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Full text of "The cell-lineage and early larval development of Fiona marina, a nudibranch mollusk"

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marina, a nudibranch mollusk. 1 ' 
by dana brackenridge 'casteel, ph.d. 



Material and Method. 


Earlier Work on Opisthobranch Development. 

Maturation and Fertilization. 

The Unsegmented Egg. 

First Cleavage. 

Second Cleavage. 

Origin of Germ Layers. 

Segregation of the Ectoblast. 

Segregation of the Ento-Mesoblast. 

Segmentation of the Entoblast. 
Cleavage History of the Ectomeres. 

The First Quartet. 

The Second Quartet. 

The Third Quartet. 


Closure of the Blastopore. 

The Velum. 

Later Velar Development. 

Head Vesicle. 

Nerve and Sense Organs. 
Cerebral Ganglia. 
Otocysts and Pedal Ganglia. 

Excretory Organs. 

The Enteron. 

Stomodseum and Mouth. 

Shell Gland and Foot. 

Larval Musculature. 
Change of Axis and Form of the Developing Organism . 
Table of Cell-Lineage. 


The study of the cleavage and early larval history of Fiona marina 
(Forsk.) 2 embodied in this paper was undertaken with the view of 

1 Contribution from the Zoological Laboratory of the University of Penn- 

' J Dr. H. A. Pilsbry, of the Academy of Natural Sciences of Philadelphia, has 
kindlv assisted me in identification. 


obtaining, as far as possible, an exact knowledge of the development 
of this Opisthobranch, in order that certain doubtful points regarding 
the embryology of Mollusks in general, and this group in particular, 
might be better understood. Fiona has proved in many ways a diffi- 
cult object for study, but in certain respects offers advantages to the 
investigator. The exact origin of the germ layers as they arise in the 
segmenting egg has been particularly sought throughout the cleavage 
history, while in later stages attention has been directed to the rise of 
larval organs from their particular protoblasts where these could be 
definitely determined . Where this has been found impossible, approxi- 
mate results are given. Certain questions have presented themselves 
both at the beginning and during the progress of this work, some of 
which may here be indicated briefly. Though it has not been my pur- 
pose to consider particularly the mechanics of cleavage, this phase 
of development has been borne in mind, and in certain instances 
discussed. Comparisons are made between the nearly equal cleavage 
of Fiona and the more unequal segmentation of many other molluscan 
and annelid an eggs. The manner of origin of the germ layers is nat- 
urally a point of cardinal interest to the cell-lineage worker, since by 
this method of investigation the most exact results are possible and 
very definite comparisons with other forms may be made. The exact 
derivation of the middle germ layer has been sought particularly. 
Has it a single or double mode of origin? If both "primary" and 
"secondary" mesoderm be present, which is "larval" and which forms 
permanent organs? How is the mesoderm segregated from the two 
primary germ layers? In the study of larval structure and develop- 
ment the excretory organs are of much interest, since widely diverse 
views are held regarding the mode of origin and the significance of 
both primitive and definitive molluscan kidneys. The axial relations 
between ovum and larva and the relations of the early cleavage planes 
to the median plane of the larva and adult are points of great interest. 
How and when does bilaterality first appear? When does trosion 
first become manifest and what is its immediate cause? These and 
other questions have arisen and have been borne in mind during the 
progress of the work. Unfortunately material for the study of later 
larval stages and metamorphosis has not been obtainable, so that a 
complete record of development from ovum to adult has been impos- 

The work was begun in the early summer of 1901, at the Zoological 
Laboratory of the University of Pennsylvania, and continued, together 
with general graduate study, during the two following years at this 


University, as well as throughout the two intervening summers at the 
Woods Hole Marine Biological Laboratory. 

I am glad to acknowledge the many courtesies extended to me at 
both institutions. I am particularly indebted to Prof. Conklin, at 
whose suggestion the work was undertaken, and it is a pleasure to ex- 
press here my sincere appreciation of the valuable assistance which 
he has given me b} r way of suggestion and kindly criticism. 

Material and Methods. 

For the material upon which this study has been made, I am 
indebted to Drs. E. G. Conklin and M. A. Bigelow, by whom it was 
collected at Woods Hole, Massachusetts, during the summers of 1897 
and 189S. The Nudibranchs were found spawning upon floating 
gulf-weed in Vineyard Sound, taken to the Laboratory and kept in 
aquaria for some weeks, where they spawned prolifically and where, 
from day to day, the eggs were collected and preserved. They were 
fixed in Kleinenberg's stronger picro-sulphuric solution and Boveri's 
picro-acetic for one-half to three-quarters of an hour and washed in 
50 and 70 per cent, alcohol, as is the usual custom. Living material 
upon which to study the breeding habits of the animals has not been 
accessible to me, though search has been made in the same locality 
during the last two summers. This lack of the living adult animals 
and embryonic stages has been a considerable drawback, as it is par- 
ticularly desirable that one investigating the developmental history of 
an organism should be able to observe its physiological activities and 
thereby verify conclusions gained through purely morphological work. 
The material at hand has been amply sufficient for carrying the work 
up to the stage of the free-swimming veliger, but not to the metamor- 
phosis. It is my hope that in the near future material for the study 
of later stages and of the metamorphosis into the adult may be 
obtained, as many questions relative to the fate of larval organs must 
remain unanswered until this be accomplished. 

Contrary to the conditions found among some other Nudibranchs, 
the gelatinous mass surrounding the egg-capsules does not become 
greatly hardened upon fixing, for upon being brought into water the 
jelly usually dissolves, leaving the eggs free in their individual capsules. 
The eggs may be sectioned without removing the jelly, as it cuts 
without difficult}-. Both whole mounts and sections were stained in 
Delafield's hsematoxylin diluted with six to ten times its volume of 
distilled water and slightly acidulated by the addition of a trace of 
HC1, or Kleinenberg's stronger solution after the method of Conklin. 


This stain gives a reddish tint which differentiates the nuclei with great 
distinctness. Iron hematoxylin proved entirely unsatisfactory for 
sections of both early and late stages, for even in the old veligers almost 
all the cells are found to contain small yolk spherules which take up 
the hsematoxylin so strongly and hold it so tenaciously that nuclei and 
cell walls are indistinguishable. Eggs which have just been stained 
and mounted are not favorable objects for study, but they should, 
if possible, stand for some time, the longer the better, until they gradu- 
ally become more transparent by the penetration of balsam. Indeed, 
the most favorable slides are a few put up at the time the material 
was collected. Ety the addition of a little cedar oil to the balsam, or 
by moistening the edges of the cover with xylol at the time of using, 
it is always possible to roll the eggs by moving the cover — a very 
necessary process in cell-lineage work. Most of the observation and 
drawing was done with the aid of a Leitz objective 7, ocular 4, a 
Zeiss camera being used, with the paper at table level and plates re- 
duced as indicated. A -^ Leitz immersion was also used for obser- 
vation when necsseary. 


As a matter of convenience and for the sake of uniformity, I have 
followed the system used by Conklin (1897) with but slight variation. 

A cleavage is oblique to the right when the upper daughter cell lies 
to the right of an imaginary observer whose body corresponds in posi- 
tion to the primary egg axis, his head being at the animal pole and 
facing the cell considered ; vice versa, a division oblique to the left is 
one in which the upper cell lies to the observer's left. In the first 
instance the cleavage is dexiotropic, in the'second Iceotropic (Lillie, 1895). 

The term "quartet" is used to designate a generation of cells or 
their derivatives given off from the four cells meeting in the center of 
the vegetative pole, regardless of their fate. The different quartets 
are designated by coefficients placed before the letter indicating in 
which of the four quadrants the cells lie, while the cell generations are 
marked by exponents which follow the letter. The upper cell resulting 
from a cleavage is, in all cases, indicated by the smaller exponent; 
thus, 2b 11 indicates the upper cell in B quadrant of the second quartet 
arising from the division of 2b 1 , while 2b 12 is the lower. When the 
spindle lies in a horizontal direction or, in other words, when the cleav- 
age plane is meridional, the cell which lies to the right is given the 
smaller exponent, to the left the larger. The capital letters A, B, C, 
anil D are reserved for the four cells which meet at the center of the 


vegetative pole ("macro-meres") and from which the "micromere* ' 
arise; for these latter the small letters a, b, c and d are used. Child 
(1900) and Treadwell (1901) have been followed in giving coefficients to 
the macromeres also, to indicate their generation, this being desirable 
when dealing with an egg in which, after the first few cleavages, the 
"macromeres" are large in name only. "Thus A, B, C, and D form 
the four-cell stage. At their next division from A arises 1A and la; 
from B, IB and lb, etc.; 1A then divides into 2A and 2a, while la 
divides into la 1 and la 2 " (Treadwell). 

Earlier Work on Opisthobranch Development. 

A rather large number of older investigators have worked upon 
Nudibranch larval development. Grant (1827) described the veligers 
of JEolis and Doris. In 1837 Sars discovered that the young of Tri- 
tonia, Doris and JZolis possess a nautiloid shell; additional researches 
by the same investigator appeared in 1840 and 1845. Loven (1839) 
described a number of Nudibranch larvae together w T ith those of other 
mollusks. Alder and Hancock's magnificent monograph upon the 
British Nudibranchs appeared in 1845 and contains a good general 
account of the results thus far obtained upon the subject of Nudi- 
branch embryology. Reid in 1846 published an interesting paper upon 
the breeding habits of Doris, Goniodoris, Polycera, Dendronotus, Doto, 
etc., together with the constitution of the larvse. An account of the 
embryology of Tergipes by Nordman appeared in the same year. An 
extremely thorough account of the development of the Tectibranch 
Actwon by Vogt also appeared in 1846. In 1848 Koren and Danielssen 
described the early stages of a number of Nudibranchs from the Nor- 
wegian coast. Schneider (1858) described the veliger of Phyllorhoe. 
Keferstein and Ehlers (1861) gave an account of some of the develop- 
mental stages of jEolis. 

The later investigations of Langerhans (1873), Lankester (1875), 
Trinchese (1880-1-7), Lacaze-Duthiers and Pruot (1887), Rho (1888), 
Mazzarelli (1892-3-5), Heymons (1893), Viguier (1898), Carazzi (1900), 
Guiart (1901), and other works upon Opisthobranch embryology, 
together with those of importance pertaining to the remaining mol- 
luscan groups, Annelids and Platodes, will be considered during the 
course of this paper. 

A good general account of spawning habits of Nudibranchs is found 
in Alder and Hancock's "Monograph of the British Nudibranchiate 
Mollusca" (1845). 


Maturation and Fertilization. 

It is not the purpose of this paper to discuss in detail the maturation 
processes of the egg, but a few words in that connection may not be 
amiss. Maturation appears to have begun at the time of laying, since 
the first polar spindle is already formed in all eggs examined. In 
fig. 1 the chromosomes have moved to opposite ends of the first matu- 
ration spindle, and at a slightly later period, fig. 2, the sperm may be 
seen making its way through the yolk globules toward the upper pole. 
In a large number of sections examined the sperm is seen to have 
entered at some point below the equator of the egg, though apparently 
never directly at the center of the vegetative pole. The chromatin 
of the sperm nucleus is but slightly evident at this time, but astral 
radiations are strongly marked in the surrounding cytoplasm. The 
clear more protoplasmic substance of the egg becomes aggregated 
principally around the first polar spindle and in the neighborhood of 
the sperm nucleus, though long strands of finely granular protoplasm 
extend through nearly the entire egg, forming the astral rays. The 
yolk, which is in the form of rather small yolk globules, encroaches 
closely upon these centers, but is not, as a rule, found within them. 
As the first polar body arises, the upper surface of the egg becomes 
distinctly indented immediately above the first polar spindle and from 
this depression the first polar body emerges, bearing with it the distal 
end of the first maturation spindle, which rises as a whole toward the 
upper surface of the egg. During this process the sperm nucleus and 
aster remain in relatively the same position as before. There appears 
to be no telophase to this division, but without entering into a rest stage 
the second polar body is given off. This arises from the same place as 
the first, pushing the latter farther outward or somewhat toward the 
side (PI. XXI, fig. 3). Both finally lie in the slight depression at the 
surface of the egg. The female nuclear elements still left within the 
egg then come to rest, at first lying closely against the cell wall below 
the polar bodies. The first polar body does not divide again immedi- 
ately and may never do so, though usually at a later period three are 
found. If it remains undivided the first polar body exceeds the second 
in size. 

With the close of maturation the sperm nucleus is seen to have moved 
upward through the yolk; its chromatic elements have become more 
evident several large nucleoli being present. The same is true of 
the female pronucleus. They now approach each other, and come to lie 
with their nuclear walls closely appressed (fig. 4), the egg nucleus lying 


above and the sperm, which is the smaller, below. The clear granular 
protoplasm of the egg together with the sphere material surrounds both 
nuclei. The upper surface of the egg has resumed its former rounded 
outline, pushing the polar bodies farther outward. Their connection 
with the egg does not appear to be a very intimate one for they do not, 
in most cases, maintain at a later period any fixed relation to the poles 
of the egg and so are of little value in orientation, though they are 
often found in the apical region. 

Unsegmented Egg. 

The unsegmented egg of Fiona averages in diameter 80 micra with 
polar axis slightly less. The two polar bodies lie at the animal pole. 
Though the ovum is rather densely yolk-ladened, the yolk globules are 
of such small size that in future cleavages they tend to become more 
equally distributed among the resulting blastomeres than is the case 
with eggs containing yolk in larger spheres. The yolk which en- 
croaches upon the more protoplasmic environs of the nucleus consists 
of smaller globules, but otherwise its distribution throughout seems 
quite equal. 

The universal distribution of yolk to all the cells of the segmenting 
egg of Fiona is probably to be correlated with the smaller size of the 
individual yolk globules. It is safe to infer that each yolk body in an 
egg, whether it be small or large, is surrounded by a thin layer of 
protoplasm. In eggs containing a relatively larger number of yolk 
globules or, in other words, where they are small in size, a greater 
amount of cytoplasm will be distributed throughout the egg, when 
compared with that aggregated around the nucleus, than is the case 
when the single aggregations of yolk are large. When this is the case 
and division occurs the whole mass will be more influenced by nuclear 
and cytoplasmic divisional activity than when the cytoplasmic con- 
stituents are more definitely separated from the yolk. Just what this 
activity is we do not know, but a comparative study of eggs showing- 
large macromeres with those like Fiona, in which cleavage is more 
equal, will, I think, show that in the former case the individual yolk 
masses are much larger than in the latter, thus allowing for greater 
cytoplasmic influence where more finely divided yolk is found. The 
more equal division of cells naturally results in a wider spread of yolk 
through the developing organism, and it might also be added, as a corol- 
lary to this, that the absorption of more finely divided yolk is doubtless 
much more readily accomplished than where large globules are found, 
thus rendering it possible that such a wide distribution should occur in 
cells not alimentary in function. 


Before segmentation the nucleus lies but slightly above the center 
of the egg, having moved downward with its surrounding mass of 
granular protoplasm. An extremely thin and easily ruptured vitelline 
membrane surrounds the egg, and on account of the delicacy of this 
membrane no micropyle is present. Usually one but often two or 
three eggs lie together within a roomy egg capsule, containing also a 
fluid substance which does not coagulate in reagents. In unstained 
fixed material, and also doubtless in the living state, the eggs are quite 
opaque from the yolk which they contain. 

First Cleavage. 

The first cleavage is initiated by nuclear rupture and increased evi- 
dence of stellar radiation. With the formation and elongation of the 
spindle the surrounding yolk spherules give place to the more proto- 
plasmic constituents of the cell which form the immediate nuclear 
environs. The spindle as it elongates moves somewhat farther down- 
ward in the egg and lies but slightly above the equatorial plane. In 
length it measures about half the diameter of the egg. From the first 
constriction is almost equally marked all around the egg, though 
slightly greater at the animal pole. After the chromosomes have 
separated and are moving toward the opposite ends of the spindle, one 
end appears somewhat higher than the other (fig. 5), a position which 
would indicate a spiral trend of cleavage; but this is not evident in 
the telophase and completed division, for in the two-cell stage the 
nuclei lie directly opposite each other. 

As in the usual history of cleaving eggs, the resulting blastomeres 
are at first much rounded, but as their nuclei form they become closely 
pressed together, forming a flattened contact surface between which 
no cleavage cavity exists (fig. 6). The nuclei, together with their 
surrounding cytoplasm, again approach the upper surface of the egg 
and lie at rest just beneath the surface on opposite sides of the polar 
bodies. There is no evidence in their position to indicate a "virtual" 
rotation before the next cleavage, as is the case in Crepidula (Conklin, 
1897) . The daughter nuclei of the first cleavage becomes much dilated, 
containing several nucleoli suspended in the chromatin network and 
surrounded by clear nuclear fluid. 

The two blastomeres thus formed are equal or so nearly equal in 
size that they present to the observer no mark of distinction, and it 
can only be conjectured which will form the anterior and which the 
posterior region of the larva. Indeed, not until the appearance of 
the mesentodermal cell at the close of the twenty-four-cell stage can 


this distinction be drawn, for until that time all quadrants appear 
identical, though doubtless cytoplasmic and nuclear differentiation is 
present. As a result of this similarity of all the quadrants the figures, 
until the appearance of the mesentoderm cell, have of necessity been 
labelled arbitrarily. Of course, even in the two-cell stage lateral may 
be distinguished from terminal areas, for by following succeeding 
cleavages and marking the relation which the lower polar furrow bears 
to the first cleavage plane and the later relation of both to the median 
plane of the embryo, it can be determined that the first cleavage plane 
is obliquely transverse to the median plane. But not until a later 
period does posterior become distinguishable from anterior end. 

In the formation by first cleavage of two cells of equal size, Fiona 
agrees with a large number of Mollusks and Annelids, among the former 
of which may be mentioned Ischnochiton (Heath, 1899), Neritina 
(Blochmann, 1881), Crepidula (Conklin, 1897), Ercolania (Trinchese, 
1880), Tethijs (Viguier, 1898), Planorbis (Rabl, 1879, and Holmes, 
1900), Limax (Kofoid, 1895, and Meissenheimer, 1896), and among the 
latter Lepidonotus (Mead, 1897) and Podarke (Treadwell, 1901). 

Unequal cleavage appears to occur as commonly as equal among 
Opisthobranchs, examples of which are Acera (Langerhans, 1873), 
Aplysia (Blochmann, 1883; Carazzi, 1900), Umbrella (Heymons, 1893) 
and Philine (Guiart, 1901). 

Second Cleavage. 

The second cleavage results in four cells of approximately equal 
size. The spindles which precede it lie at right angles to the first 
cleavage spindle, and nearly parallel to each other, the left end of each, 
however, being slightly higher than the right, showing the laeotrophic 
character of the division. As cleavage proceeds this tendency becomes 
more marked, the upper or left-hand cells (A and C) lying higher than 
the right (B and D) . In consequence of this the second cleavage planes 
do not meet in a line at the vegetative pole, but a portion of the original 
first cleavage plane unites them in the ventral polar furrow ("Quer- 
furche" or "Brechungslinie"), the cells B and D being in contact below, 
while A and C never meet at the lower pole. At the upper pole no fur- 
row is present in Fiona, the four cells all joining in a common central 
point. As is the rule among Annelids and Mollusks in which the 
second cleavage is lseotropic, the ventral polar furrow taken in connec- 
tion with the first cleavage plane, bends to the right when viewed from 
the animal pole, and, vice versa, it turns to the left if considered as a 
part of the second cleavage plane. Fiona is no exception to the above 


rule, and by observing the position of this furrow the first and second 
cleavage planes may be kept distinctly in mind until outwardly visible 
differential changes in the quadrants present other landmarks for orien- 

Origin of Germ Layers. 
Segregation of the Ectoblast. 

By the next three divisions in which the four macromeres participate 
the entire ectoblast arises. 

First Quartet. — The spindles which precede the appearance of the 
first quartet of micromeres lie at first nearly radial, their prox- 
imal ends being distinctly higher than the distal. As a rule, all 
four spindles do not show the same stage of karyokinetic activity, 
though irregularities of this nature are not as yet greatly marked 
(fig. 9). As division proceeds they turn in a dexiotropic direction and 
with associated cytoplasmic constrictions four small cells are given 
off toward the animal pole (PL XXII, figs. 10, 11). These, the first 
quartet of micromeres, are in size about one-fourth that of their 
parent macromeres. As they round out in shape they are pushed 
farther toward the right, and finally come to lie in the furrows to 
the right of the large cells from which they arose. With the com- 
pletion of cleavage the whole egg again takes on a decidedly rounded 
contour, the micromeres changing materially in shape, becoming 
more flattened on their outer surfaces and sharp-angled below to 
fit the indentations between the macromeres (fig. 14). 

Second Quartet. — The second quartet arises lseotropically, thus regu- 
larly alternating in direction of cleavage with the first. The derived 
micromeres are but slightly smaller than the underlying cells from 
which they arise and are pushed strongly toward the left as they are 
given off. By this movement the four cells of the first quartet are 
also carried somewhat to the left, though the rotation is not great. 
All the second quartet cells are alike in size, there being no sign of 
increase in D quadrant, as is the case with many Annelids and some 
Mollusks; nor is there marked difference in their time of origin, though 
in future cleavages of the egg irregularities in the time at which divi- 
sions occur in similar cells of the four quadrants become more and 
more marked. In cytoplasmic structure these cells appear to differ 
little from their parent macromeres, though probably they contain 
less yolk. Their ultimate position is opposite and beneath the divi- 


sion walls of the first quartet, but they do not appear to become so 
flattened as their predecessors (figs. 13, 14). 

The Trochoblasts. — Before the macromeres again divide the first 
quartet is seen to be in process of cleavage. There result eight cells 
of nearly equal size, the more peripheral being slightly smaller than 
those at the apical pole. The spindles which precede division are 
laeotropically directed, and the lower cells are pushed downward and 
outward between the second quartet cells and just above the macro- 
meres (figs. 15, 16). These "primary trochoblasts" or "turret cells" 
do not again divide until about sixty cells are present (PI. XXV, 
figs. 33, 38), when they have become considerably flattened and lie 
between the arms of the forming ectoblastic cross. The fate of these 
very characteristic cells will be discussed later. 

Third Quartet and First Division of Second Quartet.— The first 
division of the second quartet and the third division of the macro- 
meres occur simultaneously. Each second quartet cell forms two 
of equal size by a distinctly dexiotropic cleavage, the spindles being 
from the first inclined in that direction. As may be seen in figs. 17 
and 18, these cells do not all divide at exactly the same time, and this 
lack of regularity is also characteristic of the macromeres. By this 
division of the second quartet the eight cells of the first are pushed back- 
ward dexiotropically so that, in relation to the macromeres, they occupy 
the same place as when given off. The division of the macromeres 
results in the four cells of the third quartet, They arise in a dexiotropic 
maimer and are equal in size to the four cells left at the lower pole. 
From this stage on these latter are "macromeres" in name only, being 
equalled in size by the third quartet and but slightly larger than the 
eight derivatives of the second. Nor, indeed, do .the macromeres 
appear at this stage to contain much more yolk than the micromeres. 
At a later period they are easily discernible from the micromeres by 
their clear yellow appearance, but as the latter divide much more rap- 
idly and by growth distribute the yolk which they contain over a 
larger area, while much of it is doubtless absorbed, the preponderance 
of this material in the individual cells of the endoderm and the larger 
cells of the mesoderm as well is easily explained. As has been men- 
tioned before, in the larva the amount of yolk in ectodermal struc- 
tures is quite considerable, showing its wide and universal distribu- 
tion throughout the entire organism. 
The twenty-four-cell stage has thus been reached and as yet the egg 


is radially symmetrical (PI. XXIII, fig. 19). At the center of the upper 
pole lie four " apical" cells, while the " trochoblasts " or "turret cells" 
extend from them into the angles between the second and third quartet 
cells. The third quartet and first generation of second quartet lie 
between them and the macromeres beneath, but from the nature of the 
cleavages do not form so marked a ring as in Crepidula or other 
Mollusks with large macromeres. The ectoblast has been entirely 
separated from the underlying macromeres, which contain all of the 
entoblast and the greater portion of the mesoblast. A small portion 
of the latter is to be derived, as will be shown later, from the third 
quartet of ectoblast cells. The egg has become somewhat flattened 
along its polar axis and within is a small cleavage cavity, which arose 
during the last few divisions and which later becomes of considerable 
size. Upon the lower surface the polar furrow remains distinct and 
offers a convenient means of orientation. 

The fact that in Mollusks, Annelids and Platodes the entire ectoblast 
is separated from the entoblast by the first three successive divisions 
in which the macromeres participate is a point of similarity of the 
highest importance in considering the question of the possible genetic 
relationships of the groups. With scarcely an exception (Dreissensia, 
Meissenheimer, 1901) this is accomplished by regularly alternating 
spiral cleavages. In most cases the first three quartets of micromeres 
are small protoplasmic cells and differ widely from the yolk-ladened 
macromeres, and this is particularly true of the first series being corre- 
lated with the later history of the cells which compose it, since in all 
cases they form the apical pole and the sense organs of the larva. 
Where much yolk is not present, or the spherules are small, more equal 
cleavage results, so that the macromeres are reduced in size; as exam- 
ples may be cited many Pulmonates (Planorbis, Physa, Limncea, Limax) 
and Lamellibranchs ( Unio, Cyclas, Dreissensia) , Chiton and Ischnochiton 
among the Amphineura, Trochus for the Prosobranchs and the Opistho- 
branchs Teihys and Fiona. The same is true of many Annelids 
(Podarke, Amphitrite, Clymenella, Arenicola, etc.). 

Both in size of cells and rate and direction of division the egg of 
Tethys (Viguier, 1898) exactly parallels that of Fiona up through the 
twenty-four-cell stage. The same may be said of Aplysia (Carazzi, 
1900, and Georgeovitch as corrected by Carazzi, 1900), except for the 
larger size of the macromeres, particularly the anterior ones, and Ca- 
razzi's statement that the trochoblasts arise from division of the first 
quartet — "con fusi distintamente dessiotropici." Such is, however, 
not the case, as his figures show. Carazzi has evidently, in some 


unaccountable way, become confused with regard to the direction of 
cleavage of these cells, for in another place, after quoting Conklin's 
statement regarding the trochoblasts of Crepidula, that these cells 
"continue to rotate in a clockwise direction," he adds "E la sua fig. 16 
mostra i fusi dessiotropic". As any one acquainted with cell-lineage 
work can see by reference to the figure mentioned, the upper ends of 
the spindles all lie to the left of the lower, and if there be any question 
as to the ultimate Inotropic direction of these cleavages a glance at 
Conklin's fig. 17 removes all doubt. In Trochus (Robert. 1903), Crepi- 
dula (Conklin, 1897) and Fiona the trochoblasts are given off by divi- 
sion of the four cells of the first quartet before the second quartet cells 
divide. In the case of Trochus the second quartet is just being formed 
when the trochoblasts divide. Moreover, Trochus shows no rest stage 
at twenty-four cells as do the other two, for while the third quartet is 
forming and the second is dividing for the first time all eight cells of 
the first quartet again divide, and these cleavages are followed by re- 
newed division of second quartet cells. The mesoblast cell, 4d, does 
not form in Trochus at this time but much later (sixty-four-cell stage), 
while in Crepidula and Fiona it appears immediately after a short rest 
period following the twenty-four-cell stage. The sequence of cleavages 
of Planorbis (Holmes, 1900) up to the twenty-four-cell stage closely 
follows Crepidula and Fiona. 

Segregation of Ento-Mesoblast. 

After a period of rest during which no cells are dividing and twenty- 
four are present in the egg, cleavage occurs in one of the macromeres. 
This macromere corresponds to that which has heretofore been arbi- 
trarily designated 3D, and from this period onward the four quartets 
may be definitely distinguished. The division is Inotropic and the 
larger daughter cell, 4d, will later gradually sink into the segmental ion 
cavity, forming a depression at the posterior end of the vegetative 
surface in the angle formed by the macromeres 3C and 4D, and other- 
wise bounded by 3d, 3c and the derivatives of 2d. 4d is thrown toward 
the left and, therefore, in the direction of the median plane, though at 
first it does not lie quite in that plane but slightly to the left of it or, 
in terms of spiral cleavage, to its right (PI. XXIV, fig. 24). In con- 
tradistinction to conditions found in heavily yolk-ladened eggs, this 
cell takes on from the beginning the position of a middle germ layer 
coming shortly to lie within the cleavage cavity, though, as will be seen 
later, its derivatives do not all appear to be mesodermal h~f character. 
Aftei all three quartets and also the macromeres with the exception 


of 4D have divided, and when there are present about 44 cells (fig. 25), 
4d or, as it hereafter will be designated more usually, the mesento- 
blast, ME, divides dexiotropically into cells of equal size. Before their 
next cleavage occurs the egg contains about seventy cells (fig. 42). By 
this division, which is bilateral, one small cell arises anteriorly from 
each of the large ones (figs. 42, 49). The small cells, E 1 and E 2 , corre- 
spond to the "Primary Enteroblasts" of Conklin, and will be so desig- 
nated. Considerable variation may be observed in different eggs as to 
the later position of these cells, as in some they appear to have moved 
backward along the sides of the large cells, Me 1 , Me 2 , from which they 
arose, but, as a rule, they remain in close relation to 4D, and always in 
later stages may be seen associated with the derivatives of this cell, from 
which it is hard to distinguish them (PI. XXIX, figs. 71, 73). The large 
cells soon divide again into almost equal parts, though the posterior 
and dorsal pair (mV, m 2 z 2 ) are slightly smaller (fig. 71). These latter 
soon divide again, giving rise to two small cells, z 1 and z 2 , which are 
posterior to the larger (fig. 73). Just before this cleavage the two 
cells M r e x , M 2 e 2 divide, giving rise anteriorly and toward 4D to two 
small cells, e 1 and e 2 (corresponding to the "Secondary Enteroblasts" 
of Conklin), which lie close to the first pair of small cells, E 1 , E 2 , the 
four forming a group of little cells with deeply staining nuclei in close 
contact with 4D, 5C and 5B. Behind them lie the large cells M 1 , M 2 . 
In the nomenclature used these would correspond to "Mesoblastic 
Teloblasts," but before they begin to function directly as such each 
again divides, giving off a small cell laterally, and these two cells appear 
to be dorsally directed toward the cleavage cavity above and to the 
sides of the enteron, but may remain associated with E 1 , E 2 , e 1 and e 2 . 
However this may be, the mesoblastic teloblasts soon begin to divide, 
giving off an irregular row of cells which extend around the gastrula 
laterally. The cells m 1 and m 2 also behave in a similar manner, their 
derivatives being closely associated with those of the large teloblasts. 
In figures 80, 81 and 82 only the derivatives of the latter are shown, 
the other lying dorsal to them. As the teloblasts and the cells m 1 
and m 2 divide they diverge laterally and leave behind and between 
them the smaller cells E 1 , E 2 , e 1 , e 2 , closely associated with the posterior 
elements of the enteron. When these cells are first given off they 
lie decidedly above the level of the enteric invagination projecting 
upward into the cleavage cavity, and while in this position might well 
be characterized as mesodermal elements; but later they change their 
position, slipping in between the teloblasts and the posterior cells of the 
enteron, and by the time the teloblasts begin to separate and wander 


toward the sides of the gastrula these small cells, which have been 
derived from 4d, lie nearer the ventral surface than the cells which 
form the bottom of the invaginating enteron and closely appressed 
against the posterior boundary of this region. The small cells z 1 , z 2 , 
which are the posterior derivatives of the division of mV, m 2 z 2 , also 
continue to lie near the median line in the posterior region of the 
gastrula, closely pressed and flattened against the ectoderm. 

The later history of the enteroblasts, which I believe are concerned 
in the formation of the intestine, will be discussed in connection with 
the development of the enteron. 

In comparing the mesoblast formation of Fiona with that of other 
forms, Crepidula will be considered first, since in this Prosobranch 
4d was first found to contain both entoblastic and mesoblastic material 
(Conklin, 1897). Here 4d arises when twenty-four cells are present 
and by a Inotropic division. This cell soon cleaves dexiotropically 
into two of equal size. At the next cleavage there result in Crepidula 
four cells of similar size, the posterior and lower pair being the first 
enteroblasts, while in Fiona it is the anterior smaller cells which are 
entoblastic. At the next cleavage in Crepidula the large cells Me 1 , Me 2 , 
which still contain both mesoblast and entoblast, give off smaller 
purely mesoblastic cells anteriorly (m 1 , m 2 ), while in Fiona the larger 
posterior cells give rise posteriorly to similar cells, though they may 
not be purely mesoblastic. The next cleavage of M^ 1 , M 2 e 2 in Cre- 
pidula completely segregates mesoblast and entoblast, the cells of 
the latter lying posterior to the mesodermal elements. This division 
separates two more small enteroblasts in Fiona, which here lie with 
the first enteroblasts anterior to the large cells, M 1 , M 2 ; each gives 
rise to another small cell anteriorly in Fiona which may be entero- 
blastic, otherwise from this period on they function as teloblasts of 
the mesoderm. 

From the above comparison it is evident that if we consider the 
position of the mesodermal and endodermal constituents of 4d in 
connection with the segmented egg as a whole, directly opposite 
conditions are found. In Crepidula the derivatives of this cell form 
mesoderm anteriorly and laterally, entoderm posteriorly, while in 
Fiona the reverse is the case. But in both forms, if we consider the 
position of the enteroblasts not in relation to the egg as a whole, but 
only in connection with the macromeres with which they are to be 
associated, it will be seen that in both Crepidula and Fiona these cells 
are directed toward the posterior region of the cells 4D, 4C, or their de- 
rivatives, and that the reverse relations of the enteroblasts and meso- 




blasts in Crepidula and Fiona is the direct result of epibolic gastrulation 
in'the one case, embolic in the other, which is in turn caused by the 

quantity and nature of the yolk which the 
macromeres contain. An intermediate 
condition is found in Xereis (Wilson, 
1898). Text-figure 1 (a) shows a sagittal 
section through the cleaving egg of Crepi- 
dula after one enteroblast has been sepa- 
rated from the mesoblast. The ectoblast 
has here but half covered the yolk, and 
the entoblastic element is thrown down- 
ward and backward in the direction in 
which it must go if it follows the ecto- 
derm over the yolk, and finally reaches a 
position posterior to the blastopore as 
that structure is closing (Conklin's fig. 
61). In Nereis, text-figure 1 (b), the ec- 
toderm has advanced much farther over 
the yolk when the enteroblasts arise, and 
here we see that these elements are also 
directed downward but at the same time 
anteriorly. The next and last step in 
their change of position is illustrated by 
Fiona, text-figure 1 (c), in which, on ac- 
count of its invaginate gastrula, the en- 
teroblasts are not only anteriorly directed, 
but also at first He higher than the cells 
from which they arose. 

In Trochus (Robert, 1903) the meso- 
blast arises at about the sixty-four-cell 
stage by a Isotropic division which sepa- 
rates the very large cell 4d from 4D. 
This cell divides dexiotropically and 
equally when eighty-nine cells are present. When there are one hun- 
dred and eighteen cells, each of the two derivatives of 4d divides, and 
of the resulting four cells the anterior pair are the smaller. Later 
the two larger posterior cells divide. Robert has not found endo- 
dermal elements to arise from 4d, r but does not reject the possibility 
of such a condition. 

As might be expected from their close relationship, a nearer corre- 
spondence in the cleavage series is found when we compare Fiona with 

Fig. 1. — Sagittal sections 
through the gastrulse of 

(a) Crepidula (Conklin) , 

(b) Nereis (Wilson) and 

(c) Fiona. The entero- 
blasts are lined, the meso- 
blastic cells stippled. 


Umbrella, although Heymons' conclusion regarding the fate of the 
descendants of 4d is at wide variance with the conditions which are 
found in Fiona. After the cleavage of 4d into equal parts, Heymons 
states that two small cells are given off from these, so that they he in 
the posterior region of the macromeres. It is very evident from his 
figures that these cells, which would correspond to E 1 , E 2 of Fiona, 
at first He quite dorsal to the enteron and in the cleavage cavity. The 
large cells next divide nearly equally, the most posterior being slightly 
smaller and corresponding in size and origin to nrV, m 2 z 2 . These 
latter shortly change their position in Umbrella exactly as in Fiona, 
for, says Heymons, "Bald beginnt eine interessante Lagerungsver- 
schiebung einzutreten. Es rucken namlich die hinteren Zellen weiter 
nach dem animalen Pol hin und legen sic vollkommen auf die vorderen 
auf". While this rearrangement is occurring and after its completion 
two and later other small cells are given off by the large underlying 
cells toward the smaller cells originally budded forth. Exactly the 
same process occurs in Fiona— compare Heymons' figs. 23 and 24 with 
my fig. 71. Heymons' smaller cells M', M' (corresponding to m'z 1 , 
m 2 z 2 of Fiona), which have moved toward the animal pole of Umbrella, 
do not appear from the account to divide again so quickly as 
in Fiona, but that they later divide teloblastically is evident. 
As has been mentioned before, the small anterior cells of Umbrella, 
which correspond to E 1 , E 2 . e 1 , e 2 , of Fiona, at first he entirely within 
the segmentation cavity. Figures of later stages, however (Heymons' 
fig. 29), show that they then lie at a level with the posterior cells of the 
enteron (D, A'. C", etc.), and are directly between these and the anal 
cells. The same relative position is taken by the corresponding cells 
of Fiona. 

In interpreting the results of Heymons the above point of view is 
somewhat different from the comparison of Conklin between Umbrella 
and Crepidula, in which he suggests a resemblance and possible simi- 
larity of origin between the enteroblasts of Crepidula and the telo- 
blastic cells M, M. M', M'. of Umbrella. In both these "are large cells 
containing a considerable quantity of yolk, about equal in size and 
grouped in a characteristic way" ; but the same may be said of the 
similar cells of Fiona, yet they have no part whatever in the formation 
of the enteron, though from their appearance I was led to think such 
might be the case before a knowledge of their later history proved 
otherwise. The explanation of the whole matter lies in the axial 
change which the derivatives of 4d have imdergone in the forms con- 
sidered. The posterior macromeres (particularly D) of Umbrella are 


relatively small, the same result being here obtained as in Fiona, in 
which the entoblastic elements are produced from the anterior rather 
than from the posterior side of the teloblasts. If any of the descend- 
ants of 4d of Umbrella described by Heymons are entoblastic in nature 
they are those which arise in this way, and these are the cells which 
must be compared with the enteroblasts of Crepidvla and the small 
anterior cells in Fiona. 

Viguier (1898) describes and figures the formation of the mesoderm 
in Tethys ftmbriata as similar to that of Umbrella, and a comparison of 
figures will show almost exact correspondence. Like Heymons, 
Viguier does not consider the derivatives of 4d to be other than meso- 
dermal in fate. 

Carazzi (1900) derives both mesoderm and endoderm from the 
cell 4d ("EM") of Aplysia. He states that the cleavage which forms 
this cell is dexiotropic in direction, and such appears to be the case 
from his figures. The cell 3 A of Aplysia is larger than the others, 
thus throwing 3D so much to the right of the median line that a dexio- 
tropic cleavage is necessary to place the mesentomere upon this line. 
The divisions of 4d which follow are identical with those of Fiona, but 
Carazzi's conclusions regarding the fate of the remaining blastomeres 
are quite different. Four pairs of small cells are derived from the two 
large cells and lie anterior to them. These correspond in position to 
the four (or more?) enteroblasts of Fiona, but by Carazzi are described 
as mesodermal. Two larger cells have been given off posteriorly and 
correspond to nrV, m 2 z 2 of Fiona. From each of these a small cell 
buds forth posteriorly, the two lying near the ectoderm. These small 
cells are, according to Carazzi, enteroblasts, and go into the intestine. 
Cells similar to these in origin and, for the time at least, in position are 
found in Fiona (z 1 , z 2 ) lying closely pressed against the ectoderm in 
the posterior region of the gastrula. They are small in size, and at a 
later time I have found it impossible to distinguish them from many 
small mesodermal cells which crowd that region of the gastrula. If 
they do not shift their position, they would naturally become involved 
in the formation of the distal end of the intestine either directly, as 
lining cells of that organ, or as muscle cells for its walls. One cannot 
help feeling in comparing the development of the two forms and noting 
the great similarity in the history of the early derivatives of 4d that 
their fate is also the same ; and the same might also be said of the small 
anterior elements which Carazzi indicates as mesodermal. 

Lillie (1895) concluded that in Unio the derivatives of 4d were 
entirely mesoblastic. The two teloblasts give origin to two small cells 


anteriorly which he near the enteron and are probably concerned in 
the formation of splanchnic musculature. Similar conditions are found 
to exist in Dreissensia, according to Meissenheimer (1901). 

Among the Pulmonates the work of Rabl (1879) is confirmed by 
Holmes (1900), who finds that all the derivatives of the primary meso- 
blast are mesoblastic in fate. More particularly he states that the 
two bilaterally placed teloblasts give rise to a pair of small cells ante- 
riorly, after which the large cells divide into equal moieties. Wier- 
zejski (1897) says of Physa fortinalis, "Dass der Modus der Bildung 
eines Theiles des Mesoderm bei Physa , desjenigen aus der Urmesoderm- 
Zellen fast ganz derselbe ist wie ihn Heymons fur Umbrella eingehenden 
dargestellt". In the last stage described the mesoderm consists of 
twelve cells, a group of six small cells anteriorly placed, behind which 
are a pair of ''Urmesoclerm-Zellen" from which they arose, while behind 
and above lie two other rather large mesoderm cells which have given 
off a pair of small cells posteriorly. Both in sequence of origin, in 
relative position and in size this group corresponds to the similar 
series in Aplysia and Fiona; but Wierzejski ascribes a mesodermal 
fate to the whole. 

In Lirnax Meissenheimer (1896) describes the cleavage of 4d to a 
stage in which there are four cells, the anterior pair of which are the 
smaller. In fate they serve as anlagen for mesodermal struc- 
tures. Similar conclusions were also reached by Kofoid (1895) on 

Heath (1899) has accurately traced the origin of the mesoblast in 
Ischnochiton at the seventy-two-cell stage, and its later cleavage into 
cells of equal size which lie bilaterally. At a more advanced stage 
two more divisions were noted giving origin to small cells dorsally and 
anteriorly. Heath was unable to determine whether these cells were 
purely mesodermal or partly endodermal. 

Mead (1897) describes for the Annelid Arenicola two small cells 
budded off from the bilaterally situated pair of mesodermal cells, and 
by further division of the large teloblasts these cells are seen later lying 
at the ends of the mesodermal bands and appear to be mesodermal in 
fate. The same conclusions were reached regarding Clymenella, 
though in this case the lineage has not been traced so far. In this 
Annelid the divisions of M 1 , M 2 result in cells of nearly equal size, a 
condition which may indicate a variation in later stages. 

In 1897 Wilson, having reinvestigated the history of the second 
somatoblast of Nereis, discovered that the two small cells budded from 
the teloblasts toward the enteron, to which in his earlier paper (1892) 


a mesoblastic fate was assigned, are entoblastic in nature, and the same 
he thinks probably to be true of Aricia and Spio. 

Child (1900) has found for Arenicola that 4d after its first cleavage 
forms mesoblastic teloblasts, from which later arise two bilaterally- 
placed mesoblastic bands ; all these cells are mesoblastic in fate, and it 
is evident from his figures and discussion that he does not find here any 
entoblastic material. Though in Sternapsis the lineage was not fol- 
lowed so far as that of Arenicola, Child reaches the same conclusion, 
and particularly in the latter case he states that the mesoblastic cell 
is "purely protoplasmic and without yolk". 

In the Annelid Podarke (Tread well, 1901) 4cl arises, together with 
the other members of the fourth quartet, at the sixty-four-cell stage 
and is equal in size and appearance to them. It sinks inward with 
the invagination which forms the enteron, divides and lies in close 
connection with the endodermal cells. By this division from the 
larger cells four small cells are given to the enteron, while the remaining 
two are purely mesodermal. 

Torrey (1902), in a preliminary on the cytogeny of Thalassema, 
assigns to the two small cells arising from the teloblasts the fate of 
enteroblasts, in a similar manner as in the Annelids above considered. 

Segmentation of the Entoblast. 

Shortly after the origin of the mesentoblast 4d, when the egg contains 
forty-one blastomeres, all the "macromeres" except 4D are seen to be 
dividing lseotropically (fig. 24), with the result that three large cells, 
4a, 4b, 4c, are given off from their respective macromeres. These 
cells are slightly greater in size than those centrally grouped, but are 
not so large as the cell 4d, and on this account we find that of the four 
cells, 4A, 4B, 4C and 4D, the last is the smallest, nor does it again 
divide until over one hundred and fifty blastomeres are present. 
The position of the fourth quartet may be seen in fig. 25 and 
those following. When the egg contains over eighty blastomeres, 
4A, 4B and 4C again divide into equal moieties, the outer three of which 
(5a, 5b, 5c) lie to the right of the central group. All these cells have 
become much flattened and form a comparatively thin roof over the 
segmentation cavity, into which as yet invagination has not begim. 
The mesentoderm has sunken completely beneath the external layer 
and extends forward as far as the center of the cavity (figs. 45, 57). 
At a muchUater period, when there are nearly one hundred and fifty 
cells present, 4a, 4b and 4c again divide (figs. 71, 72, 73), giving off 
small cells to the left and outwardly (4a 1 . 4b 1 , 4c l ). The invagination 


to form the enteron has already begun by the depression of the smaller 
cells which lie in the center of the vegetative pole, while the small cells, 
E\ E 2 , e 1 , e 2 ,. at the anterior end of the teloblasts have become drawn 
into the posterior region of the invagination (except for some varia- 
tion, an instance of which is shown in fig. 72), where at this time they 
help to close that portion of the gastral pit. As the primary enteric 
cells sink into the cleavage cavity the small cells, E 1 , E 2 , e 1 , e 2 , come 
into close connection with the posterior edges of 5C, 5D, 4a. Thus a 
more or less complete cup-like invagination is brought about among 
the entomeres, in which the smaller elements lie at the bottom with 
the larger (4a 2 , 4b 2 , 4c 2 ) between, and the small cells which have arisen 
from these latter lying peripheral to them. Above, toward the ven- 
tral surface, lie small cells of the second and third quartets around the 
blastopore opening. 

In the formation of the enteric cells the manner in which the fourth 
quartet arises appears to be characteristic of a number of Opistho- 
branchs. This quartet is in Umbrella (Heymons, 1893), Aplysia 
(Blochmann, 1S83; Carazzi, 1900) and Tethys (Viguier, 1898), as well 
as in Fiona, larger than the macromeres remaining at the center of 
the vegetative pole. 

The further development of the enteron will be discussed later. 

Cleavage History of the Ectomeres. 

As has been seen, the ectoblast arises immediately after the four- 
cell stage by the three successively alternating cleavages in which the 
macromeres participate, giving rise respectively to the First, Second 
and Third Quartets of micromeres. The cleavage history of these 
cells will now be taken up and their ultimate fate, as far as can be 
determined, considered. 

The First Quartet. 

The formation of the "turrets," la 2 -ld 2 , and the "apicals," la'-ld 1 , 
leading to the radially symmetrical twenty-four-cell stage, has already 
been considered. Shortly afterward the apical cells divide in a dexio- 
tropic direction, thus alternating with the preceding cleavage, and by 
this division the four "basal" cells of the ectoblastic cross arise, while 
between these and the central point of the egg lie the four small apical 
cells from which they were derived (fig. 23). Before this cleavage had 
occurred the upper and dextral cells of the second quartet had in each 
quadrant given off a small cell in a lseotropic direction (fig. 21), which 


after the formation of the basals occupy positions just peripheral to 
them and slightly to the left. These four small second quartet ele- 
ments are the "tip" cells of the cross, 2a n -2d n , and together with the 
basals and apicals form the ectoblastic cross. 

From the time of its formation and until a late period of cleavage 
the cross of Fiona is a distinctly dexiotropic structure, the apicals of the 
four arms lying to the right of their respective tips. The cross is thus 
at the time of its formation (fig. 23) composed of twelve cells, of which 
the apicals are the central, is radially symmetrical and its anterior and 
posterior arms lie very near to, if not exactly in, the median plane of 
the future embryo. In the future history of this structure the tip 
cells will for convenience be described in connection with the rest of the 
cross, since they are so closely connected with it. 

Before further cleavage occurs in the first quartet the second and 
third quartets and the macromeres show marked karyokinetic activity, 
the number of cells in the egg having increased to nearly sixty. The 
basal cells and the turret cells or trochoblasts then divide simultaneously 
(fig. 33), though considerable variation in time occurs in different eggs 
and in different quadrants, it being, however, universally observed 
that Id 12 divides last of the basals. It may be noted in this connection 
that in all species of Crepidula examined except C. adunca the division 
in the basal cell of the posterior arm is delayed for a much longer period. 
The direction of cleavage of the basals Id 12 and lb 12 is Inotropic and 
so alternating with the last, those of the other two doubtful; la 12 
usually shows a lseeotropic to radial position of spindle, while in lc 12 
variations are present all the way from Inotropic to dexiotropic. After 
examining a large number of eggs the occurrence of this irregularity 
was more strongly confirmed, and it thus appears that in this cell, 
lc 12 , there is a strong tendency, more marked in some cases than in 
others, toward non-alternation with resulting bilaterality of cleavage 
in relation to its opposite cell, la 12 . In Crepidula, Planorbis and Neri- 
tina the cleavage of all these basal cells is non-alternating, while in 
Umbrella it is regularly alternating. 

In Fiona it would appear that we have an intermediate condition in 
which, though regular alternation is found in the anterior and posterior 
basal cells, the two lateral, particularly lc 12 , show a tendency toward 
non-alternation under the influence of approaching bilaterality. It 
is just at this time that the first distinctly bilateral cleavages occur in 
two cells of the third quartet in the two posterior quadrants, 3d 1 and 
3c 1 (figs. 31, 32), and this suggestion of bilateral divisions of the cross 
may be correlated with them. However, the influence toward bilater- 


ality must be very slight, as the radial symmetry of the upper pole is 
not disturbed to any appreciable degree. 

By the divisions of the basal cells above described each arm of the 
cross is composed of four cells — an outer tip cell (2a n -2d n ), next to it 
the "middle" cell (la 122 -ld 122 ), an inner "basal" cell (la 121 -ld 121 ), which 
is larger than its sister middle cell, and an apical (la 11 -ld 11 ). 

Synchronously with the cleavage of the basals occurs that of the 
turrets, the cell of this series in each quadrant dividing into two of 
nearly equal size, the outer being the smaller. All divisions are 
dexiotropic and alternating with those by which these cells arose 
(fig. 33). 

Comparing the cleavage of the turrets with conditions found in other 
forms, it will be noted that considerable variation exists. While in 
Fiona these cells divide when there are about sixty blast omeres in 
the whole egg, in Umbrella (Heymons) approximately seventy are 
present; like Fiona all four turrets divide at relatively the same time. 
In Crepidula the anterior trochoblasts do not divide until there are 
over one hundred cells in the egg, and Conklin states that he believes 
the posterior ones never divide. The trochoblasts of Trochus (Robert) 
arise very early, at the sixteen-cell stage, and have all divided when 
there are thirty-two cells present. In Planorbis Holmes finds them in 
division at about forty cells, and Limax (Kofoid) shows a similar con- 
dition. In Unio (Lillie) there are about fifty cells, while in Ischno- 
chiton (Heath) but thirty-two, when the "primary trochoblasts" of the 
latter form divide. Thus Fiona appears to occupy an intermediate 
position in relation to these and other molluscan forms in which the 
time of cleavage of these cells has been determined. 

Division next occurs in the cross at a stage of about eighty-four 
cells and results in the division of the apicals into eight small cells, 
of which those lying centrally form the "apical rosettes" (la m -ld lu ), 
while the outer series are the "peripheral rosettes" (la 112 -ld 112 ) of 
Conklin. Direction of cleavage is Isotropic, and of the resulting cells the 
outer are the larger (PI. XXVII, fig. 53). Shortly after the rosette series 
are established the basal cells of all arms divide again, the posterior 
one last. In the anterior quadrant the spindle and resulting cells, 
lb 1211 and lb 1212 , lie radially in the lateral arms, the division of lc 121 is 
Inotropic, that of la 121 dexiotropic, again showing bilateral influence, 
while in Id 121 the spindle is so strongly turned in Isotropic direction 
that the resulting cells lie transversely across the posterior arm (figs. 
56, 62). While this last cleavage of the basals is being accomplished 
a similar process is seen in the four inner trochoblasts (la 21 -ld 21 ), result- 


ing in eight cells of equal size and occurring at relatively the same time 
in all four quadrants. 

With the completion of the above-described divisions the large num- 
ber of cells of similar size at the upper pole of the egg makes their exact 
lineage difficult to follow, so that it is desirable to make here some com- 
parisons with the structure and development of the cross and trocho- 
blasts in other forms, and to bring together the results already obtained 
before proceeding to more uncertain ground. In formation the cross 
of Fiona arises in the same manner as in Umbrella and Planorbis, by 
the completion of the tip cells before the basals ; and in this it differs 
from Neritina and Crepidula, where the tip arises shortly after division 
has occurred to form the four basal cells. In Trochus the tips are 
relatively late in appearing, as the basals have completed their cleavage 
before these cells arise. At the first cleavage of the basals another 
striking similarity to Umbrella is found, for in this Opisthobranch the 
cleavage is lseotropic, while in Crepidula and Neritina it is dexiotropic, 
thus breaking the law of alternating cleavages ; and likewise in Planorbis 
with reversed type the division is lseotropic and non-alternating with 
the preceding. Trochus shows an extremely marked lseotropic division 
of these cells, so much so, in fact, that the resulting cells lie almost 
transversely. In Fiona the anterior and posterior basals are distinctly 
lseotropic in origin and so regularly alternating, while considerable varia- 
tion is found in the lateral arms, a radial type often occurring with lc 12 , 
sometimes showing a decided dexiotropic direction of spindle. It 
would appear from this variation in the lateral arms that Fiona shows 
tendencies toward bilaterality in the first quartet at this time, and such 
a condition would be in harmony with the bilateral cleavages of the 
third quartet cells, 3c 1 and 3d 1 , occurring just previously. However, 
the radial symmetry of the cross as a whole appears not to be dis- 
turbed appreciably, so that though these variations may show either a 
tendency toward bilaterality or toward entire reversal in all quadrants, 
as is found in Neritina, Crepidula and Planorbis, this influence has not 
as yet become sufficiently marked to affect the radial symmetry of 
the upper pole of the egg to any appreciable degree. In discussing the 
lack of alternation of these cleavages in Crepidula as opposed to alter- 
nation in Umbrella, Conklin suggests "upon this difference the future 
recognizability of the cross in the last-named cases {Crepidula and 
Neritina) depends". In Umbrella the lseotropic division of the basals 
is much more marked than in Fiona, but even in the latter case Conk- 
lin's prediction is in part, at least, fulfilled, as the cross of Fiona, after 
a slightly older stage than thus far described, becomes so irregular that 


its component cells are neither among themselves distinguishable nor 
may they be definitely separated from the surrounding blastomeres. 
Of course, this is largely due to the multiplication of the trochoblasts 
and the similarity in size of most of the cells upon the upper surface of 
the egg, yet the Inotropic twist given to the basal elements at their 
initial cleavage is largely responsible for that irregularity of contour 
which so early marks the outlines of the cross. The peripheral ends 
of the arms of the cross of Fiona become strongly twisted to the left, 
and as the structure becomes older the ends tend to bend around in 
that direction to a marked degree, greatly confusing their component 
cells with those arising by multiplication of the trochoblasts. Up to 
the stage shown in fig. 53 the cross has, with the exception of a slight 
tendency toward variation in the first division of the basals, been 
radially symmetrical, but at the next cleavage of the basals the cell 
of this series in the posterior arm divides so that its daughter cells 
lie transverse to the longitudinal axis of this arm. In the anterior 
quadrant this division produces cells which lie radially, while in C 
quadrant the cleavage is Inotropic, in A dexiotropic. 

The first indication of transverse splitting of the arms is thus seen 
to occur in the basal cell of the posterior quadrant, In Crepidula the 
reverse is the case, the anterior and lateral arms alone increasing in 
width, while the posterior later elongates by radial cleavages. In Fiona 
all the arms become longitudinally split at a later period. The inner 
and outer rosettes have not yet arisen in Crepidula when the splitting 
begins in the cells, la-b-c 122 , while in Fiona they are present and the 
egg contains many more cells, the basal cells of the anterior and lateral 
arms having again divided in such a manner that these arms are length- 
ened before increase in breadth occurs. The same is true of Planorbis. 
The early splitting of the arms of the cross in Crepidula is probably in 
part due, as Holmes suggests, to the fact that, through pressure, they 
have become much wider and tend to divide in a direction opposite to 
this elongation. It might also be suggested that the extreme breadth 
of the cross of Crepidula and the early transverse division of its anterior 
and lateral arms may be correlated with the presence of a large amount 
of yolk which must be covered by the ectoblast, while in the posterior 
region the extensive multiplication of the elements of the second quar- 
tet obviates the necessary broadening of the arm which reaches in that 

The transverse cleavage of the anterior and lateral arms of the cros 
of Fiona occurs shortly after the initiation of a similar process in the 
posterior arm, but it has been found impossible to trace the lineage 


of all the cells accurately though, after lateral extension has occurred, 
the structure may be demarkated from the trochoblasts and underlying 
second quartet cells. In fig. 75 its structure and probably cell deri- 
vation may be seen. Holmes finds for Planorbis that the tip cells 
divide in a transverse direction first, while in Crepidula the middle 
cells are the first to cleave. The tips appear to divide last in Fiona. 
In the posterior arms after the first transverse division most of the cells 
divide obliquely across the arms, and in this way the arm becomes longer 
than the other three. While the cross is increasing in lateral extension 
the outer turret cells of all quadrants divide, so that the four groups 
each consist of four cells of equal size (fig. 75) lying in the angles 
formed by the arms of the cross. 

The apical pole of the egg at this period shows a slight depression 
in the region of the rosette series. It is but transient and disappears 
with the elongation of the gastrula. A similar depression has been 
observed in Neritina, Crepidula and Trochus. Whether the structure 
is normal in Fiona is yet doubtful. Robert insists that such is the case 
with Trochus. 

The entire formation of the cross of Trochus is peculiar. The basals 
have arisen and divided before the tips appear, and this division of 
the basals is so directly lseotropic as to be practically transverse. At 
the next cleavage these two cells form an oblong group of four in each 
arm. The tips which lie peripherally to these groups next divide, 
the cleavages of 2a 11 and 2c 11 being bilateral, the first of this nature to 
occur in the egg. 

From the cases cited above of the manner of formation of the 
ectoblastic cross of Mollusks, it will be seen that this characteristic 
structure shows great diversity of details throughout the group, 
though fundamental similarity is evident. Some of the probable 
causes of such variation are (1) varying amounts of yolk, leading 
to early lateral extension of the arms in those forms possessing 
yolk-ladened entomeres, and (2) differences in the manner and rate 
of development of the trochoblasts, correlated with the later structure 
and functional importance of the locomotor organ to which they 
largely give rise. The radial arrangement of blastomeres around the 
apical pole of the cleaving egg is primarily the result of successively 
alternating spiral cleavages, and a similar arrangement may be expected 
in eggs which exhibit this mode of division. A definitely marked cross 
does not always arise from such an arrangement of blastomeres, as, 
for example, in Polyclad cleavage, so that this but suffices as a partial 
explanation. Regarding the form of the crosses of Mollusks and 


Annelids Conklin says: "The cross and rosette series are the direct 
result of the position, size and shape of their constituent cells". The 
original position of cells resulting from regularly alternating spiral 
cleavages is a function of that mode of division. The shape of cells 
depends largely upon the relations which they bear to one another. 
Their size is not so easily explained, and upon this factor depends, to a 
large extent, the varying forms of crosses met with in different in- 
stances. If it be supposed that the original arrangement of the upper 
pole cells of Mollusk and Annelid eggs was radial in form, the modifi- 
cations which have arisen in the two groups may, in part at least, be 
referred directly to the size of the cells comprising that area. The 
importance and early development of the trochoblasts of Annelids 
has resulted in encroachment upon that area which in the segmenting 
eggs of these forms corresponds to the cross region of Mollusks. As a 
result the "intermediate" series of Annelids, corresponding to the 
molluscan cross cells, lack the prominence characteristic of the same 
cells in the latter group. Moreover, it is interesting to note that such 
a Mollusk as Ischnochiton, which in the development of its trocho- 
blasts and prototroch shows a condition intermediate between Mol- 
lusks and Annelids, also exhibits a cross winch is intermediate in 
character. Though the trochoblasts have been taken here as an ex- 
ample of the influence which variation in size or rate of division may 
have upon the primitive arrangement of blastomeres in the spirally 
cleaving egg, it is doubtless true that other cells may in like manner 
undergo modifications which will result in similar rearrangements. 

Thus it may be concluded that the group of cells constituting the 
cross owes its radial arrangement primarily to the form of cleavages 
by which it arose, but that the cross as a definitely marked structure 
is the result of variations in the size, shape and rate of division of the 
cells comprising or surrounding it, these variations leading, on the one 
hand, to the formation of the molluscan cross: on the other, to the 
annelid an. 

Second Quartet. 

While the egg is yet radially symmetrical and its blastomeres num- 
ber twenty-four, the original second quartet cell of each quadrant has 
divided in a dexiotropic direction into cells of equal size. After the 
mesentoblast has arisen, but before the basal cells of the cross are 
formed, all of the second quartet cells divide in a Isotropic direction, 
the upper four giving off the four tip cells (2a n -2d u ) toward the upper 
pole, while the lower four give origin to small cells resembling the 


tips in size, which are directed toward the vegetative pole (PI. XXIII, 
figs. 21, 22, 23, PI. XXIV, fig. 24). 

The second quartet at this time consists of four similar groups of 
cells, each group consisting of two large cells, 2a 12 -2d 12 and 2a 21 -2d 21 , 
tying together, with the smaller cells above and below. The two large 
cells in all four quadrants, 2a 12 -2d 12 , 2a 21 -2d 21 , next divide almost 
simultaneously. The direction of cleavage of the right upper cells 
(2a 12 -2d 12 ) is dexiotropic, and of the resulting cells the upper (2a 121 - 
2d 121 ) are slightly larger than the lower (2a 122 -2d 122 ), the divisions being 
identical in all four quadrants. Synchronously with these divisions 
cleavage spindles appear in the other large cells of the second quartet 
(2a 21 -2d 21 ). Of the resulting cells the lower are much the smaller. 
In direction the cleavages are probably all Isotropic and therefore 
non-alternating, though in C and D quadrants the spindles are almost 
meridional in position, and the cleavages horizontal. Figures 28, 29, 
30, 31 and 32 show these divisions in the different quadrants. 

The lack of alternation found in the above instance may be explained 
as the direct result of the relative sizes of the foregoing derivatives of 
the second quartet and the positions in which they lie. By an exami- 
nation of fig. 30 it will be seen that should the two large cells, 2c 12 and 
2c 21 , have divided in the same direction a diagonal row of cells would 
have been the result, with great pressure against one another and upon 
the cells in the first and third quartets at the ends of the row. Lack 
of alternation in direction of cleavage in one of the cells would relieve 
this pressure, and this is the actual condition found. Such an expla- 
nation appears to fit this individual case of non-alternation, but no 
generalization may be made, as in many other instances the cleavage 
of blastomeres appears to follow no rules of mutual pressure and can 
be explained on no grounds so simple. 

Division again occurs in this quartet at a stage of about eighty cells 
and great variation in time is marked in their occurrence. 

The following table shows the average sequence observed in the 
different quadrants, though any one egg may show marked variation 
from the tabulated result : 




























(or 22) 

The table should be read : In A quadrant 2a 121 cleaves first, 2a 211 second, 
2a m third and 2a 212 fourth. In B quadrant, etc. Cleavages in A 
quadrant are found in figs. 50, 58 and 63; in B, figs. 52 and 59; in C, 
figs. 44, 48, 54, 60 and 65; in D, figs. 47, 51 and 61. 

The divisions of 2a 121 -2d 121 are Isotropic in all quadrants, of 2a 211 -2d 211 
universally dexiotropic, of 2a 212 -2d 212 everywhere dexiotropic, while 
variation is found in the direction of cleavage in the cells 2a 122 -2d m . 
Of these latter a decidedly Isotropic direction is found in B quadrant, 
horizontal to dexiotropic in D, horizontal to Isotropic in A and ap- 
proximately horizontal in C. With regard to the size of the derivative 
cells, it may be said in a general way that variation is evident. More 
particularly considered the following conditions are found to prevail. 
The divisions of 2a 121 , 2c 121 , 2d 121 result in cells of equal size, while in 
the case of 2b 121 the upper cell 2b 1211 is much smaller than 2b 1212 ; 2a 211 . 
2b 211 , 2d 211 form upper small and lower larger parts, while 2c 211 divides 
equally; 2b 212 , 2c 212 , and 2d 212 show similar divisions into upper small 
and lower large cells, while 2a 212 remains so long undivided that its 
derivatives are uncertain ; 2a 122 -2d 122 divide equally. 

As a result of the foregoing cleavages the second quartet contains 
in all approximately forty cells. The irregularities which have char- 
acterized the preceding divisions are increased in number as cleavage 
continues, though until a much later period all four quadrants show 
relatively the same number of cells for this quartet. If figs. 67-70, 
representing the different sides of the same egg, be examined it will 
be seen that in A quadrant 2a 1212 has divided dexiotropically, while 
2a 2112 has divided horizontally; quadrant B shows no further multi- 
plication of elements ; in C quadrant, 2c l2U is in process of division, while 
2c 2m and 2c 2112 have both given off small cells toward the upper pole; 
D quadrant remains as before. 


At a stage in which there are six cells of the second quartet in each 
quadrant in Crepidula these groups very closely resemble the similar 
ones of Fiona. When there are four cells in each group in Crepidula 
the larger middle pair divide and, as in Fiona, one of them shows lack of 
alternation ; but in Crepidula the direction of the cleavage is slightly 
Isotropic in the right cell and dexiotropic in the left, while just the 
opposite is true of Fiona. Planorbis shows a group of second quartet 
tsells in each quadrant, which may be said in this sinistral form to be 
almost the mirrored image of the same cells of Fiona, though the tips 
and the corresponding cells at the lower pole are somewhat larger in 
Planorbis, which probably accounts for their earlier division in that 
form. The large second quartet cells of Trochus, as in Fiona, show 
lack of alternation in the left cells of the series (2a 21 -2d 21 ), while the 
right (2a 12 -2d 12 ) show regular alternation. The early cleavages in the 
second quartet of Tethys (Viguier, 1898) closely parallel those of 
the same series in Fiona. Viguier has mistaken the lower elements of 
this quartet, 2a 22 -2d 22 , for members of the fourth, as Robert has pointed 
out. Further note of the errors in this paper will not be taken here, 
since they have been so thoroughly discussed by Robert. Heymons 
(1893) for Umbrella shows the second quartet series up to a stage of 
six cells in each quadrant, and here also similar conditions are found. 
Carazzi (1900) figures the egg of A plysia, where each quadrant contains 
four second quartet cells, and here also is a marked similarity to the 
other forms considered. The second quartet of Fiona maintains a 
radial symmetry for a much longer period than Planorbis, this being 
the result of similar cleavages in all four quadrants for a much later 
period than in that Pulmonate. The same may be said of Umbrella 
and Crepidula, and, as Holmes suggests, this phenomenon is probably 
correlated with the earlier development and larger size of the head 
vesicle of Planorbis than of the corresponding structure of Crepidida, 
Umbrella or Fiona. 

The Third Quartet. 

Of the three quartets the third is the first to show evidences of 
bilateral divisions. When the egg has cleaved into twenty-four 
blastomeres this quartet has but one cell in each quadrant, and those 
cells do not divide until after the second cleavage of the second quartet. 
They then all divide in a Isotropic direction, but the resulting cells 
are not of the same size in the different quadrants. 3a and 3b produce 
cells of equal size, while 3c and 3d give rise to small cells in the direction 
of the vegetative pole with very large ones above, thus forming an 


additional landmark for distinguishing anterior from posterior quad- 
rants (PI. XXIV, fig. 25). The larger cells of the posterior quadrants, 
3c 1 and 3d 1 , divide next; the spindle in 3c 1 being dexiotropic and 
alternating, that of 3d 1 lseotropic and n on -alternating; and this lack 
of alternation in one of the large cells of the third quartet, taken in 
connection with the regular alternation of the similar cell on the oppo- 
site side of the posterior region of the egg, establishes the first bilat- 
eral cleavage (PI. XXV, figs. 31, 32,34). Both upper and lower cells 
of A and B quadrants are the next third quartet elements to divide, 
the direction in all cases being dexiotropic or in some instances nearly- 
meridional (figs. 37, 40, 41). The lower cells, 3a 2 and 3b 2 , always divide 
before the upper, 3a 1 and 3b 1 , and in all cases cleavage is equal, a group 
of four similar cells arising in each of the two anterior quadrants. 
In the posterior quadrants cleavage occurs next in 3d 12 . 3d 11 , 3c 12 and 
3c 11 . It will be remembered that when these cells were formed it was 
through a lseotropic and non-alternating division of 3d 1 and a dexio- 
tropic and alternating division of 3c 1 , thus producing a bilateral cleav- 
age of similar cells of opposite sides. Xow the cells 3c 11 and 3c 12 
again divide dexiotropically, thus showing lack of alternation, while 
3d 11 and 3d 12 again exhibit distinct lseotropic cleavage and a second 
failure to alternate. Thus arise in each posterior quadrant two very 
small cells, 3c 112 , 3c 122 and 3d 112 , 3d 122 , lying below the large ones, 3c 111 , 
3c 121 , 3d 111 and 3d 121 (PI. XXVI, figs/ 43, 44, 45, 47). After these 
cleavages about eighty blastomeres are present (figs. 67, etc.). When 
this number has increased to slightly over a hundred, 3a 21 , 3a 22 , 3b 21 
and 3b 22 , each gives off a small cell toward the vegetative pole by cleav- 
ages which appear horizontal (PI. XXVII, figs. 57, 59), and these divi- 
sions are followed by equal and probably horizontal cleavages in the 
posterior quadrants of the large cells, 3c 111 , 3d 111 and 3c 121 and 3d 121 ,, 
the former pair always dividing before the latter (figs. '61, 66), so that 
each posterior group contains seven cells, of which three are small 
and he nearest the blastopore, being bounded externally by four large 
cells, 3c 1111 , 1112 , 1211 , 1212 , and 3d 1111 , 1112 , 12u , 1212 respectively. 

The history of the third quartet of Fiona thus far given adds another 
to the number of Mollusks in which it has been found that bilateral 
cleavages first appear in the posterior quadrant, and more particularly 
in the cells of the third quartet. 

The initial divisions of these cells in Umbrella appear from Heymons' 
description to be nearly radial, but his figures show that in the case of 
3c and 3d cleavage is lseotropic. The lower products of these cleavages 
are all smaller than the upper, in which they parallel only the posterior 


quadrant cells of Fiona. Moreover, these cells, 3c 1 and 3d 1 , divide 
again before the anterior ones as in Fiona, and these cleavages are the 
first bilateral divisions described. It would appear from Heymons' 
figures that the two cells next the median plain lie higher than the 
outer, and this is the condition found in Fiona. If such be the case, 
these two forms stand in contradistinction to Crepidula, in which the 
median pair are the lower. The cells 3c 11 , 3d 11 are the protoblasts 
of Heymons' excretory cells, and it will be seen later that 3c 11 serves 
a similar purpose in Fiona. It is interesting to note that Conklin says 
of 3c 11 and 3d 11 that they are "large and clear" and "have the same 
characteristics in Crepidula" , though he does not know their fate. 
Heymons describes divisions at a later stage in the anterior quadrants, 
while in the posterior 3c 11 and 3c 12 , 3d 11 and 3d 12 give rise by horizontal 
divisions to small cells which lie next to 3c 2 and 3d 2 — these latter in 
exact correspondence with Fiona. 

Of this quartet Holmes says of Planorbis; "The first cleavage forms 
a transition from the spiral to the bilateral type, and subsequent 
cleavages show a bilateral character in a more marked degree. 
At nearly the same time the lower pair of cells in the two anterior 
quadrants and the upper pair of cells in the posterior quadrants divide 
in a nearly horizontal direction into equal moieties. Later the upper 
pair of cells in the anterior quadrants divide in the same direction as 
the lower pair. The lower pair of cells in the two posterior quad- 
rants remain undivided until a much later stage". These divisions 
closely follow those of Fiona, and the same may be said of subse- 
quent ones. 

In Aplysia (Carazzi) the two third quartet cells of each anterior 
quadrant divide into equal moieties, while in the posterior quadrants 
small cells are given off toward the vegetative pole ; the same is true 
•of Fiona. At the next divisions of 3c 1 and 3d 1 "si dividono con fusi 
transversali, cioe con divisione bilaterale," while 3a 1 and 3b 1 remain at 
rest. Viguier (1898) for Tethys describes the initial division of all 
the four quartet cells as "suivant des plans sensiblement radiaux", 
the resulting two cells in each quadrant being equal. Later cleavages 
of this quartet in Fiona will be considered under the discussion of 
gastrulation and secondary mesoderm formation. Bilaterality appears 
late in the cleavage of Trochus. The first divisions of this nature do 
not occur until the ninety-seven-cell stage, and are concerned with the 
cells 2c 11 and 2a 11 . This is the first violation of Sachs-Hertwig's law 
of alternatingly perpendicular cleavages. The cleavages of the third 
quartet are very tardy in this Prosobranch, for when there are as many 


as one hundred and fifty cells present this quartet consists of but four 
cells in each quadrant. 


With the beginning of gastrulation, marked differences appear in 
the cleavages of the quadrants and the radial symmetry of the egg as 
a whole gives place to a more and more distinct bilaterality. In 
the posterior region, particularly among the cells of the second quartet, 
great divisional activity and growth takes place; while the same series 
in A. C and B quadrants show relatively slight increase when compared 
with the derivatives of 2d. It has been impossible to follow the line- 
age, except in particular instances, from the time these cleavages 
begin, as most of the cells of the gastrula of Fiona are so similar in 
size and appearance and the number becomes so great that individual 
identification is limited to special cases. However, by continued 
observation of successively developing stages one becomes familiar 
with the cell groups which will later give rise to various organs and, 
aided by a few landmarks, may in most cases follow the organogeny 
with approximate if not absolute certainty. 

An examination of figs. 69 and 70 will show that 2b 1212 and 2b 2112 
have divided again, and shortly afterward cleavage occurs in a num- 
ber of other cells, 2b 22 , 2b 2111 , etc. The upper cells of the third quartet 
in the anterior quadrants lie at first well toward the upper surface, 
but as invagination proceeds these move around toward the lower side, 
while an increasing number of second quartet elements are found sepa- 
rating the first from the third quartet at the anterior as well as the 
posterior end of the gastrula. Meanwhile the second quartet cells 
in the median posterior region (derivatives of 2d) have multiplied very 
rapidly, and by causing increase in the surface area of the gastrula in 
this region have pushed the apical pole several degrees forward. Not 
only have the posterior second quartet cells increased in numbers but 
also in size, marking out at an early period the region from which the 
shell gland will develop. The second quartet groups which lie laterally 
below the ends of the lateral arms of the cross also grow in extent and 
numbers, this being more particularly true of those which abut upon 
the enlarging cells of the same series in D quadrant. 

The history of the third quartet has thus far been followed to a stage 
when its members in each anterior quadrant number six, of which 
four are large and two small cells, while in each posterior quadrant the 
group comprises seven cells, three of which are small and four large. 
By approximately horizontal cleavages of the upper cells in the two 


anterior quadrants four cells of equal size are formed in each quadrant, 
and as the blastopore continues to narrow these cells migrate as a 
group in each of the two anterior quadrants, approaching the blasto- 
pore and slipping over the cells 3b 211 and 3b 221 , 3a 211 and 3a 221 , which lie 
between them and the smaller cells of the same series (PI. XXIX, 
figs. 68, 69). During this period the third quartet blastomeres of the 
posterior quadrants remain as before. 

The blastopore thus becomes entirely surrounded by the second and 
third quartet elements, of which the third are much more numerous, 
having the small cells 2a 22 -2d 22 or their derivatives wedged in between 
them on the median and transverse line. The gastrula, taken as a 
whole, is much flattened dorso-ventrally and is at first shorter in its 
longitudinal than transverse axis. The blastopore assumes a slit-like 
form, its longitudinal axis corresponding to the future longitudinal 
axis of the embryo. 

The next important change to be observed is the origin of the 


As the cells 3a m , u2 , m , 122 and 3b 111 , u2 , m , 122 continue to move 
toward the blastopore, the cells which they are covering over 3a 211 , 
3a 221 and 3b 2U , 3b 221 , sink downward into the segmentation cavity. 
As this occurs they all four divide, giving rise externally and in the 
direction of the blastopore to four small cells, 3a 2112 , 3a 2212 and 3b 2112 , 
3b 2212 , while the larger daughter cells continue to retreat beneath 
the overgrowing ectoderm (fig. 74). These larger cells, 3a 2111 , 3a 2211 , 
3b 2lu and 3b 22U , are the source from which the secondary mesoderm 
is derived. They later divide, as may be seen in fig. 78, and begin at 
once to form two bands of several cells each, which lie in the antero- 
lateral region of the gastrula and later in the anterior head region of the 


Since the discovery by Lillie in 1S95 of mesoderm which arose from 
the ectoderm in the Lamellibranch Unto, various other cell-lineage 
workers have arrived at similar conclusions concerning other forms. 
As is well known, Lillie found that the larval musculature of the Glo- 
chidium arose from a cell of the second quartet, 2a, which in cleavage 
gives rise to a cell toward the segmentation cavity, the descendants of 
which are mesodermal in fate. Conklin's results, published in 1897, 
gave evidence that in the Gasteropod Crepidula ectodermal mesoderm 
arose in three quadrants, in this case also from the second quartet (2a, 
2b and 2c), but appearing much later then the "larval mesoblast" of 
Lillie, so late, in fact, that the exact cell origin could not be traced. 




In 1897 Wierzejski showed that in the sinistral Pulmonate Physa sec- 
ondary mesoblast arises from certain derivatives of the third quartet 
(3c and 3b), and similar conclusions were reached in the same year for 
Planorbis by Holmes, 3c and 3b here also giving rise to cells which sink 
into the segmentation cavity. 

, The formation of the secondary mesoderm in Fiona is strikingly 
similar to its manner of origin in Planorbis, as described by Holmes. 
The following diagram (text-figure 2), showing the cleavage history of 
the ectomesomeres of the two forms, indicates how close a comparison 
is possible. 


OB 00 

Fig. 2. — Diagrams showing the manner of formation of secondary mesoderm 
in (a) Planorbis (Holmes) and (6) Physa (Wierzejski) and Fiona. The cells 
3ontaining secondary mesoderm are stippled. 

It will be noted that four cells of each anterior quadrant are meso- 
dermal in Planorbis, while in Fiona only two have this fate, the 
smaller cells, 3a 2m , 2m , and 3b 2112 , 2212 , of Fiona remaining in the ecto- 
derm. For Physa Wierzejski came to similar conclusions, but here 
there is even closer correspondence, for the cells 3b 2112 , 2212 and 3c 2112 , 2212 
of Physa remain in the ectoderm exactly as they do in Fiona. Accord- 
ing to the nomenclature used by these two investigators secondary 
mesoblast arises from quadrant B and C, while in the dextrally cleaving 
egg of Fiona it comes from quadrant A and B. Holmes and Wierzejski 
have attempted to use the same sequence of lettering for sinistral 
forms as that commonly employed for the dextral, and have thus been 
led into error, Holmes particularly arguing for a non-homology of 
cells upon this score. When the dextral or clock-wise sequence is 
employed for a sinistral form this difference in designation necessarily 
results if the cell which is to give rise to the entomesoblast be labelled 
D. The more natural and logical method is to label the cells of a 
sinistral form in an anti-clock-wise sequence, as Cramptan (1894) has 


very wisely done for Physa. Robert (1903), in his excellent paper on 
the development of Trochus, which has just reached this laboratory, 
calls attention to the above and confirms opinions which had already 
been embodied in this paper. Animals which are sinistral, or reversed 
in their larval and adult stages, develop from eggs which are likewise 
reversed in their cleavage, and the designation of the blastomeres of 
the egg should coincide with the condition of the adult, if any homology 
of cells exists. The eggs of sinistral Gastropods have probably at an 
early stage in their ovarian development undergone complete cyto- 
plasmic and nuclear inversion, for only by such a process can the 
reversed condition of the larvse and adults be understood or the reversal 
of direction of the cleavage spindles be explained, and if such an inver- 
sion be postulated, corresponding reversal of sequence in nomencla- 
ture must ensue. 

Meissenheimer (1901) describes in Dreissensia a cell lying in the 
cleavage cavity just under the First Somatoblast derivatives, but 
which, he says, does not come from this group, though he is sure it is 
of ectodermal origin. It later divides and forms muscle fibers. Simi- 
lar conditions appear to be present in Cyclas (Zeigler, 1885). In the 
fresh-water Prosobranch Paludina teloblastic pole cells are not found. 
Scattered mesenchyme cells occur, and Tonniges (1896) states that these 
have been produced from cells which lie in front of the blastopore. 
If this be the case, the formation of mesoderm in Paludina is similar 
to that of the secondary mesoderm of other Mollusks. 

In Dinophilus (the cleavage of which is, from work now being done 
in this laboratory by Dr. J. A. Nelson, typically annelidan in character) 
Schimkewitsh (1895) appears to have recognized ecto-mesoblast, for 
he says: " Gleichzeitig (with the proliferation of Urmesodermzellen) 
aber findet auch eine Immigration der Ectodermzellen in der Vorder- 
theil des Embryos statt, und es wird durch diese Zellen eine Mesem- 
ehymanlage gebildet". 

In the Annelid Aricia, Wilson (1897) discovered mesoderm arising 
from the two posterior quadrants which could not be derived from the 
pole cells, and which he located as coming from "either the second or 
third quartet" (i.e., from c 3 and d 3 or from c 2 and c 3 ). These conclu- 
sions were strengthened by a preliminary account of Treadwell (1897) 
on the cell lineage of Podarke, in which he derives secondary mesoblast 
from the third quartet (3a, 3c and 3d), and these results are confirmed 
in a later and more elaborate paper (1901). The account of the meso- 
derm formation given by Eisig (1898) for Capitella differs widely from 
the results of most workers on annelidan and molluscan embryology. 


Here the definitive mesoblast is said to arise from 3c and 3d, which 
would be in correspondence with "Wilson's "ecto-mesoblast," while what 
Eisig considers ''larval" or "secondary" mesoblast comes from that 
portion of 4d which Wilson and Treadwell found in Nereis and Podarke 
to form part of the wall of the enteron. These results have, it seems 
justly, been called in question, though the careful investigation from 
which they spring certainly gives credence to their accuracy. Tread- 
well (1901) has called attention to certain figures (PI. XXXIX, fig. 42, 
to PI. XL, fig. 49) of Hatschek on Eupomatus, which show "scattered 
muscle cells in the upper hemisphere of the larva, which could hardly 
have come from the feebly developed mesoderm bands at the posterior 
end of the body", and suggests that they are of secondary origin; and 
he likewise calls attention to the figures of Drasche (1884) for Pomato- 
ceros which show similar conditions, though neither of these investiga- 
tors appears to have realized their significance. In a preliminary paper 
on the development of the mesoblast in Thalassema, Torrey (1902) 
derives ecto-mesoblast from all three quartets. "In all there are 
at least twenty primary cells of this character, but of them only ten, 
arising from the first and third quartets, develop into functional mesen- 
chyme, while at least ten degenerate and are finally absorbed by the 
entoblasts." The greater part of the functional ecto-mesoblast comes 
from three cells of the third quartet (3a, 3c and 3d) which correspond 
closely to those which produce secondary mesoblast in Podarke. All 
of the cells arising from the second quartet and which sink into the 
segmentation cavity are rudimentary and in the end entirely degen- 
erate, thus recalling Wilson's similar conclusions regarding the "rudi- 
mentary" cells of the definitive mesoblast of Aricia and Spio. At 
least six derivatives of the seven ecto-mesoblast cells which Torrey 
derives from the first quartet have a similar fate. 

The mesoderm of Platodes, Annelids and Mollusks has of late years 
been subject to much study, and various theories have been propounded 
regarding the significance of the manner of formation of the middle 
germ layer of these groups. Without entering into a prolonged dis- 
cussion with regard to this question, a few of the more general points 
may be mentioned. The results above tabulated and my own observa- 
tions lead to the conclusion — which is, of course, not here stated as new 
— that the primitive mesoderm of these groups is represented by that 
which arises from the ectoderm, and which is alone found in the Poly- 
clad (Wilson). The suggestion of Wilson that upon this hypothesis 
ecto-mesoblast might well be found arising from all three quartets of 
ectomeres has just been verified by the work of Torrey, and shows that 


in this respect Thalessema presents an ancestral condition similar to 
that of the Poly clad, though this does not necessarily imply close 
genetic relationship. Moreover a descending series may be formed 
both among Annelids and Mollusks of forms in which the presence of 
ecto-mesoblast gradually merges into conditions in which it has totally 
disappeared, showing that in these groups ectodermal formation of 
mesoderm is on the decline. The increasing number of cases reported 
in which ecto-mesoblast is larval in fate tend also to support this con- 
clusion, nor do the results of Meyer, showing that much of this building 
material is used for adult structures, offer a serious objection, since it 
is a well-known fact that nature is not prodigal of the living substance 
on which it works, and the secondary application of ancestrally obsolete 
material is a fact of almost universal occurrence. Nor can I see that 
the later origin of ecto-mesoblast necessarily indicates its late phylo- 
genetic appearance, as some have argued, since the early origin of 
ento-mesoblast, if associated with the future elongation of the animal, 
might well be supposed to be directly explained by the precocious 
segregation of this layer in those forms in which its development is 
so intimately connected with future growth and development. The 
early appearance and teloblastic growth of ento-mesoblast in the pos- 
terior region of Annelids and Mollusks has directly led to decrease of 
the radially appearing mesoblast. The Polyclad, which shows no 
endo-mesoblast, has failed to develop such a formation, though a 
tendency in that direction may be appearing, being marked by the 
bilateral division of one of the endodermal derivatives (Wilson). 
The fact that ecto-mesoblast as well as ento-mesoblast has been shown 
among Annelids to arise from the same quadrant (Aricia, Podarke, 
Thalassema) argues, it seems to me, conclusively for an entirely 
separate mode of origin of the two. 

Closure of the Blastopore. 

With the segregation of the secondary mesoblast changes appear in 
the form of the gastrula. Heretofore its shape has been broadly oval, 
the antero-posterior axis being the shortest, but at this period two 
regions of growth become manifest leading to marked change of form. 
The multiplication and growth of cells of the second quartet in the pos- 
terior region increase in activity, ever pushing forward the apical pole 
area, while at the same time the region just anterior to the apical pole 
is seen to be rising from the surrounding surface, forming a pointed 
projection, the summit of which lies at the anterior end of the forward 
arm of the cross (PI. XXX, figs. 78, 79). 


Synchronously with these changes the blastopore continues to de- 
crease in size, being narrowed by overgrowth of cells in that neighbor- 
hood. It will be seen by the examination of fig. 78 that the large 
cells of the third quartet in the anterior quadrants (3a 111 , m , m , 122 and 
3b m , 112 , m , 122 ) are all encroaching farther upon the smaller cells of 
the same series, which have been crowded beneath them at the edge 
of the blastopore. Posteriorly, derivatives of the third quartet have 
completely surrounded the blastopore by the division and migration 
backward of the small cells 3c 2 and 3d 2 , while more laterally the re- 
maining small cells of this quartet and their neighboring larger cells 
are crowding around the depression. The second quartet cells, 2a 22 
and 2c 22 , or their derivatives, yet lie in the lateral corners; but as 
closure of the blastopore proceeds they are crowded from this position 
by encroachment of the third quartet both from before and behind, 
which finally (fig. 79) join each other on the sides. In the anterior 
median plane, however, a cleft yet remains between the large third 
quartet cells, and after the inner of these large cells have divided, as 
shown in fig. 79, cells of the second quartet, represented by the deriva- 
tives of 2b 22 , still occupy the space between them and there bound the 
blastopore. Throughout this process the greatest extension of the third 
quartet is manifest in the area covered by the posterior third quartet 
groups, and this is doubtless connected with the disappearance from 
the ectoderm in the anterior groups of the secondary mesoblast. The 
blastopore closes from behind forward, to which process the larger 
number of third quartet cells in the ectoderm of the posterior region 

The posterior surface of the gastrula is now covered by large cells 
of the third quartet, and in the median region by second quartet 
elements. On the right posterior surface (left when seen from ventral 
surface, fig. 79) may be seen one very large cell. Ex. (3c 1111 ), which will 
later become the principal excretory cell of the larva. The region 
anterior to the blastopore has been formed from the second quartet 
cells of B quadrant which have been pushed backward by posterior 
and apical growth, space being left for them through the shifting of 
the large cells of the third quartet already described. The second 
quartet cells of B quadrant have shown comparatively little division 
or growth, and thus appear to occupy a relatively smaller space than 

The blastopore of Crepidula (Conklin) is surrounded by second and 
third quartet cells, all quadrants contributing. The same is true for 
Ischnochiton (Heath). In Trochus (Robert) third quartet cells are 


mainly concerned in the closure of the blastopore, though the deriva- 
tives of 2a 22 -2d 22 also bound the narrowing opening. Planorbis 
(Holmes) shows a very similar condition, with the exception that 2d 22 
is crowded out. In Fiona all second quartet cells but a few at"[the 
anterior edge of the blastopore are excluded before the opening closes. 

The Velum. 

In its earlier stages the velum of Fiona is so ill-defined on the upper 
surface of the developing larva that its study has proved most diffi- 
cult, and though more time has been spent upon this region than any 
other portion of the developing organism the results have not been 
as satisfactory as could be wished. living material would have been 
of great value, and the lack of it has been a source of much regret. 
After the breaking up of the cross the whole external surface of the 
gastrula, and particularly the anterior end, is characterized by cells of 
small and nearly equal size, among which there appear scarcely any 
cells whose size would give them prominence, or cell rows or distinctly 
marked groups. 

In the last stage described under the discussion of the develop- 
ment of the first quartet the area covered by this series of micromeres 
represents nearly the whole upper surface of the flattened gastrula (fig. 
75). The four arms of the cross are split transversely, while in the 
angles between them lie the four groups of turret cells, each group 
consisting of four cells of equal size. In axial relation the anterior and 
posterior arms correspond to the direction of the median plane, while 
the lateral are respectively right and left. The whole first quartet 
area is completely surrounded and separated from the third bj^ deriva- 
tives of the second. By an increased growth of D quadrant of this 
series the apical pole and its surrounding area is moved forward in 
the direction of the blastopore, while at the same time growth of first 
and second quartet elements in the neighborhood of the tip of the ante- 
rior arm of the cross causes that region to become raised, until some- 
what later the pointed anterior end so characteristic of many Opistho- 
branch larvae is produced (figs. 78, 79, 96). The visible cause of the 
evagination of the ectoderm at this point may be found in the direc- 
tions taken by spindles of the dividing cells which produce it, as in 
most cases they are radially or diagonally directed toward the point 
of greatest elevation. At this time the archenteron is roughly trian- 
gular in outline, the anterior point of the triangle being marked by 


the large cell 4b 2 , which remains for a long time in this position and is 
closely pressed up into this anterior cone. It may thus be possible that 
the pointed anterior end of the larva is caused by the shape of the 
enteron, upon which the outer layer is moulded. 

At first the terminal point of elevation corresponds in position to 
the tip of the anterior arm, and is thus formed by derivatives of 2b 11 
and neighboring cells. At a somewhat later period the continued 
growth of the shell gland area pushes the whole apical region forward, 
so that eventually (figs. 95, 98, 100) this point is carried farther down- 
ward on the anterior surface. At the same time continued growth 
has increased the extent of the whole apical region, so that the anterior 
end becomes more rounded than pointed, and finally (figs. 101, 102), 
when the veliger stage is just being approached, a broad rounded con- 
tour characterizes the anterior as well as the posterior end of the larva. 
It is while these changes are taking place that the first evidence of a 
distinct velar area appears. Early in this period of forward movement 
the anterior trochoblasts may be seen to the right and left of the ante- 
rior end of the forward arm, being distinguished from the derivatives 
of the second quartet by their smaller size and compact arrangement. 
They thus, with the tip cell and two other cells behind them (probably 
lb 1221 , lb 1222 , derived by transverse splitting of the middle cell), form 
an irregular row across the anterior edge of the first quartet area 
(fig. 76). Laterally the posterior ends of this semicircle are joined by 
cells in the region of the tips of the lateral arms and thus meet the 
posterior trochoblast groups. These latter have grown larger than 
their corresponding cells in the anterior quadrants, and so are almost 
indistinguishable from second quartet elements which lie beneath 
them. On this account it soon becomes impossible to separate them 
from these cells, and so at a later period, when the velum in this region 
becomes marked, I am unable to state how much of it is derived from 
the trochoblasts, though the little evidence at hand indicates that they 
form the largest portion of it. With change of axis the anterior end of 
the velum is carried forward (PL XXXVII, figs. 95, 98), and the forward 
end comes upon a level with the antero- ventral surface. A lateral view 
(fig. 98) shows an irregular row of nuclei (cell outlines are usually in- 
distinct) running downward and backward from the anterior median 
point, and becoming lost as it continues posteriorly. This row, which 
has arisen from the anterior trochoblasts, derivatives of the middle 
and tip cells of the anterior arm and probably tip cell derivatives of 
the lateral arms, will be designated V 1 . Below this band of cells 
another irregular row may be distinguished composed entirely of second 


quartet cells which have lain nearest the first quartet area, and this 
row, the first appearance of which is indicated in figs. 97 and 98, will 
be designated V 2 , since it corresponds in general to the same cells in 
Crepidula which are designated by that term. Unfortunately the 
cells in this region have for some time presented no distinguishing 
marks, without which exact derivation is precluded by their number, 
but from their positions these lower cells probably correspond to deriva- 
tives of 2b 121 , 122 , 2n in the anterior group, and similar cells in the 
lateral. At a later period (fig. 101) these rows tend to unite to form 
an irregular line several cells in breadth, distinguishable only by their 
nuclei. As the stomodseal invagination progresses the velar rows 
are drawn forward and downward in that direction, and by the growth 
of the head vesicle they are also pushed downward laterally. It is 
probable that elements of the second quartet which lie still lower than 
those already mentioned become involved in the preoral velar area, 
either functioning directly as ciliated velar cells or taking part in the 
development of the underlying region of the expanding velar ridge. 
At the period represented in fig. 103, two irregular rows of nuclei 
may be observed in the anterior cephalic region above the stomodseum, 
and these correspond in origin to the rows V 1 and V 2 above mentioned . 
The postoral velar area is but faintly demarkated in the preparations 
studied and crosses the ventral region just behind the stomodseum. 
The cells comprising it are doubtless, in the median region, derived 
from the third quartet, to which are added second quartet elements 
more laterally where the postoral velum joins the preoral. 

A portion of the velum does not in Fiona curve sharply toward the 
apical pole, as in the case of Crepidula, where an anterior branch is 
formed, but the whole extends backward around the head vesicle, so 
that this part corresponds in position to the posterior branch of Crepi- 
dula. This difference will be evident if a comparison is made between 
figs. 78 and 82 of Crepidula and fig. 108 of Fiona. In the latter in- 
stance it will be seen that the apical pole lies far forward from the pos- 
terior ends of the velar edge, while in Crepidula the anterior branch 
curves inward toward the apex, while the posterior branch continues 
backward around the whole head vesicle, as does the entire velum of 

In Crepidula Conklin (Supplementary Note, p. 204) finds that the 
median anterior portion of the first velar row (V 1 ) probably arises 
from the divided tip cells of the anterior arm, while laterally this row 
is continued by the trochoblasts and cells at the ends of the lateral 
arms. The second row in its mid-ventral region is probably "derived 


from the cell identified provisionally as 2b 22 , which lies just beyond 
the median cells of the first row", and he adds, "I have not been able 
to determine whether any part of the second velar row arises by sub- 
division of cells of the first; if not this row may include a few of the 
third quartet (3a lu and 3b 111 , fig. 56) at the points opposite the anterior 
turrets". It also seems probable (Supplementary Note, page 204) that 
the cells 2b 12211 , 2b 12212 lie outside the first velar row. Fig. 79 shows 
two large cells between the first and second velar rows, and they appear 
to represent the major portion of these cells. Smaller derivatives 
from them may join 2b 22 in forming the median part of the second velar 
row (V 2 ). Conklin thus finds that the preoral velum arises from " a few 
cells of the first quartet, many of the second and possibly a few of the 
third". I do not believe that the third quartet becomes involved 
in the preoral portion of the velum of Fiona, though doubtless cells 
from this series are closely connected with it in the stomodseal region 
and help in the formation of the postoral velum. It will be remembered 
that in Crepidula secondary mesoblast is derived from the second 
quartet, while in Fiona it is furnished by the anterior groups of the 
third, and in this process the large cells of this series, which have hith- 
erto lain well up on the sides of the gastrula, migrate over the under- 
lying mesoblastic elements and thus become far removed from the 
region where the velum first appears. The formation of secondary 
mesoderm in the most anterior second quartet group of Crepidula 
has doubtless the same effect of lessening the external area of the 
quartet in that region, while the neighboring third quartet cells would 
lie relatively higher in this form than in Fiona. So when the second 
velar row forms in Crepidula it will lie relatively lower in the second 
quartet group (2b 22 ) and more probably involve third quartet cells, 
as Conklin states it probably does. 

Regarding the lineage of the velum of Planorbis, Holmes says that 
"the tip cell (of the anterior arm) divides as far as I can deter- 
mine, but once, and the two daughter cells become pushed apart by 
the cell lb 1211 , which forms the median cell of the upper row. These 
cells extend to the anterior trochoblasts on either side, but in later 
stages they may sometimes be separated from them by cells which 
wedge in from below". The anterior trochoblasts follow these cells 
posteriorly, but Holmes states that the tip cells of the lateral arms "do 
not form a part of the prototroch but enter into the formation of the 
head vesicle". In this Planorbis differs from Fiona. Blochmann 
states that the right and left tip cells enter the velum of Neritina. 
The lower cells in the prototroch Holmes derives from the second 


quartet, though he adds that at a later period cells are joined to the 
prototroch from below, the lineage of which is obscure. 

In Ischnochiton, the larva of which is, in its velar aspects, remarkably 
like the trochophore of Annelids, Heath finds that the prototroch is 
composed of trochoblasts, of "accessory trochoblasts" (derived from 
the original basal cells of the molluscan or intermediate girdle cells 
of the annelid an cross) of the tip cells in the anterior and lateral arms, 
while in the posterior arm the tip cells go into the ventral plate, the 
gap in the trochal ring being there bridged by derivatives of the median 
cell of that arm of the cross. Thus in this annelid-like form of larva 
none but derivatives of 2a 11 , 2b 11 and 2c 11 from the second quartet 
form the trochal ring. 

The prototroch of Trochus (Robert) is composed of twenty-five 
cells, sixteen of which comprise the trochoblasts, six represent the 
divided tip cells of A, B and C quadrants, while the other three are the 
cells 2a, b, c 12111 . A very exact and close comparison may here be 
made with the prototroch of the Annelids Amphitrite, Arenicola and 
Clymenella, particularly with the former, for, as Robert says, "Vingt- 
deux ont indetiquement la meme origine et la meme disposition que 
celles de Amphitrite; le trois autres (2a, b and c 12U1 ) sont des derives 
des cellules correspond antes de la meme AnnelideP 

Among Annelids Wilson has found that the prototroch of Nereis 
arises entirely from twelve of the sixteen primary trochoblasts, there 
being no contribution from the second quartet. All sixteen of the 
primary trochoblasts enter the prototroch of Amphitrite and Clymenella 
(Mead), as is also the case with Arenicola (Child) and Podarke (Tread- 
well). Regarding the close resemblance between the trochophore of 
Ischnochiton and those of the Annelids, Heath says: "The origin, 
development and fate of these cells (primary trochoblasts) is pre- 
cisely similar to the primary trochoblasts in Ischnochiton. The second 
quartet in Amphitrite, Clymenella and Arenicola furnishes three cells 
in each quadrant except the posterior, which enter the prototroch. 
Two of the three are homologues of the divided tip in Ischnochiton, 
while the third corresponds to a post-trochal cell". 

If now we compare the derivation and ultimate structure of the 
annelidan prototroch with the typical molluscan velum some inter- 
esting causal relations appear. At the time of its functional activity 
the prototroch of Annelids is apparently a radially symmetrical struc- 
ture. Among the Mollusks we find, as a rule, a velum strongly devel- 
oped anteriorly, with a considerable area of weakly ciliated ectoderm 
between the ends of its posterior arms. There are numerous excep- 


tions to this typical molluscan velum, Ischnoehiton and Trochus for 
examples, in which the trochal ring is as complete as among the Anne- 
lids. Returning now to the developmental history of the two groups 
certain variations are found which, when viewed in the light of func- 
tional larval structure, appear as a natural result of the divergent 
forms of the larvae, these variations having been precociously thrown 
backward upon the cleaving cells of the ovum. In Amphitritc, Areni- 
cola and Clymenella among the Annelids, and Ischnoehiton and Trochus 
representing the more primitive Mollusks, all the primary trochoblasts 
(la 211 , 212 , 221 , 222 , etc.) in all quadrants go into the prototroch, while in 
Nereis the same occurs with the exception of four, which may for all 
four quadrants be designated la 221 ; these are not functional in this 
manner, but are pushed inward and form part of the cephalic vesicle. 
In Crepidula only the anterior trochoblasts help form the preoral velum 
(la 22 , la 21 , lb 22 , lb 21 ), and the same is true of Planorbis and possibly 
also of Fiona. Accessory trochoblasts (la 1221 , la 1222 , etc.) form a part 
of the prototroch of Ischnoehiton in all quadrants, while in Podarke the 
cells la 1222 , lb 1222 , lc 1222 , corresponding to three of the above series, aid 
in the formation of the prototroch (" secondary trochoblasts" of Tread- 
well). In Planorbis Holmes finds that the cell lb 1211 is the "anterior 
median" cell of the prototroch, but does not find similar conditions in 
any other quadrants. None of these elements which are, of course, 
derivatives of the annelidan outer intermediate or molluscan middle 
cells (with the exception of lb 1211 of Planorbis, which comes from the 
inner basal) are found in the antero-lateral portion of the prototroch 
of Amphitritc, Arenicola, Clymenella and Nereis. In all the above 
forms except Nereis elements from the second quartet are also added 
to the prototroch, and these may be designated with Treadwell "ter- 
tiary trochoblasts". In Amphitritc, Arenicola and Clymenella the 
prototroch is increased in A, B and C quadrants by the cells 2a 111 , 
2a 112 , 2a 121 , etc. In Podarke 2a 112 and 2a 121 in A quadrant, and similar 
cells in B and C, function in like manner, while Ischnoehiton shows the 
same, for 2a 111 , 2a 112 , etc., enter the prototroch from the anterior and 
lateral quadrants ("secondary trochoblasts" of Heath). Of Hy- 
droides Treadwell says: "Cells are added from the lower hemisphere". 
For the prototroch of Trochus Robert derives the three cells from the 
second quartet in A, B and C quadrants (2a 111 , 2a 112 . 2a 1 -' 111 , etc.). Com- 
ing to those Mollusks which possess a typical veliger, more cells are 
found to be contributed by the second quartet, particularly in the 
anterior quadrants. In Crepidula the tip cells of the anterior and 
lateral arms go into the first velar row, while below numerous cells are 


added, so that the second row contains " probably a few cells of the first, 
many of the second and possibly a few of the third quartet". The 
velum of Planorbis is rudimentary in structure but shows the same gen- 
eral type of development as Crepidula, and here in like manner second 
quartet cells are added. The tip cells of the lateral arms, according 
to Holmes, do not enter the prototroch, but cells of the same series 
below them function in this manner. In the anterior region both tip 
cells and those lying beneath them from the second quartet enter into 
the prototroch. 

From this short comparison of the lineage of the trochal area in 
Annelids and Mollusks, it will be seen that as in the functional larval 
form the typical molluscan velum shows greater anterior development 
than the prototroch of Annelids, so also cells taken from the segmented 
egg to complete the velum in this region exceed in number those des- 
tined to form a similar area of the annelidan trochophore. To do this 
the second quartet has become greatly encroached upon in furnishing 
necessary building material for this structure in those Mollusks whose 
larvae show strong anterior velar development, and in Crepidula the 
third quartet also possibly becomes involved. It is natural to conclude, 
as indeed the facts show, that those Mollusks which in the structure of 
their larval prototrochs show great similarity to the homologous struc- 
ture of the Annelid trochophore, will exhibit a similar lineage of the 
cells constituting the larval organs compared — examples, Ischnochiton 
and Trochus. 

Later Velar Development. — With continued invagination of the 
stomodaeum and constriction of the foot, the velar area, which has 
-thus far been marked only by an irregular double row of cells 
extending around the anterior half of the head vesicle and losing 
itself in the posterior portion of that larval organ, becomes more 
prominent and takes on the bilobed outline so characteristic of 
the anterior end of veliger larvae. At first the velar lobes are 
merely rounded swellings gradually rising from the upper sides 
of the head vesicle and curving around, downward and inward 
toward the stomodaeum (fig. 105). The cells in this region do not 
as yet exhibit that differentiation which later marks the promi- 
nent ciliated margin from the underlying region. But as the lobes 
begin to constrict beneath and become more prominent (fig. 106), those 
cells which lie on their most peripheral surface show marked increase 
in size, and the ciliation which hitherto has been uniform and weakly 
developed becomes more prominent in these cells. They may now 


ridgf e an7lhn, in f I I TV" *" TOUnded ^ of th » "P«»ding 
Z,V 4 g a * firS * thlS Series of celb is indistinctly marked 

elements. j?igs. 106, 107 and 108 (PI YYYv. «.i. • 

stages i„ the evocation of these! f he^iyl lu^^TZ 
velar edge, and sections, as figs. 9! and 92 (PI. ixxil) t pirti cu IaT 
show the great increase in size which now marks them PartlCU ' ar ' 
Loincidently occurs the expansion of the velar lobes to form the 

atThe LT it°bel e ' ar T ^ *~ tad » the ^7^ 

at tne time it becomes free-swimmino- As thp t»io, - i 

becomes deep Iy notched be.ow wher^hf ,l7 ~ p 2e1^ 

ZZZ : theT the ,T th ' ^ thlS gr ° Wth ta "»*" andTrllht 
vlwTof the , S , 6 aS WelL FigS - 109 and 1W - ^ and dorsal 
views of the same vehger, show the condition of development of the 

area that the former apical point (animal pole) lies. 

Head Vesicle. 

lying within the trochoblasts and ends of the arms of the cross « n rf 
its greatest extent is covered by cells which lie posterior to the lateral 

d velopedCT' t'/t; SUCh " 1S foUnd * C « i-o hem 
midtiXd' t ° » t ° UbtIeSS * he SamC Cdls are I""* they have 
Z IfTr ? f ° greater extent than in CrepUMa or PtoLte 


pies the point of greatest anterior extension, while the tip region of 
the anterior arm through which the velum runs lies ventral to the apex 
in the direction of the blastopore (figs. 95, 98). At the same time the 
head end becomes rounded by increased growth of the cephalic area. 
The four original apical cells, as shown in figs. 75 and 76, divide soon 
after and again at a stage represented by fig. 95, so that this region, 
which in Crepidula is in the fully developed veliger still marked by four 
apicals (la 1111 , etc.), here comes to consist of at least twelve very small 
cells, among which no regularity of arrangement is sufficiently marked 
to be of value in orientation. These cells are extremely difficult 
to distinguish from numerous other cells of like form and structure 
which cover the anterior surface of the head vesicle. The apical group 
continues its forward migration in relation to the larva as a whole and, 
as it appears, pushes aside some of the cells which have arisen from 
divisions of the inner and outer basals of the anterior arm, for at a 
later period (fig. 108) the apical group lies close against the first velar 
row. Either such a shifting occurs or the basals become involved in 
the development of the velum. In fig. 108 a row of cells may be 
distinctly observed in which the nuclei are particularly large, extending 
laterally from the apical point. My first thought on seeing them was 
that they were a part of the velum, but after definitely locating the 
position of the apex and following the later history of the velum, it is 
clearly seen that this row never enters into the latter structure, but 
represents in its cell-lineage derivatives of cells of the lateral arms of 
the cross. No ciliation has been discovered in the apical area, and such 
structures are certainly not strongly marked, though without examin- 
ing living material a denial of the possible presence of such structures 
would scarcely be conclusive. 

Nerve and Sense Organs. 

Cerebral Ganglia. — The cerebral ganglia arise at a stage about 
corresponding to fig. 105, though they do not become well marked 
until somewhat later (fig. 108). During this period cells may be 
seen proliferating inward from the ectoderm of the head vesicle 
in the two regions which lie lateral from the apical area. A row 
of cells with large nuclei are at this time plainly visible running 
laterally from the apex, and it is along the anterior side of these 
cells that the ganglia first arise. This row has been identified as 
coming from the lateral arms of the cross, and cells lying between it 
and the anterior portion of the first velar row are from the same source. 


Later many of these large cells also divide and go into the ganglia. 
Thus it will be seen that the two cerebral ganglia arise from elements of 
the two lateral arms, the anterior rosettes, and probably also from some 
cells of the anterior arm which have been pushed laterally by the 
advance of the apex and lie in the region where the ganglia develop. 
The tip cells of the lateral arms certainly do not take part in the forma- 
tion of the ganglia, as they lie too far laterally and probably go into 
the velum. Where no large cells, the definite lineage of which is known, 
are left as landmarks, it is obviously impossible to give absolute deri- 
vatives for the ganglionic rudiments. Comparing, however, the above 
approximate derivation with other Mollusks which have been studied 
in this connection similarities are evident. In Crepidula the ganglia 
"very probably arise from the lateral extensions of the anterior arms". 
Holmes has been able to state very definitely the manner of origin of 
these ganglia in Planorbis, as here they are surrounded by conspicuous 
cells. He says: "The tip cells of the lateral arms and the cells lying 
immediately above them do not enter into the formation of these 
masses; with the exception of these, two cells in each arm, all the cells 
in the lateral arms of the cross, the cells of the anterior.arm, except the 
tip and basal cell, and the central region of the cross, except the four 
apicals, and the two cells lying in front of them, enter into the forma- 
tion of these rudiments". 

Otocysts and Pedal Ganglia. — The otocysts appear at a consider- 
ably earlier period than the ganglia which innervate them or the 
cerebral ganglia. They are first seen as slight invaginations on 
the sides of the foot slightly below the stomodaeal invagination, 
and at a stage shown in figs. 103 and 104 have developed to 
deep pits, the openings of which have become much constricted. As 
these constrictions narrow, the two otic vesicles arise and are con- 
nected with the external ectoderm by strands of cells which re- 
sulted from the constriction of the outer portion of the invaginations. 
Somewhat later the pedal ganglia are seen slightly external to the 
otocysts in position. These ganglia arise in part from the strands 
which connected the otocysts with the ectoderm, and in part from other 
cells proliferated from the ectoderm in the same region. At first the 
cerebral ganglia are not connected with each other by a commissure 
nor with the pedal ganglia, but later cells grow out and meeting con- 
nect the cerebral ganglia together, while between cerebral and pedal 
ganglia like connectives arise, probably both ganglia contributing 
cells to their formation. These connectives are very large (fig. 94), 


and the whole cephalic nervous system is much concentrated. Behind 
the pedal ganglia and somewhat higher dorsally may be distinguished, 
particularly in older larvae, the rudiments of the pleural ganglia, which 
also appear to have arisen by delamination of the ectoderm and lie in 
close association with cerebral and pedal ganglia. A very heavy 
commissural strand connects the two pedal ganglia, and the whole 
nervous system of the larva foreshadows in its compact structure the 
adult condition, individual ganglia being difficult to distinguish. Figs. 
92 and 94 show sections through this region at a somewhat later 
period than figs. 88 and 89. Eyes have not developed to a functional 
condition in the oldest larvae observed. Sections of these show pig- 
ment granules within cells lying close to the cerebral ganglia, and in 
some cases these cells lie around a slight invagination of the ectoderm — 
the first evidence of optic organs. 

Excretory Organs. 

The large excretory cell which lies on the right side of the larva and 
forms the chief member of a group of similar greatly vacuolated cells 
lying in that region arises from the third quartet in the C quadrant, 
and from its large size and conspicuous appearance its complete history 
is known. Returning to a segmentation stage, in which the egg con- 
tains about one hundred and tw T enty cells (fig. 70), it will be seen that 
the third quartet group in C quadrant contains seven cells. Divisions 
next occur in the three large cells, 3c 1212 , 3c 1112 and 3c 1211 (fig. 77). The 
cell 3c 1111 does not divide with these, nor does it ever again divide, but 
continues its growth, soon becoming the largest element in the ecto- 
derm. As gastrulation proceeds this large cell, 3c 1111 (Ex.), the origin 
of which is thus established, appears at the right of the elongating 
gastrula (left of figs. 78,, 79) and with the closure of the blastopore lies 
midway between dorsal and ventral surfaces, as shown in figs. 98 and 
99. It has become much larger, when compared with its neighboring 
cells, both from lack of division and by actual growth. As the veliger 
takes form this cell becomes] yet more marked (fig. 102), and when 
the shell gland has become prominent (fig. 104) it is seen lying in a slight 
depression surrounded by small cells which are in an active state of 
division. As the foot arises and the cephalic end of the veliger is 
differentiated from the body, the large excretory cells move upward 
along the body just posterior to the pedal groove, on the right side, 
this change of position being a natural sequence of the general torsion 
of that region (figs. 105, 106). The intestine has also become well 
developed by this time as a solid strand of cells connecting the pos- 


terior end of the enteric cavity with the ectoderm, and this latter point 
of contact is just below the large excretory cell. Fig. 88 shows a sec- 
tion through this region, showing the excretory cell to be much vacuo- 
lated and to lie for the most part below the ectoderm. At a consider- 
ably later stage (figs. 109, 110) its position and structure are shown 
just before the veliger escapes from the egg capsule. A large nucleus, 
which usually contains several small nucleoli and having the general 
appearance of nuclei in cells which have for a long time remained 
undivided, lies at the lower end of the cell. The cytoplasm is greatly 
vacuolated and at its peripheral end, where it meets the exterior, is 
seen a deep pit with constricted mouth. This appears to function 
as an intra-cellular duct, for it comes into connection at its inner end 
with the large vacuoles which fill the cell. Just above and anterior 
to the large cell is a group of smaller ones which contain darkly stained 
nuclei and pigment granules. One of these, the largest, also contains 
vacuoles and lies nearest the cell 3e im . In somewhat older larvae one 
or two of these smaller cells, which lie close to 3c 1111 , have increased 
much in size, become greatly vacuolated and appear to function as 
their larger neighboring cell. These smaller accessory excretory cells 
are also doubtless of ectodermal origin and. since they lie between the 
principal one and the blastopore, are doubtless derived from the same 

In addition to the excretory cells above described others of a similar 
nature are found in the larva of Fiona. Sections (figs. 90, 91) of fairly 
well-developed veligers show two cells (Nph) nearly symmetrically 
placed on the two sides of the body just behind the constriction sepa- 
rating head from body region. These cells contain large nuclei and 
their protoplasm is clear and greatly vacuolated. In a slightly older 
stage (the oldest larvae examined) yellowish-brown granules are very 
evident, lying in the meshwork of the vacuolated cytoplasm. The 
cell on the left side (fig. 91) lies just to the side of and slightly higher 
than the otocyst of that side, being closely associated with its ganglia, 
while the one on the right side (fig. 90) lies higher and is in close prox- 
imity to the smallest cells of the large excretory organ of that side. It 
may be distinguished from the cells of this organ by its clear cytoplasm 
and the color of the granules lying in it. In later stages another cell 
of similar nature may be seen beside the one on the right side, but 
only one has been observed on the left. The origin of these cells is not 
known. In earlier stages cells of slightly smaller size lie in the regions 
which they later occupy, but cannot be distinguished in structure 
from neighboring mesodermal elements. However they lie close to 


the ectoderm and may have come from that source. The later fate 
of these cells is unknown, but as they are increasing in size they prob- 
ably function as imp* irtant larval organs. They will here be designated 
"nephrocysts," for they correspond to cells of similar position and 
structure described by Trinchese (1881) for the larva of Ercolania 
and other Nudibranchs, by whom an excretory function was ascribed 
them. Older and living material is desirable before making definite 
statements regarding the nature and function of these apparently 
similar larval organs of Fiona. 

Numerous investigators have seen and described with various inter- 
pretations the excretory organs of larval Opisthobranchs. As early as 
1839 Loven observed the anal kidney in Xudibranch larvse, but did 
not recognize its function, though indicating that it was probably an 
undeveloped sexual organ. Likewise Sars (1840) described a similar 
structure in the veliger of Tritonia, which, together with the large endo- 
dermal cell which lies near it, he associated in common function with 
the liver lying on the opposite side of the enteron. In Molis like 
structures were found. Later (1845) he distinguished the vacuolated 
excretory cell and its neighboring pigmented cells, classing the whole 
as a reproductive anlage. Reid (1846) observed a like structure in a 
number of Nudibranchs (Doris, Pohjcera, Doto, etc.), considering it 
to be probably the heart from contractions which he saw it undergo. 
In Vogt's very thorough paper on Actceon, appearing in 1846, the excre- 
tory organ is somewhat neglected, though his figures indicate its pres- 
ence. Nordman in the same year described this organ in Tergipes, 
and referred a reproductive significance to it. Schneider (1858) also 
found it in PhyUirhoe, but assigned no definite function. Langerhans 
(1873), having observed in the living larva? of Doris and Acera cells in 
the anal region which contained concretions, and from which drops 
were extruded considered the organ to be of an excretory nature. 
In 1875 Lankester found similar conditions in Aplysia, and con- 
sidered the organ to have arisen either from intestinal cells near 
which it lay or from the ectoderm. 

Trinchese (1SS1) described an "anal gland for Ercolania which is 
strongly pigmented and lies on the right side of the body". This he 
believed arises from three or four mesodermal cells which acquire 
pigment and by their division form the organ in question. The same 
was found in Amphorina , Bcrgia and Doto, in the last case being paired. 
In addition to the anal excretory organ, Trinchese also found in the 
above forms two "rini primive" in the dorsal region under the ecto- 
derm, one right and the other left. These he described as vesicular, 


spherical or ovoid bodies having a lower part full of transparent 
liquid, in which lay concretions of a yellowish color. These he denomi- 
nated "nephrocisti" (nephrocysts) and ascribed to them a mesodermal 
origin, since they have no connection with the exterior. Haddon 
(18S2) found a mass of cells on the right side of Janthcria and Philine, 
near the anus in Elysia on the left side, and in Pleurobranchidium on 
both sides. In 1SSS Rho found similar organs in Chromodoris which 
he stated arise from a few mesoderm cells containing numerous con- 
cretions and excreta which indicate their functional value. He con- 
cluded that this structure corresponds to the right Prosobranch 
kidney, considering the left to be rudimentary. Lacaze-Duthiers 
and Pruvot (1SS7), in a paper on Opisthobranch embryology, described 
the anal organ of Aplysia, Philine, Bulla. Pleurobranchns, Doris and 
members of the family jEolididae, stating that in origin it is entirely 
ectodermal and that it was none other than an " anal eye." This eye, 
it was claimed, becomes strongly developed in the blind larvae and 
later atrophies as true eyes appear. It stands in connection with a 
cell-mass, ganglionic in nature, the "asymmetrical centrum" of 

Mazzarelli (1892) came to some very different conclusions from work 
on Aplysia. He believes the organ in question to have neither the 
structure nor function of an eye, and, moreover, it remains present 
in the larvae after eyes are developed. From its position and structure 
it is doubtless a kidney. He derives it from paired rudiments which 
originally were closely associated with the endodermal elements of 
the aboral pole (mesentodermal cells) and which later, separating, 
wander into the blastocoel cavity and, after torsion begins, first the 
left and then the right come to lie in the neighborhood of the anus and 
together form a small cavity which acquires communication with the 
exterior. This unpaired kidney is homologous to the kidney ("mere") 
which in many Prosobranchs is found in the same place and, as is 
well known, forms the anlage of the definitive kidney. Mazzarelli, 
therefore, concludes that the anal kidney of the Opisthobranch larva 
is a secondary kidney ("secondare mere"), while the primitive kidney 
of these Mollusks is already known (the "nephrocisti" of Trinchese). 
The anal kidney is but the anlage of the definitive kidney, which in 
this case corresponds not to the right but to the left adult kidney of 
the Prosobranch. 

Heymons (1893) has carefully described the conditions found in 
Umbrella. The excretory rudiment is here at first paired and arises 
from the cells 3c 11 , 3d 11 , which sink somewhat below the surface and 


divide several times, one cell in each group remaining large. Thus 
the excretory cells of Umbrella are ectodermal in origin. In further 
history Heymons finds that the large cell of the left side decreases in 
prominence and finally is indistinguishable from those surrounding 
it, while the right continues to enlarge and, with the torsion of the larva, 
is carried higher on that side. Later a second large cell appears by 
the side of this one, which Heymons thinks cannot represent the 
original left cell, as this would presuppose too great a migration, but 
rather one of those associated with the original right, the growth of 
which has been delayed. The function of a larval excretory organ is 
assigned only to this group of cells by Heymons. 

In 1895 Mazzarelli, after a study of the development of a large num- 
ber of forms (Philinc, Gastropteron, Acta?on, Oscairius, Pleurobranchas, 
Tethys, Archidoris, Apiysia, Hermaa, Janus, Polycera and Haminca), 
came to the conclusion that the anal organ of Loven, Sars, Pruvot, 
Lacaze-Duthiers and others was not, as Lacaze-Duthiers, Pruvot and 
Heymons maintained, of ectodermal origin, but rather mesodermal, 
arising from two large and other smaller mesoderm cells which become 
pigmented and which by a slight ectodermal invagination acquire 
an external opening. In later development he finds these cells form a 
connection with the pericardium, which has arisen from a mesodermal 
mass closely connected with them. Therefore, he concludes that 
the anal kidney of the Opisthobranch larva is not homologous with 
the head kidney of the Prosobranchs, but from its origin, position and 
relation (particularly in connection with the pericardium) it is none 
other than the anlage of the definitive kidney of the adult And 
also, since it lies to the left of the rectum, it corresponds to the 
kidney of the Gastropods which possess but one, and to the left kidney 
of those with two. Viguier (1898) describes the anal kidney of Tethys, 
distinguishing an excretory lumen, around which are grouped several 
cells; he does not indicate its origin. 

Among the Prosobranchs externally situated larval excretory organs 
appear to have been found generally. Salensky (1872) has described 
such bodies filled with concretions lying upon the side of the body in 
Calyptrcea and Nassa. Bobretzky (1877) found the same in Fusus, 
these cells lying behind the velum and without an underlying ectoder- 
mal layer. This latter condition is placed in doubt by McMurrich 
(1886). Similar organs to the above were found in Fissurella by 
Boutan (1885), while in Capulus (v. Erlanger, 1S93) a single large 
ectodermal cell, probably excretory in function, was found on each 
side of the body behind the velum. For Crcpidula Conklin (1S97) has 


minutely described a group of ectodermal cells lying laterally just 
behind the velum and probably arising from the second quartet; they 
become much vacuolated, filled with darkly stained granules and be- 
fore metamorphosis separate from the ectoderm and are lost. Erlanger 
(1892) concluded that the larval kidney of Bythinia was partly ecto- 
dermal and partly mesodermal, and had no connection with the 
definitive kidney of the adult. The earlier results of Butschli (1877) on 
Paludina as well as Bythinia were enlarged by Erlanger (1891-2), 
showing that in these fresh-water Prosobranchs the larval kidney 
was formed from inner mesodermal and outer ectodermal por- 

Rabl (1879) established a mesodermal origin for the primitive kidney 
of Planorbis, and Holmes (1900) in his late work confirms the same. 
Fol (1879) derived the larval kidney of Planorbis entirely from the 
ectoderm. Wolf son (1880) described the larval kidney of Limnoea 
as arising from a large velar cell on either side which migrates inward, 
retaining connection with the exterior through an intra-cellular duct. 
Meissenheimer (1898) says of Limax, we have "in der urniere ein rein 
ekto-dermales Gebilde vor uns, zu dem das Mesoderm auch nicht den 
geringsten Beitrag geliefert hat." From his figures and discussion it 
appears very evident that in this form the primitive kidney is purely 
ectodermal in origin. In 1899 Meissenheimer published his investiga- 
tions on the "Urniere der Pulmonaten" (of the Basommatophora, 
Ancylus, Physa, Planorbis, Limncea, and of the Stylommatophora, 
Succinea, Helix, Avion, Limax). In both these groups he shows the 
larval kidney to be entirely ectodermal in origin and similar in struc- 
ture, the urinary tube of the latter group being many-celled, while in 
the former but four cells comprise it. In both a ciliated cell or cells 
closes the inner end of the tube, and for this reason Meissenheimer com- 
pares the primitive kidney of the Pulmonate with the end cells of the 
water vascular system of the Platyhelminthes. 

Among the Lamellibranchs Hatschek (1880) describes the larval 
kidney of Teredo as probably both ecto- and mesodermal in origin. 
In the single left primitive kidney of Cyclas, Stauffacher (1897) found 
a similar though more complicated structure arising from both ecto- 
dermal and mesodermal elements. 

Meissenheimer (1901) finds that in Dreissensia pohjmorpha the larval 
kidneys arise from ectodermal cells wholly, each of the two being formed 
from a few in-wandering cells. The structure is more simple than that 
of the Pulmonates and Meissenheimer suggests that it may be the 
ground type of the group. This might then be described as an ecto- 


dermal invaginftting tube with the end closed by a vacuolated heavily 
ciliated cell. 

From the above account of some of the more important observations 
and conclusions upon the nature and origin of the larval excretory 
organs of the Lamellibranchs and Gastropods (and of the latter more 
particularly of the Opisthobranchs), one is strongly impressed with the 
feeling that much more work must be done upon these organs of mul- 
luscan larvse before we are ready to come to definite conclusions 
regarding their mutual relations and homologies, if such exist. Nor 
has the investigation recorded in this paper brought forward facts 
which justify an immediate solution of the problem. The anal kidney 
of Fiona doubtless corresponds to the similar structure described for 
so many members of the Opisthobranchia, but its derivation is totally 
different from the results obtained by some of the more recent and 
careful workers in this group. 

Mazzarelli's conclusions regarding its mesodermal origin, resulting 
from investigations upon a large number of closely related forms, are 
very different from mine. There is no point regarding the cytogeny 
of Fiona of which I am more certain than that the group of cells con- 
stituting the anal kidney is of ectodermal origin, and one member 
of the group (the largest, 3c 1111 ) has been traced through every step 
of its history, from the initial cleavages which produce it to its functional 
condition upon the right side of the veliger larva at the time of hatching. 
In this respect my results are entirely in accord with those of Heymons 
for Umbrella and, except for the function assigned to the resulting 
organ, agree closely with Lacaze-Duthiers and Pruvot's derivation 
of the same structure from ectodermal cells. With regard to the 
fate of this organ, the work of Rho and Mazzarelli appears to show con- 
clusively that it becomes metamorphosed into the kidney of the adult, 
and the latter's comparison of this organ with the adult kidney of 
those Gastropods which possess but one. or with the left of those with 
two, is in entire accord with the generally accepted opinion upon this 
subject. Unfortunately material has not been available for a study 
of the metamorphosis of Fiona. But on a priori grounds it should 
be similar in all essential features to the above-mentioned processes of 
development in closely allied forms. The metamorphosis of the anal 
kidney of the larval Opisthobranch into the definitive kidney of the 
adult might seem, at first sight, fair grounds on which to doubt its 
ectodermal origin, since the latter structure has generally been con- 
sidered to be a mesodermal derivative. But if in this connection be 
considered the recent results of Meissenheimer, who derives the adult 


kidney and allied structures of Limax and Dreissensia, representing 
two distinct molluscan groups, from ectodermal rudiments, after an 
investigation .which bears every evidence of care and accuracy, the 
possibility at least of a similar manner of formation among the 
Opisthobranchs must be granted. 

So little is as yet known of the " Nephrocysts" of Trinchese that any 
discussion of their significance and possible homologies must of neces- 
sity be largely hypothetical. An exact knowledge of their derivation 
and structure would be of the utmost value. In Fiona when first seen 
they lie in the cleavage cavity, but whether they have wandered there 
from the ectoderm or are from the first mesodermal in character is 
yet an unsolved problem. Should they prove to be of ectodermal 
origin their position might justify a close homology with the Proso- 
branch larval kidney, and possibly also with those of the Pulmonates 
and Lamellibranchs, since Meissenheimer has indicated the larval 
kidneys of the two latter groups to be of ectodermal origin, and his 
work is supported by the earlier investigations of Wolfson and Fol. 
Should these nephrocysts prove entirely mesodermal there is yet a 
possibility of their similarity to the larval kidneys of the Prosobranchs, 
Lamellibranchs and Pulmonates, through the investigations of Biitschli 
and Erlanger for the Prosobranchs, Rabl and Holmes for the Pulmo- 
nates and Hatschek for the Lamellibranchs, who derived the primitive 
kidney of members of these groups in part or entirely from mesodermal 
elements. However, the structure of the nephrocysts of Opistho- 
branchs is very different from the primitive renal organs of the groups 
above cited, for, as far as is known, they appear wholly enclosed in 
the schizocoel with no external ducts. The fact of their very rudi- 
mentary structure suggests an explanation for the great development 
reached by the anal kidney. When we consider that in other groups 
possessing true larval excretory organs the anlage of the definitive 
kidney does not develop into a condition of functional activity until 
after metamorphosis, while among Opisthobranch larvse, even before 
the time of hatching, certain cells of this structure are actively con- 
cerned in the work of excretion, the causal relation between rudimen- 
tary structures on the one hand and advanced development on the 
other is brought forcibly to mind. The nephrocyst of the Opistho- 
branch is not a prominent or well-developed structure, and with its 
phylogenctic decline precocious development has arisen in the rudi- 
ment of the definitive kidney, resulting in functional activity in a 
part at least of its formative elements long before development of 
the adult organ. 


There is yet another possible explanation of the renal organs as 
found in Opisthobranch larvae which will be stated but briefly, since a 
preponderance of hypothesis over fact is always to be regretted. It 
is generally conceded that whether the anal kidney be of mesodermal 
or ectodermal origin its rudiment is at first a paired structure, one 
part of which may fail to develop into a renal organ (Heymons) or 
unite with the other (Mazzarelli). The nephrocysts are paired struc- 
tures, one lying close to the anal kidney, the other in an almost similar 
position on the opposite side of the body. It is possible that the nephro- 
cyst of the right side is but a part of the anal kidney of that side, while 
that of the left represents the degenerate whole of the rudiment of that 
side. In this case, of course, true larval kidneys would be wanting. 

The Enteron. 

As the archenteron arises from the cleaving entoblast it presents, 
when viewed from the vegetative pole, an irregular depression, the 
bottom of which lies considerably below the edge of the blastopore. 
The macromeres, 5A, 5B, 5C and 4D, are at the bottom of this pit, with 
5a, 5b and 5c lying peripherally from them, while above these and next 
to the ectoblast come 4c 2 , 4b 2 , 4a 2 and the smaller cells 4c 1 , 4b 1 and 4a 1 . 
In the posterior region are found the small cells E 1 , E 2 , e 1 , e 2 (entero- 
blasts) which have arisen from 4d. The fifth quartet and all the 
macromeres are the next cells to divide, this resulting in enlargement 
of the wall area of the enteron, and by this division into smaller ele- 
ments closer contact between the blastomeres results. Hitherto the 
entoblasts have been much rounded (except those meeting directly 
in the center), and have lain together in a very irregular manner, 
particularly after invagination began. With diminution in size and 
rearrangement of these cells a distinct cavity with closed dorsal wall 
arises (fig. 80). At the anterior end lies the large cell 4b 2 , while pos- 
teriorly and laterally are found the two large cells 4a 2 , 4c 2 ; between 
and behind them are the entcroblasts. At first the enteron is longer 
on the right side (left of figures), the cell 4c 2 lying more posterior than 
4a 2 , this being the natural result of the division which early separated 
the large mesentomere from 4D of that side and the lack of growth 
and division in this latter cell for so long a period. But as development- 
proceeds and the whole enteron grows in antero-posterior extent it will 
be noted that 4a 2 , which is a very large cell and easily distinguishable, 
gains in its backward course upon the opposite cell of like lineage (4c 2 ), 
comes to lie opposite to it and later more posterior (figs. 80, 81, 82). 
This process is the beginning of the torsion of the intestine, and is appa- 


rently to be explained in at least its first manifestations as the direct 
result of increase in growth of one side over the other. After 4a 2 lies 
considerably more posterior than the derivatives of the large cell, 
which before lay opposite it (4c 21 , 4c 22 , fig. 81), the cell 4b 2 is seen to 
be undivided as yet and still at the anterior median point of the 
enteron, showing that the change of position of 4c 2 relative to its 
opposite cell has been the result of greater increase in the area of the 
left over that of the right enteric wall. 

During this process 4a 2 has not been observed to divide and it main- 
tains its large size throughout. On the opposite side 4c 2 has divided 
into cells of equal size and divisions are continued in this region, result- 
ing in the thinning of that portion of the enteric wall and an equaliza- 
tion of the size of the cells which compose it. With the continued 
growth of the enteron 4a 2 is moved still more posteriorly and finally 
toward the right (left of figs. 82, 83). In fig. 84. which represents 
the enteron in optical section at a stage about corresponding to fig. 104, 
4a 2 is seen lying directly in the median line. Above, in the anterior 
median portion of the enteron, is a group of large yolk-ladened cells 
which have been derived from 4b 2 and its neighboring cells. This 
group will soon shift somewhat to the left and become the rudiment of 
the liver. 

As was seen before, the small cells E 1 , E 2 , e\ e 2 , which were separated 
from the anterior end of the mesentoderm, at first lie between 4a 2 and 
4c 2 . An actual section at this stage parallel to the ventral surface 
(fig. 85) shows that the inner of these cells are yet in contact with the 
enteric cavity. I am confident that the cells in this figure marked 
"enteroblasts" represent mesentoblastic derivatives. Their history, 
position, size and the structure of their nuclei, which are small and 
darkly stained, correspond to these cells. With the increase in extent 
of the left side of the enteron and, after the closure of the blastopore, 
by its continued growth, these enteroblasts, which may be distinguished 
from their neighbors by their darkly staining nuclei and their smaller 
size, become pushed from the median plane toward the right side as the 
large cell 4a 2 advances around to a more and more posterior position 
(fig. 83). Finally, when 4a 2 itself lies on the median line, these cells lie 
entirely to the right and are more posterior than those which have come 
from 4c and oc. A slightly diagonal actual section, as fig. 86, shows the 
large cell 4a 2 in the median plane. Just behind it and slightly to the 
right are shown in the section five small cells lying closely pressed 
between 4a 2 and the shell-gland invagination behind. These cells 
correspond in position and in appearance to the small enteroblasts 


of fig. 85. If we now examine section fig. 87, which is taken through 
a veliger slightly older than that shown in fig. 104, the relation of the 
enteron to its surrounding structures may be observed. The largo 
entodermic cell, 4a 2 , has been successively traced through preceding 
stages from its origin on the left side of the archenteron to its final 
position on the right of the enteric cavity, as is shown in the figure. 
Just posterior to this will be noted a mass of cells connecting the enteron 
with the ectoderm. The nuclei of these cells are compact and deeply 
staining, and the cytoplasm is decidedly clearer and contains less 
yolk than that of the cells directly surrounding the enteric cavity. 
Moreover, their position beside the large cell 4a 2 and now, through the 
torsion which the enteron has undergone, their later position some- 
what posterior to this cell, indicates the probability of their correspond- 
ence with the " enteroblasts " of fig. 86 (PI. XXXI) and earlier stages, 
in which the identity of these cells is unquestioned. 

It is proper in this place to consider again the results of Carazzi's 
work on Aplysia and its relation to the mesentodermal history of 
Fiona. It will be remembered that Carazzi's account of the lineage 
of 4d up to a stage when its derivatives number twelve cells exactly 
parallels my results on Fiona, but regarding the fate of these cells 
there is lack of agreement. The anterior small cells of Aplysia are 
believed to be purely mesoblastic. while at least four of them in Fiona 
appear, from the preceding account, to be entodermal in nature. 
Carazzi, however, derives endoderm from the two small posteriorly 
directed cells (e, e 1 of Aplysia) which correspond to z 1 . z 2 of Fiona. 
These latter cells were last seen lying at the posterior end of the gas- 
trula of Fiona closely pressed against the ectoderm. At a later period, 
when a large number of mesodermal elements lie in this region, the z 1 , 
z 2 cells become indistinguishable from these. Sections of later stages 
(fig. S7) show two cells which are larger and clearer than the entero- 
blasts and which lie against the ectoderm where the intestinal mass 
touches it. They may represent the cells z\ z 2 , but of this there is 
no evidence except that given above. Anal cells are not a marked 
feature of the developing embryo of Fiona, but at this time sections 
in particular show two cells of somewhat larger size than the surround- 
ing ectodermal elements, against which the forming intestine abuts 
and which are doubtless comparable to the anal cells of other forms 
(fig. 87, An.C). 

It will now be seen that the portion of the enteron lying most 
posterior and close against the shell-gland invagination has been 
derived from the cells which formed the bottom and the left side of 


the original archenteric invagination (5B, 5b, 4C, 5C, 5c, 4D, 5A) 
while dorsally and anteriorly are seen more yolk-ladened elements 
whose origin may be traced to the large entoderm cell 4b 2 and those 
around it. The stomodaeal invagination breaks through at a much later 
period between the descendants of 5a and 4b and their neighboring 
cells, which have been turned in an anterior direction, while doubtless 
cells from 4c and 5b also push in upon this region with the closure of 
the blastopore. By the torsion which the enteron has undergone the 
upper mass of large yolk-ladened cells is moved more and more to the 
left, while in like manner 4a 2 turns to the right. While this is occurring 
the invaginating shell-gland has pushed the anterior and posterior 
walls of the enteron very closely together, both enteric and cleavage 
cavities being practically obliterated (fig. S6). When this structure 
evaginates the enteron again opens out and has then lost its elongated 
form, being rounded with its wall cells in close contact (fig. 87). 

In Umbrella as well as in Fiona 4b 2 occupies the anterior end of the 
enteric mass pushing up into the pointed apex of the gastrula, and the 
same is true of Aplysia in which there are but two large blastomeres, 
though according to Blochmann's nomenclature such does not appear 
to be the case. In later stages the positions of the large cells of the 
fourth quartet of Umbrella and Fiona are identical. The intestine of 
Umbrella is said to be formed by C" and D" (5c and 5d), which, as Hey- 
mons did not take into consideration an entoblastic contribution from 
4d, correspond fairly well to the conditions found in Fiona, where these 
cells he just at the place of origin of the intestine and may well take 
part in its future development. The cell-lineage of the archenteron 
of Crepidula is given as follows: "The four macromeres form the roof 
of the archenteric cavity. The cells of the fifth quartet form its lateral 
boundaries, arching the cavity on all sides save the posterior. Here 
the archenteric cavity runs backward between the cells 5C and 5D (5c 
and 5d) nearly to the posterior boimdary of the egg. The cells of the 
fourth quartet come together on the ventral side of the archenteron, 
forming its floor anteriorly and ultimately giving rise to some of the 
many small cells which form that part of the mesenteron, adjoining 
the stomodseum." The intestine arises from the posterior lower right 
region of the enteron as a tube-like evagination, formed from the entero- 
blasts derived from 4d and neighboring small endodermal cells and 
ending blindly against the ectoderm. Later it elongates and the end 
is carried somewhat upward along the right side by trosion of the larva. 
It contains a lumen from the first. As the stomach begins to enlarge 
it is seen to be bounded by large cells dorsally and anteriorly in its lower 


regions. As development proceeds it is elongated, its posterior end 
being ventraUy directed and turned toward the right. The develop- 
ment of the liver of Crepidula comes later, being retarded by the 
great amount of yolk. 

The next change in the development of the enteron of Fiona may 
be observed in fig. 105, which represents a veligcr in which the ali- 
mentary canal is beginning to become differentiated into several parts. 
Anteriorly is seen the stomodseum, which has as yet not broken through 
but touches the wall of the enteron. Above and to the left of this 
point of contact is a decided lobing of the wall of the enteric cavity, 
formed of the large yolk-ladened cells which at an earlier period lay in 
the anterior region of the archenteron. This is the rudiment of the 
liver, and as development proceeds the invagination becomes larger and 
more constricted at its base, forming a rounded lobe upon the left dorsal 
wall of the enteric canal. Behind the rudiment of the liver the enteron 
has widened into a capacious sac which is larger at its upper anterior 
end, the walls of the whole being formed of rather small cells which are 
yet rich in yolk. This is the stomach, and it ends blindly against the 
intestinal mass behind and to the right. The intestine is yet a solid 
strand of cells connecting the posterior end of the stomach with the 
ectoderm. With the growth of the veliger this strand has become 
more slender, elongated and turned forward, its distal end lying well 
up on the side of the body behind the constriction which forms the 
foot. The huge excretory cell lies just dorsal to this point (figs. 106, 
107). In figs. 90, 91, 92 and 93, which represent coronal sections of 
a veliger somewhat older than figs. 105 and 106, and slightly more 
mature than that of fig. 107, it will be seen that the intestine is still a 
solid strand of cells, and that the oesophagus is as yet not in open con- 
nection with the rest of the alimentary canal. An examination of a 
considerably older larva (figs. 109, 110) shows a very small lumen, 
just beginning to form in the center of the intestinal strand, but as 
yet no communication between oesophagus and enteric cavity. 

Stomodamm and Mouth. 

As the blastopore narrows (fig. 79) it becomes entirely surrounded, 
except at the anterior end, by third quartet cells. At the anterior 
point second quartet cells from 2b 22 and 2b 212 lie along the edge also. 
Figures of a later stage (as 97, 9S) show the blastopore as a mere 
rounded opening, its edges and walls below thickly set with darkly 
nucleated cells, and when complete closure occurs a plug of these cells 


may be observed upon lateral optical section dipping down from the 
region of closure to the enteron beneath. These cells have come largely 
from the third quartet of all four quadrants, and represent the smaller 
cells of this quartet which lay nearest the open blastopore. This 
condition exists but for a short time, for soon a broad pit may be ob- 
served in this region occupying txactly the place where the blastopore 
closed. As it forms the cells which have been invaginated to form the 
blastopore-plug open out again so that a blind pit results, the lower 
surface of which is formed by those cells which were first pushed inward 
as the blastopore was closing, and correspond to the second and third 
quartet elements which are shown in fig. 79 surrounding the blastopore. 
The stomodaeal invagination continues to increase in depth by growth 
and division of the cells which already form it and by further invagina- 
tion of surrounding cells, so that, as the form of the veliger begins to ap- 
pear (figs. 103, 104, 105, 106), second and third quartet cells from all 
the quadrants lying in the region probably become involved. , At first 
the stomodseum is broad and shallow, but as it increases in depth it 
narrows and becomes more dorsally directed at its inner end. In 
section, fig. 90, and in drawings of the oldest veliger shown (figs. 109, 
110), the stomodaeal invagination has as yet not formed an open con- 
nection with the enteron, but shortly afterward this occurs, at which 
time the stomodaeum is much elongated. Union is established with the 
stomach pouch just below the opening of the large liver lobe. 

Fiona agrees with a large number of Mollusks in which the blasto- 
pore closes and the stomodseum forms at the same point. Among 
them may be named Nassa (Bobretzky), Neritina and Aplysia (Bloch- 
mann), Elysia (Vogt), various iEolididse (Trinchese), Doris (Langer- 
hans), Crepidula (Conklin), Planorbis (Holmes) and Trochus (Robert). 
In Patella (Patten), Fusus (Bobretzky), Pteropods and Heteropods 
(Fol) and Limncca (Lankester) the blastopore is said to remain open 
and pass over directly into the mouth. 

Shell-gland and Foot. 

If one examines the segmenting egg somewhat later than such a 
stage as shown in fig. 73, it will be observed that the posterior has con- 
siderably outstripped the anterior region in extent and that, together 
with numerous divisions, the cells have also enlarged considerablv in 
size. The area which lies along the median line, and so is derived from 
the second quartet, shows most plainly this rapid increase in extent. 
and it is here particularly that the cells themselves become greatlv 


enlarged and prominent. This is the region of posterior growth, and 
from this area arise both the shell-gland and the foot. 

Taking up first the history of the former of these two organs, it will 
be found that in a stage represented by figs. 95 and 98 the whole 
area between the blastopore and the end of the posterior arm of the 
cross shows karyokinetic activity, but particularly in the region marked 
Sh.G. the cells have increased considerably in size. As growth con- 
tinues these cells upon the upper and posterior surface of the gastrula 
protrude above the level of the ectoderm, the area which they cover- 
having the appearance of a rough cobble-stone pavement; but some- 
what later they settle down and form a smooth surface. The center of 
this area, which now lies just opposite the region of the stomodaeum, 
begins to invaginate, pushing the enteron before it and reducing its 
cavity, so that there results a deep pit which, growing in size below, con- 
stricts above, and around which are several rows of large granular cells 
(fig. 102). Such a condition lasts but a short time, for soon the invagi- 
nated area opens outward, the whole forming a large thick-walled 
cap upon the posterior end of the veliger, constricted around its edge 
and merging abruptly with the thin-walled ectoderm anterior to it (fig. 
104). As growth proceeds the shell-gland spreads and becomes much 
thinner, while the larval shell appears as a secretion of the large cells 
which compose it. As the shell continues to extend over the veliger 
its outer edge is marked by several rows of large cells, which by their 
secretive activity lay down the substance which forms the shell (figs. 
105, 106, 107). Almost from its origin as a distinct structure the shell- 
gland is slightly displaced to the left side of the body, and as it increases 
in extent this lack of bilateral symmetry becomes more marked (fig. 

The ventral prominence which develops into the foot arises some- 
what later than the shell-gland, and the cells which go into it come from 
the second quartet of D quadrant and the third quartet of C and D 
quadrants. The large ectodermal excretory cell, which in the larva 
lies just behind the foot, serves as a guide to show that much of the 
foot, like this cell, arises from C quadrant of the third quartet; and 
though no such landmark is present on the other side, the early history 
of the two quadrants are so similar that we may reasonably suppose a 
like origin from the third quartet for the left side of the foot. Lillie 
has derived the foot of Unio from cells of the second quartet, and Conk- 
lin appears to have done the same for Crepidnla. Holmes states for 
Planorbis that as the cells immediately behind the blastopore are of 
third quartet origin, probably the "median portion of the anterior end 


of the foot is derived from some of these cells". Robert describes a 
similar condition for Trochus. In Fiona not only the median portion 
but also much of the lateral area certainly comes from the third quartet. 
The foot here does not arise as a paired swelling as in Patella (Patten), 
Fulgar (McMurrich) and Trochus (Robert), but shows from the first 
a median protuberance which increases in size and later becomes 
broadened and flattened (figs. 103, 108, 110). Its upper surface is 
covered with numerous cells, but they are not arranged to form a 
conspicuous cell-plate as in Crepidula. Large cells mark its lower 
surface and they soon begin to secrete the operculum. 

Larval Musculature. 

It is particularly unfortunate that for a study of the muscles of the 
velum no living material has been available, as without this many 
points of interest must of necessity be lost. When the veliger breaks 
from its capsule it presents an appearance shown in figs. 109, 110, 
though it should be remembered that in fixed material, from which 
the drawings were made, the muscles must be much contracted. The 
whole posterior region is swollen into a huge transparent vesicle, at the 
anterior end of which lies the contorted alimentary canal. In dotted 
outline is represented the probable position of the cuticular-like shell 
before shrinkage. In a larva of such age one of the most characteristic 
features is a large dorsal retractor muscle, which has its posterior point 
of attachment well to the left of the dorsal side of the posterior vesicle. 
It runs forward and branches just before reaching the liver lobe, its 
two anterior ends becoming attached to the alimentary canal and the 
body wall in the region of the oesophagus. In structure it is composed 
of large spindle-shaped interlacing cells, which are flattened dorso-ven- 
trally, giving the muscle a band-like form. In function this muscle 
doubtless acts as a retractor for the anterior and particularly the upper 
portion of the cephalic region. A dorsal view of the same veliger 
shows two lateral muscles, the right and left retractors of the foot, 
which arise about midway back on the sides of the posterior vesicle 
and extend forward through the lower part of the neck region, to end 
in branching fibers in the foot. That of the right side is larger than 
the left, and in earlier stages (figs. 105, 106) is much thicker than later 
and relatively larger. In figs. 105 and 106 is shown a small muscle 
(Vl.R.) extending from the dorsal neck region to the velar folds where 
it branches greatly. Other similar retractor muscles of the velar 
lobes extend from the walls of the alimentary canal and the body wall 


outward into the velar area branching extensively. Fine interlacing 
fibers are also found in the foot in older stages. 

Returning to the period marked by fig. 105, the dorsal retractor 
muscle is seen to be a short thick strand of cells extending from the 
shell region to the enteron near the position of the liver. It is here 
already branched and runs along the sides of the alimentary canal. 
The right retractor of the foot is, as shown, a very heavy cell strand 
which unites the foot with the lower dorso-lateral portion of the shell. 
A view from the left side would show a muscle occupying a similar 
position, but in this case much thinner (fig. 107 shows their relative 
sizes at a slightly later stage). Even at this early period the dorsal 
retractor is posteriorly attached to the left of the median line. 

Bearing in mind the distinction of Lillie and others between primary 
mesoblast (ento-mesoblast) and secondary mesoblast (ecto-mesoblast 
or larval mesoblast), the attempt has been made to distinguish between 
these two sources of muscular tissue in the developing larva of Fiona , 
with, however, but partial success. The velar retractors, which lie in 
the region of the head vesicle, are formed from secondary mesoblast. 
Those cells which we have seen cut off from the third quartet in the 
two anterior quadrants lie in the antero-lateral region of the gastrula, 
and may for some time be distinguished from the primary mesoblast 
cells. When at an early period spindle-shaped muscle fibers appear 
in this region, their origin from these cells can scarcely be doubted. 
The component elements of the dorsal retractor are hard to distinguish. 
When this muscle first appears at a stage about midway between figs. 
104 and 105, several large cells lie wedged in between the rounded 
wall of the enteron and the ectodermal area in the upper region of the 
shell-gland. The evidence is strong that these cells at least are from 
the primary mesoblasts. At this time, however, other cells extend 
along the enteron, connecting the compact posterior group with the 
loosely lying spindle-shaped elements of the velar retractors. They 
doubtless help form the more anterior portion of the dorsal retractor 
and, lying as they do so close to where secondary mesoblast was formed, 
may be derivatives of it. The two retractors of the foot and the inter- 
lacing fibers of that organ itself are doubtless composed of cells which 
have come from 4d. From the above account it is seen that a true 
"larval mesoblast" is found in Fiona, since much at least of the mus- 
culature of the velum, a purely larval organ, is derived from this 
secondary mesoblast. 

No organ in any way comparable to a larval heart is to be found in 
the oldest veli^ers which I have studied. 

1904.] natural sciences of philadelphia. 391 

Change of Axis and Form of the Developing Organism. 

The egg at the time of laying is spherical. With the division into 
four cells the primary egg axis, running between the centers of the 
animal and vegetative poles, becomes shorter than the diameter of the 
equatorial plane. As segmentation proceeds this relation persists 
(fig. 14), and with continued division the formation of a large cleavage 
cavity becomes more pronounced. Until the cleaving egg reaches a 
stage of over sixty cells its surface, when viewed from either pole, 
appears almost perfectly rounded, but shortly after this its antero- 
posterior axis becomes shorter than the lateral (figs. 45, 56, 74), this 
relation holding until increased growth in the posterior and anterior 
quadrants causes elongation in that direction. Until about a stage 
shown in fig. 74 the primary egg axis, running from the center of the 
animal to the center of the vegetative pole, follows a straight line. 
Immediately after this, accentuated growth of the posterior region 
initiates a bending of this axis, which finally results in its complete 
folding upon itself, or a rotation through ISO degrees. A sharply 
pointed anterior projection arises (fig. 78), while at the same time the 
posterior dorsal region is rapidly increasing in extent and changing 
the embryonic axis. As the gastrula elongates the apical pole is moved 
forward, and by the time the first velar row becomes distinct the origi- 
nal polar axis has become so bent upon itself as to form an angle of 
nearly 90 degrees (figs. 95, 98). With the continued multiplication 
of cells in the head region that portion of the larva changes from its 
originally pointed shape into a rounded though not prominent head 
vesicle, while at the same time the opposite end is rounded by continued 
growth of second and third quartet elements (figs. 100, 101). The orig- 
inal polar axis will be seen in these figures to have moved through about 
135 degrees. In the next stage, represented by figs. 102 and 104. 
the head vesicle has reached its largest relative size when taken in 
connection with the veliger as a whole. Comparing these figures with 
those which have gone before, a marked increase will be seen in the 
antero-posterior depth, and if this be considered in connection with the 
great change of axis the enormous growth of the posterior region will 
be evident. It is generally conceded that the head vesicle of mol- 
luscan and annelidan larvae is of functional importance in serving as a 
float. In Fiona the head vesicle is never large and prominent and a 
substitute may reasonably be expected. With the differentiation of 
the velar lobes and foot the shell-gland may in figs. 105. 106 and 107 


be seen to be rapidly spreading over the posterior region. As this 
is being accomplished it also grows greatly in size, producing the enor- 
mous posterior vesicle which in figs. 109 and 110 extends far behind 
the internal organs of the body. The importance of such an organ 
must be considerable and, taken in connection with the early decrease 
in size of the head vesicle, strongly suggests that its functional value 
is similar in kind to that usually ascribed to the anterior or head 
vesicle of other larvse. 

In all older veligers figured the original polar axis has become com- 
pletely bent upon itself, a rotation of 180 degrees having occurred. 
With regard to the median plane of the future embryo, the first cleavage 
plane is obliquely transverse to this plane. When the mesoderm is 
formed it is thrown over toward this median plane, and from the first 
is approximately bilateral in position (figs. 24, 31, 34). The elements 
of both entoblast and ectoblast, which in late stages of cleavages lie 
on the median plane, appear to be derived from cells of the early 
cleavages which occupied similar positions. Little rotation, if any, 
is apparent other than a certain amount of irregularity found in all 
portions of eggs with equal or nearly equal cleavage. 

Conklin describes for Crepidula an entire rotation of the ectoblastic 
cap at the time when the anterior and lateral cells of the fourth quartet 
arise. Heymons shows a similar rotation in Umbrella. Such a change 
of axis in the germ layers does not occur in Fiona, nor is there necessity 
for it. The large macromeres of Crepidula and Umbrella are here 
represented by small cells, which do not modify the positions of the 
germ layers at the time of their origin nor necessitate supplementary 


Maturation begins at the time of laying. Two polar bodies are 
given off, the first of which may or may not divide. The un- 
segmcnted egg of Fiona is rich in yolk, the spherules being com- 
paratively small. In shape the egg is round, but slightly flattened 
in the direction of its polar axis. One to three eggs are found in a 
roomy egg capsule. 

The early cleavage is strictly spiral after the dextral sequence. The 
first quartet of micromeres are much smaller than the macromeres, 
but with succeeding divisions the cleavage becomes equal in character. 
After the four macromeres are formed they give rise to successive 
quartets of micromeres. The first three quartets contain all the ccto- 


blast. The mesoblast arises in part from the fourth quartet cell 
of D quadrant. The remaining fourth quartet cells and all the 
macromeres are entoblastic, as is also the case with a small portion 
of 4d. 

The first quartet of ectomeres give rise to the trochoblasts and ecto- 
blastic cross. To the latter structure are added as "tips" the upper 
cells of the second quartet in all quadrants. The cross is radially 
spiral in symmetry, and does not increase in breadth by transverse 
splitting of its arms until a comparatively late period. Cells from the 
first quartet form the head vesicle, cerebral ganglia and eyes, and a 
portion of the first velar row. 

The second quartet has a similar cleavage history in all four quad- 
rants until a stage of about 150 cells. In later development the 
elements of this quartet in D (posterior) quadrant show great 
increase in size and divisional activity, initiating the posterior point 
of growth, with resulting bending of the embryonic axis. Cells from 
this area form the shell-gland and median portion of the foot. A 
large number of second quartet cells from the anterior and lateral 
groups aid in the formation of the velum. The more ventral elements 
of B quadrant help to close the blastopore. 

In the third quartet bilateral cleavages first appear in the posterior 
quadrants (cells 3c 1 and 3d 1 ). Secondary mesoblasts arise from the 
anterior quadrant groups of this quartet (cells 3a 2111 , 3a 2211 and 3b 2111 > 
3b 2211 ). The large anal excretory cell (3c 1111 ) and its associated cells 
are derived from C quadrant of this quartet. Third quartet cells 
surround the blastopore as it closes, with the exception of a small 
anterior portion; much of the stomodseum and the lateral portions 
of the foot come from third quartet elements. 

The mesoblast of Fiona is derived from two sources, ento-mesoblast 
from 4d and ecto-mesoblast from the third quartet in A and B quad- 
rants. The greater amount comes from 4d and forms teloblastic 
bands in the posterior region of the gastrula. The secondary mesoblast 
(ecto-mesoblast) is largely "larval" in fate, since much of it goes to 
form the muscles of the velum. From the history of 4d it appears 
that this cell contains both mesoblastic and entoblastic derivatives, 
the latter taking part in the formation of the intestine. 

As is the case with many Opisthobranchs, the gastrula is sharply 
pointed anteriorly, the apical point at first lying at the end of the 
anterior arm of the cross. 

The blastopore at the time of closure is surrounded by third 
quartet cells, except at its anterior edge, where second quartet cells are 


present. The stomodaeum later forms at the point where the blasto- 
pore closed. 

The shell-gland at first forms a deep invagination, which later opens 
out and covers the posterior end of the veliger with a cap of large 
cells which soon begin to secrete the shell. From the first the shell 
is slightly shifted toward the left, and this asymmetry becomes more 
marked with continued growth. With the enlargement of the shell 
a conspicuous posterior vesicle results. 

The foot arises as an unpaired swelling below the stomodaeum. Its 
under surface later secretes an operculum. 

The first velar row is formed from the anterior trochoblasts (A and 
B quadrants), the tips of the anterior arm of the cross, and possibly 
from other cells of the first quartet in this region. The second velar 
row is derived from underlying cells of the second quartet. A post- 
oral velar area is but slightly marked. In later development the velum 
becomes bilobed and broadly expanded. 

A prominent head vesicle is not present in the older veligers, and 
with this may be correlated the development of a large posterior vesicle. 
No apical sense-organ has been found, nor are distinctly marked apical 
plates present. The cerebral ganglia appear in the angles between 
the anterior and lateral arms of the cross. Otocysts are formed by 
invaginations of the ectoderm upon the sides of the foot, and pedal 
ganglia appear closely associated with them. The eyes are late in 
appearing and are intimately connected with the rudiments of the 
cerebral ganglia. 

The anal kidney of the larva is derived from the ectoderm, coming 
from 3c 1111 and associated cells. With the torsion of the larva this 
group is shifted farther to the right, and eventually lies well up on the 
right side of the veliger above the anal opening. Primitive ex- 
cretory cells are also found lying in the body cavity laterally behind 
the velum. 

The enteron is formed by invagination of the entomeres, which at 
first form an elongated sac; with the evagination of the shell-gland 
this becomes rounded. The liver is derived from large yolk-ladened 
cells lying at the anterior end of the enteron, and later the rudiment 
of this organ becomes turned toward the left side. Torsion of the 
enteron results from lengthening of the left side and is caused by 
increased growth of that region. The intestine is at first a solid thick 
cell-strand and is composed largely of entoblasts from 4d; it later 
elongates and acquires a lumen. 

1904.] natural sciences of philadelphia. 395 


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Beitrage zur Kenntniss der Gasteropoden. Ibid., Bd. XXXVIII, 1883. 

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Mittheilungen iiber Ban und Entwicklung einiger marinen Prosobranchier. 

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Etudes sur le developpement des gasteropodes pulmones faites au labora- 

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Etudes sur le developpement des Gasteropodes pulmones. Ibid., Tom. 

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Georgevitch, P. Zur Entwicklungsgeschichte von Aplysia depilens. Anat. 

Anz., Bd. XVIII, 1900. 

Carazzi und seine Kritik. Ibid., Bd. XIX, 1901. 

Grant, R. E. On the Existence and Uses of Cilia in the Young of Gastropodous 

Mollusca, and on the cause of the Spiral Turn of Univalve Shells. Edinb. 

Journ. Sci., Vol. VII, 1S27. 


(JriART. J. Contribution a l'etude des Gasteropodes Opistobranches et au 
particulier des Cephalaspides. Mem. Soc. Zool France, Tom. XIV, 1901. 

Haddon, A. C. Notes on the Development of Mollusca. Quart. Jour. Mic. Sci., 
Vol. XXII, 1882. 

Hatschek, B. Ueber die Entwicklungsgeschichte von Teredo. Arb. Zool. 
hist. Wien, Bd. Ill, 1880. 

Entwicklung der Trochopora von Eupomatus. Ibid., Bd. VI, 1885. 

Heymons, R. Zur Entwicklungsgeschichte von Umbrella mediterranea. Zeit. 

Wiss. Zool., Bd. LVI, 1893. 
Heath, H. The Development of Ischnochiton. Zool. Jahrb. Abth. Anal u. 

Ontog., Bd. XII, 1899. 
Holmes, S. J. The Cell-Lineage of Planorbis. Zool. Bull, Vol. I, 1897. 

Secondary Mesoblast in the Mollusca. Science, Vol. VI, 1897. 

Reversal of Cleavage in Ancylus. Am. Nat., Vol. XXXIII, 1897. 

The Early Development of Planorbis. Jour. Morph., Vol. XVI, 1900. 

Keferstein, W. Malacoza Cephalophora. Bronn's Klass u. Ord., Bd. Ill, 

Keferstein, W., und Ehlers, E. Beobachtungen tiber die Entwicklung von 

vEolis peiigrina. Zool. Beitrage, 1861. 
Kofoid, C. A. On the Early Development of Limax. Bull. Mus. Comp. Zool. 

Harv. Coll., Vol. XXVII, 1895. 
Koren und Danielssen. Bemaerkninger til Molluskernes Udvkling. Nyt. 

Mag. }. Nat. Videnskab., Christiania, 1848. 
Lacaze-Duthiers, H. Memoire sur l'anatomie et l'embryogenie des Vermets. 

Ann. Sci. Nat. Zool, Tom. XIII. 1860. 
Lacaze-Duthiers et Pruvot. Sur un ceil anale larvaire des Gasteropodes 

Opisthobranches. Comp. rend. Acad. Sci. Paris, Tom. CV, 18S7. 
Lang, A. Die Polycladen des Golfes von Neapel. Fauna u. Flora des Goljes 

Neap., Bd. XI, Monographia, 1884. 
Langerhans, P. Zur Entwicklung der Gastropoda Opisthobranchia. Zeit. 

Wiss. Zool, Bd. XXIII, 1873. 
Lankester, E. R. Observations on the Development of the Pond Snail 

(Lymnseus stagnalis) and on the Early Stages of Other Mollusca. Quart. 

Journ. Mic. Sci., Vol. XIV, 1874. 

Contributions to the Developmental History of the Mollusca. Phil. Trans. 

Roy. Soc. London, Vol. CLXV, Pt. I, 1875. 

On the Coincidence of the Blastopore and Anus in Paludina vivipara. 

Quart. Journ. Mic. Soc, Vol. XVI, 1876. 

On the Originally Bilateral Character of the Renal Organs of Prosobranchs, 

etc. Ann. Mag. Nat. Hist, Vol. VII, 1881. 

Lillie, F. R. The Embryology of the Unionida?. Journ. Morph., Vol. X, 

Loven, S. Bidrag till Kaennedwmen af Molluskernas utwickling. Kongl. 

Vetenskaps Acad. Handl, 1838, Stockholm, 1841. 
Mazzarelli, G. Intorno al preteso occhio anale delle larve degli Opistobranchi. 

Rend. R. Accad. Lincei, Vol. I, Fasc. Ill, 1892. 

Monografia delle Aplysiidae del Golfo di Napole. Mem. Soc. Ital, Tom. 

IX, 1893. 

Bemerkungen iiber die Analniere der freilebenden Larven den Opistho- 

branchies. Biol Centralbl, Bd. XVIII, 1898. 
McMtjrrich, J. P. A Contribution to the Embryology of the Prosobranch 

Gasteropods. Stud. Biol. Lab. J. Hop. Univ., Vol. 1886. 
Mead, A. D. The Early Development of Marine Annelids. Journ. Morph., 

Vol. XIII, 1897. 
Meissenheimer, J. Entwicklungsgeschichte von Limax maximus. I. Fur- 

chung und Keimblatterbildung. Zeit. Wiss. Zool, Bd. LXII, 1896. 

Entwickslungsgeschichte von Limax maximus. II. Die Larven-periode. 

Ibid., Bd. LXIII, 1898. 

Zur Morphologie der Urniere der Pulmonaten. Ibid., Bd. LXV, 1899. 

Entwicklungsgeschichte von Dreissensia polymorpha Pall. Ibid., Bd. 

LXIX, 1901, 

Meyer, E. Studien fiber der Korperbau der Anneliden. Mitth. Zool. Stat. z. 
Neapel, Bd. XIV, 1901. 


Nokdman, A. v. Essai d'une Monographie de Tergipes Edwardsii. Ann. Sci. 

Nat. Zool, Tom. V, 1846. 
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Rabl, C. Die Ontogenie der Siisswasser-Pulmonaten. Jena. Zeit. Naturw., 

Bd. IX, 1875. 

Ueber den Entwieklung der Tellerschnecke. Morph. Jahrb., Bd. V, 1879. 

Beitrag zur Entwicklungsgeschichte der Prosobranchier. Sitzunq. k. 

Akad Wiss. Wien, Bd. LXXXVII, Abth. Ill, 1883. 

Reid. J. On the Development of the Ova of Nudibranch Mollusks. Ann. Man. 

Nat. Hist., Vol. XVII, 1846. 
Rho. F. Studii sullo sviluppo della Chromodoris elegans. Atti R. Accad. 

Sci. Napoli, Vol. I, 1888. 
Robert, A. Recherches sur le developpement des Troques. Compt. rend. 

Acad. Sci. Paris, Tom. CXXVII, 1898. 

La segmentation dans le genre Trochus. Ibid., Tom. CXXXII. 1901. 

Recherches sur le developpement des Troques. Arch. Zool. exv. gen 

Ser. Ill, Tom. 2, 1902. 
Salensky, W. Beitrage zur Entwicklungsgeschichte der Prosobranchien. Zeit. 
Wiss. Zool., Bd. XXII, 1872. 

Zur Entwicklungsgeschichte von Vermetus. Biol. Centralbl. Bd. V, 18S6. 

Sars, M. Zur Entwicklungsgeschichte der Mollusken und Zoophyten. Arch 

Naturgesch., Bd. Ill, 1873. 

Beitrage zur Entwicklungsgeschichte der Mollusken. Arch. Naturgesch., 

Bd. VI, 1S40. (Supplementary paper on Nudibranchs, Ibid., Bd. XI, 

Schimkewitsch, W. Zur Kenntniss des Baues und der Entwieklung des Dino- 
philus vom Weissen Meere. Zeit. Wiss. Zool., Bd. LIX, 1895. 

Schneider, A. Ueber die Entwieklung der Phyllirhoe bucephalum. Arch 
Anat. u. Phtjs., Vol. XXV, 1858. 

Smallwood, W. M. Notes on the Natural History of some Nudibranchs. Bull. 
Syracuse Univ., Ser. IV, No. 1. 

Stauffacher, H. Eibildung und Furchung bei Cyclas cornea. Jena Zeit f 
Naturw., Bd. XXVIII, 1893. 

Tonniges, C. Die Bildung des Mesoderms bei Paludina vivipara. Zeit. Wiss. 
Zool, Bd. LXI, 1896. 

Treadwell, A. L. The Cell-Lineage of Podarke abscura. (Preliminary Com- 
munication.) Zool. Bull., Vol. I, 1897. 

The Cytogeny of Podarke obscura. Journ. Morph., Vol. XVII, 1901. 

Trinchese, S. I primi momenti dell' evoluzione nei Molluschi. Atti R. Accad 

Lincei, Mem. Vol. VII, Roma, 1880. 

Per la fauna marittima italiana. ^Eolididse e Familie affini. Ibid.,\o\ XI 

Roma, 1881. 

Ricerche anatomische ed embriologiche sulla Flabellina affinis. Mem. R. 

Accad. Sci. dell' Instituto d>, Bologna, Tom, VIII, 1887. 

Torrey, J. C. The Early Development of the Mesoblast in Thalassema. Anat. 

Am., Bd. XXI, 1902. 
Viguier, C. Contribution a Petude du developpement de la Tethys fimbriata 

Arch. Zool. exp. gen., Tom. XVI, 1898. 
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Sci. Nat. Zool, Tom. VI, 1846. 
Whitman, C. O. The Inadequacy of the Cell Theory of Development. Woods 

Holl Biol. Led., 1893. 
Wierzejski, A. Ueber die Entwieklung der Mesoderm bei Physa fontinalis 

Biol. Centralb., Bd. XVII, 1897. 
Wilson, E. B. The Cell-Lineage of Nereis. Journ. Morph., Vol. VI, 1892. 

Considerations on Cell-Lineage and Ancestral Reminiscence. Ann. New 

York Acad. Sci., Vol. XI, 1898. 

Wolfson, W. Die Embryonale Entwieklung des Limnaeus stascnalis. Bull. 

Acad. Sci. St. Petersbourg, Tom. XXVI, 1880. 
Ziegler, E. Entwieklung von Cyclas cornea. Zeit. Wiss. Zool, Bd XLI, 1SS5. 


proceedings of the academy op 

Table of Cell-Lineage, 
a. quadrant. 



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Arrows pointing to right, dexiotropic direction of cleavage; to left, Ino- 
tropic; double-headed arrows, horizontal; double-headed bent arrows in history 
of 4d, bilateral cleavages with relation to cells of opposite side. 


Table of Cell-Lineage, 
b. quadrant. 



proceedings of the academy of 

Table of Cell-Lineage. 
c. quadrant. 


1904.] natural sciences of philadelphia. 

Table of Cell-Lineage. 

d. quadrant. 



Reference Letters. 

Ap Apical point. 

An.C Anal cell. 

Bl Blastopore. 

C.G Cerebral ganglion. 

Dr.R Dorsal retractor muscle. 

Ebl Enteroblasts. 

E.C Large enteric cell. 

En Enteron. 

Ex Large anal excretory cell. 

Ft Foot. 

Int Intestine. 

L Liver. 

Lt.R.Ft Left retractor muscle of 


Mo Mouth. 

M.F Muscles of foot. 

Nph.L Left nephrocyst. 

Nph.R Right nephrocyst. 

Oes (Esophagus. 

Op Operculum. 

Ot Otocyst. 

P.B Polar Body. 

P.C Pedal Commissure. 

P.G Pedal ganglion. 

Rt.R.Ft Right retractor muscle of 


Sec.M Secondary mesoderm. 

Sh.E Edge of shell. 

Sh.G Shell-gland. 

St Stomodseum. 

Stom Stomach. 

Tel Teloblast. 

V 1 First row of velar cells. 

V 2 Second row of velar cells. 

Vl.L Velar lobe. 

VI. R Retractor muscle of velum. 

Note. — In the drawings the trochoblasts are represented with stippled nuclei; 
upper pole views show the ectoblastic cross in heavy outline. Plates I-XI, 
XIII, and Figs. 101-104 of PI. XIV are reduced £ from original drawings, the 
remaining figures £. Figs. 7, 36, 39, 46 and 55 have been omitted from Plates. 

Explanation of Plates XXI-XXXV. 

Plate XXI, Fig. 1. — Section of egg of Fiona marina, showing first maturation 

Fig. 2. — Section. First polar body being given off. Sperm nucleus with 
astral rays below. 

Fig. 3. — Section. Rise of second polar body. Enlargement of sperm 
nucleus and astral rays. 

Fig. 4. — Lateral view of entire egg. Approach of male and female pro- 

Fig. 5. — First cleavage; figure seen from side. 

Fig. 6. — Completion of first cleavage, as seen from above. The two polar 
bodies lie between the nuclei. 

Fig. 8. — Completion of second cleavage, seen from upper pole. A polar 
furrow is present at the vegetative but not at the animal pole. 

Fig. 9. — Upper pole view, showing spindles which institute third cleavage. 

Plate XXII, Fig. 10. — Dexiotropic ' turning^of spindles of the first quartet, 

with constriction and rounding out of these cells. Lateral view. 
Fig. 11. — Same egg as fig. 10, seen from above. 
Fig. 12. — Lateral view of slightly older egg than Fig. 10, showing compact 

grouping of blastomeres after division. 
Fig. 13. — Completion of fourth cleavage, laeotropic in direction, by which 

the second quartet is separated from the macromeres. 
Fig. 14. — Lateral view of same egg as fig. 13. 
Fig. 15. — Lseotropic division of first quartet, by which the "turret cells" 

(trochoblasts), la 2 , lb 2 , lc 2 , Id 2 , arise. In following figures the turret 

cells and their derivatives are indicated by stippled nuclei. 
Fig. 16. — Lateral view of same egg as fig. 15. 
Fig. 17. — First cleavage of the cells of the second quartet (dexiotropic). 

The rhacrom ires are about to give off the third quartet by dexiotropic 



Plate XXIII, Fig. 18. — Slightly later stage (lateral view) than fig. 17. Divi- 
sion of the second quartet is about completed. 

Fig. 19. — Animal pole view of egg, in which the divisions shown in figs. 17 
and 18 are fully completed. 24 cells. 

Fig. 20. — Vegetative pole view of egg slightly older than fig. 19, showing 
spindle which initiates the separation of 4d. 

Fig. 21. — Transition stage between 25 and 33 cells (seen from animal pole). 
All eight cells of the second quartet are dividing lasotropically, the 
upper four forming the "tip" cells of the cross (2a 11 , 2b 11 , 2c 11 , 2d 11 ). 

Fig. 22. — Same egg as fig. 21, seen from vegetative pole. The laeotropic 
division of 3D, forming 4D and 4d (ME), is completed. 

Fig. 23. — Animal pole view of egg containing 41-44 cells, la^ld 1 have 
divided in a dexiotropic direction the "apicals" (la n -ld u ) and the 
"basals" (la 12 -ld 12 ) of the" ectoblastic cross." 

Plate XXIV, Fig. 24. — Same egg as fig. 23, seen from vegetative pole. All 
the third quartet cells have divided lseotropically. The macro- 
meres, 3A, 3B, 3C, are dividing in a similar direction to complete the 
fourth quartet. 

Fig. 25. — View of vegetative pole of egg slightly older than fig. 24. The 
formation of the fourth quartet is completed and the mesentomere 
4d (ME) has divided into right (ME 1 ) and left (ME 1 ) halves. 

Fig. 26. — Lateral view from B quadrant of an egg same stage as fig. 25. 

Fig. 27. — Lateral view of an egg, D quadrant, same stage as fig. 25. 

Fig. 28. — Animal pole view of an egg showing, (1) dexiotropic divisions of 
2c 12 , 2a 12 , 2b 12 ; (2) lseotropic division of 2b 21 ; lseotropic to horizontal 
division of 2c 21 . The trochoblasts, lc 2 , Id 2 , are also beginning to 

Fig. 29. — Same egg as fig. 2S, seen laterally from B quadrant. 

Fig. 30. — Same egg as fig. 28, lateral view of C quadrant; 3c 1 is cleaving in 
a dexiotropic direction. 

Fig. 31. — Vegetative pole view of same egg as fig. 28, showing bilateral 
divisions of 3c 1 , 3d 1 . 

Plate XXV, Fig. 32. — Lateral view D quadrant, slightly older stage than fig. 
28, showing bilateral divisions of 3c 1 , 3d 1 . 

Fig. 33. — Upper pole view of same egg as fig. 32, showing cleavage in three 
of the "basal" cells of the cross, lb 12 is dividing in a lseotropic 
direction; in lc 12 the spindle is dexiotropic to radial in position; in 
la 12 lseotropic to radial spindle. The turrets, la 2 and lb 2 , show dexio- 
tropic cleavage. About 60 cells. 

Fig. 34. — View of vegetative pole of somewhat older stage than fig. 31 ; 
3a 1 , 3a 2 , 3b 1 and 3b 2 have all divided in a dexiotropic manner. 

Fig. 35. — Same stage as fig. 34, lateral view of C and D quadrants. 

Fig. 37. — Lateral view of A quadrant, showing dexiotropic division of 3a 2 . 

Fig. 38. — Upper pole view, showing completion of cleavage forming "basals" 
(la 121 -ld 121 ) and "middle" (la 122 -ld 122 ) cells of cross. 

Fig. 40. — Slightlv older stage than preceding, showing completed cleavage 
of 3b 1 . 

Fig. 41. — Same egg as fig. 40, A quadrant. 

Plate XXVI, Fig. 42. — View of vegetative pole of egg with about 68 blas- 
tomeres. ME 1 and ME 2 are dividing bilaterally. 

Fig. 43. — Lateral view, A and D quadrants of egg with about 75 blastomeres, 
showing dexiotropic cleavage of 2a 211 and laeotropic divisions in 
3d 11 , 3d 12 . 

Fig. 44. — Same egg as fig. 43, seen from C quadrant. 3c" and 3c 12 are divid- 
ing dexiotropicallv. 

Fig. 45. — View of vegetative surface of egg with about 80 cells. The 
mesentomeres have divided into two small cells, E 1 and E 2 , and 
two large, Me 1 and Me 2 . 

Fig. 47. — Same egg as fig. 45, lateral view of D quadrant. 


Fig. 48. — Same egg as fig. 45, lateral view of C. quadrant. 
Fig. 49. — Lateral view of D quadrant in egg of about 86 cells. 2d 121 is 
dividing laeotropically ; 2d 2U and 2d 212 have divided dexiotropically. 
Fig. 50. — Lateral view of same egg as fig. 49, showing A quadrant. 

Plate XXVII, Fig. 51. — D quadrant, a lateral view. Me 1 and Me 2 all dividing 

Fig. 52. — Lateral view, B quadrant of same, egg as fig. 49. 
Fig. 53. — Upper pole view of egg of about 86 cells. The "apical" (la lu - 

ld 111 ) and "peripheral" (la 112 -ld 112 ) rosettes have been formed by 

lseotropic cleavages. 
Fig. 54. — Same egg as fig. 51, seen from side (C quadrant). 
Fig. 56. — Upper pole view of an egg of approximately 106 cells. The basal 

cells, la 121 / lb 121 , lc 121 , have divided; Id 121 is dividing with spindle 

transverse to posterior arm of cross. The two inner posterior 

trochoblasts (lc 21 , Id 21 ) are dividing bilaterally. 
Fie;. 57. — Vegetative pole view of same egg as fig. 56. Completed division 

of Me 1 , Me 2 into IVFe 1 , M 2 e 2 and m'z 1 , m 2 z 2 . 
Fig. 58. — Same egg as fig. 56, showing A and D quadrants on lateral view. 
Fig. 59. — Same egg as fig. 56, principally B quadrant. 

Plate XXVIII, Fig. 60. — Same egg as fig. 56, lateral view of C quadrant. 

Fig. 61. — Lateral view, D quadrant, same egg as fig. 56. 

Fig. 62. — Upper pole view of egg slightly older than last series (over 115 
cells). All the interior trochoblasts have divided, and the completed 
transverse division of the basal cell of the posterior arm of the cross 
is shown. 

Fig. 63. — Same egg as fig. 62, showing A quadrant on lateral view. 

Fig. 64. — Lateral view, same egg as fig. 62, B quadrant. 

Fig. 65. — Lateral view, same egg as fig. 62, C quadrant. 

Fig. 66. — Lateral view, same egg as fig. 62, D quadrant. 

Fig. 67. — Egg of about 125 cells, lateral view, C quadrant. 

Plate XXIX, Fig. 68. — Same egg as fig. 67, lateral view of A quadrant. 

Fig. 69. — Same egg as fig. 67, lateral view of B quadrant. 

Fig. 70. — Slightly later stage than fig. 67, lateral view of C quadrant. 

Fig. 71. — Entomeres and mesomeres from egg of over 150 cells, seen from 
vegetative pole. 

Fig. 72. — Entomeres and mesomeres of egg about stage of fig. 71. 

Fig. 73. — Entomeres and mesomeres, seen from vegetative pole of egg slightly 
older than the two former stages. 

Fig. 74. — Vegetative pole view of about same stage as fig. 73, showing the 
overgrowth of the "secondary" mesoblasts (ecto-mesoblasts, 3a 2111 , 
3a 2211 , 3b 2111 , 3b 22n ) by other cells of the third quartet. 

Fig. 75. — Upper pole view, about the same stage as fig. 74, showing trans- 
verse splitting of the arms of the cross and division of outer trocho- 

Plate XXX, Fig. 76. — Upper pole view of somewhat later stage than fig. 75, 

showing increase in breadth of cross area. 
Fig. 77. — Lateral A'iew of stage similar to fig. 75, showing large excretory 

cell (3c llu ) and neighboring cells. 
Fig. 78. — Vegetative pole view of gastrula with closing blastopore, showing 

pointed anterior end and complete overgrowth of the ecto-mesoblast. 
Fig. 79. — Somewhat older gastrula than preceding figure. 
Fig. 80. — Optical section (parallel to ventral surface) of gastrula of about 

the stage shown in fig. 79. 

Plate XXXI {except figs. 81-2), Figs. 81-84. — Optical sections, similar in direc- 
tion to that of fig. 80, through successively older gastruke. showing 
torsion of the enteron through increase in area of the left side (right 


of figures). Fig. 84 represents a section taken through a young 

veliger about the stage of that shown in fig. 104. 
Fig. 85. — Actual section through a gastrula similar in age to fig. 80 and in 

same plane. 
Fig. S6— Actual section (sagittal) through a gastrula about the age shown 

in fig. 95. 
Fig. 87.— Actual section (about 30° to the right of the sagittal plane) 

through a young veliger slightly older than as shown in fig. 104. 

Plate XXXII (except fig. 94), Figs. 88-89.— Actual sections (nearly horizon- 
tal) through a veliger about the stage shown in fig. 105, showing 
cerebral and pedal ganglia, pedal commissure and otocysts ; also large 
excretory cell on right side of larva and large enteric cell on same 
side of enteron. 

Figs. 90-93. — Four successive horizontal actual sections through a veliger 
slightly older than that shown in fig. 107. 

Fig. 94. — Nearly horizontal actual section through veliger of same age as 
preceding series, showing nerve ring around oesophagus. Un PI. 

Plate XXXIII, Fig. 95. — Gastrula, seen from right side, showing first indica- 
tion of the first velar row (V 1 ). 

Figs. 90-97. — Upper and lower sides respectively of gastrula of the same 
age as shown in fig. 95. 

Figs. 98-99. — Lateral (right) and lower sides of a veliger slightly older than 
that shown in figs. 96-97. 

Fig. 100. — Left side of gastrula somewhat older than that shown in the two 
preceding figures. 

Plate XXXIV, Figs. 101-102.— Anterior and right-lateral views of larva mid- 
way between gastral and veliger stages. The deep invagination of 
the shell-gland (Sh.G.) has formed and the stomodseal pit (St.) is 
well marked. 

Figs. 103-104. — Anterior and right-lateral views of a young veliger. The 
shell-gland has opened outward, the foot (Ft.) is becoming evident 
and the velar lobes are just beginning to appear. 

Fig. 105. — -Veliger, seen from right side, somewhat older than the preceding 
one, showing further development of velar lobes and foot, developing 
shell, differentiation of enteron and larval musculature. 

Fig. 106. — Slightly older veliger than fig. 105, seen from right side. 

Plate XXXV, Figs. 107-10S. — Dorsal and anterior views of the same veliger 
somewhat older than fig. 106. The shell and the velar lobes show 
considerable advance in development. 
Figs. 109-110. — Right-lateral and dorsal views of the same veliger just 
before hatching. The dotted lines represent the probable shape of 
the posterior vesicle before shrinkage. 



Last summer (1903). through advantages offered by the new Bio- 
logical Station in Bermuda, I was able to collect the shells on which 
this paper is based. In the study of the material I owe much to Dr. 
H. A. Pilsbry, of the Academy of Natural Sciences of Philadelphia. 

It will be necessary in the discussion of the fossils to compare them 
with the species that are now native, in the looser sense, to the islands. 
In drawing the line between these and the snails supposed to have 
been brought by commerce, I shall follow Dr. Pilsbry 's latest paper on 
the "Air-breathing Mollusks of the Bermudas." 2 I shall also rule 
out all the littoral species, including Truncatella, because the fossil 
beds were not situated where such shells could be expected. 

The most unsatisfactory feature of work on Bermudian fossil land 
shells is the difficulty in determining the ages of the various deposits. 
The rock of Bermuda is exclusively solidified dunes of calcareous sand, 
and the soil is the rust-colored residue of the weathered rock. In 
weathering, the surface of the rock becomes completely broken up 
into pockets and crevices packed with the earth. It is estimated 3 
that every inch of earth must represent eight or nine feet of rock eroded, 
and thus when it is possible to judge of the average depth of soil formed 
over a deposit, that depth can be made an index of the age of the 

Probably the oldest good fossiliferous deposit that I examined is 
collecting locality No. 807 (see Map No. 3) of the Bermuda Biological 
Station, at a hard-stone quarry on the west side of Knapton Hill, 
about midway between Hotel Frascati and "Devil's Hole." At this 
point a layer of eight or ten inches of red earth containing shells was 
covered by an ancient dune. The dune has become hard limestone, 
and its top has been eroded until now the red earth in its pockets must 
represent a layer averaging not less than six inches in thickness. The 
series of Pcecilozonites that we took from this bed is very incomplete, 
and the fossils of all the genera are poorly preserved, but notwith- 
standing this we are able to recognize at least eleven species and sub- 

1 Contributions from the Bermuda Biological Station for Research, No. 2. 

2 Trans. Conn. Acad., Vol. X. 

3 A. E. Verrill. Trans. Conn. Acad., Vol. XI, p. 400. 


species. These are enough to identify its fauna with that of another 
deposit, locality No. 806 (see Map No. 2), where the shells are abundant 
and well preserved, but with no external evidence by which to estimate 
their age. This locality is another hard-stone quarry, where the 
excavations have uncovered a number of crevices and a cavern of 
considerable size. The shells are in stalagmitic conglomerate at the 
mouth of the cavern, and in the crevices, and also in the earth that 
fills certain of the pockets. They may represent a considerable period 
of time, but there is no way to distinguish any difference in age. 

Another deposit at the same locality as the one last mentioned is a 
horizontal band of slightly reddish rock about half-way up the face of 
the quarry, and from two to three inches thick. This is part of the 
rock out of which the cave and pockets were eroded, so that the shells 
here are very much older than the others at No. 806; but here, again, 
there is no basis for a comparison with the date of No. 807. The re- 
mains here are obscure casts of Pcecilozonites circumfirmatus and of 
what appear to be Vertigo and Carychium. 

I collected from three other beds in this neighborhood what seem to 
represent the same formation as the pockets of No. 806. 

The first of these, locality No. 814, is a newly opened quarry just 
south from Coney Island. A red-earth pocket -here contained a fine 
series of Pcecilozonites nelsoni, very large, but wanting the most ex- 
treme examples of both the elevated and the depressed variations. 
There are also fossiliferous conglomerates in caverns at this quarry, 
but they are composed of gravel too fine to contain Pcecilozonites 

The best fossil specimens of Pcecilozonites reinianus came from local- 
ity No. 815, near Harrington House. They are noticeably larger than 
the recent specimens. No. 816, near 815, but on the shore of Castle 
Harbor, has large numbers of Pcecilozonites bermudensis zonatus and 
Pcecilozonites reinianus, the former associated with Pcecilozonites 
nelsoni in a conglomerate. 

Bifidaria rupicola, found in the red earth of No. 806, may perhaps be 
an importation subsequent to the formation of No. 807, and Strobilops 
fmbbardi, found at the same place, possibly may not have been a per- 
manent resident; but we can safely assume that all the other species 
from the above localities belong to the epoch of the red-earth streak 
at No. 807. The remaining three deposits from which I collected are 
clearly much more recent than No. 807. These are in sand pits, in 
the nearly pure sand of partially solidified dunes. None of them have 
any clear signs of red earth, either about them or overlying them. 


The shells at these places are so perfectly preserved that even the term 
"semi-fossil" seems a misnomer for them. Probably the sand pre- 
serves them by saturating the water with lime before it reaches them. 

One of these shell deposits, locality No. 818, on the land of Mr. Benja- 
min Trott, in Tucker's Town, is only from 8 to 36 inches below the 
surface. The P. nelsoni were mostly in the upper foot of the deposit, 
where the bank is thoroughly solidified by the rain ; but a few inches 
lower the sand is still loose enough to be scraped out with a strong hoe. 

The two localities last to be mentioned, Nos. 808 and 809, are essen- 
tially alike. They face the Devonshire marshes on the northwest 
side — 808 near the north end and 809 close to the barracks. The sand 
in these dunes appears to have drifted from near the present line of 
the north shore — a consideration which may yet give a clue to their 

The following are my records of fossil and semi-fossil shells in these 
localities : 

Locality 807. 



" CIRCUMFIRMATUS, | , , . . 

,, r Intergraded. 



" bristoli. One specimen. 

Thysanophora hypolepta. 

succinea bermudensis. 
Vertigo numellata. 


Carychium bermudense. 

Casts in the Rock, Locality 806. 

pcecilozonites circumfirmatus. 



Cave and Pockets, Locality 806. 
Pcecilozonites nelsoni. Both extremes in height of spire. 





Thysanophora HYPOLEPTA. 
succinea bermudensis. 
Strobilops hubbardi. 
Bifidaria rupicola. One specimen. 
.Vertigo numellata. 


Carychium bermudense. 

Locality 814- 



" nelsoni. In crevices. 


r In one pocket. 

In crevices. 
sis zoxatus, 1 
reinianus, I In stalagmitic conglom- 



Locality 815. 
Pgecilozonites bermudensis zonatus? Small fragments only. 


Locality 816. 



" reinianus. (None kept in the collection.) 


Locality 818 (Sand Pit). 





Bifidaria servilis. One specimen. 
Carychium bermudense. 

Locality 808 (Sand Pit). 




Bifidaria rupicola. One specimen. 

Carychium bermudense. 

(Polygyra microdonta? One immature specimen, which may have 

crawled into the sand in recent times. We shall give it no further 


Locality 809 (Sand Pit). 


" circumfirmatus. (None kept in collection.) 

Succinea bermudensis. (None kept in collection.) 
Carychium bermudense. 
Pupoides marginatus. One specimen. 

These lists include all the known fossils except Pcecilozonites dalli. 
Outside of Pcecilozonites, the species that do not appear in deposit 
No. 807 are : 

Strobilops hubbardi. 
Bifidaria rupicola. 
" servilis. 

Pupoides marginatus. 

The last two of these appear only in the sand pits, and are in all 
probability later importations. The first two, found at No. 806, may 
also have arrived after No. 807 was covered up, but the fossils at No. 
807 are so poorly preserved that we cannot presume upon the absence 
of these species. Ignoring these doubts, we may combine and re- 
arrange the lists from Nos. 807 and 806 — the more ancient fossils — 
mentioning after each species the habitat of its nearest relatives in 
other countries, as follows: 

Pcecilozonites nelsoni. 

" nelsoni callosus. 










Vertigo numellata. \ Eastern North America. 


Carychium BERMUDENSE. J 

4 Zonitoides minusculus. North America and West Indies. 
4 Bifidaria rupicola. Florida, Cuba. 
4 Strobilops hubbardi. Florida, Jamaica. 
Thysanophora hypolepta. West Indies. 
Succinea bermudensis. West Indies. 

Total, 17 forms, 14 of them probably peculiar to Bermuda. For 
comparison we have the following recent species,' supposedly not im- 
ported by man : 

Pcecilozonites bermudensis, ] 


CIRCUMFIRMATUS, J Remnant of the fossil fauna> 


Thysanophora hypolepta, 
Succinea bermudensis, 
5 blfidaria rupicola. 
5 pupoides marginatus. 
5 Thysanophora VORTEX, 


Helicina CONVEXA. 

North America, West Indies. 

West Indies. Five species. 

Total, 13 species, 6 of them probably peculiar to Bermuda. 

Dr. Pilsbry's conclusion, from the anatomy of Pcecilozonites, that the 
oldest importations to Bermuda came from continental America, is thus 
confirmed by a large majority of the fossil forms. Bermuda, at the time 
of the No. 807 deposit, was characterized by not less than five genera 
of continental affinities, of which at least one had been resident long 
enough to have developed new generic characters and a respectable 
diversity of species. The abundance of the individuals, too, and the 
size and variability of some of the species, seem to show that the island 
was not inhospitable to continental genera at that epoch. There were 
not only the large extinct species Pcecilozonites nelsoni and Pcecilozo- 
nites cupula, but larger varieties also of Pcecilozonites bermudensis and 

4 Species not peculiar to Bermuda. 

5 Species not peculiar to Bermuda. 


Pcecilozonites reinianus than are now living. The largest specimens 
even of Pcecilozonites circumfirmatus and Succinea bermudcnsis are 
among the fossils. These snails must have found more food than there 
is now on the uncultivated ground. There is also geologic evidence 
that they belonged to a more prosperous epoch than the present. Prof. 
Heilprin reports that in excavations for one of the docks, specimens 
of Pcecilozonites nelsoni were brought up from a peat deposit at a depth 
of forty feet below water. A rise of the land sufficient to put these 
shells ten feet above sea-level (see Map No. 1) would multiply the land 
area eight or ten times, changing it from a narrow ridge, hardly two 
miles wide at its widest, into an elliptical area, including, it is true, some 
large lagoons, but in all about ten miles across and more than twenty 
miles long. A large, protected interior valley would then receive the 
fertile soil that is now washed into the lagoon by every storm. It 
would not surprise me if the deposits at locality 807 should be shown 
to date from the period of this Greater Bermuda, but a person need 
hardly wait for this proof before supposing that the indigenous con- 
temporaries of Pcecilozonites nelsoni were also characteristic of Greater 

In spite of their evident prosperity, I do not think it could be proved 
that these snails lived under any densely shading vegetation. The 
humidity at Bermuda makes such a shade less necessary for snails 
than it is in many places. I have often seen Succinea bermudcnsis 
clinging to grass and to trunks of trees in such situations that I imagine 
an American summer day would have desiccated them. The tract 
about Prospect Hill (No. 809) must have been desolate, unshaded land 
when the hills were growing dunes, yet the sand here (localities 808 
and 809) contains numerous well-developed specimens and quite a 
variety of species. These must either have lived where they are found, 
or else have been blown there from some place almost equally wind- 

The extinction of species that were able to prosper on those barren 
parts of the island seems to me a strange occurrence. If, as I believe 
is probable, the sand for these dunes came from near the present north 
shore, then the island must have had very nearly its present shape 
and size when these snails were alive. Thus when the Greater Bermuda 
sank, the change seems to have set new dunes in motion across this 
section of the Lesser Bermuda; and Pcecilozonites zonatus, Cary- 
chium bermudense and Euconulus turbinatus not merely survived the 
subsidence, but even formed a considerable population on the parts of 
the remaining island that were most damaged by the changing condi- 


tions. How many other species still survived in the less altered sec- 
tions it is impossible to say. It is hardly possible to prove that even 
the set of fossils from No. 806 belong to any earlier date. Indeed we 
might draw an analogy between Bifidaria rupicola at No. 806, which 
may be one of the later arrivals, and Bifidaria senilis at No. 818 and 
Pupoidcs marginatus at No. S09, either of which we can hardly hesitate 
to treat as recent arrivals. But however this may be, the sand-pit 
deposits are against the supposition that the Carychium and its hardier 
associates were exterminated merely by the increasing barrenness of 
the island. We should be in a better position to discuss the other 
causes if we knew whether these species survived till after the West 
Indian arrivals had begun to take possession of the land. The West 
Indies snails, especially Polygyra microdonta, of Bahama, are at present 
much the commonest of the "native" snails, and it may be that then- 
special fitness for the more barren land of the new Bermuda made them 
deadly competitors to the old species. The newer formations at the 
west end of the islands, which I had not the time to visit, ma}^ perhaps 
be the ones in which to look for evidence on this question. 

Notes and Descriptions. 

Thysanophora vortex Pfr. 

Living animals quite abundant under stones; but I looked in vain 
for fossil specimens. Greater Antilles, Bahamas, Southern Florida. 
Thysanophora hypolepta ' Shuttl.' Pils. 

I found more examples of this than of Z. ?ninusculus among the 
fossils, but among the living snails Z. minusculus seems to be far more 
abundant. It is supposed to be indigenous. 

Polygyra microdonta Desh. 

Excluding importations from Europe, this species is the one now most 
in evidence. It is partial to the coarse native grass, but is to be found 
almost everywhere. I was surprised not to find any indubitable 
specimens of this in the sand pits. I hope other collectors will look 
for it. Bahamas. 
Strobilops hubbardi Brown. 

An adult and an immature specimen, from locality 806. The adult 
is somewhat larger than the usual size on the continent. Alt. 1.2, 
diam. 2.S mm. Habitat, the Gulf States and Jamaica. 

Vertigo numellata n. sp. PI. XXXVI, fig 6. 

Shell rimate, minute, elliptical or bluntly pupiform, yellowish- 
corneous, faintly striate, of 5 rather convex whorls; the diameter 
through the body whorl not much greater than that through the whorl 


preceding. A prominent, whitish, inflated ridge, appearing like a 
second peristome, occurs behind the peristome. Aperture propor- 
tionately more contracted than that of V. ovata; set with a parietal. 
an angular and a columellar lamella; and with two palatal and a basal 
fold. The palatal folds are prominent, the upper one slightly double- 
topped, the lower one more immersed and entering spirally. The 
parietal lamella is stout and blunt; the angular lamella smaller and 
thinner; the columellar lamella and the basal fold low and blunt. 
Peristome rather thin, expanded, and notched opposite the upper 
palatal fold, as in V. ovata. 

Alt. 1.7, diam. .9 mm. 

In one specimen there appears a slight suprapalatal denticle. A 
considerable number of smaller, more globose specimens seem to belong 
to this species. One of these from locality 806 measures 1.4 x .9 mm. 

I have assumed that this species is more closely related to V. ovata 
than to any of the species reported from the West Indies. 

Localities 806 and 807; the type from 806. 

This is the common fossil Vertigo. 

Vertigo marki n. sp. PI. XXXVI, fig. 7. 

Shell rimate, ovate, yellowish-corneous, faintly striatulate; whorls 
nearly 5, rather convex. Apex obtuse, but not rounded like that of 
Vertigo numellata. The inflated ridge inconspicuous, whitish, crowded 
close to the peristome. Aperture ovate, much longer than in Vertigo 
numellata, set with four denticles, of which the parietal lamella is the 
largest. The lower palatal fold denticular, smaller than that of Vertigo 
numellata and less immersed ; the upper palatal fold minute ; and the 
columellar lamella broad and low. The peristome is expanded, white, 
strongly thickened within, hardly notched at the upper palatal fold. 

Alt. 1.9, diam. 1 mm. 

Named in honor of Dr. E. L. Mark, of Harvard, Director of the 
Bermuda Biological Station for Research. 

This species is somewhat suggestive of V. tridentata, but is a little 
slenderer, with a longer aperture, and a heavy white peristome. 

Locality 806; doubtful specimens from S07. 
Bifidaria rupicola Say. 

One specimen each from localities S06 and 808, and several recent 
specimens. Dr. Pilsbry reminds us that the Bermudian form has a 
thicker lip than the others of this species. Cuba, Florida. 
Bifidaria servilis Gld. 

One specimen from locality 818, and a few recent. Cuba and other 
West Indian islands. 


Bifidaria jamaicensis C. B. Ad. 

The commonest of the recent Pupidse, but I failed to find it fossil. 
Greater Antilles. 
Pupoides marginatus Say. 

I got one indubitable specimen from locality S09, but it went to 
pieces in my hands. I found only two or three recent ones. Mr. 
Owen Bryant, who was collecting at the same time, found a larger 
number. Eastern and Central North America, and some West Indian 
Carychium bermudense n. sp. PI. XXXVI, figs. 11, 12. 

Shell almost regularly tapering, corneous-white, imperforate, finely 
striate; whorls about 5, increasing regularly, those of the spire very 
convex, with deep sutures. Aperture quite oblique, obstructed by a 
small parietal and a very minute, deeply placed columellar lamella. 
Peristome broadly expanded and reflexed, thickened within by a white 
callus, with a slight groove on its front face, and developed inward 
to form a prominence slightly above the middle of the outer margin 
(near the position of the upper palatal fold in Bifidaria). 

Alt. 1.8, diam. .9 mm. 

This species is very dissimilar to the slender Carychium jamaicense. 
The shape of the aperture allies it more nearly to Carychium exiguum 
of North America, but its heavy peristome is quite its own. 

It is one of the most abundant fossil species, occurring in the red 
earth of localities 806 and 807, and even in the sand that fills the larger 
shells in the sand pits. 

Poecilozonites nelsoni (Bid.). 

Hijalina nelsoni Bid., Ann. Lvc. N. H. of N. Y., XI, 1S75, p. 78. 

P. nelsoni Pilsbry, Proc. Acad. Nat, Sci. Phila., 1888. p. 290. 

P. nelsoni v. Mart., Sitzungsber. Ges. Nat. Freunde, Berlin, 1889, p. 201. 

The typical form of this species is, I suppose, the large, moderately 
elevated form. This is represented among my specimens from locality 
814, where the variation in dimensions is as follows: 

Alt, 29 


. 39 mm 















23 (estimated) 35 
The way these lay, piled together in a little pocket, compels the 
supposition that they lived at about the same time, and their varia- 


tions in outline show what may occur in a single intergenerant colony. 
The specimens from locality 806 show even greater differences, of 
which the following are the extremes: 



Diam. 34 mm, 







I should like to suggest the name discoides, merely as a convenient 
term by which to know the variation represented by the last two shells 
(PL XXXVI. fig. 4). I must say, however, that this suggestion 
would be unfortunate if it resulted in the division of the series ob- 
tained from locality 814. It seems to me, rather, that some physio- 
logical peculiarity has destroyed the diagnostic value of the elevation 
of the spire. The upper whorls differ less than the lower, and in the 
most elevated forms the suture of the later whorls is much below the 
keel of the preceding whorl, as if the slant of the spiral had been 
abnormally diverted downward. 

Poecilozonites nelsoni var. callosus n. var. PI. XXXVI, fig. 5. 

Shell smaller than the typical form, shiny, with heavy ribbed striae, 
colored with a broad yellowish-brown peripheral band on a white 
ground. Whorls a trifle more than nijae, increasing regularly and very 
gradually. The suture does not change its character nor become de- 
flected from the peripheral line of the preceding whorl. The usual 
peripheral angle is almost obsolete. The base has a stronger angle 
about the umbilical perforation than is usual in the species. The 
peristome is greatly thickened on the inside from 1 mm. at the suture 
to fully 2 mm. near the columella. A prominent callosity covers the 
parietal wall of the aperture. 

Alt. 24, diam. 33 mm. 

The combination of small size and large number of whorls is charac- 
teristic. The ratio of height to diameter is more constant than in 
the typical form, and the tendency to produce the callosity is very 

Type from locality 818, others from 818 and 807. 

The stability of the variety, occurring as it does in the oldest and 
the latest formations, is the most interesting tiling about it. It is also 
my excuse for regarding such slight distinctions in a remarkably vari- 
able species. 

I suppose the color patterns of Pcecilozonites nelsoni were essentially 
the same as those on the living Poecilozonites bermadensis. For ex- 
ample, the type specimen of callosus probably had a dark brown band 


on a background of a yellowish cuticular color. The depressed speci- 
men which is figured has traces of a subperipheral band, a supra- 
peripheral line,, and radial flaming above this line. This flamed pat- 
tern appears in several of the flat specimens. 

Poeoilozonites cupula n. sp. PI. XXXVI, fig. 2. 

Shell solid, dome-shaped, with somewhat flattened base, perforate, 
strongly striate; pale, shiny-corneous, with subsutural and subper- 
ipheral bands of darker color, and faint traces of two narrow band.-; 
on the periphery. Whorls 7f , a little convex, increasing slowly; the 
last vaguely angulate at the periphery. The aperture is somewhat 
quadrangular on account of the straight, vertical columella and the 
peripheral angle. The peristome is simple, thin, with the columellar 
margin reflexed. 

Alt. 13 Diam. 16 mm. 

Locality No. 806. 

Other specimens measure: 

Alt. 13.5 

Diam. 16.5 mm 









The last specimen has 8f whorls. 

The type was selected as the best-preserved specimen, not as the 
most representative example. The majority of the specimens have 
a more rounded base and periphery, giving the peristome a more oval 
contour. The height of the shell and the absence of a keel distinguish 
it readily from P. bermudensis zonatus, and the very round dome 
and less angulate periphery separate it from immature specimens of 
Poeoilozonites dalli n. sp. PI. XXXVI, fig. 1. 

Shell elevated, with rounded apex and convex base, perforate. Its 
surface is polished, with incremental lines less pronounced than those 
of P. cupula; milky- white, with a yellowish-brown band below the 
periphery a. d a line above the periphery. The first four whorls are 
translucent whitish. Whorls 1\; all but the final whorl are flat as if 
keeled, that one has a blunt peripheral ridge, below which it is deeply 
rounded. The aperture is quite oblique, round-lunate. The peris- 
tome is simple, except at the columella, which it joins without an angle, 
but the columellar margin is reflexed, partly covering the perforation. 

Alt. 8.5 Diam. 7.3 mm. 



Another specimen has the height 10, cliam. 7 mm., and is composed 
of 9 whorls. It shows more of the brown and less of the white color. 

The extreme variability of P. cupula leaves it debatable whether 
this may not be a dwarf race of that species. 

No specimens of this form were found last summer, and it is through 
the courtesy of Dr. William H. Dall of the National Museum, that I 
am able to describe and figure it. The specimens came to him without 
labels, so that we are left to conjecture their age. The slender specimen 
is so glossy and brightly colored that Dr. Dall doubts whether it can 
be a fossil, but it seems to me the simpler hypothesis to suppose that 
it was preserved in the sand in the same manner as the type of P. nelsoni 
callosus, which it so closely resembles in color and polish. The shell 
sand seems to be a complete protection from destructive agents. On 
this hypothesis it had originally about the color of Poecilozonites ber- 
Poecilozonites bermudensis Pfr. 

Pilsbry, Proc. Acad. Nat. Sci. Phila., 1888, p. 289; 1889, p. 85. 

The typical variety seems to be of recent origin. It is distinguished 
from the fossil by a less rounded upper surface, less flattened apex, 
larger umbilical perforation, and usually smaller number of whorls. 
My largest specimen I found on Rabbit Island, Harrington Sound, 
buried under drift sand at some time previous to the cultivation of 
the island. It measures alt. 13, diam. 24.5 mm. The largest and 
smallest living mature shells measure as follows : 

Alt. 14.5 Diam. 20. mm. 
14 22 

10 16.5 

An average fully adult specimen measures : 

Alt. 11 Diam. 20 Umb. 1.7 mm. 

and has a trifle more than 7 whorls. 
Poecilozonites bermudensis var. zonatus Verr. PI. XXXVI, fig. 3. 

This differs from the type of the species in possessing an almost 
uniformly curved upper contour line, an almost flat apex, and a more 
constricted umbilicus. The keel is distinct, as in the recent form. 
Whorls 7f, The aperture is surrounded by callous thickenings as in 
P. nelsoni callosus. Alt. 13.5, diam. 23, umb. 1 mm. 

Specimens come from localities Nos. 806, 808, 814, 816 and 809. 

The extremes from locality No. 808 are : 

Alt, 16 Diam. 22.5 mm. 

15 25 

12.5 20.5 Umb. 1 mm. wide. 


Thus the smallest adult is quite equal to the average recent shells. 
A few selected specimens of the fossil and recent shells can hardly be 
distinguished. Many of the fossils do not have the callosity. 

Locality 816 has great quantities of these shells so firmly cemented 
together that most of them are worthless as specimens. They have 
the peculiar spheroidal upper surface, but the perforation is wider 
than in the series from locality 808 — not so wide, however, as in the 
recent. Several specimens here occur below some fragments of Pcecilo- 
zonites nelsoni in stalagmite, apparently showing that they were there 
previous to the extinction of nelsoni. 

Broken and immature specimens from locality 808 show that the 
umbilicus was not much narrower than that of the recent variety until 
the last whorl had commenced to grow. The peculiar contour is also 
less noticeable prior to the last whorl. Thus in their smaller number of 
whorls, their less rounded contour, and their larger umbilicus, the 
present snails seem like an undeveloped or degenerate race of the 
former species. . 

It is possible that this fossil variety is what Pfeiffer (Monographic! , 
I, p. 80) mistook for Helix ochroleuca Fer. 

Poecilozonites reinianus Pfr. 

Helix reiniana Pfeiffer, Malak. BL, XI, 1863, p. 1. 

P. reinianus Pilsbry, Proc. Acad. Xat. Sci. Phila., 18S8, p. 290; 1889, p. 85. 

I found this species in every deposit examined except No. 807. 

Further search would doubtless show it there also. At locality 815 

many fine specimens were embedded in stalagmite. They show the 

typical color-pattern, with the dark marks changed as usual to reddish, 

and the lighter ground to ivory-yellow. 

The largest specimen from No. 815 measured.... Alt. 7 Diam. 13 mm. 

The largest from No. 808 12 

The largest from No. 806 11.5 

The largest from the pocket at No. 814 .... .-. 11 

The largest recent, lent by Mr. Bryant 6 11.3 

My largest recent 5 10.3 

From Town Hill (locality 819) come some good specimens of var. 
goodei Pils. Examples of these measure: 

Alt. 4 Diam. 10 Umb. 4 mm. 

3.5 9.3 3.4 

3.7 10 4 

The species is not so uniformly common as Pcecilozoniles circwn- 
firmaius, but is very abundant in some places, for example, near 
locality 806. It would be interesting to learn whether its place in the 
economy of nature is different from that of the following species. 


Pcecilozonites circumfirmatus Redf. 

Helix circumftrmata Redfield, Ann. Lye. X. H. of X. Y., VI, p. 16. 
Pcecilozonites circumfirmatus Pilsbry, Proc. Acad. Xat. Sci. Phila., 1888, p. 

The modern variety comes from both formations at locality 806, 
and from 814 and 808. Those from locality S08 are some of them more 
keeled than is now usual. A series of poor specimens from No. 807 
seem to bridge the gap from these to var. discrepans. 

This species has lost less in size than the others of its genus. My 
largest fossil, coming from locality 808, has alt. 7, diam. 12 mm. My 
largest recent shell has alt. 7, diam. 11.5 mm. I think the fossils 
average larger than the adults of the recent shells, but it is not 
easy to eliminate the immature of either. 

Poeoilozonites circumfirmatus v?- discrepans Pfr. 
Helix discrepans Pfr., Malak. Bl., 1864, p. 1. 

Localities 807, 818 and two specimens of doubtful identity from 806. 
Some from 818 are extremely flat and carinate, one of them having 
alt. 4.8, diam. 10.5 mm. If this were the only locality that yielded 
the variety it would undoubtedly rank as a distinct species. 

I should like to raise the question whether Pcecilozonites discrepans 
is not one of the extinct varieties. I believe it has not been treated 
as such heretofore, but none were found last summer any more recent 
than those from this sand pit. 

Euconulus turbinates n. sp. PI. XXXVI, figs. 8, 9, 10. 

Shell acutely conic, with contour very slightly convex; minutely per- 
forate, thin, glistening yellowish-corneous, closely striate, and sculptured 
with microscopic spirals. Apex rounded off abruptly. Whorls 7£, 
not convex, narrow, the last strongly angulate at the periphery. 
Suture simple, hardly impressed. Base rather fiat, not excavated. 
Aperture almost quadrangular, but with the angle at the columella 
indefinite. Columella slightly curved, the columellar margin narrowly 
refiexed. Alt. 3.4, diam. 2.8 mm. (from locality [807) ; diam. 3 mm. 
(from locality 808). 

From localities Nos. 807, 806, 814, 816, 808, and 818. 

The above description is a composite. The general form is described 
from the specimen from locality 807, but the sculpture is that of the 
best specimen from 806, which should, perhaps, be considered the type, 
and the base and aperture are taken from the specimen from 808. 
From 814 comes the longitudinal section of one 3.8 x 2.8 mm., with an 
.unusually convex contour. 

The genus Eucomdus is of course, not wholly satisfactory for this 


Zonitoides minusculus Binn. 

Locality 807, and recent. Its abundance in the one deposit and 
absence in the others is a little surprising. 

Zonitoides bristoli n. sp. PI. XXXVI, fig. 13. 

Shell resembling Zonitoides minusculus in general form, but much 
smaller, only moderately umbilicate, white, costulate, and densely 
sculptured with spiral lines ; composed of 3 convex whorls. Apex 
somewhat elevated. Aperture lunate, the outer and basal margin 
more uniformly curved than in Zonitoides minusculus, and the preced- 
ing whorl cutting out a greater arc. Peristome simple, thin. Costulse 
regularly spaced, coinciding with growth lines. The spaces between 
them crowded with fine striae. A close, regular, spiral sculpturing 
crosses these lines and gives the costulse a slightly tubercular appear- 

Alt. .7 Diam. 1.17 mm. 

Named in honor of Dr. C. L. Bristol, of New York University, Associate 
Director of the Bermuda Biological Station for Research. 

One specimen from each of localities 807 and 818; the type from the 
latter place. 

Succinea bermudensis Pfr. 

| S. bermudensis Pfr., P. Z. S., 1857, p. 110; Monographia, IV, p. 817. 
S. barbadensis Pilsbry, Trans. Conn. Acad., X, p. 502. 

Localities 807, 806, 818, 808, 809 and recent. In the absence of alco- 
holic specimens of S. barbadensis I have given up that name and re- 
turned provisionally to the name bermudensis. Its presence as a fossil 
makes it not unlikely that it may be proved distinct from S. barbadensis. 
This is another species that was formerly larger than now. The largest 
fossil, from locality 808, measures alt. 13, diam. 7 mm. The largest 
out of 30 recent specimens lent b} r Mr. Bryant has alt. 12, diam. 6.3 mm. 
Helicina convexa Pfr. 

If this species were indigenous we could expect it to be as abundant 
formerly as it is now. Instead of that it seems to be entirely absent 
from the beds I examined. The evidence seems to me strong that its 
real home is elsewhere. 



Map 1. 


Bermuda Island. 




Map 2. 








S A 


















■ I 


























" li 



















mSftj Sr ' 




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Reference to Plate XXXVI. 

Figures 2 to 5 are natural size; the others are variously enlarged. 

Plate XXXVI Fig. I.— Poecilozonites dalli. 

Fig. 2. — Poecilozonites cupula. Locality 806. 

Fig. 3. — Poecilozonites bermudensis zonatus. Locality 808. 

Fig. 4. — PcBcilozonites nelsoni form discoides. Locality 806. 

Fig. 5. — Poecilozonites nelsoni callosus. Locality 818- 

Fig. 6. — Vertigo numellata. Locality 806. 

Fig. 7. — Vertigo marki. Locality S06. 

Fig. 8. — Euconulus turbinatus. Section from compact rock, locality 814. 

Fig. 9. — Euconulus turbinatus. Locality 806. 

Fig. 10. — Euconulus turbinatus. Locality 808. 

Figs. 11, 12. — Carychium bermudense. Locality 806. 

Fig. 13. — Zonitoides bristoli. Locality 818. 


April 19. 
The President, Samuel G. Dixon, M.D., in the Chair. 

Seventy-six persons present. 

The deaths of Edwin Sheppard, April 7, and E. W. Clark, April 9, 
members, were announced. 

The Publication Committee reported that papers under the follow- 
ing titles had been offered for publication : 

"A Monograph of the Genus Denclrocincla Gray," by Harry C. 
Oberholser (April 8). 

"Post-Glacial Nearctic Centers of Dispersal for Reptiles," by Arthur 
Erwin Brown (April 11). 

Dr. E. G. Conklin made an illustrated communication on the earli- 
est differentiations of the egg, with special reference to the mechanism 
of heredity and evolution. (No abstract.) 

The following were elected members: Everett F. Phillips, Herbert 
Guy Kribs, Henry R. M. Landis, M.D. 

The following were ordered to be printed : 













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