this table included neuromeres as far posteriorly as the 
eleventh. Accepting Hoffmann’s (’94) conclusion that vertebral arches 
as far back as that which corresponds with van Wijhe’s tenth somite 
fuse into the cranium of the adult Squalus,? it would follow that neuro- 
meres I to XI would be included in the cranium. The variability 
in the number of segments added to the occipital region of the cra- 
nium in different Selachii and Ichthyopsida (Fiirbringer, Sewertzoff) 
makes the exact number in Squalus a matter of no great morphological 
importance. 

We see that the cephalic segments are highly modified segments 
altered by reduction or enlargement (possibly even by substitution and 
change of relation, as, for example, in the case of the vagus segments) of 


1 Recently confirmed by Sewertzoff (’98). 


” 


253 


NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 


NEAL 


n 
Q 
c= 
io) 
=) 
<4 
Bw 
Q 
— 
qq 
le) 
= 
4 
« 
iso} 
iF) 
Q 
1S) 


Tasty IIl.— NEUROMERES I TO XI IN SQUALUS, AND THEIR RELATIONS TO NERVES, SOMITES, 


NEUROMERES 


Somites. . 
Nerves (dorsal) 


Nerves (ventral) 
Visceral clefts 


Visceral arches 


ophth. 
prof. V (v)" 


lost 


lost mouth lost 


a oo PRP BO DS 


AND VISCERAL ARCHES. 


VII 


Ill IV 


Possibly also representing a visceral pouch. 

Fuses with the dorsal ganglion of X in later stages. 

Represented by ganglia which probably disappear in development. 
Form the first three roots of the embryonic hypoglossus nerve. 

Found in Hexanchus, Heptanchus, and Chlamydoselachus. 

Possibly represented in the two labial cartilages. 

Roman numerals bracketed indicate the theoretical nerve relationships. 


254 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


parts of the original segments. Fortunately, however, with the knowl- 
edge that neuromeres and mesomeres correspond numerically, we are 
able to see that the majority of changes which have occurred are cor- 
related ones, and therefore capable of explanation. We furthermore see 
that the greatest changes have taken place in the more anterior meta- 
meres, chiefly and primarily by the loss of the ventral parts of these 
metameres. Since the more posterior of the cephalic segments have 
indubitable metameric value, I shall discuss in detail only those anterior 
ones (viz. I to VIL) concerning which there is most disagreement among 
morphologists, beginning with the consideration of the seventh, whose 
relations are least modified. 


VIII. Primitive Relations of Cephalic Segments. 


a. RELATIONS OF ENCEPHALOMERE VII. 


Opposite the posterior constriction of this encephalomere in very early 
stages lies van Wijhe’s 6th somite, which develops embryonic muscle 
fibres and is universally considered a true somite. I therefore regard 
this as the mesomere corresponding with encephalomere VII, whose 
neural-crest cells first meet the mesoderm opposite the anterior constric- 
tion of this somite (Plate 3, Fig. 13). These cells form the Anlage of 
the anterior branch of the vagus (Urvagus), and I assume that the 
primitive relations of this nerve were with the myoseptum between the 
5th and 6th somites. The intermediate position of the Urvagus with 
respect to the myotomes and its ontogenetic union with spinal ganglia 
in some Vertebrates serves to show that there is no fundamental 
difference in this respect between cranial and spinal nerves. For rea- 
sons which will be stated in connection with the study of the relations 
of encephalomere IV, I regard the abducens (Plate 4, Fig. 21), whose 
fibres have their exit from the ventral horn of encephalomere VII, 
as representing in part the ventral nerve of this segment. Furthermore, 
I assume that the mesoderm of the 6th somite was primitively connected 
with the mesoderm of the 4th visceral arch (Plate 3, Fig. 16); because 
that somite in Ammoccetes which I regard as its exact homologue, viz. 
the 2d post-otic somite, is certainly in early stages thus connected. 
Consequently the present 3d visceral cleft bounds ventrally the visceral 
(splanchnic) portion of this segment. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 255 


6. RetatTions oF ENCEPHALOMERE VI. 


The present structure and relations of the component parts of what I 
regard as the primitive sixth cephalic segment have been considerably 
changed ccenogenetically by the development of the otic capsule. Aris- 
ing from what in all probability was primitively a sensor organ of the 
dorsal lateral line (Ayers), the great enlargement and subsequent invagi- 
nation of this capsule bring about ontogenetically the degeneration of 
the musculature of the 5th somite, whose cells, after assuming the 
elongated spindle form of embryonic muscle cells, are transformed in 
early stages into loose mesenchyma. In Ammoccetes, however, only the 
median portion of the Ist post-otic somite disappears during ontogeny, 
while the lateral portion forms the most anterior segment of the lat- 
eral body musculature (muse. lateralis capitis anterior, von Kupffer). 
Furthermore, in Squalus the development of the otic capsule causes a 
shifting backward of the point of exit of the fibres of the glossopharyn- 
geus, whose ganglion cells were proliferated from encephalomere VI; 
moreover, the fibres of this nerve may be traced in the neural tube as far 
forward as encephalomere VI, in which, it is my opinion, their nuclei lie. 
The growing ganglionic Anlage of this nerve meets the mesoderm between 
the 4th and 5th somites (Fig. 13), and I assume that it was primitively 
related, as are the dorsal nerves of Amphioxus, to a myoseptum, i. e. the 
one primitively between somites 4 and 5. The sensor fibres of this 
nerve innervate the skin of the present 2d visceral cleft (Fig. 14), 
which was, I assume, primitively inter-somitic in position and situated 
ventral to the myoseptum between the 4th and 5th somites. Its motor 
fibres innervate the splanchnic musculature of the present 3d visceral 
arch, probably a primitive relation. The abducens nerve, I believe, 
represents the primitive ventral nerve of this metamere. 


c. RELATIONS OF ENCEPHALOMERE V. 


The fourth somite, the one corresponding to the fifth cephalic segment, 
is the most rudimentary of all the cephalic somites. The phylogenetic 
loss of its musculature and the ontogenetic dissolution of its cells into a 
loose mesenchyma may be explained as due to the same cause as that 
operative in the case of the 5th somite, the development of the otic cap- 
sule. The dorsal nerve of this segment, the facialis, is inter-somitic in 
position, occupying the constriction dividing the 3d and 4th somites 
(Figs. 11-17), and its motor fibres innervate the (splanchnic) muscu- 
lature of the corresponding (2d visceral or hyoid) arch. Correlated with 


256 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


the loss of the somatic musculature of this segment, a somatic (ventral) 
nerve is wanting, and as in the case of the 6th segment I assume that 
this is to-day represented by the abducens. Since the sensor fibres of 
the facialis innervate the skin of the hyomandibular (1st visceral) cleft, 
this cleft may be believed to have been primitively situated ventral to 
the constriction between the 3d and 4th somites. I find no evidence to 
support the view that the hyoid arch represents two splanchnic segments. 


d. RELATIONS OF ENCEPHALOMERE LY, 


As has already been stated, there is no ganglionic nerve Anlage 
proliferated from encephalomere IV, and although the fibres of the major 
root of the trigeminus have their exit in early stages from its outer con- 
vexity, the probability is that such relation of nerve V is secondary, and 
therefore not of phylogenetic significance. I hold that this encepha- 
lomere, by virtue of its local thickening, affords evidence of a lost visceral 
arch, the loss of which would naturally be correlated with the loss of the 
dorsal nerve. Since, however, the disappearance of the splanchnic por- 
tion of this segment may have been due simply to a union with the 
corresponding portion of the anterior (mandibular) segment, it is also 
possible that the dorsal nerve has fused with the nerve of the anterior 
segment, the trigeminus. This conclusion seems indeed supported by 
the evidence that at least some of the fibres of the trigeminus roots have 
their nucleus in the lateral horn of this (4th) encephalomere. 

In a scheme of primitive segmental relations such as I am at present 
advocating, there is likewise difficulty in explaining the fact that the 
somite (van Wijhe’s 3d) which I assume to correspond with encephalo- 
mere IV is innervated by the abducens, whose fibres make their exit 
from encephalomere VII. The evidence which leads me to conclude 
that the abducens to-day represents the primitive ventral nerve of this 
encephalomere, as well as those of encephalomeres V, VI, and VII, has 
been partly given in connection with the study of its development ; it 
may be summarized as follows. (1) Its roots are many (4-6 in various 
Selachii) and more widely separated than those of any other nerve. 
(2) Not only do abducens fibres innervate pre-otic musculature (muse. 
rectus posterior), but fibres from this nerve may also be traced for a con- 
siderable distance in the mesoderm of the embryo posterior to encepha- 
lomere VII (Fig. 20). (3) The variability as to the place where its fibres 
emerge, as shown by comparative embryological evidence, appears to 
indicate that its relations are not limited to any single encephalomere. 
(4) Its nucleus in the ventral horn of the neural tube is greatly elongated. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 257 


(5) In Torpedo it innervates musculature (musc. rectus posterior) de- 
rived from two somites, viz. van Wijhe’s third and fourth (Sewertzoff, ’98). 
I am not able, however, to offer direct evidence that the nerve has part of 
its nucleus in encephalomere IV. I am therefore not able to exclude 
the possibility that the ventral root of a post-otic somite has been substi- 
tuted for the pre-otic ventral nerve which once innervated somite 3. 
That such a substitution of the fibres of a ventral nerve of one segment 
for those of another may take place ontogenetically, I have the following 
evidence. I find that in a Squalus embryo of 50 mm. the ventral nerve 
of van Wijhe’s 7th somite has become very rudimentary, while fibres 
from the ventral nerve of the 8th somite extend to the musculature de- 
rived from the 7th somite, which in this stage forms the most anterior 
segment of the lateral musculature. Now, if the ventral root of the 7th 
somite atrophies before the adult stage is reached, and if the muscula- 
ture derived from this somite remains the first segment of the lateral 
trunk musculature of the adult, as has been stated by van Wijhe (782) 
and Hoffmann (’94), the conclusion seems unavoidable that we have to 
do here with a substitution of a posterior nerve for one farther anterior. 
Moreover, in Petromyzon we have evidence that the first five post- 
otic myotomes of the lateral trunk musculature are innervated by the 
ventral nerves of the last two of the corresponding somites, i.e. the 4th 
and 5th post-otic, which in my opinion are homologous with the 4th and 
5th post-otic somites of Squalus (van Wijhe’s 8th and 9th). Here also 
the conclusion seems to me to be warranted that there has been a phy- 
logenetic, if not an ontogenetic, substitution of the nerves of posterior 
segments for those of more anterior segments.' We may therefore 
infer, with a considerable degree of probability, that a similar substi- 
tution of a post-otic nerve for a pre-otic one may have occurred phy- 
logenetically in the case of the abducens. Such evidence, however, 
seems to render unwarrantable the assumption of a primary and in- 
separable connection of motor nerve and muscle. Furthermore, the 
evidence that the motor nerves develop as axis-cylinder processes of me- 
dullary cells given by His (’89) for spinal nerves, and by myself in this 


1 See Neal (’97, Figure 2, p. 446) for evidence that the fibres of a post-otic ven- 
tral nerve (hypoglossus auctorum) extend into the pre-otie region with the muscle 
they innervate. It would seem a very easy matter for such fibres to come into 
nervous connection phylogenetically with the eye muscles, and especially the 
posterior of these, with which in Petromyzon they are very closely connected. 
Hatschek (’92) stated that the musc. rectus posterior becomes connected with the 
anterior of the post-otic myotomes. See evidence given by M. Fiirbringer (’97) 
and Neal (’97) upon this question. 

VOL. XXXI.— NO. 7. 8 


258 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


paper for cranial ventral nerves in Selachii, leads to the same conclusion. 
The visceral cleft which defined anteriorly the splanchnic part of the 
fourth segment is not ontogenetically evident in Squalus. Kupffer (93) 
has possibly seen evidence of a rudimentary cleft between the mouth and 
the hyomandibular cleft of Acipenser. And possibly this cleft may be 
represented in the ‘‘ Pseudobranchialrinne ” of Amphioxus. 


e. RELATIONS OF ENCEPHALOMERE III. 


As in the case of the four posterior hindbrain segments, the study of 
the development of the nerves connected with encephalomere III (Hinter- 
hirn) gives the clue to the primitive relations of this primary vesicle. 
The neural-crest cells proliferated from it pass ventrally into the man- 
dibular arch. From a part of these a large ganglion is formed (the 
Gasserian), through which pass the motor fibres, whose nucleus is, at 
least in part, in encephalomere III, to innervate the musculature of the 
first visceral (mandibular) arch. We have thus the splanchnic elements 
ofacranial segment. In the Table of Nerve Relations (p. 253) the troch- 
learis has been given as the ventral (somatic) nerve of this segment. 
The evidence in favor of this view has already been stated, and consists 
in the facts that it innervates musculature derived from dorsal (somatic) 
mesoderm, that its fibres develop as processes of neuroblasts in the neural 
tube, and that its histological relations and structure in the adult show 
it to be a purely motor nerve with motor nucleus in the ventral horn of 
encephalomere III. I regard the mouth as representing the fused visce- 
ral clefts which bounded anteriorly the splanchnic portion of this seg- 
ment. We have thus all the essential elements of a head metamere. 


f. Revations oF EncepHaLomerE II. 


From the simple dorsal expansion of encephalomere II are proliferated 
cells which pass ventrally and fuse with the skin to form the meso- 
cephalic ganglion! lateral to the Ist somite (Figs. 17 to 20). Although 
this ganglion never becomes connected with the midbrain (encephalo- 
mere II), since its fibres enter the brain through the r. ophthalmicus 
profundus V, it must in my opinion be regarded as a segmental gan- 
glion comparable with those of the following cranial nerves ; the oph- 
thalmicus profundus must likewise be considered as a dorsal nerve 
homodynamic with the succeeding cranial nerves. Its want of motor 
fibres may be explained as resulting from phylogenetic loss, since in 


1 This ganglion is homologous with the first trigeminus ganglion of Cyclostomes. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 259 


Myxinoids this nerve possesses motor fibres (J. Miller, P. Fiirbringer, 
Price), and its segmental value as a dorsal nerve seems thereby estab- 
lished. ‘The fact that the fibres of the ophth. profundus V enter the 
brain at a point posterior to encephalomere III, instead of anterior to 
it, as they should in order to conform to my scheme of segmental rela- 
tions, appears to me no serious objection. That they enter the brain 
at a point posterior to that at which the motor fibres innervating the 
mandibular musculature enter, and in consequence cross these fibres in 
the mesocephalic ganglion, is to be explained by the tendency, especially 
of the sensor cranial nerves, to enter the brain as near the otic capsule 
as possible (see Ahlborn, ’84*), and by the more conservative relations 
of the motor fibres (roots) generally. 

In my preliminary paper I placed tentatively the so called “ thalamic ” 
nerve as the possible dorsal nerve of encephalomere II. Now, how- 
ever, I question the correctness of this opinion. We certainly need 
something more than a strand of neural-crest cells which persist for 
some time in a region of constriction between encephalomeres, but 
which never assume fibrillar relation with the neural tube, to warrant us 
in assuming that we have to do with a nerve.* 

The development and relations of van Wijhe’s first somite and of the 
oculomotorius leave no doubt that in them we have the somatic ele- 
ments of a metamere. Probably no ventral or splanchnic portion of the 
mesoderm of this segment exists, consequently the r. ophthalmicus pro- 
fundus possesses no splanchnic fibres.?, In my opinion it is doubtful if 
the hypophysis may be regarded as evidence of an ancestral visceral cleft 
between segments I and II. 

However, I hold that the structural comparability of encephalomere II 
with hindbrain encephalomeres, together with the evidence of its rela- 
tion with a segmental ganglion, and of its connection with somatic muscu- 
lature by means of a ventral motor nerve, strongly favors the view that 
it is serially homologous with hindbrain encephalomeres. 


g. RELATIONS OF ENCELPHALOMERE I. 


. That which I regard as the first cephalic segment of Craniota consists 
of an encephalomere (primary forebrain) which has been shown to be 


1 Kupffer excels Miss Platt in discovering “rudimentary” nerves, but until 
we have a better criterion for a nerve than a cellular strand there is no reason 
why the number of “rudimentary” nerves should not be much larger than it is 
at present recognized to be. 

2 Possibly the skeletogenous element of the ventral portion of this segment is to 
be found in the “ maxillar Lippenknorpel” of Gegenbaur. 


260 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


morphologically comparable with the hindbrain encephalomeres. It is 
in connection with a sensor nerve, the olfactory, which appears com- 
parable with the sensor portion of a dorsal segmental nerve in so far as 
it is composed of bipolar ganglion cells which send their fibres into the 
brain and, in my opinion, are in part derived from the neural crest. 
My observations upon this point, however, are as yet incomplete. The 
want of motor fibres in the dorsal nerve of this segment is correlated 
with the want of splanchnic musculature.t. That structure which I, in 
agreement with Miss Platt (’91) and Hoffmann (’94 and ’96), regard 
as a rudimentary somite (compare Plate 3, Figs. 16, 17, cav. a.), — since 
it resembles the following somites and gives evidence of producing rudi- 
mentary muscle cells, — represents the somatic portion of this segment. 
In correlation with the want of fully developed musculature, no ventral 
somatic nerve is present. Van Wijhe (’86%, p. 680) wrote: “ Wenn der 
Olfactorius ein segmentaler Nerv wire, miisste man bei demselben das 
urspriingliche Vorhandensein eines Somiten und einer zugehérigen ven- 
tralen Nervenwurzel annehmen. Von beiden ist keine Spur vorhanden.” 
So far as the somite is concerned, it appears that in the “anterior so- 
mite”? we now have the requisite evidence. The lateness of the differ- 
entiation of the “ premaxillar Lippenknorpel” makes it seem at most 
only remotely possible that it may be regarded as the ventral skele- 
togenous element of this segment. 


h. CoMPARISON WITH THE SEGMENTATION OF AMPHIOXUS. 


A comparison of the segmentation of Squalus as shown in Table ITI. 
(p. 253) with that of Amphioxus is of interest, inasmuch as it appears 
to favor important conclusions reached by the study of Squalus alone. 
However, before stating my own conclusions concerning the primitive 
metamerism of Amphioxus and the homologies of its segments with 
those of Squalus, it will be well to review the conclusions of previous 
investigators. 

A comparison of their results may be made in the form of a ieee on 
the opposite page (after M. Fiirbringer, ’97, p. 643, slightly modified). 

While Hatschek (’92), Willey (94), and M. Firbringer (97) homol- 
ogize the mouth of Amphioxus with that of Tunicates and Craniota, but 


1 Likewise in Amphioxus the anterior dorsal nerve is generally believed to be 
purely sensor in function. 

2 Van Wijhe (’82) saw the “anterior somite ” in Galeus, but unfortunately pos- 
sessed only one embryo; he was therefore unable to express an opinion concerning 
its segmental value, based on a knowledge of its development and differentiation. 


261 


NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 


NEAL 


DABBLE LY. 


Farr or Harscuek, 792. Van Wisner, 793. Wiiry, 794. 


Single persisting mouth, the | Primary mouth (Autostoma) | Single persisting mouth, the 
homologue of the median un- | pushed toward the left and | homologue of the mouth of 
paired mouth of Tunicates | modified to form the pre-oral | Tunicates and higher Ver- 
and higher Vertebrates. pit (Raderorgan). tebrates. 


1 Unpaired mouth. 


. Perhaps the homologue of the | Probable homologue of the 
Right. Ist head cavities (1st somites) | right Ist (premandibular) 
of Selachii. head cavity of Craniota. 


Ist visceral pouch (vor- 
2 deres Entodermsiickchen) 
of Hatschek. 


Pre-oral pit, probable homo- 
Left Pre-oral pit (Riderorgan; | Pre-oral pit; also primary | logue of the left 1st (preman- 

~ " | Sinnesorgan). mouth (Autostoma). dibular) head cavity of Cra- 
niota, 


Right. | Right pseudo-branchial groove. | Club-shaped gland. Club-shaped gland. 


2d visceral pouch of Hat- 
3 schek (Ist visceral cleft of Secondary permanent mouth 
van Wijhe and Willey). Left. | Left pseudo-branchial groove. | (Tremostoma). (Spiracular 

cleft of Selachii. 


Ist (abortive) primary gill 
slit. 


2d (1st permanent) secondary 


: Ist permanent right visceral 
3d visceral pouch of Hat- Bphy: cleft. : Losti or right gill slit. 
4 schek (2d visceral cleft of | |_——|———________—_ 
van Wijhe and Willey). Left. | 1st, permanent left visceral L 2d (1st permanent) primary 
"| cleft. ORK. gill slit. 


ne Net 
th visceral cleft of Hat- | cleft. cleft. slit. 
5 schek (3d visceral cleft of 


van Wijhe and Willey). 


2d permanent left visceral | Ist permanent left visceral | 3d (2d permanent) primary 
cleft. cleft. gill slit. 


Left. 


262 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


deny its gill-cleft nature (Dohrn), van Wijhe regards it as a visceral 
cleft on the left side, antimeric to the club-shaped gland, which with 
Willey he regards as a modified visceral cleft, exactly homologous with 
the hyomandibular (spiracular) cleft of Craniota. Van Wijhe (’93, 
p- 155) finds evidence of a primary unpaired mouth in the external 
opening of the left anterior entodermic diverticulum known as the pre- 
oral pit (Riaderorgan). Homologizing the “ Gehirnanschwellung” of 
Amphioxus with the ‘ Gehirnblase” of the larve of Ascidians, he con- 
siders it impossible to homologize the mouth (tremostoma) of Am- 
phioxus with the median dorsal mouth of Tunicates, since in the former 
the mouth and its antimere are laid down immediately posterior to the 
brain vesicle, whereas in the latter the mouth arises in the median 
plane immediately anterior to the brain vesicle ; however, the visceral 
clefts of the young Ascidian larva are laid down, like the mouth of 
Amphioxus, immediately behind the brain vesicle. Moreover, van 
Wijhe holds that the mouth of Amphioxus is an organ of the left side 
only, and on the following grounds (quoted from Willey, 94, p. 178): 
“The outer muscle of the oral hood represents the anterior continua- 
tion of the left half only of the transverse and subatrial muscles. The 
inner nerve-plexus of the oral hood is formed on both sides exclusively 
from nerves which arise from the left side of the central nervous system. 
The velum is innervated entirely from nerves of the left side,” viz. 
branches from the 4th, 5tb, and 6th left dorsal nerves. 

Willey (’94) finds evidence to support his view, that the mouth of 
Amphioxus represents the median dorsal mouth of Ascidians, in the 
marked asymmetrical conditions of the larva, for which van Wijhe’s 
observations and conclusions afford no explanation. Affirming the 
asymmetry to be non-adaptive and non-advantageous (contra Korschelt 
und Heider), he concludes that it is the mechanical result of the (phy- 
logenetic) forward extension of the notochord, an extension which is 
advantageous to an animal which bores in the sand. Hatschek (92) 
and M. Fiirbringer (97) agree with Willey in this explanation as to the 
homology of the mouth of Amphioxus, but bring forward no evidence to 
support their view. There is no disagreement in homologizing the an- 
terior entodermic diverticula (vordere Entodermsackchen) of Amphioxus 
with at least part of the premandibular head cavities (1st somite of 
van Wijhe) in Craniota. 

From the foregoing review it will be seen that two very important 
questions concerning the nature and homologies of the Vertebrate mouth 
remain in dispute, viz.:— 1. Is or is not the mouth of Amphioxus to be 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 263 


homologized with the mouth of Ascidians? 2. Is it or is it not homol- 
ogous, either wholly or in part, with the mouth of Craniota? Upon the 
answer to the former question would seem to depend the settlement of 
the question whether the mouth of Amphioxus may or may not be re- 
garded as a visceral cleft, for there is no reason to believe that the 
mouth of Ascidians represents a pair of visceral clefts. Notwithstanding 
that Willey appears to have in the asymmetrical mouth of Amphioxus 
strong evidence in favor of his homology, which has also met the 
approbation of Marshall (93), I consider the different relation of the 
Tunicate and Vertebrate mouth to the brain vesicle a very serious ob- 
jection to his theory. Furthermore, the presence of a preoral intestine 
in Vertebrates, which in Squalus extends (morphologically) anterior to 
the infundibulum, —even to the neuropore, as does the intestine of 
Tunicates, — leads me to agree with Beard, Kupffer (’88 and ’91), and 
van Wijhe (’94), that in the present mouth of Vertebrates we have a 
neostoma, and also that a palewostoma homologous with that of Tunicates 
must be sought in an anterior opening of the preoral intestine. Kupf- 
fer finds evidence of this palg@ostoma in the ectodermic invagination of 
the hypophysis towards the “ Preoraldarm,” while van Wijhe finds 
it in Amphioxus, as stated in the Table, in the actual opening of 
the preoral intestinal diverticulum of the left side as the preoral pit 
(Raderorgan). 

Waiving the question as to which, if either, of these theories is cor- 
rect, I regard the mouth of Ascidians as opening at the morphologically 
anterior end of the alimentary canal; for there appears to me noth- 
ing in the literature upon Tunicates to show the presence of a preoral 
intestine in these forms. The mouth of Appendicularia, which has no 
“preoral lobe,” (though homologized by Willey with the preoral intes- 
tinal diverticula of Amphioxus and the premandibular cavities of Crani- 
ota,) has a terminal position.1 According to Willey the method of 
formation of the preoral lobe in those Ascidians possessing such is as 
follows (p. 218): ‘‘When the larva first hatches, the entoderm and 
ectoderm are in contact with one another at the anterior extremity of 
the body, just as they are in the earlier stages. Soon, however, the 
ectoderm, with the adhering papille, springs away from the endoderm 
at this point, leaving a space into which the two lateral mesodermic 


1 Willey (’94, p. 277) writes: “ Whatever the truth may be as to the precise 
systematic position and phylogenetic value of Appendicularia, one thing, to my 
mind, remains absolutely certain, namely, that it has descended from a form which 
possessed a przoral lobe, and that it has secondarily lost that structure.” 


264 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


bands force their way. In this way a special anterior portion of the 
body cavity, preoral and preenteric, is produced, and is at first com- 
pletely filled by a compact mass of rounded cells derived from the meso- 
dermic bands. . . . The anterior, or przoral portion of the body cavity, 
of which we have just traced the origin, is, and subsequently becomes in 
a still more pronounced way, the cavity of the snout, or preoral lobe.” 

On the other hand, the preoral ‘‘ head cavities” of Amphioxus, which 
Willey homologizes with the “ preoral lobe” of Ascidian larve, are formed, 
as stated by Hatschek (’81), from an abstricted portion of the preoral 
archenteron. The differences in the formation of these two structures, 
therefore, seem too striking to permit their being considered completely 
homologous with each other. 

Evidence has already been given in this paper which, in my opinion, 
makes it impossible to homologize the preoral ‘head cavities” (anterior 
entodermic diverticula) of Amphioxus with the “ premandibular head 
cavities” of Craniota. The morphologically anterior portion of the arch- 
enteron, the “anterior head cavities” (Platt), are the only structures 
in higher Vertebrates which, in my opinion, can be homologized with 
the “head cavities” of Amphioxus. Homologizing, therefore, the “an- 
terior head cavities” in these two forms, I submit on the opposite page 
for comparison with Squalus the table of the anterior eleven segments 
in Amphioxus as I interpret them. 

If we compare Tables III. (p. 253) and V., we find the following 
fundamental resemblances in the segments of Squalus and Amphioxus. 

Of the component elements of the first segment, that which I have 
regarded as the somatic element, consists of paired cavities cut off from 
the anterior portion of the archenteron. Since in both cases these 
cavities represent ventral as well as dorsal and lateral portions of the 
archenteron, it is impossible to contend that they contain only the 
mesodermic element of the segment. It seems not improbable that 
potentially they represent also the visceral-pouch element between this 
and the following segment. The opening of the left of these in Amphi- 
oxus to the exterior as the preoral pit may be regarded as evidence 
favoring this view. Moreover, M. Fiirbringer (97, p. 633) finds a late 
differentiated and rudimentary myotome, which lies anterior to the 
dorsal paired nerves II, which would, if present as stated by him, repre- 
sent the mesodermic element of this segment, and the “anterior head 
cavities”? would in consequence necessarily be regarded as modified or 
abortive visceral pouches, as held by Kupffer. Since my sections of 
Amphioxus give me no evidence of this rudimentary myotome, I hold 


b 


265 


NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 


NEAL 


SEGMENTS 


Somites 
Dorsal nerves 
Ventral nerves 


Visceral clefts (r.) 


“ “ (1.) 


Taste V.—METAMERISM OF AMPHIOXUS. 


lost 


lost 


lost 


Til IV 
Se aoe 
Ill IV 
2 3 
c.s.g.2 lost 
M.3 il 


1 Possibly representing also a visceral-pouch element. 


* Club-shaped gland. 
8 M. Mouth. 


The permanent visceral clefts are indicated by asterisks. 


V VI vil Vill IX x XI 
S| SS ee SSS SSS SS SS 
5 6 ul 8 10 
V VI VII Vill IX x XI 
5 il 8 10 
1* 2% 3% 4% 5k 6* Tx 
2* 3* 4% 5* 6* 1% 8* 


266 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


with van Wijhe, Hatschek, and Willey, that the first myotome in Amphi- 

oxus is situated behind the first two pairs of nerves, and also that the 
“anterior head cavities” of Amphioxus represent the somatic elements 
of the anterior segment in Craniota. From this point of view, the 
different fates which the two cavities in Amphioxus undergo, as well as 
the loss of musculature, is to be regarded as coenogenetic. The dorsal 
nerves in both forms are exclusively sensor in correlation with the loss 
of splanchnic musculature.’ The unpaired olfactory of Amphioxus is to 
be compared with the long persisting median connection of the neural 
tube and olfactory plate in Squalus (lobus olfactorius impar, Kupffer). 
No somatic musculature and no ventral nerves are developed. I regard 
the cerebral vesicle of Amphioxus, since it is limited posteriorly by the 
tuberculum posterius (Kupffer), as homologous with the primary fore- 
brain of Squalus, and therefore as the neuromeric element of the first 
segment.” Whether or not the visceral cleft of this segment is repre- 
sented in the “anterior entodermic diverticula” (anterior head cavities), 
I am not able to assert with any degree of positiveness. Fitirbringer’s 
discovery appears to favor this view. 

In the second segment a well developed myotome and ventral nerve 
develop. In both forms the dorsal nerve of this segment appears to be 
exclusively sensor. In Myxinoids, however, the dorsal nerve (ophthal- 
micus profundus) has motor fibres, and it appears to me not improbable 
that such will be found in its homologue in Amphioxus.? If the vis- 
ceral-cleft element in this segment is not represented in the “ anterior 
head cavities,” this may be assumed to have disappeared phylogeneti- 
cally. All the components of the third segment are present, viz. somatic 
and splanchnic musculature, dorsal and ventral nerves, and visceral clefts. 


1 The homology of the olfactorius (I) with the first paired nerve of Amphioxus has 
already been asserted by Owen (1866). The first paired nerve of Amphioxus accord- 
ing to Owsjannikow (1866) and Rabl (’89) is homologous with trigeminus; with r. 
orbito nasalis or r. I. trigemini (Huxley, 1874); with opticus (Schneider, ’79) ; with 
part of trigeminus (Rohon, 1881, and Krause, 1888) ; with ophth. prof. trig. (Hat- 
schek, 792); and with nervus apicis (van Wijhe, 793). 

2 Kupffer (’93) homologizes the cerebral vesicle of Amphioxus with the Vorhirn 
(Vorderhirn and Mittelhirn) of Craniota. 

8 The second paired nerve of Amphioxus has previously been homologized 
with part of trigeminus by J. Miiller (1842), W. Miiller (1875), and Krause (1888) ; 
with trigeminus by Goodsir (1841); with trigeminus and vagus by Quatrefages 
(1845) and Owen (1866); with facialis by Owsjannikow (1867); with opticus by 
Hasse (1876); with part of trigeminus and with facialis by Rohon (1881) ; with 
acustico-facialis by Rabl (’89); with trigeminus exclusive of ophth. profundus by 
Hatschek (’92); and with ophth. profundus by van Wijhe (’93). 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 267 


I regard the mouth of Amphioxus as homologous with the left half of 
the mouth of Craniota and the club-shaped gland as its antimere. That 
the mouth of Amphioxus as an organ of the left side is exactly homolo- 
gous with the left half of the mouth of Squalus appears to me probable 
ou the a priori ground that it is improbable that an organ of the same 
function should be twice acquired in the Vertebrate series ; and also be- 
cause the region of fusion of endoderm and ectoderm to form the mouth 
cleft is in both these forms ventral to the constrictions which separate 
the second and third mesodermic segments (lst and 2d myotomes). 
The club-shaped gland also appears as an eutodermic diverticulum below 
the constriction between the second and third mesodermic segments 
of the right side, that is, opposite the mouth diverticulum, and I there- 
fore, in agreement with van Wijhe (93), regard it as the antimeric 
gill cleft.? 

In the fourth segment the following points of resemblance are to be 
noted. Somatic musculature and a somatic ventral nerve are present. 
While in Squalus the pair of visceral clefts which bounded anteriorly 
the splanchnic portion of this segment have disappeared, leaving no trace 
behind except in the neuromere with which they were connected, in 
Amphioxus only the right visceral cleft has been thus lost. The left 
visceral cleft, however, disappears ontogenetically without leaving a trace 
behind it. A further difference in the two forms appears in the fact that, 
whereas in Squalus the dorsal nerve has disappeared (or fused with the 
trigeminus), the dorsal nerve of the left side in Amphioxus is the first 
of the nerves which innervate the musculature of the velum (van 
Wijhe). 

With the jifth segment in both forms begin the permanent visceral 
clefts. In agreement with Willey (94), I regard the first secondary cleft 
as antimeric to the second primary cleft. Their fusion with the ecto- 
derm below the mesodermic constriction between mesoderm segments 
4 and 5 (myotomes 3 and 4) is the evidence for their relation to 
this, the fifth segment. I therefore consider the first pair of permanent 
visceral clefts in Amphioxus as the exact homologues of the hyomandibu- 
lar clefts of higher Vertebrates. As has already been stated by Willey 
(94), all except eight of the primary clefts (starred in the table), which 
become paired with eight antimeric clefts, undergo atrophy. In conse- 


1 Willey (’94) gives reasons for regarding the club-shaped gland as the antimere 
of the first primary visceral cleft. His reasons are based on topographic relations 
in stages when the primitive topographic relations are considerably changed, and 
they seem to me less strong than the reasons stated by van Wijhe and myself. 


268 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


quence there is found at the end of the larval period a “ critical stage ” 
of considerable duration, when Amphioxus possesses eight visceral clefts, 
which, if the homology above be correct, are exactly homologous with the 
eight morphological clefts of Heptanchus (Selachian) and Petromyzon 
(Cyclostome). The evidence of the exact homology of the mouth and 
visceral clefts of Amphioxus at its critical period with those of Craniota 
appears to me strongly confirmatory of the truth of the exact homology 
of segments in Amphioxus and Squalus as stated above. 


z. GENERAL CONCLUSIONS, 


The exact numerical correspondence of neuromeres (encephalomeres) 
and somites has been found not to be a purely accidental one. The 
ventral motor nerves (oculomotorius and trochlearis) of two successive 
encephalomeres, viz. II and III, are connected with two successive 
somites, viz. van Wijhe’s lst and 2d, and the nerves VII, IX, and X 
(Urvagus), by their topographic relations to successive somites 4, 5, and 
6, show a similar metameric correspondence between encephalomeres 
and somites. Where correspondence does not clearly exist to-day, as in 
the case of the abducens nerve, we have developmental evidence which 
suggests how such modifications may have taken place. 

Thirteen years ago Ahlborn (84°), as a result of his examination of the 
evidence presented by van Wijhe (’82), stated it as his conclusion that 
in the head we have a dysmetameric neuromerism, which no longer 
repeats the metamerism of the mesomeres (somites), but is related to 
a series of other conditions dependent on both ectoderm and entoderm. 
Ahlborn likewise concluded that branchiomerism and mesomerism do 
not correspond. ‘Gegenbaur’s assumption, that the segmentation of the 
cranial nerves, related as they are to visceral arches, is comparable to 
the segmentation of the spinal nerves, which correspond with somites, 
still remains to be proved.” The evidence presented above certainly 
tends to make the assumed correspondence of mesomerism and branchi- 
omerism more probable, and thus indirectly to prove the homodynamy 
of the nerves which innervate mesomeres and branchiomeres. The re- 
cent evidence presented by Hatschek (’92), Kupffer (’91, 96), Price 
(96), and Miss Platt (’97) from their studies on Amphioxus, Cyclo- 
stomes, and Amphibia points in the same direction, and thus favors 
Gegenbaur’s assumption. The comparative embryological evidence 
which has been given shows, however, that the adoption of Gegenbaur’s 
view by no means necessitates the assumptions later made by him (’87), 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 269 


viz. : (1) that the head primitively ended with van Wijhe’s 6th somite ; 1 
(2) that between this and the following somite segments (dorsal as 
well as ventral) have been phylogenetically lost ; and (3) that the head 
primitively ended with the gill region. It has been shown, I believe, 
that the probable phylogenetic and actual ontogenetic disappearance of 
visceral clefts does not uecessitate the loss of the corresponding mesomere 
and neuromere. It is true that we have very good reason to infer a 
phylogenetic loss of distinctly differentiated somites and neuromeres in 
the Vertebrate series. It is also true that we find evidence of an onto- 
genetic disappearance of mesomeres. Nevertheless such evidence does 
not prove that somites have been phylogenetically lost from the occipital 
region before the group of Selachii, of which Squalus is one of the most 
primitive forms, is reached. I believe that the evidence which has 
been given of the complete metameric correspondence of neuromeres and 
mesomeres — that the Selachian embryo is in this respect an unbroken 
continuum — renders it unnecessary to assume that somites have been 
so completely lost that no traces of them appear phylogenetically in 
Selachii. It is no longer necessary to assume a palingenetic portion 
of the Vertebrate head which ended with the sixth visceral arch of 
Selachii (Gegenbaur), or an exact homology between the hypoglossus 
roots (surely a most uncertain “fixed point”) of adult Vertebrates (M. 
Firbringer). The evidence which I have given seems thus to favor 
the opinion of Sewertzoff (95), that we have “keinen Grund, vorauszu- 
setzen, dass zwischen den palingenetischen Somiten vy. Wijhe’s (I-VI) 
und den coenogenetischen (VII-IX) ein Wegfallen der Segmente statt- 
gefunden hat. Wir sehen sine vollkommen regelmissige Anlage der 
Kopfsomiten und ein eben so regelmissiges [ontogenetic] Verschwinden 
derselben.” 

I am aware, however, that the structural differences between the hind- 
brain neuromeres, e. g. IV to VII, and the neuromeres immediately 

1 The suggestion that the gill region is not confined to the head region was first 
made by Huxley (’58). I believe that direct evidence in favor of this suggestion is 
furnished by Amphioxus (Hatschek, ’92), and by Myxinoids (Price, ’96). In this 
connection, moreover, it is of interest that in my previously (’97) made homology 
the /ast visceral cleft in Ammocetes primitively bounds posteriorly the segment 
which is homologous with the /ast cranial segment (Hoffmann, 94) of Squalus, viz. 
van Wijhe’s 10th somite. Furthermore it has been shown (p. 268) that this last 
visceral cleft of Petromyzon is exactly homologous with the last visceral cleft of 
Amphioxus in its “critical stage” of development. It should, however, be noted 
that there have been published three other interpretations of homologies between 


Selachii and Cyclostomata, differing from that made by me, viz. those by Ahlborn 
(7842), by Hatschek (’92), and by Sewertzoff (’95). 


270 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


following these may seem to favor Gegenbaur’s view that the former be- 
long to a palingenetic portion of the Vertebrate head which ended 
with the 6th (van Wijhe’s) somite (bounding neuromere VII posteriorly 
and ventrally). The structural gap between the seventh and eighth 
neuromeres is not, however, so sharp that it should outweigh evi- 
dences of similarity, and especially the evidence that somites 6 and 7 
are indisputably serially homologous. I must confess that I cannot see 
that the assumption of palingenetic and ccenogenetic portions of the 
Vertebrate head has added to the clearness of our morphological con- 
ceptions, nor can I see that it is rendered necessary by any ontogenetic 
or phylogenetic evidence now in our possession. Note, furthermore, 
the disagreement of opinion as regards what is and what is not palin- 
genetic or ccenogenetic among those who have been prominent as advo- 
cates of this view, viz. Gegenbaur (87), his pupil, Fiirbringer (97), and 
Miss Platt (97). While Gegenbaur holds that van Wijhe’s 6th somite 
is palingenetic, Firbringer regards the 6th, and possibly the 5th and 4th 
somites, as cenogenetic. Miss Platt, on the other hand, believes that 
the 4th and 5th somites are palingenetic, but that the 6th somite is 
probably ccenogenetic. All this appears to me confusing and unneces- 
sary. The terms ccenogenetic and palingeuetic are purely relative 
terms. I hold the view that each metamere of the head may be re- 
garded as cenogenetic in comparison with the metameres anterior to 
it, the head gradually receiving accessions from the trunk. Gegenbaur’s 
famous “ Kritik” of 1887 appears more an attempt to establish the 
visceral arches as the essential criteria of cephalic metameres, than a 
wholly unprejudiced effort to weigh the evidence both anatomical and 
embryological which was at his command. I believe that the evi- 
dence given in the present paper tends to strengthen the generally 
accepted opinion, which Gegenbaur has sought to overthrow, that 
the mesomeres in the head, like those in the trunk, afford the most 
trustworthy criteria of metamerism. The dorsal (neuromeric and meso- 
meric) segmentation must be regarded as more conservative than the 
ventral (branchiomeric or splanchnic) segmentation. The lost elements 
are chiefly the ventral ones. Their loss has indirectly caused the losses 
in the dorsal elements, such as the disappearance of splanchnic motor 
fibres from dorsal nerves and (2) of the thickening of the lateral zones of 
encephalomeres I and II. 

It appears to me that the evidence now in our possession gives reason 
to hope for an eventual solution of the head problem, not only as regards 
the nature, but also the number of head segments. The problem, it is 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. QF 


true, is easier for occipital than for pre-occipital segments. The seriai 
homology of occipital with trunk segments is not generally questioned 
at present. A comparison of the integral parts of occipital and 
trunk metameres shows that the belief in their serial homology is well 
founded. It must, however, he admitted that occipital metameres 
show no evidence of either excretory or reproductive organs. Never- 
theless we may readily believe from the evidence of these organs in the 
gill region of Amphioxus that this is a ccenogenetic loss in the Ver- 
tebrate series. The chief grounds for belief in the homology of trunk 
and occipital metameres are these: (1) Occipital somites with their 
(2) ventral nerves are undoubtedly the serial homologues of trunk 
somites with their ventral nerves. This evidence alone has convinced 
most morphologists. But there are still other reasons. With our 
. present knowledge, we may, I think, affirm that (3) dorsal occipital (or 
cranial) and dorsal spinal nerves are serial homologues. One by one, 
since the discovery by Schneider (’79) of ventral nerves in Amphioxus, 
the differences between dorsal spinal and cranial nerves, which were 
at one time or another maintained, have been with increased compara- 
tive embryological and anatomical knowledge shown to be unessential. 
The evidence given by Schneider (’79), Hatschek (’92), and van Wijhe 
(93) shows that dorsal nerves, as seen in Amphioxus, are mixed in 
function, innervating the skin and splanchnic musculature, while ventral 
herves are motor in function, innervating somatic musculature. The 
typical cranial nerves of Craniota, viz. V, VII, IX, and X, are mor- 
phologically comparable with the dorsal nerves of Amphioxus, and are 
therefure to be regarded, as Balfour for other reasons regarded them, 
more primitive than the spinal nerves, which lack the lateral and dor- 
sal (except in Cyclostomes) cutaneous branches.! The recent researches 
of von Lenhossék (90), Ramon y Cajal, and Kdélliker, by demonstrating 
the existence of non-ganglionic fibres in the dorsal spinal nerves of 
Craniota, which by their relations must be regarded as motor in func- 
tion, have shown that in this respect spinal nerves do not differ from 
cranial. Moreover, in view of the evidence given by Goronowitsch (’92), 
Sewertzoff (’95), Neal (’97), and Miss Platt (97), it can no longer be 


1 The place of these branches has been usurped by the lateral branches of the 
vagus, as I believe has been suggested by Eisig. The advantage in greater cen- 
tralization is obvious. If it be true, and itis generally admitted, that cranial nerves 
receive cells from the skin while the spinal nerves do not, an explanation of 
this also is seen in the extension of the vagus and the concomitant loss to 
spinal nerves. 


Ate BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


truthfully said that cranial nerves differ from spinal in that the former 
extend laterad and the latter mediad of the mesomeres. We must con- 
clude that dorsal nerves were in all probability, as in Amphioxus, re- 
lated to the septa between myotomes. Finally, the distinction made by 
His, in the case of dorsal cranial nerves, between dorsal (sensor) and 
lateral (motor) roots, has, with the knowledge of the facts above stated, 
an anatomical and physiological rather than a morphological interest. 
I therefore see no escape from the conclusion that the occipital region of 
the head is not a region sw generis, and I pass to the consideration of 
the pre-occipital segments. 

To those who are deeply impressed with the differences between post- 
otic and pre-otic regions of the Vertebrate head, it is necessary to em- 
phasize the following fundamental resemblances in the segments of these 
two regions. (1) Pre-otic and post-otic encephalomeres have been 
shown to be morphologically comparable. (2) The dorsal nerves con- 
nected with these, and (3) the visceral arches which these nerves supply 
are in these two regions serially homologous. Moreover, as evidence 
pointing in the same direction, it may be stated that (4) a post-otic 
nerve inuervates pre-otic musculature. Furthermore, the serial homol- 
ogy of pre-otic and post-otie somites appears established by the fact 
that (5) a pre-otic somite (van Wijhe’s 3d somite) is a segment of the 
dorsal mesoderm. That it is such seems clear, for it is defined 
anteriorly and posteriorly by well marked constrictions (observed by 
several investigators), it becomes differentiated into myotome and sclero- 
tome, and its musculature appears first in its median wall, and becomes 
innervated by a ventral nerve (abducens) serially homologous with ven- 
tral spinal nerves. The fact that the primitively dorsal mesoderm of the 
pre-otic region grows ventrally to form the splanchnic musculature, as 
has been stated for Cyclostomes, Selachii, and Amphibia, is not a basis 
for a fundamental distinction between post-otic and pre-otic regions, 
since this is the method of formation of splanchnic mesoderm through- 
out the length of the body in Amphioxus. In this respect, as in 
respect to the nerves, the head shows more primitive conditions than 
the trunk. Since the literature of the last decade and a half shows 
little agreement of opinion as to the morphology of the eye-muscle 
nerves, more especially the oculomotorius and the trochlearis, and 
since in the preceding pages evidence has been given which tends to 
reconcile existing differences, it is important to consider briefly the 
bearing of their morphology upon that of the pre-otic segments. The 
more recent attempts to classify the eye-muscle nerves as dorsal, 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 273 


lateral, or ventral indicates that the point of view of morphologists 
is now fundamentally different from that of the older anatomists, who, 
in dealing with the question of the segmental value of cranial nerves, 
excluded the eye-muscle nerves from consideration on the ground of 
their inconstancy in appearance and distribution. Except on the part 
of Froriep, Kastschenko, and Rabl, who regard the pre-otic region as 
one sut generis, I find no tendency to revert to the view of Stannius 
(49, p. 125) that “der Parallelisirung der Augenmuskelnerven mit 
Spinalnerven stellen sich, wegen ihrer eigenthiimlichen Ursprungs- 
verhaltnisse, des ihnen zukommenden Mangels von Ganglien und der 
ausschliesslichen Vertheilung ihrer ungemischten Primitivrobren in den, 
auch ihrerseits mit Muskeln der Wirbelsiule durchaus nicht vergleich- 
baren, Muskeln eines Sinnes-Apparates so untiberwindliche Schwierig- 
keiten entgegen, dass von einer solchen nicht fiiglich die Rede sein 
kann.” However, the labors of comparative anatomists, among whom 
may be named Huxley, Gegenbaur, M. Fiirbringer, and Schwalbe, during 
the thirty years following the work of Stannius just quoted, resulted 
in so well establishing the “ Biirgerrecht”’ of the eye-muscle nerves 
that morphologists now assume that they are comparable with either 
dorsal or else ventral segmental nerves. Only a minority of anatom- 
ists, among whom may be named Schneider (’79), van Wijhe (’82), 
Beard (’85), His (’88*), Dohrn (’91), Neal (96), and M., Fiirbringer (’97), 
have regarded them as ventral segmental nerves. The weightiest well 
established evidence in favor of this view was first stated by His (’88), 
and consists in the fact that the eye-muscle nerves, at least of the adult, 
resemble ventral spinal nerves both in histological structure and in the 
situation of their motor nucleus in the ventral horn of the neural tube ; 
and also in the less well established fact that they innervate muscula- 
ture derived from segments of the dorsal mesoderm. On the other 
hand, the majority of morphologists, among whom may be named Bal- 
four (78), Marshall (’81), Dohrn (’85, ’87, 790), Gaskell (’89), Hoff- 
mann (89, ’94), Oppel (’90), Houssay (90), Platt (’91), Froriep (’91), 
Zimmermann (’91), Hatschek (’92), Mitrophanow (92, ’93), and Kupffer 
(794, ’95, 96), while in general of the opinion that the abducens is the 
homologue of one or more segmental ventral nerves, have held that 
either the trochlearis or the oculomotorius, or both, represent dorsal (or 
lateral) segmental nerves. The chief arguments in favor of this view 
consist in evidence (1) of the development of these nerves from neural- 
crest cells; (2) of a cellular or so called ganglionic structure of the 
nerves in the embryo ; (3) of transitory or permanent ganglia in con- 
VOL. XXXI.— NO. 7. 9 


274 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


nection with them; and (4) of the development of at least a part of the 
musculature innervated by them from splanchnic mesoderm. Thus 
there is to-day a distinct conflict as to the morphology of the eye-muscle 
nerves, one party to the conflict being supported by histological evi- 
dence, the other by embryological. The assumption by His (’88), that 
the eye-muscle nerves develop as processes of medullary cells (neuro- 
blasts), — which is involved in his contention that they are the serial 
homologues of ventral spinal nerves, — has never hitherto received the 
requisite embryological confirmation. In fact, the latest embryological evi- 
dence concerning the development of the oculomotorius and trochlearis 
seems quite irreconcilable with the view of Schneider (’79), van Wijhe, 
and His. In regard to the latter nerve, Hoffmann (’89, p. 338) says, 
if one disregards the fact that no ectodermal fusion takes place, “so 
gleicht,die Anlage des Trochlearis in sehr jungen Entwicklungsstadien 
[of Lacerta] vollkommen der eines segmentalen Kopfnerven, besonders 
der des Trigeminus.” Froriep also finds that the trochlearis possesses 
in early stages a ganglion, and is differentiated from neural-crest cells im 
situ. Miss Platt (’91*, p. 259) likewise states that “in Acanthias the 
development of the trochlearis in all essential respects so completely 
corresponds to that of the trigeminus and facialis, that like them it must 
be considered to combine primarily those dorsal and ventral elements 
which have separate roots in the nerves of the trunk. It can, therefore, 
not be regarded as the ventral root of another segmental nerve.” More- 
over, Kupffer (95, ’96) finds the trochlearis to possess in Ammoceetes 
both dorsal and ventral roots. 

With regard to the oculomotorius, the conclusions of embryologists 
are even more conflicting. While Dohrn (’91) finds that this nerve is 
formed by the migration of cells from the ventral wall of the midbrain, 
and considers it a motor nerve, Miss Platt (’91*) states that she has 
shown the oculomotorius to be “ undoubtedly originally sensory.” Her 
observation that the nerve develops from the ganglion toward the brain 
has been confirmed by both Mitrophanow (’93) and Sedgwick (’95). 
Nevertheless the evidence which has been stated by me in division VI. 
shows conclusively, as I believe, that all the eye-muscle nerves, oculo- 
motorius, trochlearis, and abducens, develop, like ventral spinal nerves, 
as processes from neuroblasts lying in the ventral horn of the medul- 
lary tube. Therefore, from their development, as well as their adult 
histological structure and relationships, the eye-muscle nerves must be 
regarded as the serial homologues of ventral spinal nerves. Finally, 
with the accumulating evidence given by many investigators, — among 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 205 


them Beard, Dohrn, Ayers, and Kupffer, — that the complicated sensory 
organs of ear, eye, and nose are differentiations of lateral-line sense 
organs, we may conclude that there exist no fundamental differences in 
nature between pre-otic and post-otic segments. 

The number of cephalic segments in the post-otic region (Sewertzoff, 
Fiirbringer) appears to be variable in different Vertebrates. If the 
estimate given by Hoffmann (’94) for Squalus be correct, there are six 
post-otic cephalic segments in that form. In the otic and pre-otic 
regions, I hold the number to be not greater than six,‘ and the exact 
numerical correspondence of neuromeres and somites very strongly sup- 
ports the estimate of six, which accords very closely with that made, 
upon similar but not identical grounds, by van Wijhe, Beard, Marshall, 
and Miss Platt. I cannot agree with Hoffmann (’96) and M. Fiirbringer 
(97), who— from the evidence that there is one more mesodermal seg- 
ment (viz. the “anterior ”) in Squalus and Galeus than in other known 
Selachian embryos — conclude that still other anterior mesodermal 
segments have phylogenetically disappeared, and that it is therefore 
impossible for us to estimate the number of pre-otic segments. We have 
quite as little reason to believe that somites anterior to Platt’s have disap- 
peared, as we have to believe that encephalomeres anterior to encephalomere 
I (the primary forebrain) have once existed. In the exact numerical 
correspondence of neuromeres and somites we have, not only evidence of 
the serial homology of head and trunk segments, but the means to 
determine their number in the pre-otic region. 


IX. Summary. 


I am unable to regard Locy’s ‘neural segments” as segments in the 
true sense of the word, because I| find them irregular in size, inconstant 
in number, bilaterally asymmetrical, and without definite relation to 
structures known to be segmental. They are phenomena connected 
with the proliferation and disassociation of the cells of the neural 
crest. 

The posterior boundary of the cephalic plate coincides with the 
posterior boundary of encephalomere VI, opposite which the auditory 
invagination takes place. 

Orr’s criteria for hindbrain neuromeres hold good only for the later 


1 Six neuromeres alternating with five somites. With Miss Platt (’94) I hold 
that the otic sense organ was primitively situated above the constriction between 
van Wijhe’s 4th and 5th somites. 


bo 


76 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


stages of development of S. acanthias. In the early stages of this 
animal, the neuromeres are local thickenings of the lateral zones, as well 
as dilatations of all of the zones of the medulla. As paired glangionic 
enlargements of the central nervous system, they obviously, resemble, 
except in position, the ventral chain of ganglia of Annelids. Therefore 
they cannot be explained as the passive result of mechanical shoving or 
bending. The constrictions between the neuromeres, as well as the 
crowding of nuclei in the regions of constriction, may however be, 
and most probably are, intensified by shoving or bending of the neural 
tube. 

No structural conditions are presented by the myelomeres which are 
not reconcilable with the hypothesis that their existence is dependent 
upon the presence of the mesodermal somites. If they ever possessed 
a dorsal segmentation like that of the “hindbrain neuromeres,” — and 
there is no evidence to show that they ever did, —it has been lost. 
But, though they appear of doubtful morphological value, their numerical 
correspondence with nerves and somites attests their metameric value. 

The so called neuromeres of the forebrain and midbrain (encepha- 
lomeres of Zimmermann) are not morphologically comparable with ‘ hind- 
brain neuromeres,” since they are simply dorsal or ventral expansions 
which are secondary in the time of their appearance. I hold that there 
are much better reasons — viz. on the grounds of time of appearance, 
of structure, and of relation to nerves and somites — for regarding each 
of the primary forebrain and midbrain vesicles (neuromeres I and II) as 
serially homologous with hindbrain neuromeres (neuromeres III to VII), 
than for so regarding their later subdivisions. The latter are ccenoge- 
netic vesiculations of the neural tube, and not of metameric value. 

Both dorsal ganglia and ventral nerves in the trunk develop in the 
regions of constriction between myelomeres. A comparison with the 
conditions in Amphioxus and Petromyzon shows ‘that this condition is 
not to be regarded as primitive, but that previously dorsal and ventral 
nerves alternated, the former being intersomitic in position. Such 
topographical relation is retained by some cranial nerves, viz. V, VII, 
IX, and X (Urvagus). 

The ganglionic Anlagen of four cranial nerves, viz. V, VII, IX, and X, 
are proliferated from four encephalomeres, viz. III, V, VI, and VII. 
Chiefly for this reason, but also because of the clear connection of two 
splanchnic motor roots, viz. V and VII, with two of the encephalomeres, 
I conclude that the primitive metameric relations of the latter were with 
the visceral arches. The local thickenings of the hindbrain neuromeres 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 277 


(encephalomeres) may be considered as the primitive nervous centres of 
nerves which corresponded numerically with visceral arches. If they 
were such, then one of the encephalomeres (IV) affords evidence of a 
lost visceral arch. 

Although the structure of myelomeres and encephalomeres is seen to 
be different, yet in the stages of embryonic development, where both 
are present, the latter are seen to have segmental value from the fact 
that corresponding with them there is an equal number of somites. 
These somites, as exemplified in the 3d (van Wijhe’s), are morphologically 
comparable and serially homologous with trunk somites. I conclude, 
then, that there was a primitive correspondence between neuromerism, 
mesomerism, and branchiomerism. 

The development, histological structure, and relationships of the 
eye-muscle nerves (III, IV, and VI) show them to be the serial homo- 
logues of ventral spinal nerves. Like the latter (His), they develop 
as axis-cylinder processes of neuroblasts in the ventral horn of the 
neural tube. 

Pre-otic and post-otic metameres, like their integral parts, are serially 
homologous with one another. Therefore, if the latter are serially 
homologous with trunk metameres the former must be also, Table III. 
(p. 253) summarizes my opinion as to the primitive composition of 
metameres I to VII. I regard the r. opthalmicus profundus as a seg- 
mental dorsal nerve belonging to metamere II, while the oculomotorius 
is its ventral root. The trochlearis is the ventral nerve of metamere 
III, and the abducens represents the ventral nerves of metameres IV 
to VII. 

There are five mesomeres alternating with six neuromeres in the otic 
and pre-otic regions of the Vertebrate head. Probably eleven neuro- 
meres are finally included in the head of Squalus. The evidence of the 
numerical correspondence of neuromeres and mesomeres shows that 
there is no more reason for believing that somites have been lost 
anterior to Platt’s (anterior) somite, than that neuromeres have been 
lost anterior to the primary forebrain. 

In agreement with van Wijhe, I homologize the mouth of Amphi- 
oxus with the left half of the mouth of Craniota. The first pair of per- 
manent visceral clefts in Amphioxus are exactly homologous with the 
hyomandibular clefts of higher Vertebrates. The eight visceral clefts 
possessed by Amphioxus at its “critical stage” (Willey) are exactly 
homologous with the eight morphological clefts found in some Selachii 
and Cyclostomes. 


278 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


This investigation has been made in the Zodlogical Laboratory of the 
Museum of Comparative Zodlogy at Harvard College. I gratefully ac- 
knowledge the valuable assistance and advice of its Director, Prof. E. L. 
Mark, at whose suggestion the work was undertaken. I am indebted to 
Alexander Agassiz for the privilege of studying at his private labo- 
ratory in Newport; also to Professor Mark for embryonic material of 
Petromyzon, and to Miss Julia B. Platt for embryonic material of 
Amphioxus. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS, 279 


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°92. The Histogenesis of Nerve. Anat. Anz., Jahrg. 7, pp. 290-302. 
Béraneck, E. 
*84. Recherches sur le développement des nerfs craniens chez les Lézards. 
Recueil Zool. Suisse, Tom. 1, No. 4, pp. 519-603, Pls. 27-30. 


Béraneck, E. 
°87. Etude sur les replis médullaires du poulet. Recueil Zool. Suisse, 
Tom. 4, No. 2, pp. 305-364, Pl. 14. 


Bischoff, T. L. W. 
42. Entwickelungsgeschichte der Saugethiere und des Menschen. Leipzig, 
xiv + 575 pp. 
Broman, I. 
795. Beschreibung eines menschlichen Embryos von beinahe 3 mm. Lange 
mit spezieller Bemerkung iiber die bei demselben befindlichen Hirnfalten. 
Morph. Arbeiten, Bd. 5, Heft 2, pp. 169-206, Taf. 9, 10, u. 7 Textfig. 


Burckhardt, R. 
°93. Die Homologieen des Zwischenhirndaches und ihre Bedeutung fiir die 
Morphologie des Hirns bei niederen Vertebraten. Anat. Anz., Jahrg. 9, 
pp. 152-155. 1 Fig. 


Burckhardt, R. 
°94. Der Bauplan des Wirbelthiergehirns. Morph. Arbeiten, Bd. 4, Heft 
2, pp. 131-150, Taf. 8. 


Cajal, Ramon y. 
°94. Les nouvelles idées sur la structure du systeme nerveux chez l’homme 
et chez les Vertébrés. Revue gen. sci. pures et appliquées, Anneé 5, 
pp. 141-153. 
Chiarugi, G. 
91. Sur les myotomes et sur les nerfs de la téte postérieure et de la région 


proximale du tronc dans les embryons des Amphibies anoures. (Résumé.) 
Arch. Ital. de Biol., Tom. 15, fase. II. pp. 229-239. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 281 


Chiarugi, G. 
794. Lo sviluppo dei nervi oculomotore e trigemello. Monitoire Zool. Ital., 
Anno V, No. 12, pp. 275-280. 


Clarke, S. F. 
791. The Habits and Embryology of the American Alligator. Jour. Morph., 
Vol. 5, No. 2, pp. 181-214, Pl. 9-13. 


Coggi, A. ; 
795. Il cordone gangliare anteriore in Torpedo e Pristiurus. Ricerche fati 
nel labor. anat. Univ. di Roma ed in altri labor. biolog., Vol. 5, pp. 27-41. 
Dohrn, A. 
775. Der Ursprung der Wirbelthiere und das Princip des Functionswechsels. 
Genealogische Skizzen. Leipzig, xv + 87 pp. 
Dohrn, A. 
85. Studien zur Urgeschichte des Wirbelthierkorpers. X. Zur Phylo- 
genese des Wirbelthierauges. Mitth. Zool. Stat. Neapel, Bd. 6, Heft 3, 
pp. 432-480, Taf. 23, 24. 


Dohrn, A. 
°88. Studienu.s.w. XIII. Ueber Nerven und Gefasse bei Ammocoetes 
und Petromyzon Planeri. Mitth. Zool. Stat. Neapel, Bd. 8, Heft 2, 
pp. 231-306, Taf. 10-15. 
Dohrn, A. 
°90. Bemerkungen itiber den neuesten Versuch, einer Lésung des Wirbel- 
thierkopf-Problems. Anat. Anz., Jahrg. 5, Hefte 2 u. 3, pp. 53-64, 
78-85. 


Dohrn, A. 
790%. Studienu.s.w. XV. Neue Grundlagen zur Beurtheilung der Meta- 
merie des Kopfes. Mitth. Zool. Stat. Neapel, Bd. 9, Heft 3, pp. 330-434, 
Taf. 14, 15. 


Dohrn, A. 
791. Studien u.s.w. XVI. Ueber die erste Anlage und Entwicklung der 
Augenmuskelnerven bei Selachiern und das Einwandern von Medullarzellen 
in die motorischen Nerven. Mitth. Zool. Stat. Neapel, Bd. 10, Heft 1, 
pp. 1-40, Taf. 1-5. 
Dohrn, A. 
91", Studienu.s.w. XVII. Nervenfaser und Ganglienzelle. Mitth. Zool. 
Stat. Neapel, Bd. 10, Heft 2, pp. 255-335, Taf. 15-23. 
Dohrn, A. 
*92. Die Schwann’schen Kerne der Selachierembryonen. Anat. Auz., Jahrg. 
7, pp- 348-351. 
Dursy, E. 
*°69. Zur Entwicklungsgeschichte des Kopfes des Menschen und der hoheren 
Wirbelthiere. Tiibingen, xii + 232 pp., und Atlas, 9 Taf. 


282 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Edinger, L. 
*96. Vorlesungen iiber den Bau der nervésen Centralorgane des Menschen 
und der Thiere. 5 Aufl., Leipzig, xii + 386 pp., 258 Abbildg. 


Ewart, J. C. 
*90. On the Development of the Ciliary or Motor Oculi Ganglion. Proce. 
Roy. Soc., London, Vol. 47, No. 289, pp. 287-290. 


Eycleshymer, A. C. 
°95. The early Development of Amblystoma, with Observations on some 
other Vertebrates. Jour. Morph., Vol. X. No. 2, pp. 343-418, Pls. 
18-22. 
Foster, M., and Balfour, F. M. 
°74. The Elements of Embryology. London, xix + 272 pp., 71 Figs. 


Froriep, A. 
°85. Ueber Anlagen von Sinnesorganen am Facialis, Glossopharyngeus und 
Vagus, iber die genetische Stellung des Vagus zum Hypoglossus, und 
iiber die Herkunft der Zungenmusculatur. Arch. f. Anat. u. Physiol., 
Anat. Abth., Jahbrg. 1888, pp. 1-55, Taf. 1 u. 2. 
Froriep, A. 
91. Zur Entwickelungsgeschichte der Kopfuerven. Verh. Anat. Gesellsch. 
V. (Miinchen), pp. 55-65. 
Froriep, A. 
°92. Zur Frage der sogenannten Neuromerie. Verh. Anat. Gesellsch. VI. 
(Wien), pp. 162-167. 


Froriep, A. 
°92*%. Entwickelungsgeschichte des Kopfes. Anat. Hefte, Abth. 2, Ergebnisse 
Anat. u. Entwg., Bd. 1, pp. 551-605. 


Froriep, A. 
°94. Entwickelungsgeschichte des Kopfes. Anat. Hefte, Abth. 2, Ergebnisse 
Anat. u. Entwg., Bd. 3, pp. 391-459. 


Fiirbringer, M. 
°97. Ueber die Spino-occipitalen Nerven der Selachier und Holocephalen und 
ihre vergleichende Morphologie. Festschr. zum siebenzigsten Geb. von 
Carl Gegenbaur, Bd. 3, pp. 349-788, 8 Taf. Leipzig. 


Firbringer, P. 
°75. Untersuchungen zur vergleichenden Anatomie der Muskulatur des 
Kopfskeletes der Cyclostomen. Jena. Zeitschr., Bd. 9, Heft 1, pp. 1-93, 
Taf. 1-3. 
Gaskell, W. H. 
°89. On the Relation between the Structure, Function, Distribution, and 
Origin of the Cranial Nerves; together with a Theory of the Origin of 
the Nervous System of Vertebrata. Jour. Phys., Vol. 10, No. 3, pp. 153- 
211, Pls. 16-20. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 283 


Gegenbaur, C. 
°72. Untersuchungen zur vergleichenden Anatomie der Wirbelthiere. Heft 3. 
Das Kopfskelet der Selachier. Ein Beitrag zur Erkenntniss der Genese des 
Kopfskelets der Wirbelthiere. Leipzig, 10 + 316 pp., 22 Taf. 


Gegenbaur, C. 
°87. Die Metamerie des Kopfes und die Wirbeltheorie des Kopfskeletes. 
Morph. Jahrb., Bd. 13, Heft 1, pp. 1-114. 


Goette, A. 
°75. Die Entwickelungsgeschichte der Unke (Bombinator igneus) als Grund- 
lage einer vergleichenden Morphologie der Wirbelthiere. Leipzig, vii -- 
964 pp., und Atlas (1874), 22 Taf. fol. 
Goette, A. 
90. Entwickelungsgeschichte des Flussneunauges (Petromyzon fluviatilis). 
Theil I. Hamburg u. Leipzig, 95 pp., 9 Taf. Abhandl. z. Entwg. d. Thiere, 
Heft 5. 
Golgi, C. 
°93. Intorno all’ origine del quarto uervo cerebralo (patetico o trocleare) e 
di una questione di Isto-fisiologia generale che a questo argomento si collega. 
Atti Accad. Lincei, Rendiconti, Vol. 2, Semestre 1, fase. 9 e 10, pp. 379- 
389, 443-450, 2 Fig. Also in French, Arch. Ital. Biol., Tom. 19, fase. 3, 
pp. 454-474. 
Golowine, E. 
*90. Sur le développement du systéme ganglionnaire chez le poulet. Anat. 
Anz., Jahrg. 5, No. 4, pp. 119-124, 4 Figg. 
Goronowitsch, N. 
°88. Das Gehirn und die Cranialnerven von Acipenser ruthenus. Ein 
Beitrag zur Morphologie des Wirbelthierkopfes. Morph. Jahrb., Bd. 
13, Hefte 3 u. 4, pp. 427-574, Taf. 17-28. 
Goronowitsch, N. 
°92. Die axiale und die laterale (A. Goette) Kopfmetamerie der Vogel- 
embryonen. Die Rolle der sog. “Ganglienleisten” im Aufbaue der 
Nervenstamme. Anat. Anz., Jahrg. 7, pp. 454-464. 
Goronowitsch, N. 
°93. Untersuchnngen tiber die Entwicklung der sog. “ Ganglienleisten” im 
Kopfe der Végelembryonen. Morph. Jahrb., Bd. 20, Heft 2, pp. 187- 
259, Taf. 8-11. 
Hatschek, B. 
81. Studien iiber Entwicklung des Amphioxus. Arb. Zool. Inst. Wien, 
Tom. 4, Heft 1, pp. 1-88, Taf. 1-9. 
Hatschek, [B]. 
*92. Die Metamerie des Amphioxus und des Ammocoetes. Verh. Anat. 
Gesellsch. VI. (Wien), pp. 136-161, 11 Figg. 


284 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Hatschek, B. 
°93. Zur Metamerie der Wirbelthiere. Anat. Anz., Jahrg 8, pp. 89-91. 
Hensen, V. 

64. Zur Entwickelung des Nervensystems. Arch. f. Path. Anat. u. Phys., 

Bd. 30, pp. 176-186, Taf. 8. 
Hensen, V. 

°76. Beobachtungen iiber die Befruchtung und Entwicklung des Kaninchens 
und des Meerschweinschens Zeit. Anat. u. Entwg., Bd. 1, Hefte 3-6, 
pp. 2138-273, 353-423, Taf. 8-12. 

Herrick 'C: L:- 

92. Embryological Notes on the Brain of the Snake. Jour. Comp. Neu- 

rology, Vol. 2, pp. 160-176, Pl. 15-19. 
His, W. 

°79. Ueber die Anfange des peripherischen Nervensystemes. Arch. f. Anat. 

u. Physiol., Anat. Abth., pp. 455-482, Pls. 17 u. 18. 
His, W. 

88. Die Entwickelung der ersten Nervenbahnen beim menschlichen Embryo. 
Uebersichtliche Darstellung. Arch. f. Anat. u. Physiol., Anat. Abth., Heft 
6, pp. 368-378, 8 Figg. 

His, W. 

°88*. Die morphologische Betrachtung der Kopfnerven. Eine kritische 

Studie. Arch. f. Anat. u. Physiol., Anat. Abth., Heft 6, pp. 379-453. 
His, W. 

°88>. Zur Geschichte des Gehirns, sowie der centralen und peripherischen 
Nervenbahnen beim menschlichen Embryo. Abhandl. Sachs. Gesellsch. 
Wissensch., math.-phys. Cl., Bd. 24, No. vi., pp. 341-392, Tab. 1 u. 2, 

His, W. 

°89. Die Neuroblasten und deren Entstehung im embryonalen Mark. Arch. 

f. Anat. u. Physiol., Anat. Abth., Hefte 3 u. 4, pp. 249-300, Taf. 16-19. 
His, W. 

*89%. Ueber die Entwickelung des Riechlappens und des Riechganglions und 
iiber diejenige des verlangerten Markes. Verh. Anat. Gesellsch. III. 
(Berlin), pp. 63-66. 

Hoffmann, C. K. 

°85. Weitere Untersuchungen zur Entwicklungsgeschichte der Reptilien. 

Morph. Jahrb., Bd. 11, Heft 2, pp. 176-219, Taf. 10-12, 1 Fig. 
Hoffmann, C. K. 

°89. Ueber die Metamerie des Nachirns und Hinterhirns, und ihre Beziehung 
zu den segmentalen Kopfnerven bei Reptilienembryonen. Zool. Anz., 
Jahrg. 12, pp. 337-339. 

Hoffmann, C. K. 

°90. Reptilien. V. Entwicklungsgeschichtlicher Theil. Bronn’s Klassen und 
Ordnungen des Thier-Reichs, Bd. 6, Abth. 3, III. pp. 1872-1889, Taf. 
138-170. 


bo 
Ol 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 8 


Hoffmann, C. K. 
°94. Zur Entwickelungsgeschichte des Selachierkopfes. Anat. Anz., Bd. 9, 
pp. 638-653, 5 Figg. 
Hoffmann, C. K. 
°96. Beitrage zur Entwickelungsgeschichte der Selachii. Morph. Jahrb., 
Bd. 24, Heft 2, pp. 209-286, Taf. 2-5. 


Houssay, F. 

°90. Etudes d’embryologie sur les Vertébrés. Arch. de Zool. exp. et gen., 
sér. 2, Tom. 8, No. 2, pp. 143-244, Pls. 10-13. [II. Origine et développe- 
ment du systeme nerveux périferique. pp. 178-208. ] 

Houssay, F. 

91. Etudes d’embryologie sur les Vertébrés. IV. Les fentes branchiales 
auditive, hyo-mandibulaire, spiraculaire et les somites mésoblastiques qui 
leur correspondent chez |’Axolotl. Bull. Sci. de la France et de la Belgique, 
Tom. 23, Pt. 1, pp. 55-71, Pls. 1-3. 

Huxley, T. H. 

758. The Croonian Lecture. — On the Theory of the Vertebrate Skull. Proce. 

Roy. Soe. Lond., Vol. 9, No. 33, pp. 381-457, 10 Figs. 
Kastschenko, N. 

88. Zur Entwicklungsgeschichte des Selachierembryos. Anat. Anz., Jabrg. 

3, No. 16, pp. 445-467. 
Killian, [G.] 

°91. Zur Metamerie des Selachierkopfes. Verh. Anat. Gesellsch. V. (Miin- 

chen), pp- 85-107, 25 Figg. 
Kolliker, A. 

61. Entwicklungsgeschichte des Menschen und der héheren Thiere. Leipzig, 

x + 468 pp., 225 Figg. 
Kolliker, A. ; 

°79. Entwicklungsgeschichte des Menschen und der hoheren Thiere. 2te 

Aufl. Leipzig, xxxiv + 1033 pp., 606 Figg. 
KOlliker, [A.] 

92. Ueber die Entwickelung der Elemente des Nervensystems, contra Beard 

und Dohrn. Verh. Anat. Gesellsch. VI. (Wien), pp. 76-78. 
Krause, W. 

81. Ueber die doppelnatur des Ganglion ciliare. Morph. Jabrb., Bd. 7, 

Heft 1, pp. 43-56, Taf. 5. 
Kupffer, C. 

°86. Primare Metamerie des Neuralrohrs der Vertebraten. Sitzb. Akad. 

Wissensch. Miinchen, math.-phys. Cl., 1885, Heft 4, pp. 469-476. 
Kupffer, C. 

*88. Ueber die Entwicklung von Petromyzon Planeri. Sitzb. Akad. Wis- 

sensch. Miinchen, Bd. 18, Heft 1, pp. 71-79. 


286 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Kupffer, C. 
°90. Die Entwicklung von Petromyzon Planeri. Arch. f. mikr. Anat., Bd. 
35, Heft 4, pp. 469-558, Taf. 27-32. 


Kupffer, C. 
°91. Die Entwickelung der Kopfnerven der Vertebraten. Verh. Anat. 
Gesellsch. V. (Miinchen), pp. 22-55. 


Kupffer, C. 
°93. Die Entwicklung des Kopfes von Acipenser sturio, an Medianschnitten 
untersucht. Studien zur vergleichenden Entwicklungsgeschichte des 
Kopfes der Kranioten. Heft 1. Miinchen und Leipzig, iv + 95 pp., 

9 Taf. 


Kupffer, C. 
°93". Entwickelungsgeschichte des Kopfes. Anat. Hefte, Abth. 2. Ergeb- 
nisse Anat. u. Entwg., Bd. 2, pp. 501-564. 


Kupffer, C. 
94. Die Entwicklung des Kopfes von Ammocoetes Planeri. Studien z. 
vergl. Entwg. d. Kopfes d. Kranioten, Heft 2. Minchen und Leipzig, 
79 pp-, 12 Taf. 


Kupffer, C. 
°95. Die Entwicklung der Kopfnerven von Ammocoetes Planeri. Studien z. 
vergl. Entwg. d. Kopfes d. Kranioten, Heft 3. Mimchen, J. F. Leh- 
mann, 1895. 


Kupffer, C. 
°96. Entwickelungsgeschichte des Kopfes. Anat. Hefte, Abth. 2, Ergebnisse 
Anat. u. Entwg., Bd. 5, pp. 562-618, 4 Abbildg. 


Lenhossék, M. von. 
°90. Ueber Nervenfasern in den hinteren Wurzeln, welche aus dem Vorder- 
horn entspringen. Anat. Anz., Jahrg. 5, No. 13 u. 14, pp. 360-362. 


Lenhossék, M. von. 
°91. Die Entwickelung der Ganglionanlagen bei dem menschlichen Embryo. 
Arch. f. Anat. u. Physiol., Anat. Abth., Heft 1, pp. 1-25, Taf. 1. 


Lenhossék, M. von. 
°92. Beobachtungen an den Spinalganglien und dem Riickenmark von Pris- 
tiurusembryonen. Anat. Anz., Jalhrg. 7, pp. 519-539, 19 Abbildg. 


Locy, W. A. 
°93. The Derivation of the Pineal Eye. Anat. Anz., Bd. 9, No. 5 u. 6, pp. 
169-180, 5 Figs. 


Locy, W. A. 
°94. The Optic Vesicles of Elasmobranchs and their Serial Relation to other 
Structures on the Cephalic Plate. Jour. Morph., Vol. 9, pp. 115-122. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 287 


[Locy, W. A.] 
94", Nachtrag zu dem Aufsatze von Locy in No. 5 und 6, p. 169. Anat. 
Anz., Bd. 9, No. 7, pp. 231, 232. 


Locy, W. A. 
94>, Metameric Segmentation in the Medullary Folds and Embryonic Rim. 
Anat. Anz., Bd. 9, No. 13, pp. 393-415, 11 Figs. 


Locy, W. A. 
794°. The Mid-Brain and Accessory Optic Vesicles. A Correction. Anat. 
Anz., Bd. 9, No. 15, pp. 486-488. 


Locy, W. A. 
795. Contribution to the Structure and Development of the Vertebrate Head. 
Jour. Morph., Vol. 11, No. 3, pp. 497-594, Pls. 26-30. 


Locy, W. A. 
97. Accessory Optic Vesicles in the Chick Embryo. Anat. Anz., Bd. 14, 
No. 5, pp. 113-124, 9 Figs. 
Marshall, A. M. 
°78. The Development of the Cranial Nerves of the Chick. Quart. Jour. 
Micr. Sci., Vol. 18, No. 69, pp. 10-40, Pls. 2, 3. 


Marshall, A. M. 
781. On the Head Cavities and Associated Nerves in Elasmobranchs. 
Quart. Jour. Micr. Sci., Vol. 21, pp. 72-97, Pls. 5, 6. 


Marshall, A. M. 
82. The Segmental Value of the Cranial Nerves. Jour. Anat. Physiol., 
Vol. 16, Pt. 3, pp. 305-354, Pl. 10. Also in Stud. Biol. Lab., Owens 
College, Vol. 1, pp. 125-169. 1886. 


Marshall, A. M. 
93. Vertebrate Embryology. New York and London. xxiii + 640 pp., 
255 Figs. 
Marshall, A. M., and Spencer, W. B. 
81. Observations on the Cranial Nerves of Scyllium. Quart. Jour. Micr. 
Sci., Vol. 21, No. 82, pp. 469-499, Pl]. 27. Also in Stud. Biol. Lab., 
Owens College, Vol. 1, pp. 87-123, Pls. 5,6. 1886. 


Martin, P. 
*90. Die erste Entwickelung der Kopfnervender Katze. Oesterr. Monatsschr, 
f. Thierheilkunde, Jahrg. 15. Sept., 1890. 


Martin, P. 
91. Die Entwickelung der neunten bis zwélften Kopfnerven bei der Katze. 
Anat. Anz., Jahrg. 6, Heft 8, pp. 228-232. 
McClure, C. F. W. 
89. The Primitive Segmentation of the Vertebrate Brain. Zool. Anz., 
Jahrg. 12, No. 314, pp. 435-438. 


288 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


McClure, C. F. W. 
°90. The Segmentation of the Primitive Vertebrate Brain. Jour. Morph., 
Vol. 4, pp. 35-56, Pl. 3. 


Mihalkovics, V. von. 


77. Entwicklungsgeschichte des Gehrins. Leipzig, 203 pp., 7 Taf. 


Minot, C. S. 
°92. Human Embryology. New York. xxiii-+ 815 pp., 463 Figs. [pp. 
604-606. ] 


Minot, C. S. 
°97. Cephalic Homologies. A Contribution to the Determination of the 
Ancestry of Vertebrates. Amer. Nat., Vol. 31, No. 371, pp. 927-943. 


Mitrophanow, P. 
*91. Sur la formation du systeme nerveux périférique des Vertébrés. 
Comptes Rendus, Paris, Tom. 113, No. 19, pp. 659-662. 
Mitrophanow, P. 


92. Note sur la signification metamerique des nerfs craniens. Congres 
Internat. de Zool., Session 2, Moscou, Partie 1, pp. 104-111. 


Mitrophanow, P. 
93. Etude embryogénique sur les Sélaciens. Arch. Zool. exp., sér. 3, 
Tom. 1, pp. 161-220, Pls. 9-14. 
Neal, H. V. 
°96. A Summary of Studies on the Segmentation of the Nervous System in 
Squalus acanthias. Anat. Anz., Bd. 12, No. 17, pp. 377-391, 6 Figs. 


Neal, H. V. 
°97. The Development of the Hypoglossus Musculature in Petromyzon and 
Squalus. Anat. Anz., Bd. 13, No. 17, pp. 441-463, 2 Figs. 


Onodi, A- D. 
°84. Ueber die Entwickelung der Spinalganglien und der Nervenwurzeln. 
Internat. Monatschr. f. Anat. u. Histol., Bd. 1, pp. 204-209, 255-284. 


Oppel, A. 
°90. Ueber Vorderkopfsomiten und die Kopfhohle bei Anguis fragilis. 
Arch. f. mikr’ Anat., Bd. 36, pp. 603-627, Taf. 30. 


Orr, EB. 
’°87. Contribution to the Embryology of the Lizard. jo Morph., Vol. 1, 
pp: 311-372, Pls. 12-14. 


Pinkus, F. 
°94. Ueber einen noch nicht beschriebenen Hirnnerven des Protopterus 
annectens. Anat. Anz., Bd. 9, No. 18, pp. 562-566, 4 Fig. 


Platt; +}-.B: 
°89. Studies on the Primitive Axial Segmentation of the Chick. Bull. Mus. 
Comp. Zoél. Harvard Coll., Vol. 17, No. 4, pp. 171-190, 2 Pls. 


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Platt, J. B. 
91. A Contribution to the Morphology of the Vertebrate Head, based on a 
Study of Acanthias vulgaris. Jour. Morph., Vol. 5, pp. 79-112, Pls. 4-6. 


Platt, J. B. 
91", Further Contribution to the Morphology of the Vertebrate Head. 
Anat. Anz., Jabrg. 6, No. 9 u. 10, pp. 251-265, 15 Figs. 
Platt, J; B: 
93. Ectodermic Origin of the Cartilages of the Head. Anat. Anz. Jahrg. 8, 
No. 14 u. 15, pp. 506-509. 
Platt, J. B. 
°94. Ontogenetische Differenzirung des Ectoderms in Necturus. Studie I. 
Archiv. f. mikr. Anat., Bd. 43, Heft 4, pp. 911-966, Taf. 37-42. 
Platt, J: B- 
°96. Ontogenetic Differentiation of the Ectoderm in Necturus. Study II. 
On the Development of the Peripheral Nervous System. Quart. Jour. 
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Platt, J; 5: 
°97. The Development of the Cartilaginous Skull, and of the Branchial and 
Hypoglossal Musculature in Necturus. Morph. Jahrb., Bd. 25, Heft 3, 
pp: 377-464, Taf. 16-18. 


Pollard, H. B. 
95. Ueber Labialknorpel. Verh. Anat. Gesellsch. IX. (Basel), pp. 232- 
235, 3 Abbildg. 
Prenant, A. 
*°89. Note sur lexistence des replis médullaires chez l’embryon du pore. 
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*96. Zur Ontogenie eines Myxinoiden (Bdellostoma Stouti, Lockington). 
Sitzb. Akad. Wissensch. Miinchen, math.-phys. Cl., Bd. 26, Heft 1, 
pp. 69-74. 


Price |G. iC: 
*96* Some Points in the Development of a Myxinoid (Bdellostoma Stouti 
Lockington). Verh. Anat. Gesellsch. X. (Berlin), pp. 81-86. 
Rabi; C. 
85. Bemerkung iiber die Segmentirung des Hirnes. Zool. Anz., Jahrg. 8, 
No. 191, pp. 192, 193. 
Rabl, C. 
°87. Ueber das Gebiet des Nervus facialis. Anat. Anz., Jahrg. 2, No. 8, 
pp- 219-227. 
Rabl, C. 
°89. Theorie des Mesoderms. Morph. Jahrb., Bd. 15, Heft 2, pp. 113-252, 
Taf. 7-9, u. 9 Figs. 
VOL. XXXI. — NO. 7. 10 


290 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


Rabl, [C.] 
°92. Ueber die Metamerie des Wirbeltierkopfes. Verh. Anat. Gesellsch. VI. 
(Wien), pp. 104-135, Taf. 2, u. 4 Abbildg. 
Ramon y Cajal. See Cajal, Ramon y. 
Ransom, W. B., and Thompson, D’A. W. 
°86. On the Spinal and Visceral Nerves of Cyclostomata. Zool. Anz., Jahrg. 
9, pp. 421-426. 
Remak, R. 
°55. Untersuchungen iiber die Entwickelung der Wirbelthiere. Berlin, fol. 
194 + xxxvili. pp., 12 Taf. 
Retzius, G. ; 
°94. Ueber das Ganglion ciliare. Anat. Anz., Bd. 9, No. 21, pp. 633-637, 
2 Abbildg. 
Sagemehl, M. 
82. Untersuchungen iiber die Entwickelung der Spinalnerven. (Diss.) 
Dorpat, 47 pp., 2 Taf. 
Sagemehl, M. 
°83. Beitrage zur vergleichenden Anatomie der Fische. I. Das Cranium 
von Amia calva L. Morph. Jahrb., Bd. 9, Heft 2, pp. 177-228, Taf. 10. 
Schneider, A. 
°79. Beitrage zur vergleichenden Anatomie und Entwicklungsgeschichte der 
Wirbelthiere. Berlin, viii + 164 pp., 16 Taf.. 3 Holzschn. 
Schneider, A. 
°80. Ueber die Nerven von Amphioxus, Ammocoetes und Petromyzon. Zool. 
Anz., Jahrg. 3, No. 59, pp. 330-334. 
Schwalbe, G. 
"79. Das Ganglion oculomotorii. Ein Beitrag zur vergleichenden Anatomie 
der Kopfnerven. Jena. Zeitschr., Bd. 13, pp. 173-268, Taf. 12-14. 
. Scott, W. B. 
°87. Notes on the Development of Petromyzon. Jour. Morph., Vol. 1, No. 2, 
pp: 253-310, Pls. 8-11. 
Sedgwick, A. 
92. Notes on Elasmobranch Development. Quart. Jour. Micr. Sci., Vol. 
33, Pt. 4, pp. 559-586, Pl. 35. 
Sedgwick, A. 
°94. On the Inadequacy of the Cellular Theory of Development, and on the 
Early Development of Nerves, particularly of the Third Nerve and of 
the Sympathetic in Elasmobranchii. Quart. Jour. Mier. Sci., Vol. 37, 
pp. 87-101. 
Sewertzoff, A. [N.] 
°95. Die Entwickelung der Occipitalregion der niederen Vertebraten im 
Zusammenhang mit der Frage iiber die Metamerie des Kopfes. Bull. Soe. 
Imp. Nat. Moscou, Année 1895, No. 2, pp. 186-284, Pl. 4 et 5. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 291 


Sewertzoff, A. N. 
9g. Die Metamerie des Kopfes von Torpedo. Anat. Anz., Bd. 14, No. 10, 
pp. 278-282. 
Shipley, A. E. . 
°87. On some Points in the Development of Petromyzon fluviatilis. Quart. 
Jour. Micr. Sci., Vol. 27, pp- 325-370, PL 26-29. 
Stannius, H. . 
49. Das peripherische Nervensystem der Fische, anatomisch und physiolo- 
gisch untersucht. Rostock, iv 4- 156 pp., 5 Taf. 
Strong, O. S. 
°95. The Cranial Nerves of Amphibia. Jour. Morph., Vol. 10, No. 1, 
pp. 101-230, Pls. 7-12. 
Waters, B. H. 


°91. Some additional Points on the Primitive Segmentation of the Vertebrate 
Brain. Zool. Auz:, Jahrg. 14, No. 362, pp. 141-144. 


Waters, B. Hi. 
92. Primitive Segmentation of the Vertebrate Brain. Quart. Jour. Mier. 
Sci., Vol. 33, Pt. 4, pp. 457-475, Pl. 28. 


Wiedersheim, R. 
°g0. Das Gehirn von Ammocoetes und Petromyzon Planeri mit besonderer 
Beriicksichtigung der spindlartigen Hirnnerven. Morph. Studien, Heft 1, 
pp. 8-26. Jena. 
Weedersheim, R. 
792. “Discussion” iz Verhand]. Anat. Gesellsch., VI. (Wien), p. 167. 
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793. Grundriss der vergleichenden Anatomie der Wirbelthiere. Aufl. 3, 
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Wijhe, J. W. van. 
’82. Ueber die Mesodermsegmente und die Entwickelung der Nerven der 
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86. Ueber Somiten und Nerven im Kopfe von Végel- und Reptilienembry- 
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Wijhe, J. W. van. 
86" Ueber die Kopfsegmente und die Phylogenie des Geruchsorganes der 
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Wijhe, J. W. van. 
89. Die Kopfregion der Kranioten beim Amphioxus, nebst Bemerkungen 


iiber die Wirbeltheorie des Schidels. Anat. Anz., Jahrg. 4, No. 18, 
pp. 558-566. ‘ 


292 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 


Wijhe, J. W. van. 
°93. Ueber Amphioxus. Anat. Anz., Jahrg. 8, No. 5, pp. 152-172. 


Wilder, B. G. 
°89. The Gross Anatomy of the Brain. Wood’s Reference Handbook of the 
Medical Sciences, Vol. 8, pp. 107-164. 
Willey, A. 
°94. Amphioxus and the Ancestry of the Vertebrates. Macmillan & Co., 
New York and London. xiv + 316 pp, 135 Text Figures. 
Zimmermann, W. 
°91. Ueber die Metamerie des Wirbelthierkopfes. Verh. Anat. Gesellsch., 
V. (Minchen), pp. 107-1138. 


Zimmermann, W. 
°93. [Demonstration of a reconstruction drawing of abducens, ventral roots 
of glossopharyngeus and vagus, and of the hypoglossus in a human embryo 
in the beginning of the second month.} Verh. Anat. Geselisch. VII. 
(Gottingen), p. 216. 


NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 293 


° 
DESCRIPTION OF PLATES. 


All the drawings were made with the Abbé camera lucida. Figure 40 (Plate 6) 
is, however, a reconstruction from sections and dissected specimens. The Figures 
of Plates 8 and 9, with the exception of Figures 61 and 65, are also reconstructions 
from several sections. In sagittal sections, the embryo is always viewed from the 
right side. In cross sections, it is the posterior face of the section that is shown, so 
that right in the figure corresponds to right in the section. In frontal sections, the 
dorsal face is shown, so that right in the figure here also corresponds with right in 
the section. Only Figures 2 and 3 (Plate 1) represent embryos viewed from the 
ventral side, so that what appears on the left side in these two figures is really on 
the right side of the embryo. The cells of the neural crest are in all cases colored 
blue. 


294 


BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 


ABBREVIATIONS COMMON TO ALL FIGURES OF TEXT 
AND PLATES. 


* 


T=Vil. 
exp 
2’, 8’. 


V. md. 


V. mz. 
V. opt. su. 


= 


7. opt. p’fnd. 
V. rx. maj. 
V. rx. min. 
VII. ac. 
VII. bue. 
VII. hoi. 
VII. opt. su. 


a, B, Y. 


ap. 


ax-cyl. 
brs. vse. 1-6. 
cav. a. 


cbl. 
cd. 
cl. ers. n. 
cl. ms-ce. 


cl. n’bl. 
coms. a. 
coms. d. 


Posterior limit of cephalic 
plate. 

Encephalomeres. 

Somites (van Wijhe’s). 

Cavities of head somites 
2, 3. 

Ramus mandibularis tri- 
gemini. 

R. maxillaris trigemini. 

R. ophth. superficialis 
trigemini. 

R. opth. profundus trig. 

Radix major trigemini. 

“minor trigemini. 
Ramus acusticus facialis. 
ja Duccalisn was 
hyoideus “ 
“  ophth.  superfici- 
alis fac. 

Position of frontal sec- 
tions (Figs. 36, 37, 38). 

(Fig. C.) Position of 
section (Fig. D). 

“ Anterior cavity ” (Figs. 
B, C, E). Ventral fibre 
tract (Fig. F). 

Abducens nerve. 

Dorsal aorta. 

Visceral arch 1. 

Archenteron. 

Auditory invagination 
(otic vesicle). 

Axis-cylinder process. 

Visceral pouches 1 to 6. 

Platt’s somite (anterior 

cavity). 
Anlage of cerebellum. 

Chorda dorsalis. 

Neural-crest cells. 

Cells of mesocephalic 
ganglion. 

Neuroblastic cell. 

Anterior commissure. 

Dorsal ee 


“e 


coms. p. 
coms. SU. 
ec’drm. 
en’drm. 

ent. 

Jis. vse. 1-6. 
gls-phy. 

gn. ac-fac. 


gn. fac. ; 


gn. Gas. 
gn. gls-phy. 


gn. ms-ce. 
gn. trig. 


gn. spi. 
gn. vag. 


’ fo. 


la. ct. 
la. mu. 


m-b. 

mu. ob. su. 
mu. Tt. SU. 
mu. Tt. a. 
my cel. 
myl-mer. 
my-tm. 
n-po. 
oc-mot. 
prene. 

TL. Ve 

So. 

th. n. 

thl. 


treh. 
vag. 
vn. erd. 
vs. opt. 


Posterior commissure. 
Superior “ 
Ectoderm. 
Entoderm. 
Entoderm. 
Visceral clefts 1 to 6. 
Glossopharyngeus nerve. 
Ganglionic Anlage of 
acustico-facialis. 
Ganglion of acustico- 
facialis. 
Gasserian ganglion. 
Ganglion of glossopha- 
ryngeus. 
Mesocephalic ganglion. 
* Ganglionic Anlage of the 
trigeminus nerve. 
Spinal ganglion. 
Ganglionic Anlage of the 
Vagus. 
Infundibulum. 
Lamina cutis (cutis 
plate). 
Lamina muscularis (mus- 
cle plate). 
Midbrain. 
Superior oblique muscle. 
ae rectus muscle. 
Anterior rectus muscle. 
Myoceele. 
Myelomere. 
Myotome. 
Neuropore. 
Oculomotorius nerve. 
Prosencephalon. 
Ventral root of nerve. 
Somite. 
Neural tube. 
Thalamic portion of the 
trigeminus Anlage. 
Trochlearis nerve. 
Vagus nerve. 
Vena cardinalis. 
Optic vesicle. 


he 


‘ 
i ‘ 
worse 
wis 
"yy 


eo ww 
sa ihe 
~ 
peo” iy ; 
“i 
» 


NEAL. — Nervous Syst. Squalus. 


PLATE 1. 


All figures magnified 48 diameters, and oriented on the plate with the chief axis 
horizontal, the anterior end of the embryo to the right. The embryo made trans- 
lucent was drawn in outline with camera lucida and afterwards studied as an 
opaque object. 


Fig. 1. A dorsal view of an embryo with 6 to 64 somites. The edges of the neu- 
ral plate are seen to be irregularly lobed. ‘The two deep depressions at 
the anterior end of the cephalic plate mark the position of the future 
fundus of the infundibulum. 

Fig. 2. A ventral view of the same embryo. Locy’s segments are seen as lobings 
of the ventrally recurved margin of the neural plate. 

Fig. 8. A ventral view of another embryo of the same stage of development. The 
specimen was dissected to show the chorda, a rod in the median axial 
line, on either side of which lie the somites, van Wijhe’s seventh somite 
being designated as 7. An asterisk (*) marks the posterior boundary of 
the cephalic plate. 


NERVOUS SYST. SQUALUS. 


NEAL 


teen ewe 


—— 


ith Boster 


Meisel 


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NEAL. — Nervous Syst. Squalus. 


PLATE 2. 


All figures magnified 43 diameters and oriented as in Plate 1. The outlines 
were first made from the translucent embryos with camera lucida, and afterwards 
the embryos were studied as opaque objects. 


Fig. 4. An embryo with 4 somites viewed from the dorsal side. Locy’s segments 
are seen to be confined to the “ marginal bands” of the cephalic plate. 

Fig. 5. An embryo with 10 or 11 somites viewed from the right side and partly 
from above. The posterior part of the cephalic plate is seen to be 
sharply flexed ventrad on the right side. 

Fig. 6. An embryo with 12 somites viewed from the right side. The neural folds 
in the region of the cephalic plate have not yet met in the mid-dorsal 
line. The demarcation between cephalic plate and trunk is seen to be 
sharp. The anterior three primary vesicles (encephalomeres I, I, and 
III) appear in surface study as shown in the figure. In neither this 
embryo nor in the one represented in Figure 5 do Locy’s segments 
appear. 


NEAL-NERVOUS SYST. SQUALUS. PLATE. 2 


B. Meisel lith Bostes. 


HYVNeal del. 


ee tales @ on ey 


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Neat. — Nervous Syst. Squalus. 


PLATE 3. 


All figures drawn from cleared specimens and magnified 48 diameters. The 
neural tube seen in optical sagittal section. Neural-crest cells (ganglionic Anla- 
gen) colored in blue. In all cases the embryo is viewed from the right side. 


Fig. 7. Anembryo with 14 to 16 somites. Six vesicles only appear, and these are 
included within the limits of the cephalic plate. Neural crest (trigemi- 
nus Anlage) differentiated in the region of encephalomeres II and III. 
The mesodermic constrictions dividing somites 1, 2, 3, and 4 have ap- 
peared. ‘Two visceral pouches (1 and 2) are in the process of breaking 
through the lateral plates (splanchnic mesoderm). 

Fig. 8. Embryo with 18 or 19 somites. A thickening of the lateral zones in the 
posterior part of encepbalomere III (not shown in figure) appears in 
sections of this stage. The acustico-facialis Anlage has become dif- 
ferentiated in the region of encephalomere V. The “anterior cavity ” 
(Platt’s) begins to be cut off from the mesoderm of the 1st somite (van 
Wijhe’s). 

Fig. 9. Embryo with 19 or 20 somites. A dorsal expansion now appears behind 
VI, as the first indication of encephalomere VII. Posteriorly it is 
bounded by somite 6. The constriction between van Wijhe’s 3d and 
4th somites has become obscured by the migration of cells from both 
sides of the constriction to meet the advancing Anlage of the acustico- 
facialis. 

Fig. 10. Embryo with 21 or 22 somites. The conditions remain practically un- 
changed. 

Fig. 11. Embryo with 24 or 25 somites. A ventral migration of neural-crest cells in 
the region of encephalomere VI has now begun, and the crest is now 
differentiated in the region of encephalomere VII and posteriorly. 

Fig. 12. Embryo with 26 or 27 somites. A continuous neural crest extends from 
encephalomere V into the trunk region. Thalamic portion of the tri- 
geminus Anlage clearly differentiated. 

Fig. 13. Embryo with 28 to 80 somites. At this stage all of van Wijhe’s somites 
appear clearly differentiated. The Anlagen of the acustico-facialis 
and the glossopharyngeus, differentiated from encephalomeres V and 
VI, appear topographically related to the constrictions between van 
Wijhe’s somites, 3-4 and 4-5. 

Fig. 14. Embryo with 33 or 34 somites. Trochlear portion of trigeminus Anlage 
(compare Fig. 21, trch.) clearly differentiated. The commissure con- 
necting acustico-facialis and glossopharyngeus appears dorsal to the 
auditory invagination. 

Fig. 15. Embryo with 38 or 39 somites. 

Fig. 16. Embryo with 42 somites. Platt’s “anterior somite” (cav. a.) clearly 
differentiated. The anterior cells of the vagus Anlage, proliferated 
from encephalomere VII, have become clearly differentiated in the 
3d visceral arch as the Urvagus Anlage. Two visceral clefts have 
appeared. 

Fig. 17. Embryo with 48 somites (7.5 mm.). The fifth and seventh nerves have 
assumed fibrillar relation with the neural tube. The main branches 
of the trigeminus begin to appear. 


B Meisel ith Boston. 


1 


Syst. SQUALUS | . 
gaveg. gngis phy, gnae- fac, 


NgAL-NERVOU 


PLATE 3. 


i 


gnvag. — gngis 


ATE, 
ee 


z 6 


B Meine] Unk Beste 


HNes! ed «| 
a 


een D.* i) 
- ; 


1 


pear 2: 


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Fs 
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rye: 
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es pA). 
ie 4 ime we +, 4° |; 
gilt seal a) 3 a aae 
per: ee Bh eal 
a y “ig Sled pe hthe 


Ly nv of 
Rh ee 


é 


NeaAL. — Nervous Syst. Squalus. 


PLATE 4. 


Figs. 18 and 21 magnified 23 diameters. Figs. 19 and 20 magnified 21 diameters. 


Fig. 18. An embryo with 52 somites (8 mm.). The otic capstle now lies opposite 
encephalomere VII. The thalamic and trochlear portions of the tri- 
geminus appear only as scattered clumps of cells. Posterior com- 
missure clearly differentiated. Between this andthe preceding stage 
the oculomotorius has appeared as a fibrillar process from the ventral 
wall of the midbrain, near gn. ms-ce. 

Fig. 19. An embryo with 65 somites (10 mm.). The chief peripheral branches of 
the cranial nerves have appeared; the abducens, asa process from the 
ventral wall of encephalomere VIL 

Fig. 20. Anembryo with 78 to 80 somites (16 or 17 mm.). In this stage the ramus 
opthalmicus superficialis trigemini appears to have fibrillar relation 
with the mesoderm of the 2d somite, which is growing forward. The 
fibrous process of the abducens has come into relation with the 3d 
somite, and also is seen to have a branch passing to the mesoderm pos- 
terior to its place of origin. Most of the fibres of the ramus mandibu- 
laris trigemini appear in connection with encephalomere III. 

Fig. 21. An embryo 21 or 22 mm. long. The trochlearis is now differentiated, and 
in relation with the musc. obliquus posterior. 


PLATE & 


B Meisel lith Boston. 


weg. 3 a 


7 
-. al ee 


; ed AU VAI yr 


i. wu, 
she 


Md ee cp 

- ish 
a y mes 9 ),'y 
yi 

oO eh tp Me 

> ‘ > . 


. ee E 
b a "eis ee, ae Ob 
The it aul Capt? ‘1 * yf 
7 < vt é iy 
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Van? TAT oe NY, 


hs Obie! le 


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Ps 5 ? 


U “ ns 
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‘ 


Neat. — Nervous Syst. Squalus. 


PLATE 5. 


All the Figures except 51 and 52 represent frontal sections of embryos viewed 
from the dorsal side. All except Figures 25, 32, and 35 are magnified 45 diameters. 


Fig. 


Fig. 


Fig. 


22. 


. 23. 


g. 25. 


. 26. 


. 27, 


. 28. 
ig. 29. 


. 30. 


ol. 


. 32. 


. 33. 


34. 


A frontal section of an embryo with 14 or 15 somites. Encephalomere 1V 
appears as a thickening of the lateral walls of the neural tube. No 
local thickening seen in the region of encephalomere III. 

From an embryo with 16 or 17 somites. A local thickening of the lateral 
walls in the posterior part of encephalomere III appears. 

From an embryo with 19 or 20 somites. The first four hindbrain neuro- 
meres are now seen as local thickenings of the lateral walls, the 
thickening of neuromere III affecting its posterior part only. 

From an embryo with 28 to 30 somites, magnified 75 diameters. Five 
hindbrain neuromeres are seen. The auditory invagination appears 
opposite encephalomere VI. 

From an embryo with 50 somites (8 mm.) in the region of the “ Deck- 
platte,’ showing the faintly marked expansions of the encephalomeres. 

A more ventral section of the same embryo. The encephalomeres sharply 
defined by constrictions. A secondary constriction in encephalomere 
III appears. 

A more ventral section of the same series, in the region of the lateral 
zones. The local thickenings of the encephalomeres well marked. 

A still more ventral section of the same embryo. The inner cusps be- 
tween the neuromeres appear as in the more dorsal sections (Fig. 27). 
Frontal section in the region of the lateral zones of an embryo of 15 mm. 
The structure of the neuromeres is seen to be the same as that 

described by Orr (’87) for the Lizard. 

Cross section of an embryo with 20 somites, in the region of encephalo- 

mere IV, to show the thickening of the lateral zones. 

Cross section of an embryo with 28 to 30 somites in the posterior region 
of encephalomere III. The lateral zones more markedly thickened 
than in the previous stage (Fig. 31). 

Frontal section of an embryo with 50 somites (8 mm.), killed with a mix- 
ture of chromic, picric, and sulphuric acids, showing great intensifica- 
tion of the neuromeres, as the result of contraction due to inadequate 
fixation. The drawing, however, exaggerates the phenomena, since it 
represents the nuclear regions of the medullary wall with deeper 
shading. ‘ 

Frontal section of a 19-day Swine embryo. The constrictions between 
the neuromeres are sharply defined. 

Frontal section of an embryo of Amblystoma shortly after the closure of 
the neural tube. The neural tube is sharply outpocketed in the regions 
of proliferation of the ganglionic Anlagen of nerves V and VII. No 
evidence of a thickening or outpocketing comparable with encepha- 
lomere IV appears either at this or later stages. 


fags 


PLATES: 
B Meisel lith Bosten. 


’ 


NeRvous Syst. SQUALUS. 


AUER RENN ae ATTEN et 


NEAL. — Nervous Syst. Squalus, 


PLATE 6. 


All Figures, except 40, magnified 100 diameters. Only the right half of the em- 
bryo is shown in Figures 36-39, 42, and 44. 


Fig. 36. Frontal section of an embryo with 28 to 30 somites, showing the structure 
of the neuromeres IV and V in the region of the “ Deckplatte.” 

Fig. 37. A more ventral frontal section in the same series cut in the region of the 
lateral zones. The neuromeres appear as well marked local thickenings. 
The radial arrangement of nuclei much less clearly shown than in the 
preceding section (Fig. 36). 

Fig. 88. A still more ventral section of the same series, in the region of the 
“ Grundplatte.” The inner concavity appears as in the dorsal section 
(Fig. 36). 

Fig. 39. Frontal section of an embryo with 28 to 30 somites, in the region of the 
trunk, showing the structure of the myelomeres and their relation to 
the somites. 

Fig. 40. A reconstruction from sections and dissected specimens of the anterior 
end of an embryo with 28 to 30 somites, magnified 56 diameters. The 
lumen of the neural tube is exposed so as to show the hindbrain neuro- 


mere as local thickenings of the left wall. Van Wijhe’s somites, at — 


this stage separated by clearly marked constrictions, and Platt’s an- 
terior somite, are seen. Cells, in chief part derived from the neural 
crest, are seen surrounding the mesodermic epithelium of the 1st and 
2d visceral arches. 

Fig. 41. Across section of an embryo with 28 to 30 somites in the trunk region. It 
is seen that the somites press against the ventral half of the neural tube. 
A migration of mesenchymatous cells from the sclerotome portion of 
the somite has already begun. 

Fig. 42. Frontal section of an embryo with 50 somites (8 mm.) in the trunk fees 
(ectoderm omitted), taken in the region of the points of exit of the ven- 
tral nerves. No constrictions in the ventral wall of the neural tube are 
to be seen at this stage, but the ventral nerves lie opposite the middle 
of the somites. 

Fig. 43. A more dorsal frontal section from the same series as Figure 42. Con- 
strictions in the lateral wall, opposite which the ganglia lie, show no 
corresponding ridges on the inner surface of the lateral wall. 


Se 


A) i 


‘) 


; 


8) 9709909000 
Be ade ny 


Ho 89 


HOOPLE ge ey) 
depen Soe her se ae 


_ B Meisel lith Boston, 


Neat, — Nervous Syst. Squalus. ’ 


PLATE 7. 


Figures 44 to 53 illustrate the primary and secondary subdivisions of the forebrain 
and midbrain (encephalomeres I and II). All the Figures, except 47, 48, 53, 55, and 
56, magnified 43 diameters. 


Fig. 44. A parasagittal section of a Chick embryo of 33 hours’ incubation (14 
somites). Seven primary expansions of the encephalon appear, from 
the fifth of which, as in Squalus, the Anlage of the acustico-facialis is 
proliferated. 

Fig. 45. A parasagittal section of a Squalus embryo with 18 somites. Six primary 
vesiculations (encephalomeres) are seen, all included in the region of the 
cephalic plate. Clefts in the dorsal mesoderm separate from each 
other all of van Wijhe’s somites except the 4th and dth. 

Fig. 46. <A parasagittal section of a Squalus embryo with 28 to 50 somites. Both 
encephalomeres II and III have become subdivided by constrictions, 
that of the former, however, affecting the ventral wall only. All of 
van Wijhe’s somites separated by clearly marked mesodermic clefts. 

Fig. 47. A parasagittal section of an embryo with 65 somites, magnified 23 
diameters. Two subdivisions of encephalomeres I and three sub- 
divisions of encephalomere II appear. The latter remains, however, 
dorsally a simple expansion. Nerve relation of encephalomeres II and 
VII with somites 1 and 3 shown. 

Fig. 48. A frontal section in the dorsal part of encephalomeres I and II of an em- 
bryo with 80 to 382 somites, magnified 56 diameters. Only two vesicula- 
tions appear. Relation of thalamic portion of the trigeminus Anlage 
to these shown. 

Fig. 49. Frontal section in the dorsal portion of encephalomeres I and II in an 
embryo with 19 or 20 somites. ‘Two vesiculations only appear. 

Fig. 50. A frontal section of the same embryo as that shown in Figure 48, but 
more ventral, showing the constriction in the ventral wali of the mid- 
brain. 

Fig. 51. A frontal section of an embryo with 65 somites in the dorsal part of 
encephalomeres I and II (forebrain and midbrain), showing subdivision 
(thalamic) of the former. 

Fig. 52. A more ventral frontal section in the same series, showing a constriction 
in the lateral wall of the midbrain corresponding in position with the 
posterior commissure (coms. p.). The constriction in the forebrain 
corresponds with the superior commissure. 

Fig. 53. A frontal section, showing forebrain and midbrain regions in an embryo 
of 22 mm., magnified 23 diameters. Two constrictions only appear, 
one in the region of the superior commissure, and the other in the 
region of the posterior commissure. 

Fig. 54. A cross section of an embryo with 6 or 7 somites in the region of the 
cephalic plate, showing the ventral flexure of its edges. 

Fig. 55. A cross section of an embryo at a stage when the edges of the neural 
plate are about to be raised, showing the differentiation of a neural 
crest in the anterior part of the cephalic plate. Magnified 100 di- 
ameters. 

Fig. 56. A cross section in the posterior part of the cephalic plate of an embryo 
with 9 or 10 somites, showing that migration of neural-crest cells has 
already begun. Magnified 85 diameters. 


Nervous Syst SQUALUS. PLATE 7. 


WUT gw» 


B Meisal lith Basten. 


NEAL. — Nervous Syst. Squalus. 


PLATE 8. 


Stages in the development of the nerves oculomotorius, ramus ophthalmicus 
superficialis trigemini, and ramus ophthalmicus profundus trigemini. 


Fig. 58. 


Fig. 59. 


Fig. 60. 


Fig. 61. 


A frontal section of an embryo with 55 somites (8-9 mm.). Embryo 
killed with Davidoff’s fluid. The oculomotorius appears as a cellular 
strand extending from the inner side of the mesocephalic (profundus) 
ganglion to the wall of the midbrain. Magnified 500 diameters. 
Reconstructed from three sections. 

A sagittal section of an embryo with 56 somites. Embryo killed with 
Davidoff’s fluid. Near the brain the nerve appears composed of loose 
fibrille, while peripherally it is cellular in appearance. Magnified 360 
diameters. Reconstructed from five sections. 

A combination of two parasagittal sections through the left side of an 
embryo of 16 mm. Protoplasmic processes from the ramus ophth. sup. 
trig. appear in relation with the anterior projection of the 2d cavity 
(muse. obl. superior). Magnified 70 diameters. 

A parasagittal section from the right side of an embryo with 51 or 52 
somites (8 mm.). A well marked fibril passes from the mesocephalic 
ganglion to van Wijhe’s lst somite. The oculomotorius has not yet 
appeared. 


NEAG-NERVOUS Syst. SQUALUS. 


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B. Meisel lith, Boston. 


HVNeal del. 


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NEAL. — Nervous Syst. Squalus. 


PLATE 9. 


Stages in the development of the abducens nerve. All the specimens were 
killed with Davidoff’s fluid (corrosive-acetic). All Figures are from frontal sections 
except Figure 65. 


Fig. 62. A frontal section of an embryo with 62 somites (9 mm.). Magnified 450 
diameters. Two roots are present. 

Fig. 68. From an embryo with 68 somites (10 mm.) magnified 285 diameters. 
The course of the nerve is very direct in this stage, at least in the 
specimen figured. Throughout the most of its course it is entirely 
free from nuclei. 

Fig. 64. A combination from 7 sections of an embryo with 80 somites. Three 
roots present on the side of the embryo figured. Deeply staining nuclei 
appear in close connection with the nerve, and there is some (in my 
opinion doubtful) evidence of the migration of nuclei to or from the 
neural tube. Peripherally the nerve divides into fine fibrille. Magni- 
fied 200 diameters. 

Fig. 65. A cross section of an embryo with 75 somites in the region of the 
posterior root of the abducens, magnified 285 diameters. Evidence of 
migration of nuclei (?). 


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sf of Comparative Zoology 
H3 Bulletin 
v. 30-31 
Biological 


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PLEASE DO NOT REMOVE 
CARDS OR SLIPS FROM THIS POCKET 
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UNIVERSITY OF TORONTO LIBRARY