quainted with the mode of its production ;
for it is still undetermined whether it arises by
cell formation within the primitive vitelline
OVUM.
[135]
membrane through some change in the sub-
stance of the primitive yolk, or whether it is
derived, as I am inclined to believe may be
the case in birds and some other animals, in
a space external to these parts, and more in
connection with the cellular contents of the
ovarian follicle.
In limiting, then, our comparison to the
parts of the ovum in a bird and a mammifer,
we may regard the germinal vesicles as homo-
logous in both ; the finely granular germinal
disc of the bird's ovarian egg as homologous
with the whole vitellus of the maminiferous
ovum ; the zona pellucida of the mammiferous
ovum as temporarily represented by the clear
outer band of the primitive yolk, which is seen
in the bird's ovarian ovum when of a diameter
of from T \j to 2\y of an inch ; the cellular yolk of
the bird's egg, and its enclosing vitelline mem-
brane as probably homologous with the fluid
and granular contents and lining tunica granu-
losa of the ovarian follicle of the mammifer, and
not by any means with the corpus luteum of
a later period, as has been suggested by some.
The albumen of the bird's egg has its homo-
logue perhaps in the similar deposit which
the ova of several Mammalia acquire in their
descent through the Fallopian tube. The
chorion of the ovum of Mammalia, being an
after growth, has probably no true homologue
in the bird's egg, unless we should regard the
shell membrane and shell as occupying this
place.
Many ovologists have thought it of import-
ance to establish a comparison between the
ovum or its parts, and some one or other of
those microscopic structures which, since the
publication of the discoveries of Schleiden
and Schwann, have been known as organised
cells. Schwann himself, though looking upon
the entire animal ovum as a cell, entertained
some doubts as to the exact nature of the
comparison to be instituted for its several
parts. These doubts are not yet removed,
and the progress of knowledge has tended
rather to diminish than to increase the ap-
propriateness of the comparison, more espe-
cially from the somewhat various and indefinite
nature of the bodies which are now recognised
as organised cells.
It cannot be denied that, if we regard
merely the structure of the simpler ova of
small animals, we find in them the general
characteristics of an organised cell, as these
have been usually understood; that is, we
find the external structureless vesicular cell-
wall, the internal granular contents, and the
internal nucleus or inner cell with its nu-
cleolus. But when we consider more fully
the whole history even of the most simple ova,
and extend our regard to the structure and
history of the more complex forms of ova, we
perceive many circumstances which render
the comparison with detached animal cells
inapplicable.
Leuckart remarks, in his article Zeugung,
previously referred to (p. 815.), that if we
attempt to deduce the most general result
from what has been ascertained as to the
formation of the ovum, it is this, that " the
animal ovum is formed by deposit round the
germinal vesicle." The progressive forma-
tion of the parts of the ovum, therefore, would
appear to differ widely from that which
Schwann held to occur in most cells. But
our whole knowledge of the various forms
and modes of production of cell-like struc-
tures has been extended, and has undergone
some modification since the time of Schwann ;
and there are now known to be not a few cell
structures which are formed by external de-
posit of matter round a nucleus, nearly in
the same manner as occurs in the ovum. In
this view, therefore, the simpler kinds of ova
might be regarded as examples of " deposit
cells."
The great variation in the magnitude of
different ova, and the prodigious size which
some of them attain, as compared with the
minute and generally microscopic size of the
organised cells of the animal body, cannot by
itself be received as a conclusive argument
against the cellular constitution of the ovum ;
but the complexity of its structure, its rela-
tion to fecundation, the peculiar micropyle of
the outer wall in some, the separation of the
germinal from the nutritive part of the yolk-
substance, and the new formation of cells
after segmentation in a limited or more ex-
tended space over the yolk in the interior of
the vitelline membrane, are so widely different
from any thing belonging to the history of
other cells in the animal body, that we are
forced to regard the ovum rather as a struc-
ture of a peculiar kind than as a mere modi-
fication of a cell.
The germinal vesicle it might be held, both
in its structure and its mode of origin, merits,
more justly than the whole ovum, the com-
parison with an organised cell. But even in
its history there are points of difference, and
we are still too little acquainted with the
mode and consequences of its disappearance
at the time of the maturation of the ovum, to
warrant our making more than a vague and
general comparison of the germinal vesicle to
an organised cell. In admitting that the
ovum, or its germinal vesicle, present some of
the features of the organic cellular structure,
we ought always to bear in mind that they
are the source of all the other cells from which
the animal body is developed.
The manner of the very first origin of the
germ of the ovum is still involved in obscurity,
for we only know of the existence of an ovi-
germ when the germinal vesicle has attained an
appreciable size. Whence the first germs of
the germinal vesicles proceed can as yet be
matter only of conjecture. Without enter-
ing here upon the debated ground of the
origin of organised cells in general *, I would
venture to express the opinion, that as there
is no ovigerm without a parent, so there is no
new organisation without previously existing,
and at some time or other connected, orga-
nisation. Hence, notwithstanding the appa-
* See upon this subject the very interesting and
suggestive Review by Mr. Huxley in the Brit, and
For. Med. Chir. Review for October, 1853.
[**]
[136]
OVUM.
rent isolation of the origin of cells in blastema
or intercellular substance, it might still be
held that the unseen germs of new cells con-
tained in that blastema may have derived
their origin from other cells or organised parts
proceeding from cells. And thus, in regard
to the first origin of the ova of animals, it is
fair to conjecture that the germs from which
they spring have taken their descent from
parent cells or structures derived from cells
through the organs appropriate to their form-
ation. But here observation fails to assist us
further, and we are lost in the region of
speculation.
If, however, with the reservations now
stated, it should be thought desirable to
compare the ovum to the organic cellular
structures, the germinal vesicle may be re-
garded as the simple cell of the ovum, the
whole ovum as a complex cell ; the first of
these being formed probably by expansion
from a minute point or molecule, the second
by superposition or external deposit round
the internal cell ; but both at the same time
presenting features which are peculiar to
themselves, and different from those which
characterise other cells of the animal eco-
nomy. The different and separate formation
of the germinal vesicle and yolk, which is
perceptible to some extent in the ova of most
animals, is placed in its most striking point of
view by those instances in which, as in Tre-
matode and Cestoid Entozoa, there are dis-
tinct germigenous and vitelligenous organs,
and those in which, as in Nematoidea and
Insecta, the ovary is tubular, and the forma-
tion of the several parts of the ovum goes on
progressively in different parts of the tube.
4. Phenomena attendant on the maturation
of the ovum, and its discharge from the
ovary.
The ovum naturally undergoes in the ovary
a progressive development till it arrives at the
state of maturity, when it is usually separated
from the ovary by a process of dehiscence, is
conducted through the female passages either
to be excluded or laid, as in oviparous ani-
mals, or to be retained in a uterus or other
part of the female organs in viviparous ani-
mals during uterogestation. The maturation
of the ova and their separation from the
ovary is in many animals periodical and inde-
pendent of fecundation. This natural peri-
odical separation of the ova has been termed
Ovulation by some authors.*
The change which the germinal vesicle
undergoes at the period of the maturation of
* The observations of Bischoff had long ago
shown that in the periodical dehiscence of ova
which accompanies the heat of female quadrupeds,
the ova may be detected, though unfecundated, in the
course of their descent through the Fallopian tubes
and uterus (Periodische Losreifung, &c., 1842), and
some observations appear also to have shown that
the same is the case in the human female at the
periodical return of menstruation. (See a paper by
H. Letheby, M. B. in the Philos. Trans, for 1851,
p. 57., where two cases are described in which ovules
or their remains were detected in the Fallopian
tubes of unimpregnated women who had died at or
about the menstrual period.)
the ovum has naturally attracted much at-
tention, from the hope that through the ob-
servation of this phenomenon some explanation
might be obtained of the first origin of the
germ round which, after fecundation has taken
place, the segmenting and organising stratum
is collected, from which the blastoderm is
produced ; but it must be allowed that as yet
little success has attended our efforts to de-
tect the connection, if it exists, between these
two processes. In almost all animals the
germinal vesicle is lost to view at the time of
the maturation of the ovum, and generally
before or about the time when the ovum
leaves the ovary. In large-yolked ova the
macuke of the germinal vesicle become very
numerous by their multiplication and sub-divi-
sion at an early period ; while in the small-
yolked ova, as has been observed in a few
animals at least, the increase in the number
of the maculae does not take place till imme-
diately before the diffluence or disappearance
of the vesicle. The more minute phenomena
of this diffluence are as yet very imperfectly
known. In some animals, as Mammalia and
Birds, it has been observed that shortly
before the diffluence of the vesicle, its delicate
wall undergoes a softening or approaching
solution, so as to make it impossible to
separate the vesicle entire. After this, when
the diffluence is complete, the contents dis-
appear from the situation they have previously
occupied, but what becomes of them has not
yet been determined. In some instances, as
Birds and Batrachia, it has been observed
that, after the diffluence of the germinal ve-
sicle, the germinal part of the yolk, which
previously consisted exclusively of small
opaque granules, is now mingled throughout
with clear hyaline spherules, somewhat similar
to the dispersed maculae of the germinal
vesicle ; but it is only matter of conjecture
that these clear spherules have been derived
from the germinal vesicle or its maculae.
In a few instances, as in Ascaris, it has
been thought that the entire nucleus or
macula of the germinal vesicle has remained
undivided, and it has been supposed that it
has of itself constituted the germ of the
embryo-cell, which afterwards occupies the
centre of the first segmenting mass of the
yolk, and whose progeny by division exists as
nuclei in the interior of the successively in-
creasing segments of the cleaving germinal
portion of the yolk. A recent observation
by J. Muller seems to lead the way to a
different view of this phenomenon. He has
observed * in one of the Mollusca, the Ento-
choncha mirabilis, that the germinal vesicle
does not disappear or undergo a change at
the time of the maturation of the ovum, but
remains discernible as the foundation of the
clear embryonic -cell which occupies the centre
of the yolk mass when segmentation is about
to take place. Rernak f has been led, by his
observations on the Batrachian ovum, to
* Archiv. der Physiol. 1852. Leydig in the same.
+ Untersuch. iiber die Eutwickel. der Wir-
berthiere.
OVUM.
[137]
doubt the correctness of the view hitherto
generally adopted as to the entire disappear-
ance of the germinal vesicle in that instance,
and holds it as probable that a part of it at
least remains in connection with the forma-
tion of the embryonic cell. These statements
are sufficient to show that the phenomena of
the dehiscence of the germinal vesicle and
its relation to the subsequent changes in the
ovum induced by fecundation are as yet very
imperfectly understood, and that the discovery
has still to be made of the link in the chain
of connection between the last stage of
existence of the ovigerm, and the first origin
of the nucleus round which the subsequent
organising process of segmentation begins.
But that some such connection exists, all
who have made a study of this part of the his-
tory of the ovum are inclined to believe,
5. Relation of the ovum to fecundation by
the male sperm.
The act of fecundation is necessary for the
perfection of all true ova. In the production
of gemma? or buds, in the multiplication of
nonsexual individuals, and in the various
examples of Metagenesis previously referred
to, the germs from which the new products
arise may be nucleated cells or groups of
these, and may without doubt be the descend-
ants of the original cell-germs of ova; but
for their development into the new beings
produced from them, no combination, so* far
as is yet known, with the product of cells of
a different kind, as in fecundation, is necessary.
It is otherwise with all true ova. Their
germs may be the descendants through the
ovary of an original cell-germ, from which
the animal bearing the ovary was produced ;
but for the generation of an ovum the ovigerm
must be subjected to the influence of the
sperm, and for its development there is re-
quired a new process of organisation, inaugu-
rated by segmentation, which is the invariable
consequence of fecundation, and is the first
obvious change in a fecundated ovum leading
to embryonic formation.
The developed form of the spermatic sub-
stance * is in by far the greater number of ani-
mals that of minute ovoid or rounded particles of
various form, with each of which is connected
a long and extremely delicate filament, which
moves with vivacity in a vibrating or oscil-
latory manner when immersed in water and
various bland animal solutions. There are
other less common forms of spermatozoa,
such as those of Crustacea and Nematoidea,
which have not the filamentous appendage,
and are motionless. The vibratory motion
of filamentous spermatozoa bears some resem-
blance to that of some kinds of fine cilia, and
is the most apparent indication of the active
state of their vitality.f
It is now ascertained beyond doubt that in
a number of animals the spermatozoa come
into direct contact with the yolk substance
* See the article SEMES.
t See especially the recent researches of Kolliker
on the Sperm in Zeitsch. fur, Wissensch. Zool.
v ol. vii.
and embryogerm, or with the internal con-
tents of the ovum. The actual entrance of
the spermatozoa into the ovum has been
observed in Mammalia, Batrachia, Osseous
Fishes, Insects, Nematoid Worms, some
Mollusca, and Echinodermata ; and there
have been ascertained circumstances regarding
the ova of other animals which warrant the
inference that the spermatozoa enter the
ovum in many more than those in which the
phenomenon has already been actually ob-
served. After long continued doubt and
much discussion of this point, physiologists
are therefore now generally agreed that in
all instances a direct action of the sperma-
tozoa in substance on the contents of the
egg is necessary to fecundation. The manner
of access of the spermatozoa to the interior
of the ovum is probably various in different
animals. In a few, as Trematode and Cestoid
Entozoa, the sperm is mixed with the contents
of the ovum, viz., the germinal vesicle and
yolk, at the time when these are brought
together from the separate organs in which
they are formed : in some, as the Nematoid
Worms, and probably also in some other
animals, the sperm comes in contact with the
ovum previous to the formation of an en-
veloping membrane ; in a third set it seems
probable that, as in Lumbricus, and perhaps
in some Mollusca and Hirudinea, the vitelline
membrane which had existed at an earlier
period is dissolved or removed previous to
fecundation, and the ovum or yolk substance
and germ are thus left directly exposed to
the action of the spermatozoa, which in Lum-
bricus have been observed in great numbers
penetrating the substance of the yolk.
In the majority of animals, however, the
sperm only reaches the ovum at a later stage
of its formation, when it is already covered by
the vitelline membrane or other envelopes,
and through these coverings, therefore, the
spermatozoa must pass to gain access to the
yolk and germ. In a certain number of ani-
mals the vitelline or enveloping membrane
appears to be quite entire and closed on all
sides, so that, as in Mammalia, in which
Martin Barry was the first in 1843 to perceive
with certainty the entrance of the spermatozoa
into the ovum, these bodies must in some
way, not yet fully known, pass through the
consistent wall of the enclosing membrane ;
but in other animals, as first discovered by
J. Mtiller, a special aperture or perforation of
the egg- covering exists, apparently destined
to allow of the more rapid entrance of the
spermatic bodies. This micropyk apparatus,
sometimes consisting of one, and at others of
a number of apertures, has now been observed
in several Echinodermata, in Acephalous
Mollusca, in all Insects, and in Osseous Fishes ;
and it is more than probable that it exists in
a considerable number of other animals in
which it has not yet been detected. But
still, making due allowance for the probable
extension of discovery in this direction, the
care and accuracy with which the micropyle
apparatus has since its first discovery been
sought for without success in Mammalia and
[138]
OVUM.
some other animals, in which, had it been
present, it could scarcely have escaped so
careful a scrutinj', warrant the belief that in
a certain number of animals the spermatozoa
do actually penetrate the apparently entire egg-
covering.
It is not my design to enter here upon the
consideration of the mode and nature of the
action exerted by the spermatic matter or the
spermatozoa in producing the changes of
fecundation. Upon this subject the reader
may with great advantage and interest consult
the latter part of the article Semen in this
Cyclopaedia by R. Wagner and Leuckart, the
papers of the late Mr. Newport in several
recent volumes of the Philosophical Trans-
actions, and the learned article by Professor
Leuckart on Generation contained in the
fourth volume of R. Wagner's Handbuch der
Physiologic. I will only remark in passing
that from Mr. Newport's and other researches
it appears that while the actual mixture of an
appreciable quantity of the spermatic sub-
stance is necessary to induce fecundation, the
extreme rapidity with which the action takes
place, the minuteness of the quantity of the
spermatic matter which is sufficient to induce
it, and the fact now observed in a variety of
instances that the spermatozoa which have
entered the ovum remain apparently little
changed for a considerable time after the
changes of the ovum consequent on fecunda-
tion have made some progress, lead to the
conclusion that there is something in the
nature of this action inconsistent with the
idea that it is one of mere combination in
substance of the developed contents of the
male and female generative products. But
whether this is to be referred to the class of
" contact actions " of which themselves so
little is known, or to what other kind of
action it may be compared, the ascertained
facts do not enable us in the least to deter-
mine. The almost universal presence of vi-
bratory motion in the spermatozoa during the
time in which they retain their fecundating
power, naturally led physiologists to connect
that motion with the fecundating action ; but
on the other hand, the occasional, though rare
examples in which the spermatozoa are en-
tirely motionless, seem sufficient to cause the
rejection of the view that the force which
produces the vibratory motion is identical
with that which calls forth the series of
histogenetic and organogenetic changes which
result from fecundation.
' But the consideration of this subject would
lead us into the discussion of the whole
question of vital forces, which in its present
unsatisfactory state it is desirable to avoid.
The physiologist agrees, for the sake of con-
venience of expression, to adopt the terms of
power, property, force, &c., to denote the con-
ditions necessary for the occurrence of certain
actions or changes. He employs the term
vital force merely as the indication of the
supposed cause or causes of an ascertained
regular sequence of vital phenomena ; but all
philosophical accuracy rejects the idea of any
unseen separate and single force which is at
work in bringing about the sequence in ques-
tion. The fecundating power of the semen
is an expression used only for convenience
to denote the invariable sequence or relation
as cause and effect which has been observed
to subsist between the contact of spermatic
matter with the ovum, and the changes in the
latter which follow on the act of fecundation.
We might with as much propriety have given
a name to a separate power residing in the
egg or its germ which render it susceptible of
fecundation, as of a special power belonging
to the semen by which that susceptibility
of the ovum is acted upon. The efficient
cause of the process of fecundation can only
be educed, as in all physical as well as vital
changes, from a perfect knowledge of all its
phenomena, and the statement of the efficient
cause of such actions is only the expression
of the most general and best known law to
which a full acquaintance with the phenomena
enables them to be reduced. Fecundation is
to be regarded as a purely vital change, seeing
that it takes place only in the usual conditions
of vitality ; but, like all other vital changes,
it appears more probable that a variety of
conditions of the organic matter rather than
any one known property or condition are
necessary for its occurrence.
In endeavouring to deduce the most ge-
neral phenomena which accompany this re-
markable change, it may be said that fe-
cundation consists essentially in the mutual
action of two different organised bodies,
which are respectively formed from two
different cells ; the ovigerm and the sperm-
germ. If we may form any general con-
elusion from what may be so well observed
in Nematoid Worms, the development of the
ovum and the spermatic cells from their re-
spective germs is remarkably similar, for in
both the internal cell is developed from a
minute molecule from within, while the ex-
ternal part is deposited from without. The
spermatozoa are formed in connection with
the nucleus or nuclei of the sperm-cell ; and
the germinal part of the ovum, though it con-
sists mainly of the granular part of the yolk,
which is directly formative, very probably
comprehends also in some shape or other the
effused contents of the germinal vesicle. In
this way, then, we may conjecture that in the
act of fecundation the products of the original
cell-germs meet and combine or mutually
influence each other. The cell-germs, then,
are the links in the chain of organic connec-
tion between either or both the parents and
the progeny capable of being developed
from the fecundated ovum. Such a view,
though still in a great measure speculative,
seems to be in accordance with the facts
known as to the perfect transmission of the
structure and qualities of either or of both
parents to the offspring.'
6. Immediate effects of fecundation on the
ovum ; segmentation, and first changes of the
ovum related to the commencement of em-
bryonic development.
It does riot come within the scope of the
present article to describe in detail the pro-
cess of fecundation, or the phenomena which