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by
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. -
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
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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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Zeitschr. f. Biologie, Bd. XXI. pp. 411-428. d/so, Gesammelte AbhandL.,
Bd. II. pp. 1-23.
Roux, W.
94. “Discussion” of Ziegler’s Paper on “ Furchung unter Pressung,”
Verhandl. Anat. Gesellsch., 8te Versamml., p. 151.
Roux, W. :
94" Jie Methoden zur Hervorbringung halben Froschembryonen und zum
Nachweis der Beziehungen der ersten Furchungsebenen zur Medianebene
des Embryo. Anat. Anzeiger, Bd. IX. pp. 248-282. 4/so, Gesammelte
Abhandl., Bd. Il. pp. 940-986.
Roux, W.
95. Ueber den “Cytotropismus” der Furchungszellen des Grasfrosches
(Rana fusca). Arch. f. Entw.-mech., Bd. I. pp. 43-68, 161-202, Taf.
I.-II1., 3 Figg.
Ryder, J. A.
93. The Growth of Euglena viridis when confined principally to two Dimen-
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
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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
; "
a =
ae
7 "
¥ co
f
= a
Ly
}
1
4
" ,
Ms
ea ie hem we ; 4
oS) a ; Yi Pm . ies a
od ny yy eke cat hg Abate
: Fy P , re fe art
a MY i q 13 2
= aa A ae ae i
iy OF Dia = at
ie hp ee
~ t
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 |
ead war init © ; 3
| 4
r y . :
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
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 |.
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H.Emerton del, Heliotype Printing Co.
Iconaxius caribbeus Fax.
Glyphocrangon neglecta Fax.
1-4
9-6
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(ON, BLAKE” CRUSTACEA
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merton del.
1-3 Acanthephyra affinis Fax.
4-7 Prionocrangon pectinata Fax.
8-10 Lophogaster longirostris Fax.
Hehotype Printing Co.
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‘
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. ) 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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Kellogg, V. L.
94. The Taxonomic Value of the Scales of the Lepidoptera. Kansas Uniy.
Quart., Vol. 3, p. 45-89, pl. 9-10.
Dis? BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY.
Kirby, W. F.
‘71-77. A synonymic Catalogue of Diurnal Lepidoptera. London, 5+ 690
pp. Supplement. London, p. 691-883.
Leydig, F.
°55. Zum feineren Bau der Arthropoden. Miiller’s Archiv, Jahrg. 1855
p. 376-380, Taf. 15-18.
Mayer, A. G.
°96. The Development of the Wing Scales and their Pigment in Butterflies
and Moths. Bull. Mus. Comp. Zool. Harv. Coll., Vol. 29, p. 209-236, 7 pls.
Ménétriés, E.
63. Descriptions des nouvelles Espéces de Lépidoptéres. St. Petersbourg.
[New Heliconidae. }
Merrifield, F.
94. Temperature Experiments in 1895 on several Species of Vanessa and
other Lepidoptera. Trans. Ent. Soc. London, 1894, p. 425-438, pl. 9.
Moore, F.
°90~96. Lepidoptera Indica. London, 2 Vols., 190 pls.
Miiller, F.
‘78. Notes on Brazilian Entomology. Trans. Ent. Soc. London, 1878, p. 211—
223. [Odors emitted by Butterflies and Moths. ]
Miiller, F.
‘79. Ituna und Thyridia. Ein merkwiirdiges Beispiel von Mimicry bei
Schmetterlingen. Kosmos, Bd. 5, p. 100-108, fig. 1-4. English translation
by Raphael Meldola. Trans. Ent. Soc. London, 1879, p. 20-29 Proc., fig. 1-4.
Miiller, W.
°86. Siidamerikanische Nymphalidenraupen. Versuch eines natiirlichen
Systems der Nymphaliden. Zool. Jahrbiicher, Bd. 1, p. 417-678, Taf.
12-15.
Poulton, E. B.
°85. The essential Nature of the Colouring of Phytophagous Larvae (and
their Pupae); with an Account of some Experiments upon the Relation
between the Colour of such Larvae and their Food-plant. Proc. Roy. Soc.
London, Vol. 38, p. 269-315. ;
Poulton, E. B.
87. The experimental Proof of the Protective Value of Colourand Markings
in Insects in Reference to their Vertebrate Enemies. Proc. Zool. Soc.
London, 1887, p. 191-274.
Poulton, E. B.
°90. The Colours of Animals. International Sci. Series, 67. New York,
13 + 560 pp.
Poulton, E. B.
°93. The experimental Proof that the Colours of certain Lepidopterous
Larvae are largely due to modified Plant Pigments derived from Food,
Proc. Roy. Soc. London, 1893, p. 417-450, pl. 5-4.
Ridgway, R.
°86. A Nomenclature of Colors for Naturalists. Boston, 129 pp., 17 pls.
Rippon, R. H. F.
°89-°93. Icones Ornithopterorum: A Monograph of the Rhopalocerous Genus
Ornithoptera, or Bird-wing Butterflies. London, parts 1-5, Plates.
MAYER: COLOR AND COLOR-PATTERNS. 253
Schiffer, C.
89. Beitriige zur Histologie der Insekten. Zool. Jahrbiicher, Morph. Abth.,
Bd. 3, p. 611-652, Taf. 29-30. [ Reference, p. 647-652. ]
Schatz, E., und Rober, J.
°85-"92. Die Familien und Gattungen der Tagfalter. (Exotische Schmetter-
linge von Staudinger und Schatz.) Fiirth, 2. Theil, 2 -+ 284 pp., 50 Taf.
Scudder, S. H.
*88—’89. The Butterflies of the Eastern United States and Canada with special
Reference to New England. Cambridge, 3_Vols., 24 4+ 1958 pp., 89 pls.
Seitz, A.
89. Lepidopterologische Studien im Ausland. Zool. Jahrbiicher, Syst. Abth.,
Bd. 4, 771-779, 905-924.
Semper, G.
86-92. Die Schmetterlinge der Philippinischen Inseln. Wiesbaden, 2 Bde.
Snellen, P. C. T., en Leeuwen, J. van, Jr.
87. Bijdrage tot de Kennis der Lepidoptera van het Eiland Curacao.
Tijdsch. voor Ent., Deel 30, p. 9-66, pl. 1-5.
South, R.
89. Notes on some Aberrations in the Genus Vanessa. Entomologist,
Vol. 22, p. 217-221, pl. 8. [Fig. 7.]
Srnka, A.
84. Eine neue Athyrtis. Lepidoptera: Fam. Heliconidae. Berlin. Ent.
Zeitschr., Bd. 28, p. 163-165.
Srnka, A.
85. Neue Siidamerikanische Danaidae und Heliconidae. Berlin. Ent.
Zeitschr., Bd. 29, p. 121-130, Taf. 1.
Staudinger, O.
82. On three new and interesting Species of Rhopalocera. Proc. Zool. Soe.
London, 1882, p. 396-398, pl. 24. [New Heliconidae. ]
Staudinger, O.
8488. Exotische Tagfalter. (Exotische Schmetterlinge von Staudinger
und Schatz.) Fiirth, 1. Theil, 333 pp. 100 Taf. [New Heliconidae. ]
Urech, F.
91. Beobachtungen itiber die verschiedenen Schuppenfarben und die zeit-
liche Succession ihres Auttretens (Farbenfelderung) auf den Puppen-
fliigelchen von Vanessa urticae undio. Zool. Anzeiger, Jahrg. 14, p. 466-
473.
Urech, F.
92. Uber Eigenschaften der Schuppenpigmente einiger Lepidopteren-
Species. Zool. Anzeiger, Jahrg. 15, p. 299-306:
Urech, F.
93. Beitrage zur Kenntniss der Farbe von Insektenschuppen. Zeitschr. f.
wiss. Zool., Bd. 57, p. 306-384.
Van Bemmelen. See Bemmelen, J. F. van.
Wallace, A. R.
67. [Theory of Warning Coloration.] Trans. Ent. Soc. London, Ser. 3,
Vol. 5, p. 80-81, Proc.
Wallace, A. R.
‘89. Darwinism. London and New York, 16 +494 pp. [p. 255-267. ]
254 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY.
Walsingham.
*85. On some probable Causes of a Tendency to melanic Variation in
Lepidoptera of high Latitudes. Trans. Yorkshire Union, pt. 5, p. 113-140.
Cf. Nature, Vol. 31, p. 505.
Watkins, W.
91. Ornithoptera trojana, Staudinger. Entomologist, Vol. 24, p. 178-179,
pl. 4.
Weismann, A.
°82. Studies in the Theory of Descent. Translated and edited by Raphael
Meldola. London, 2 Vols., 28 -+ 729 pp., 8 pls. [p. 1-158.]
Weymer, G.
75. Exotische Lepidopteren. Stettiner Ent. Zeit., Jahrg. 36, p. 368-385,
Taf. 1-2.
Weymer, G.
*84. Exotische Lepidopteren. II. Stettiner Ent. Zeit., Jahrg. 45, p. 7-28,
Taf. 1-2. ;
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.
=|
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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.
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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.
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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.
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Necharutis
ithinia Bates:
Nechanitis
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Necharuitis
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Nechaniiis
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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
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Heliconiiis
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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
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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.
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XXX.
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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
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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.
x ow
S Kw
g S ng
A) ew me
S 8 iF =
hatte ~ : & S Ess
SS S R ie C :
~ K
do % ~
.) »® ® ;
% oe
he dB,
g
- —_—. )
< J
—
Wi TTT
e &§ =
= Fos S +
fe is :
= BS S :
Lae ‘
tL,
¥ eae SARA,
. 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
sf |
- «|
Y |
a |
‘a
aty
a%d
A)
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.
Fig.
Fig.
Fig.
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.
i)
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H
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 | —
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.”
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ave 4
i tee
e fii
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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.
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‘Albatross ” Fa. LBA « Lsopoda LPL: VI.
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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” ~~.
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and only dorsal mesoderm,*
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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
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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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bo
Ol
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°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.
Wiedersheim, R.
793. Grundriss der vergleichenden Anatomie der Wirbelthiere. Aufl. 3,
Jena, xx + 695 pp., 387 Figg., 4 Taf.
Wijhe, J. W. van.
’82. Ueber die Mesodermsegmente und die Entwickelung der Nerven der
Selachierkopfes. Natuurk. Verh. d. K. Akad. Wissensch. Amsterdam, Deel
22, 50 pp., 5 Taf., 1883. Also separate, Amsterdam, 1882, 50 pp., 5 Taf.
Wijhe, J. W. van.
86. Ueber Somiten und Nerven im Kopfe von Végel- und Reptilienembry-
onen. Zool. Anz., Jahrg. 9, No. 237, pp. 657-660.
Wijhe, J. W. van.
86" Ueber die Kopfsegmente und die Phylogenie des Geruchsorganes der
Wirbelthiere. Zool. Anz., Jahrg. 9, No. 238, pp. 678-682.
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.
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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.
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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
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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.
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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 &
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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.
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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. ‘ oO
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