published in 1844.* The ova of the Sepia are
deposited singly, but are attached in numbers
close together by pedicles to the stalks of
Algse and other marine productions. Those
of Loligo arc arranged in sm?ll masses, in
which a number are enclosed in a general bag
or covering of gelatinous matter, which is at-
tached along with others of the same kind by
means of pedicles. I have found those of
Sepiola also thus enclosed in small pyriform
capsules.
The ovum of Cephalopoda possesses a firm
laminated external covering or chorion, which
in some is darkened on the surface by the
colouring matter or ink, in others is trans-
* Entwickelungs-gesch. der Cephalopoden, 4to.
Zurich, 1814.
parent and' colourless. Immediately within
this outer membrane is situated a structureless
vitelline membrane, containing the mass of
yolk-substance, which is separated from the
membrane by a slight interval. It appears to
be ascertained that the chorion is formed by
superposition on the surface of the ovum dur-
ing its descent through the oviduct.
In the ovary the ova are contained in slender
capsules, attached to the rest of the ovary
by narrow pedicles. When ripe the ova escape
from the capsules, in some species by an ir-
regular laceration, in others by a more regular
and defined opening, and, falling into the cavity
of the ovary, pass thence into the oviduct,
through which they are finally excluded. Fe-
cundation is believed to occur soon after the
escape of the ova from their ovicapsules or
in the earlier part of their descent through
the oviduct ; but this process has not, so far
as I am aware, been directly observed.
The ova of the common Sepia officinalis
have an oval form, one end being much nar-
rower than the other. It is at this the pointed
extremity or narrow pole of the egg that the
germinal vesicle is situated, while the egg is in
the ovary, close under the vitelline membrane;
and it is at this part also that, at a subsequent
period, the process of segmentation and the
first formation of the embryo take place. The
narrow end is therefore the germinal pole.
This extremity of the egg is always turned to
the opposite side from the pedicle of the cap-
sule, which is attached to the middle of the
blunt or wider end.
One of the most remarkable peculiarities in
these ova, is the extraordinary change which
the outer part of the yolk and the vitelline
membrane undergo during the greater part
of the time occupied by the growth of the
ovum in the ovary. This change, of which
the appearance had been known to some
previous observers, was first accurately-
described and explained by Kolliker. From
his observations it appears that at first
the ovarian ova are quite smooth on the
surface, and that at the time of complete ma-
turity of the ovum, or after its escape from
the ovary, the vitelline membrane and surface
of the yolk are also quite smooth ; but that
in the intervening time, that is, during the
greater part of the period of its growth, the
surface of the yolk is indented or marked with
peculiar grooves, into which folds of the vitel-
line membrane pass so as to line them to the
bottom, somewhat after the manner in which
the pia mater descends into the sulci of the brain,
but without the same convoluted form. This has
been represented by Kolliker in the Sepia, and
1 have observed it in this genus, and have con-
firmed in every particular that author's state-
ments as to this change. It appears that
at first these inflections of the yolk and
membrane begin as longitudinal folds, extend-
ing between the wide and narrow poles of the
ovum, and, gradually increasing, become at
last so deep as almost to meet each other in
the interior of the yolk. Subsequently they
are traversed by more numerous depressions.
[106]
OVUM.
which subdivide them ; and as these cross folds
are formed the longitudinal ones beome gradu-
ally shallower. The surface of the egg then
presents the reticulated appearance which is
shown in fig. 70.* On making a section
through such an egg, hardened in alcohol or
any other suitable reagent, it is easy to per-
ceive that the ovicapsule takes no part in the
inflections, but that they consist entirely in the
grooving of the yolk, and the corresponding
bending into the grooves of the vitelline mem-
brane. This state is maintained till the ovum
is approaching maturity, when the depth of
the grooves or folds speedily diminishes; and
these come at last to be completely effaced in
those ova which have left the ovicapsule.
In Loligo, it is stated by Kolliker, there are
only the longitudinal folds. No satisfactory
opinion has been offered as to the cause of
this peculiar structure.
Fig. 70 *.
Ova of the Sepia. (From Kolliker.')
A. Three ovarian ova of the Sepia in somewhat
different stages of advancement attached by their
pedicles to the ovary, and represented several times
magnified. They all show the reticulated mark-
ings on the surface produced by the folding in of
the vitelline membrane ; g, the place of the germi-
nal vesicle and possibly also of a micropyle at the
Bonall pole of the egg, in which segmentation after-
wards occurs.
B. Direct view of this germinal pole of one of
the ova, showing the absence of the folds towards
the centre in which the germinal vesicle is situated.
c. Cross section of one of the ova, showing at o
the unfolded or smooth ovarian capsule, and at v m
the folded vitelline membrane.
Towards the narrow pole of the ovum, the
folds now described become less marked ; and
they are entirely absent just at the pole itself,
so that the germinal vesicle may be seen in the
smooth space which is left between them. At
this place I had some expectation to find an
aperture of the nature of a micropyle ; and I
accordingly sought for it in some specimens of
the ovarian ova of Sepia which I had pre-
served in alcohol, but without success, per-
haps on account of the opacity produced in
the membranes by the alcohol, and the adhe-
sion of the yolk substance to them. Professor
Kolliker has since informed me that he believes
the micropyle to exist in these ova, which I
think extremely probable.
The germinal vesicle, according to this ob-
server, remains entire and visible till the ova
are mature, as may be seen by the examination
of specimens hardened in alcohol. It dis-
appears just about the time of the ova leaving
the ovarian capsule ; but in several instances
he found it still remaining in ova that were
already free. It was always gone in those ova
which had regained the smoothness of their
exterior.
The yolk-substance of the mature ova con-
sists entirely, excepting immediately at the
seat of the germinal disc, of corpuscles some-
what similar to the vitelline tablets of the Frog's
egg. At an earlier period there are heaps of
fine granules of the same size, from which the
corpuscles are therefore probably formed. In
the earliest stage the vitelline substance is
entirely composed of fine molecules the pri-
mitive yolk which appear to be formed in
the same manner as in other animals.
The Cephalopoda furnish a remarkable ex-
ample among the Invertebrata of a very limited
or partial segmentation. This process, upon
the detailed description of which I will not
enter here, usually commences in a spot either
in or more frequently near to the germinal
pole, by the formation of the primitive groove
which extends across the disc. The forma-
tion of the second groove, which crosses the
first, the production of other radiating
grooves, the separation of annular sets of
segments from the periphery, and the suc-
cessive steps of the process which follow, are
probably determined by the same circum-
stances which have been referred to as re-
lated to this phenomenon in the cicatricula of
the Bird's egg.
Gasteropoda. In the greater number, if not
almost all, of the remaining Mollusca, the
ova differ greatly from those of the Cephalo-
poda, and approach more nearly to those I
have classed under the groups possessing the
small or middle sized yolk, which is princi-
pally or entirely formative, and which under-
goes a more or less complete segmentation.* In
the Pteropoda and in those of gasteropodous
Mollusca, which have the male and fe-
male organs in the same individual, there is
a remarkable combination of the ovary and
testis in a single hermaphrodite organ, usually
* The genus Sagitta, among the Pteropoda, is,
however, probably an exception to this statement,
as in it, according to Darwin, the embryonal part
of the yolk is distinct from the rest, or rather covers
it like a ring.
U07J
jenital Organs of Phyllirhoe bucephalum, one of the Hermaphrodite Gasteropoda.
Gegenbaur.*)
(From H.MiiUerand
A. The compound or hermaphrodite organs dissected out and represented several times magnified ;
o t, the two productive organs each composed of ovigerous and seminiferous parts ; v d, the common
excretory ducts for both kinds of organ ; v s t the seminal vesicle ; , the uterus ; p, a part of the
penis ; c, the common external vent.
B. One of the lobes of the common productive organ laid open and more highly magnified.
Towards the surface o o, the ova are seen in different stages of development in the ovarian stroma ; in
the interior t t, the substance of the testis with spermatic cells and spermatozoa in various degrees
of advancement ; some of the filaments being very long ; v d, the common excretory duct for ova and
spermatozoa.
enclosed in the liver, the nature of which was
for along time involved in obscurity, and occa-
sioned much doubt and difficulty to naturalists.
The explanation of this peculiar structure we
owe first to H. Meckel*, and Leuckart-f- ; and
more recently H. Meckel and Gegenbaur have
described this organ particularly in one of the
heteropodous Mollusca, viz. Phyllirrhoe buce-
phalum.J The outer part of this curious organ
constitutes the ovary, the inner the testis ; and
the products of these respective organs, in
leaving the seat of their first formation, pass
together into an inner common cavity, and
thence downwards in the excretory duct.
There is, therefore, a common outlet for both.
The ova and spermatozoa most frequently
pass out at different times ; but occasionally
both these reproductive elements are seen to-
gether in the passages. It seems probable
therefore that they in general meet for im-
pregnation only in 'the lower part of the pas-
sages ; but this apparently is not yet fully de-
termined, and the modes of union may be
* Miiller's Archiv. for 1844, p. 483., see plates
xiv. and xv.
t Zur Morphologic und Anatomie der Geschlechts
Organ. 1847, p. 128.
I Zeitsch. fur Wissen. Zool. vol. v. p. 355. pi. xix.
various in different genera or families. At
all events, the primitive ova and spermatozoa
seem to come into contact with each other
previous to the addition of the enveloping
membrane. *
The ova of the Mollusca are in general of
small size. The yolk consists of a viscid al-
buminous substance, containing suspended in
it minute granules, and a variable quantity of
coloured oil globules. The germinal vesicle
is proportionally of considerable size ; and the
macula is distinct and granular. Leuekart*f*
states that he and Nordmann have ascertained
that in Lymneus, the first part of the ovum
which is formed is the germinal vesicle, that
the yolk-substance begins as a clear trans-
parent albuminous matter surrounding the
germinal vesicle, and, as we have seen in various
other animals, the granular yolk matter is
gradually deposited in this clearer part of the
vitelline substance, occupying at first princi-
* The hermaphrodite gland exists in the Ptero-
poda, Apneusta, Nudibranchia, Infero-branchia, Tec-
tibranchia, and Pulmonata. The Mollusca which
have separate sexual organs belong chiefly to the
orders Cyclobranchia, Scutibranchia,Tubulibranchia,
and Cirnbranchia, some Heteropoda, Pectinibranchia,
and operculate Pulmonata or Cydostoma.
t Article Zeugnng, p. 800.
[108]
OVUM.
pally its outer part. The vitelline membrane
does not exist at first, but seems to be formed
at a later period by the consolidation of an
external layer of the primitive yolk substance.
The time of the disappearance of the ger-
minal vesicle has not been determined in
many of these Mollusca. Previous to seg-
mentation a phenomenon occurs, which has
now been observed in a large number of ani-
mals, but which first attracted special atten-
tion in the gasteropodous Mollusca; viz., the
separation of one or more clear hyaline liquid
globules of considerable size from the surface
of the yolk substance, into the space be-
tween it and the vitelline membrane. This
was first observed by Dumortier*, and de-
scribed by Pouchetf, by Van Beneden in
the Aplysia J, by Nordmann in Tergipes Ed-
wardsii $, by C. Vogt in Actoeon || , and by
various others. A precisely similar phenome-
non has also been observed in some of the
Vertebrata, as in Mammalia by Wharton
Jones, Barry, and Bischoff, and in Batrachia
by Newport. But though this separation of
one or more hyaline globules from the yolk-
substance at the time of segmentation appears
to be a very general accompaniment of that
process, it must be confessed that its import,
either in connection with fecundation or de-
velopment, has not yet been ascertained.
Acephala. In Acephalous Mollusca the
ova are generally of small size, the yolk-sub-
stance principally finely granular, the germinal
vesicle clear, with a distinct macula, which last
not unfrequently presents the form of a double
or elongated biscuit-shaped particle. The
vitelline membrane is distinct and possesses
considerable strength ; and there is generally
a considerable space occupied by clear fluid
between it and the surface of the yolk. The
most interesting feature of the ova of these
Mollusca is the funnel-shaped aperture
which most of them possess, leading through
the vitelline or external membrane into the
space occupied by the yolk. This aperture,
styled micropyle by J. Miiller in the Ho-
lothuria, the first instance in which it was
discovered, in 18591F, was observed in the
ova of Unio and Anodonta by Leuckart **
and Keber. ff The latter author supposed
that he had observed the penetration of a
spermatozoon into the ovum through this
aperture, and has described with great form-
ality and minuteness all the phenomena which
he conceived were related to that process.
Although Keber was correct in asserting the
existence of the micropyle in these Mollusks,
* Embryol. des Mollusques, in Annal. des Scien.
Nat. for 1837, p. 136.
f Id. lib. for 1838, vol. x. p. 63. See also Pou-
chet's further observations in his work, Theorie
positive de 1'Ovulation spontanee, pi. xvi.
J Annal. des Scien. Nat. 1841, p. 126.
Id. lib. 1846, p. 147.
|| Sur PEmbryol. des Mollusques Gasteropodes, id.
lib. 1846, p. 33.
f Archiv. 1852, p. 19.
"* Article Zeugung, p. 801, Ann. 1853.
ft De Introitu spermatozoorum in ovula, &c.
Konigsberg, 4to. 1853.
Fig. 112
Ova of Unio in different stages of development.
(A. B. C. and D., from Hassling ; E. from Keber.}
A. The early stage of the ovum, when the ger-
minal vesicle alone is distinguishable lying in a
bulging part of the ovarian substance.
B. The same somewhat more advanced; the
ovicapsule and vitelline membrane have assumed
the pediculated form, and the yolk granules sur-
round the germinal vesicle.
c. The ovum now enlarged and spherical in form,
the yolk granules increased in quantity, and the
pedicle narrowed so as to form a short micropyle
tube ; s, the small body taken by Keber for a sper-
matozoon, existing long previous to the occurrence
of fecundation.
D. The ovum, &c. at a later stage ; g, the ger-
minal vesicle ; v, the yolk ; t/, the separated portion
of the yolk ; s, as in c. now enlarged.
E. A nearly similar stage of the ovum as figured
by Keber. Some of the contents of the separated
portion of the yolk are escaping through the micro-
pyle aperture ; s, Keber's alleged spermatozoon.
it appears that the body described by him as
spermatozoon cannot have been of that nature,
seeing that it has been proved by other
observers that the appearance on which
Keber's supposition was founded existed
long before fecundation, and remained
long after the commencement of embryonic
formation in the same condition.*
The existence of a similar aperture or micro-
pyle in several other Acephalous Mollusca
has been ascertained by the recent investiga-
tions of various authors ; but the actual en-
trance of the spermatozoa by the aperture,
has not, so far as I aware, been satisfactorily
observed. There seem, however, to be suffi-
cient grounds for believing that in the Ace-
phala, as in other animals in which it is found,
the micropyle is immediately related to the
process of impregnation, by affording a ready
access of the spermatozoa to the yolk through
the more resistent membranes of the ovum.
The accompanying figures from Keber and his
critic Hessling give a sufficiently clear view of
* Hessling, in Zeitsch. fiirWissensch. Zool., 1854,
vol. v. p. 380. ; and Bischoff, "Wiederlegung des von
Dr. Keber bei den Naiaden, &c. Giessen, 4to. 1854.
OVUM.
[109]
the nature of this structure in the mature
ovum of Anodonta.
In this family of Mollusca the micropyle
forms a small but very apparent funnel-shaped
projection from the surface of the outer mem-
brane ; and its hollow nature may easily be
ascertained by the fact that the fluid and
granular yolk-substance may be forced through
it from within. The yolk ball is placed ex-
centrically within the vitelline membrane, the
inner surface of which it touches just at the
place where the micropyle is situated.
From a variety of observations, it has been
Fig. 73*.
Structure and Fonnation of Ova in Acephala.
(From Lecaze Duthiers.}
a. Portion of the ovary with three pediculated
ovicapsules and contained ova from Cardium rusti-
cum, magnified 400 diameters ; the micropyle is
afterwards formed at the place where the pedicles
are detached from the secreting coeca of the ovary.
b. Unripe ovum of Spondylus gaederopus magni-
fied 170 diameters, showing the remains of the cap-
sule at the upper part, and the projection of the
vitelline membrane at the same place where the
micropyle is situated.
c. Ripe ovum of the same burst by pressure,
showing the escape of some yolk granules through
the micropyle and into the space between the yolk
and the outer membrane. In^this and the previous
figure the double state of the macula is represented.
shown that the micropyle of the Acephalous
Mollusca owes its origin to the early pediculated
attachment of the ovum. This has been fully
brought out by the observations of Hessling
in Unio and Anodonta, of Leydig in Venus,
and of Lecaze Duthiers in Cardium and some
other genera.* From these observations it
appears that the ova first arise in the ova-
rian stroma by the formation of the ger-
minal vesicles, as in most other animals, each
vesicle possessing a distinct single macula.
These vesicles come very soon to be surrounded
by some of the primitive or finely granu-
lar yolk, which gradually increases in quantity.
These parts are from a very early period en-
closed by a membrane which may be regarded
as vitelline, but which is differently disposed
from that in any of the animals previously re-
ferred to ; for, instead of having a regular and
complete spheroidal or vesicular form, this
membrane is elongated at one part into a
pedicle, so as to give the whole of the early
ova a pyriform shape, and so as to attach them
to the ovarian substance by the pediculated
parts of the vitelline membrane. In Venus de-
Fig. 74*.
Ovarian ova of Venus decussata. (From Leydig.')
a. A group of five ova in their earliest stage
projecting from the ovary in their pediculated
capsules : the germinal vesicles with single macula,
the vitelline granules, vitelline membrane, and
ovicapsule are all distinct.
b. Two ovicapsules within which at a more ad-
vanced stage the ova have become detached from
their pedicles, the remains of which at the upper
ends of the ova form the micropyle. A considerable
amount of albumen has been deposited between the
ovum and the ovicapsule.
* Ann*! des Scien. Nat 1854, ii. p. 155.
[no]
OVUM.
cussata, according to Leydig*, the ova are ar-
ranged in aggregated pediculated groups, from
which it seems probable that they are originally
produced in numbers by the multiplication or
division of multiple germs, somewhat in the
same manner as will afterwards be stated to
have been observed by Meissner among the
Gordian Nematoid Worms. An albuminous
layer is afterwards formed externally, and may
be instrumental at last, from its increasing
thickness, in separating the ovum from its pe-
diculated attachment to the ovary. There
seems therefore to be little doubt that in these
Mollusca, and in a certain number of other
Invertebrate animals in which the micropyle
has been observed, this apparatus is produced
by the remains of an original or early ovarian
pedicle. In the Unio and Anodonta it is
certainly not formed by the peculiar process
of development from within the ovum, which
has been elaborately described by Keber.-j*
It will afterwards be shown, however, as in-
deed may be concluded from what has already
been stated in regard to osseous fishes, that
in other animals the micropyle may arise in
other modes and without the early existence
of the pedicle now described.
When the ova are detached by the rupture
of the pedicle in the Acephala, they lie, in
different stages of advancement, but all pro-
vided with the micropyle, in the general ovarian
cavity. The coverings of the Acephalous
ovum appear to be composed at least of
two layers, of which the inner may per-
haps be looked upon as the vitelline mem-
brane, the outer as a chorion ; but sufficient
data have not yet been furnished to determine
the homological rank of these membranes.
The early connection, in a pediculated form,
with the ovarian stroma might point to a
different view of their nature. Leydig states
that while in Unio and Anodonta the albu-
men is deposited within the membranes, in
Venus it is added externally. The micropyle
appears to be closed previous to the com-
mencement of embryonic development.
Arthropoda. The ova of Articulate animals
might with most propriety be classed with
the large-yolked group, at least as regards the
ova of Insecta, Arachnida, and the higher
Crustacea. In addition to the germinal vesicle
and finely granular yolk-substance, they all
contain a large proportion of clear or oil glo-
bules of considerable size ; and the process of
segmentation is generally limited to a small
portion of the yolk surface. The ova of
.these three classes present, however, many
subordinate differences in their structure and
mode of production, which renders it neces*
sary to give a short separate account of them
in this place.
Insecta. The ova of insects are more
especially distinguished by the extraordinary
varieties of their external form and appearance.
These varieties affect, however, principally,
or depend upon modifications of the external
M tiller's Archiv. 1854, p. 320.
Hessling and Bischoff, loc. cit
covering, chorion or shell-membrane, as it has
been called. They differ also from those of
most other animals in a frequent departure
from the regular symmetrical form. Some
are nearly hemispherical, others more oval ;
many are somewhat bent in an antero-posterior
direction *; many present the most curious
elevations and irregularities on their external
surface reticulated ridges or fringes, and de-
pressions, tubercles, hairs or spines, or other
long processes, sometimes single, at other
times in numbers. These modifications
of the external coverings of the eggs of
Insects appear to have reference chiefly to
the protection of the ova from the effects of
external injury, and to serve various mechani-
cal purposes connected with their deposition
and attachment ; but they are not, in most at
least, attended with any important varieties in
the internal structure, which, on the whole,
presents considerable uniformity throughout
the whole class. The ova of all insects, we
shall afterwards see, are provided with one or
more apertures corresponding to the micro-
pyle. t
All recent observers agree that, in the ova
of Insects, in addition to the external shell-
covering, there is a delicate transparent vitel-
line membrane. The germinal vesicle is of pro-