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1904.] NATURAL SCONCES OF PHILADELPHIA. 325
THE CELL-LINEAGE AND EARLY LARVAL DEVELOPMENT OF FIONA
marina, a nudibranch mollusk. 1 '
by dana brackenridge 'casteel, ph.d.
Outline.
Introduction.
Material and Method.
Nomenclature.
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.
Gastrulation.
Ecto-Mesoblast
Closure of the Blastopore.
Organogeny.
The Velum.
Later Velar Development.
Head Vesicle.
Nerve and Sense Organs.
Cerebral Ganglia.
Otocysts and Pedal Ganglia.
Eyes.
Excretory Organs.
The Enteron.
Stomodseum and Mouth.
Shell Gland and Foot.
Larval Musculature.
Change of Axis and Form of the Developing Organism .
Abstract.
Table of Cell-Lineage.
Introduction.
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-
svlvania.
' J Dr. H. A. Pilsbry, of the Academy of Natural Sciences of Philadelphia, has
kindlv assisted me in identification.
326 PROCEEDINGS OF THE ACADEMY OF [April,
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-
sible.
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 327
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.
328 PROCEEDINGS OF THE ACADEMY OF [April,
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.
Nomenclature.
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 3£9
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).
330 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 331
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.
332 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 333
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
334 PROCEEDINGS OF THE ACADEMY OF [April,
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-
tation.
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-
1904.] NATURAL SCIENCES OF PHILADELPHIA. 335
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
336 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 337
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
22
338 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 339
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-
340
PROCEEDINGS OF THE ACADEMY OF
[April,
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.
1904.] NATURAL SCIENCES OF PHILADELPHIA. 341
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
342 PROCEEDINGS OF THE ACADEMY OF [April,
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
1901.] NATURAL SCIENCES OF PHILADELPHIA. 343
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
Limax.
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)
344 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 345
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
346 PROCEEDINGS OF THE ACADEMY OF [April,
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-
1904.] NATURAL SCIENCES OF PHILADELPmA. 347
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-
348 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 349
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
direction.
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
350 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 351
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
352 PROCEEDINGS OF THE ACADEMY OF [April,
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 :
1904.
NATURAL SCIENCES OF PHILADELPHIA.
353
1st.
2d.
3d.
4th.
2a
121
211
122
212
2b
121
211
212
122
2c
121
211
212
122
2d
211
121
212
122
(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.
23
354 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 355
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
356 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 357
as one hundred and fifty cells present this quartet consists of but four
cells in each quadrant.
Gastrulation.
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
358 PROCEEDINGS OF THE ACADEMY OF [April,
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
Ecto-Mesoblast.
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
larva.
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.
1904.]
NATURAL SCIENCES OF PHILADELPHIA.
359
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.
dS>®
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
360 PROCEEDINGS OF THE ACADEMY OF [April,
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.
1904.] NATURAL SCIENCES OF PHILADELPHIA. 361
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
362 PROCEEDINGS OF THE ACADEMY OF [April,
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).
1904.] NATURAL SCIENCES OF PHILADELPHIA. 363
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
conduces.
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
previously.
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
364 PROCEEDINGS OF THE ACADEMY OF [April,
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.
Organogeny.
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 365
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
366 PROCEEDINGS OF THE ACADEMY OF [April,
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
Fiona.
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 367
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
368 PROCEEDINGS OF THE ACADEMY OF [April,
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-
1904.] NATURAL SCIENCES OF PHILADELPHIA. 369
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
24
370 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 37 1
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
372 PROCEEDINGS OF THE ACADEMY OF [April,
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.
1904.] NATURAL SCIENCES OF PHILADELPHIA. 373
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),
374 PROCEEDINGS OF THE ACADEMY OF [April,
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-
1904.] NATURAL SCIENCES OF PHILADELPHIA. 375
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
quartet.
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
376 PROCEEDINGS OF THE ACADEMY OF [April,
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,
1904. J NATURAL SCIENCES OF PHILADELPHIA. 377
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
Lacaze-Duthiers.
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
378 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 379
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-
tions.
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-
3S0 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 3S1
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.
382 PROCEEDINCxS OF THE ACADEMY OF [April,
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-
1904.] NATURAL SCIENCES OF PHILADELPHIA. 383
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
384 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 385
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
25
3S6 PROCEEDINGS OF THE ACADEMY OF [April,
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 387
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
388 PROCEEDINGS OF THE ACADEMY OF [April,
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.
107).
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 389
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
390 PROCEEDINGS OF THE ACADEMY OF [April,
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
392 PROCEEDINGS OF THE ACADEMY OF [April,
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
rearrangement.
Abstract.
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-
1904.] NATURAL SCIENCES OF PHILADELPHIA. 393
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
394 PROCEEDINGS OF THE ACADEMY OF [April,
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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Holmes, S. J. The Cell-Lineage of Planorbis. Zool. Bull, Vol. I, 1897.
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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,
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1904.] NATURAL SCIENCES OF PHILADELPHIA. 397
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Ziegler, E. Entwieklung von Cyclas cornea. Zeit. Wiss. Zool, Bd XLI, 1SS5.
398
proceedings of the academy op
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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.
1904.] NATURAL SCIENCES OF PHILADELPHIA.
Table of Cell-Lineage,
b. quadrant.
399
400
proceedings of the academy of
Table of Cell-Lineage.
c. quadrant.
[April,
1904.] natural sciences of philadelphia.
Table of Cell-Lineage.
d. quadrant.
401
402 PROCEEDINGS OF THE ACADEMY OF [April,
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
foot.
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
foot.
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
spindle.
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-
nuclei.
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
cleavage.
1904.J NATURAL SCIENCES OF PHILADELPHIA. 403
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
divide.
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.
404 PROCEEDINGS OF THE ACADEMY OF [April,
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
bilaterally.
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-
blasts.
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 405
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.
XXXI.
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.
406 PROCEEDINGS OF THE ACADEMY OF [April,
THE FOSSIL LAND SHELLS OF BERMUDA. 1
BY ADDISON GULICK.
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
deposit.
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.
1904.] NATURAL SCIENCES OF PHILADELPHIA. 407
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
nelsoni.
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.
408 PROCEEDINGS OF THE ACADEMY OF [April.
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
age.
The following are my records of fossil and semi-fossil shells in these
localities :
Locality 807.
PCECILOZONITES NELSONI.
" NELSONI CALLOSUS.
" CIRCUMFIRMATUS, | , , . .
,, r Intergraded.
DISCREPANS. »
EUCONULUS TURBINATES.
ZONITOIDES MINUSCULUS.
" bristoli. One specimen.
Thysanophora hypolepta.
succinea bermudensis.
Vertigo numellata.
" MARKI?
Carychium bermudense.
Casts in the Rock, Locality 806.
pcecilozonites circumfirmatus.
Vertigo.
Carychium?
Cave and Pockets, Locality 806.
Pcecilozonites nelsoni. Both extremes in height of spire.
" BERMUDENSIS ZONATUS.
" REINIANUS.
" CIRCUMFIRMATUS.
1904.] NATURAL SCIENCES OF PHILADELPHIA. -409
PCECILOZONITES CUPULA.
EUCONULUS TURBINATUS.
Thysanophora HYPOLEPTA.
succinea bermudensis.
Strobilops hubbardi.
Bifidaria rupicola. One specimen.
.Vertigo numellata.
" MARKI.
Carychium bermudense.
Locality 814-
PCECILOZONITES NELSONI,
" REINIANUS
" nelsoni. In crevices.
" BERMUDENSIS ZOXATUS,
r In one pocket.
In crevices.
sis zoxatus, 1
reinianus, I In stalagmitic conglom-
CIRCUMFIRMATUS,
EUCONULUS TURBINATUS
erate.
Locality 815.
Pgecilozonites bermudensis zonatus? Small fragments only.
" REINIANUS.
Locality 816.
PCECILOZONITES NELSONI.
" BERMUDENSIS ZONATUS.
" reinianus. (None kept in the collection.)
EUCONULUS TURBINATUS.
Locality 818 (Sand Pit).
PCECILOZONITES NELSONI CALLOSUS.
" REINIANUS.
" DISCREPAXS.
EUCONULUS TURBINATUS.
ZONITOIDES BRISTOLI.
SUCCINEA BERMUDENSIS.
Bifidaria servilis. One specimen.
Carychium bermudense.
Locality 808 (Sand Pit).
PCECILOZONITES BERMUDENSIS ZOXATUS.
" REINIANUS.
410 PROCEEDINGS OF THE ACADEMY OF [April,
PCECILOZC ) N I TES CIRCTJMFIRMATTJS.
EUCONULUS TURBINATUS.
SUCCINEA BERMUDENSIS.
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
notice.)
Locality 809 (Sand Pit).
PfJECILOZONITES BERMUDENSIS ZONATUS.
" REINIANUS.
" 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.
" CUPULA.
" BERMUDENSIS ZONATUS.
" REINIANUS.
" CIRCUMFIRMATUS.
" DISCREPANS.
1904.]
NATURAL SCIENCES OF PHILADELPHIA.
411
EUCONULUS TURBINATUS.
ZONTTOIDES BRISTOLI.
Vertigo numellata. \ Eastern North America.
" MARKI.
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, ]
" REINIANUS, j
CIRCUMFIRMATUS, J Remnant of the fossil fauna>
'ZONITOIDES MINUSCULUS, g eyen ^^
Thysanophora hypolepta,
Succinea bermudensis,
5 blfidaria rupicola.
5 pupoides marginatus.
5 Thysanophora VORTEX,
5 POLYGYRA MICRODONTA,
3 BlFIDARIA SERVILIS,
5 BlFIDARIA JAMAICENSIS,
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.
412 PROCEEDINGS OF THE ACADEMY OF [April,
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
Bermuda.
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-
swept.
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-
1904.] NATURAL SCIENCES OF PHILADELPHIA. 413
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
414 PROCEEDINGS OF THE ACADEMY OF [Apiil
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.
1904.] NATURAL SCIENCES OF PHILADELPHIA. 415
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
islands.
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
Diam
. 39 mm
28
37
27
41
27
40
26
35
25
39
23.5
36
23
41.5
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-
416 PROCEEDINGS OF THE ACADEMY OF [April,
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:
Alt.
34
Diam. 34 mm,
31
33
19
37
19.5
39
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
marked.
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
1904.] NATURAL SCIENCES OF PHILADELPHIA. 417
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
12.5
17
13
19
13
20
15
15.5
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
nelsoni.
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.
27
418 PROCEEDINGS OF THE ACADEMY OF [April,
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-
mudensis.
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.
1904.] NATURAL SCIENCES OF PHILADELPHIA. 419
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.
420 PROCEEDINGS OF THE ACADEMY OF [April,
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.
291.
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
species.
1904.] NATURAL SCIENCES OF PHILADELPHIA. 421
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-
ance.
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.
422
PROCEEDINGS OF THE ACADEMY OF
Map 1.
[April,
Bermuda Island.
1904.]
NATURAL SCIENCES OF PHILADELPHIA.
423
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collecting stations.
1904.] NATURAL SCIENCES OF PHILADELPHIA. 425
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.
426 PROCEEDINGS OF THE ACADEMY OF [Apifl,
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 :
PROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXI.
/•*M
-4#
CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXII.
CASTEEL ON FIONA MARINA.
PROC ACAD. NAT. SCI. PHILA. 1904.
PLATE XXIII.
CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 190 i.
33 e r
PLATE XXIV
*F ME'
CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 1904.
2d " ed'" ! c '
PLATE XXV.
Jd" dt>" j!>
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CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXVI
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CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXVII.
,< " JJt ' j," " ' ,, •"• JJ.
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CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXVIII.
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CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXIX.
ed""
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CASTEEL ON FIONA MARINA.
'ROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXX
CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXXI.
S-Sh.6.
uc>
An.C
CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXXII.
CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXXIII.
Ap. SS.
Sh.&.-%
-31.
h- 28.
tfA G.
Str-
6,6.
CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXXIV.
CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXXV.
CASTEEL ON FIONA MARINA.
PROC. ACAD. NAT. SCI. PHILA. 1904.
PLATE XXXVI.
4
11 12
GULICK. FOSSIL LAND SHELLS OF BERMUDA.
S)
V