any constant law which seems more certainly



OVUM.



[131]



than others to result from all recent researches
into the history of organic nature, it is this
necessary connection by descent of one being
or set of beings from another.

In all animals, with the exception of the
simplest tribes already referred to, the descent
from parent to offspring is through a product
formed and perfected only by the concurrence
of male and female organs ; but we are still
at a loss to determine whether the unseen
germinal bodies by which the Protozoa are
reproduced are of the same or of a different
nature. The structure of some of these ger-
minal bodies as described in the earlier part
of this article (p. 7., &c.), bears a very great
resemblance to that of true ova ; but yet the
sexual distinction of the parent animals has
not yet been discovered. The recent re-
searches of naturalists indeed show that our
whole knowledge of the history of the Pro-
tozoa may be considered as only in its infancy.
The discoveries as to the encysted stage of
existence among the Vorticellae and Gre-
garinae and others, the phenomena of conju-
gation observed in Gregarina and Actino-
phrys, the entire knowledge lately gained of
the form, structure, and habits of the Fora-
minifera, all point to important future dis-
coveries and modifications of our hitherto
crude and imperfect views of these tribes of
beings, and must make us refrain from at-
tempting at present to form any opinion or
even conjecture as to the modes of their re-
production ; while at the same time the recent
discoveries as to the existence of the sexual
distinction in the simplest forms of plants
encourage the hope that ere long the repro-
duction of the Protozoa may, in a similar
manner, be removed from the obscurity in
which it now lies hidden. It does not appear
necessary from these considerations that our
definition should make any direct reference
to animal bodies of the nature of which our
knowledge is still so imperfect.

The result of development from a fecun-
dated ovum in all vertebrate and in a con-
siderable number of invertebrate animals, is
the formation of an embryo which, by a pro-
cess of progressive growth, arrives at matu-
rity, and assumes the form, structure, and
habits, either, as the case may be, of a her-
maphrodite animal, or of the parent of either
sex. In a certain number of these instances,
as in Batrachia, Insects, Crustacea, and others,
growth is not altogether continuously pro-
gressive, but is subject to one or more breaks
or changes as it were, which are marked by
some change in the mode of life, or some
difference in structure of the individual. To
such marked changes in the course of the
development or growth of an individual ani-
mal proceeding from a fecundated ovum, the
name of Metamorphosis is given.

But from the facts narrated in the earlier
part of this article, it appears that in a cer-
tain number of the invertebrate animals, such
as those which have been referred to under
the heads of Echinodermata, Polypina, Aca-
lepha, Tunicate Mollusca, Trematode and



Cestoid Entozoa, Annelida and Insecta, a
very different result may, either regularly and
constantly in some, or only occasionally in
others, attend the first development from the
fecundated ovum. To this modification of
the developing and reproductive process the
appellations of Alternate Generation or Meta-
genesis have been given, of which terms the
latter may perhaps be the most appropriate.
The phenomena which have been described
under this head are so very various, that it is
difficult, if not impossible, to give a short and
general statement of their nature. The dif-
ference between this and the better known
form of direct generation may, however, be
stated nearly as follows : In the Metagenetic
form of reproduction the individual formed
by the development of the fecundated ovum
is generally different in aspect, structure, and
mode of life from the parent or parents by
which the ova were produced ; this individual,
or zoi'd, though possessed, in many instances,
of an organisation and of powers which fit it
for the efficient performance of many of the
most important acts of independent animal
existence, is yet wanting in the attribute of
perfect animal maturity, viz., the sexual or-
gans and activity, and is consequently incapa-
ble by itself of an act of true generation, or,
in other words, of the formation of fecundated
ova, by which alone the species can be per-
manently reproduced. In such instances,
then, it is only by the formation from these
intermediate beings of others which are sexu-
ally perfect, that the generative act can be
repeated. There are two phenomena re-
quiring to be distinguished in connection with
the most common forms of this process ; the
one the frequent multiplication of the im-
perfect intermediate beings, or zoi'ds ; and the
other the production either directly or by a
succession of acts of development from the
intermediate beings of those which are sexu-
ally perfect, or which resume the form be-
longing to the parents from which the fecun-
dated ova were derived. It seems proper,
therefore, to distinguish between an act of
true sexual generation, and that by which
new beings are formed from the intermediate
individuals (or so-called nurses of Steen-
strup, or zoi'ds of other authors) ; the first
consisting invariably in development from a
fecundated ovum ; the second being probably
more analogous to a process of budding or
gemmation from a parent stock. It must be
confessed, however, that we have still much
to learn regarding the phenomena of this pro-
cess, before we can form any general notion
of its nature. The whole subject is replete
with the deepest interest not only in connec-
tion with the history of reproduction, but in
its influence, as stated in some parts of the
preceding article, on the whole range of zoo-
logical classification and distinction. Our
extended definition comprehends an allusion
to these phenomena.

Lastly, the ovum may be considered as
having two phases or stages of existence ;
the one in connection only with the female
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[132]



OVUM.



parent or female organ, in which the greater
part of the organised material first to be
employed in development is provided, and in
which the ovum arrives at a certain stage of
maturity ; and the other in its relation to fe-
cundation, or to the influence of the product
of the male by which its developing powers
are awakened or called forth. The mature
ovarian ovum may therefore, in one sense, be
looked upon as complete, if we regard only its
own structure ; but here its progress would
be arrested without the occurrence of fe-
cundation, and if we view it, therefore, with
reference to its more important destination as
the means of continuing the animal species, the
ovum can only be regarded as perfect when that
hitherto inscrutable change has been effected
on its substance by admixture with the minute
elements of the sperm in fecundation. The
constancy of this law in the whole animal king-
dom, with the exception of those of the Pro-
tozoa already referred to, makes it proper that
our definition should make reference to fecund-
ation as the means of perfecting the ovum. To
the nature of this process itself a further al-
lusion will hereafter be made.

2. Recapitulation of the most general facts
ascertained by the comparison of the ova of
different animals.

The ova of animals in their complete state
may be considered as consisting ot two sets
of parts which are of very different relative
importance in connection with the develop-
ment of the embryo : the first of these sets of
parts belong to the ovarian ovum, and are
formed previous to their quitting that organ ;
the others are subsequently formed, and may
be looked upon as accessory. These last often
present great varieties, so as to cause the ex-
ternal form and appearance of the ova of ani-
mals to differ widely, while the ovarian part
much more nearly corresponds. To this
ovarium ovum we shall principally confine our
present remarks.

An extended comparison of the ovarian
ova of animals belonging to almost every
family of the animal kingdom has shown that,
notwithstanding great differences in size, and
some variation in form and structure, they all
agree in consisting of three essential and nearly
similar parts before the period of their detach-
ment from the ovary : these are, 1st, The in-
ternal nucleated cell or germinal vesicle with
its macula or maculae ; 2nd, The vitellus, or
yolk-substance ; and 3rd, The enclosing vesi-
cular envelope, or vitelline membrane. In all
animals there is, also, a general similarity in
the manner in which these parts are formed and
combined so as to constitute the ovarian ovum ;
the germinal vesicle is the first produced, and
may be regarded as the ovigerm ; the yolk-
substance next gradually envelopes it or is
deposited round the germinal vesicle, and in
general the vitelline membrane which encloses
the whole is the latest formed.

The most marked differences among the
ova of animals are connected with the struc-
ture of the yolk and the relation which it bears
to the formation of the germinal part out of



which the embryo is afterwards developed*
Founding upon this difference, three groups'
two principal and one subordinate, may be
distinguished among the ova of animals:
1st, The group of small-yolked ova, to which
belong those of Mammalia and a considerable
number of invertebrate animals, such as most
Mollusca, the lower Crustacea, most Anne-
lida, the Entozoa, Rotifera, Echinodermata,
Acalepha, and Polypina. In this group, the
ovum is generally of small or of moderate
size, as a whole ; the vitelline substance con-
sists entirely or chiefly of fluid with fine gra-
nular particles, and the entire yolk undergoes
segmentation : the entire yolk mass, therefore,
is directly formative, or is employed from the
first in the formation of the blastoderm or
organised substratum in which the embryo is
developed : the germinal vesicle is in this
group usually of small size, and has only a
single macula, or one composed of very few
particles.

The second principal group comprehends
the large-yolked ova, such as those of Birds,
Scaly Reptiles, Cartilaginous Fishes, and the
Cephalopoda, to which, perhaps, may be added
Insects, Arachnida, and most Crustacea. In
this group, the largely developed yolk contains,
suspended in its basement, homogeneous sub-
stance, two kinds of organised corpuscles, viz.,
1st, A certain portion of the small granular
part, similar to that of the small yolked ova,
which occupies a limited but determinate place
in the ovum, and in its centre the germinal ve-
sicle is situated ; and 2nd, A larger portion of
spherules, cell-like or other corpuscles of greater
magnitude. It is the first or finely granular
portion only which is immediately germinal,
or which is subject to segmentation and forms
the basis of the blastoderm ; the remainder,
or large cellular portion, is only secondarily
employed in supplying nourishment to the
embryo or its accompanying organised parts
in the progress of their development. In the
ova of this group, therefore, we distinguish
the formative or directly germinal portion of
the yolk-substance from the indirectly nutritive
portion. In these ova, the germinal vesicle is
also proportionally large, and the contents of
the vesicle, though consisting in the earliest
stages of their formation of a single macula,
or of a very small number, very soon become
converted into very numerous maculae, or into
a fine granular pulp.

The third, or subordinate group, may com-
prehend the ova of Amphibia, or scaleless rep-
tiles, and osseous fishes, to which, perhaps,
may be added some of the invertebrate ani-
mals mentioned under the second group. The
ova of this group are intermediate in their
structure between those of the first and se-
cond : they have certainly the greatest affinity
with the large-yolked group, but there are
many gradations among the ova of this kind,
among allied species of animals, and it is chiefly
on the ground of the incompleteness of the
segmentation that I have thought it proper to
arrange them in a separate group.

It may be remarked further, that in all ani-



OVUM.



[133]



mals, whatever may be the ultimate structure
of the yolk, the primitive yolk, or that which
is first formed, is invariably of the finely gra-
nular kind, the cellular or large corpuscular
yolk-substance is of later formation. These
two parts remain distinct from each other, and
the finely granular or formative yolk is that in
which the germinal vesicle is invariably im-
bedded. In those instances, such as the Bird,
Reptile, &c., in which the large cellular yolk
greatly preponderates over the formative yolk-
substance, the latter assumes in the later
stages of formation the shape of a flatfish
disc on one side of the greater mass of the
yolk, with the germinal vesicle placed in its
centre.

The vitelline membrane presents some va-
rieties in structure, being in some animals
very delicate and homogeneous; in others,
as Mammalia, remarkably thick, tough, and
elastic, but without visible structure ; in a
third set, exhibiting peculiar structure, such
as the finely tubular perforations of the ex-
ternal membrane of the fishes' ovum, or the
radiated markings in the ova of Holothuria
or Cestoidea ; but in these last three in-
stances the vitelline membrane is probably
associated with additional layers of substance
derived from a different source from that
which forms the homogeneous membrane.

A remarkable peculiarity has recently been
discovered in the enclosing membrane of the
ovarian ovum of some animals, in the aper-
ture or micropyle which has been observed
in osseous fishes, insects, some Crustacea*,
the Acephalous Mollusca, some Annelida,
Holothuria, and some other Echinodermata.
There seems reason to believe that a similar
aperture exists in the ovum of Batrachia and
Cephalopoda ; and it is very probable that it
may yet be discovered in other animals. At
the same time it is right to state that in
Mammalia and several other animals it has
been most carefully sought for without suc-
cess. This aperture appears to be designed
to admit the spermatozoa into the cavity of
the ovum, or into contact with the yolk-sub'
stance and germ, in those instances especially
in which the egg coverings are thick and
touiih, and fecundation is late of occurring.

The relation of the ova to the ovaries or
organs in which they are produced, exhibits
considerable varieties in different animals.
1. The most common is that in which the
germs of the ova arise within minute close
follicles or vesicles, which are imbedded in
the more or less solid or loose stroma of the
ovary ; the follicle enlarging with the ovum
as its other parts are added till the period of
of maturity, when, periodically, the follicles
open for the escape of the ova. 2* In a
second form, as in Nematoid Worms and
Insects, the germs of the ova are produced
free in the upper part of an ovarian tube,

* It has been inadvertently stated in a preceding
part of this article (p. 116.) that the micropyle had
not been observed in the ova of Crustacea, whereas
Meissner has ascertained its presence in that of
Gamraarus. (See his Memoir in Zeitsch. fur Wissen.
Zool. vol. v. p. 284.)



and the yolk-substance, &c. are added gra-
dually as the egg germs descend through suc-
cessive portions of the tube : here no true de-
hiscence is necessary to allow of the escape
of the ova. 3. In a third form, as in Trema-
tode and Cestoid Entozoa, distinct organs are
provided for the formation of the ovigerms
and the yolk-substance, and these last are
brought together and combined into the sphe-
rical form of an ovum in another part of the
genital apparatus. 4. In the greater number
of animals the germs for each ovum appear
to arise singly, and the ova are thus isolated
from the first ; but it would appear that in
some animals these germs arise in groups,
perhaps by development from a common
germ, so that they are from the earliest period
connected together by pedicles. Yet, with
all these differences, there is to be perceived,
on the whole, a general similarity in the plan
of formation of the parts of the ovum itself in
different animals. This plan may be generally
stated as follows.

The germinal vesicle is universally the first
part of the ovum which makes its appearance ;
it does not appear to be nucleated or to pos-
sess its macula from the first in all instances,
and this macula cannot therefore be regarded
as the centre of its formation. The germinal
vesicle is generally at first only a minute
point ; it soon enlarges, however, and either
possesses from the first, or at a very early
period acquires, its macula or nucleus. In
animals with the solid follicular ovary, each
follicle is occupied by a single ovum, which
begins within it as a minute germinal vesicle.
The delicate wall of the follicle is also per-
ceptible at the same time as the ovigerm ; in-
deed, there is reason to believe that it even
precedes the commencement of the formation
of the ovum, though this is a point not yet
fully determined. In those animals, on the
other hand, in which the ovary is tubular, the
ovigerms appear, in some instances at least, to
arise in groups within cells ; and it may be a
question whether these cells bear to the ovi-
germs arising within them the relation of the
ovarian follicles of solid or closed ovaries.
Whether this be so or not, that relation is in
most instances speedily changed, as the ova
soon become free, or, in others, are attached
by a pedicle to a common stalk.

The wall of the ovarian follicle consists at
first of an extremely delicate vesicular mem-
brane, which is the same as that to which the
name of ovicapsule or ovisac has been given*
At a very early period, and while the ovum con-
sists of no more than the germinal vesicle, the
homogeneous wall of the follicle is lined with
a layer of flat cells somewhat analogous to
some forms of epithelium : this is the com-
mencement of the structure which in Mam-
malia afterwards forms the tunica granulosa,
and the fluid and cellular contents of the
Graafian follicle. It appears to have various
destinations in different animals.

The second stage in the formation of the
ovum is the deposit of the vitelline substance
around the germinal vesicle. In most ani-
mals the yolk-substance, when it first begins
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[134J



OVUM



to be formed, is scarcely granular, and in some
instances quite clear, consisting of a viscous
blastema, and as it increases separating the ger-
minal vesicle within from the ovarian follicle,
which expands proportionally. Very soon,
however, and in many animals indeed from
the first, fine opaque granules make their ap-
pearance, as if by precipitation or deposit, in
the clearer basement substance, and thus the
primitive yolk-substance of the ovum in all
animals is formed. In most instances there
is a time during which the ovum, consisting
of germinal vesicle, with a small quantity of
primitive yolk, exists, without any other co-
vering than that given to it by the ovarian
follicle ; but as the deposit of the finely
granular yolk increases, and at a very variable
period in different animals, the vitelline mem-
brane is formed round its exterior. The ad-
dition of this covering may be regarded as the
third stage in the formation of the ovum.
The manner of the origin of the vitelline
membrane has not yet been accurately ob-
served ; and it is probable (as will be hereafter
stated) that the coverings known under this
name may have different modes of origin ; but
if we restrict our attention at present to such
simple ova as those of Mammalia, I believe it
may be stated as extremely probable that the
so-called zona pellucida which constitutes the
vitelline membrane of the Mammiferous ovum,
takes its origin by the consodidation of the
superficial part of the basement substance of
the primitive yolk.

It appears probable that in the large-yolked
ova, such as those of the bird, the vitelline
membrane, which we find enclosing the whole
mass of the yolk, owes its origin to a dif-
ferent source ; and I am inclined to believe
that in this and in many other animals the
membrane which we term vitelline, as being
the immediate investment of the yolk, is not
of the same nature with the zona pellucida,
or the simple homogeneous vesicle of the
smaller ova, but rather a structure of later
formation, which owes its origin to the fusion,
or amalgamation, or to some other change in
the outermost layer of cells which form the
nutritive yolk of these animals.

In connection with this view, it is import-
ant to remark, that at that earlier stage of
formation of the bird's egg when it consists
entirely of formative or primitive yolk, there
is an approach to the formation of a zona, in
the existence of a very distinct, clear, and
consistent marginal portion of the yolk blas-
tema, from which the yolk granules seem to
retire. When the large cellular or nutritive
yolk is formed, this temporary zona seems to
disappear, and to be replaced externally by
the permanent vitelline membrane already
mentioned.

In those animals in which the ovigerms
arise by development within cells so as to be
connected in groups (Gordiacei), and in some
others, the vitelline membrane, or a substitute
for it, seems to be formed from the earliest
period in a different manner from that now
described.

The germinal vesicle is unimacular in ge-



neral in the small-yolked ova, and multima-
cular in the large-yolked ova, and also in the
intermediate kinds. In the latter it is rare to
observe the earliest stage in which the ma-
cula is still single : the multiplication of the
maculae takes place with remarkable rapidity,
and apparently by a process of endogenous
development, or possibly by division. The
ultimate destination of these macula is still
a subject of doubt.

3. Morphology of the ovum ; homology of
its parts, and relation of the ovum to other
organic structures.

Should the views be correct which have
now been stated with regard to the relations
of the parts in the mature ovarian ovum, and
the manner in which they are formed, it will
be apparent that a strict homology or ana-
tomical correspondence can be pointed out in
regard only to some of the parts which are
recognised under similar designations, as re-
spectively belonging to the ova of different
animals. All physiologists will probably be
disposed to look upon the germinal vesicle
or ovigerm as corresponding or homologous
in the ova of all animals, and, notwithstanding
the great differences known as to its more
simple or multiple condition, the same view
may also be taken of the structure known as
nucleus or macula. The primitive or finely
granular yolk-substance, more especially that
which immediately surrounds the germinal
vesicle, and is afterwards employed in the
formation of the blastoderm or embryogerm,
seems also to have a similar origin, structure,
and relation in all animals. But beyond this
it is more difficult to trace the homological
correspondence ; for under the names of
cellular yolk-substance and vitelline mem-
brane it appears that there have been brought
together parts of which the origin, structure,
and relations may be dissimilar in different
animals. There seems at least to be sufficient
reason, from what is already known of the
varieties of the enclosing membrane, or so-
called vitelline membrane, to establish a dis-
tinction between several forms of that struc-
ture; as, for example, between the vitelline
membrane, which exists from the earliest
period as a pediculated sac in connection
with the ovarium, as in Holothuria ; that
which is derived from the extension of the
wall of the original germ-cell in grouped ova,
such as have been described by Meissner in
Gordiacei ; that which is later formed round
the ovum of Mammalia as a zona pellucida,
by the consolidation of the outer layer of the
primitive basement substance of the yolk ;
and that which in the bird and other animals
whose ova are similarly constituted, appears
to derive its origin in part, at least, from
coalesced cells corresponding to those of the
tunica granulosa of the ovarian capsule on
the exterior of the cellular yolk.

With regard to the cellular yolk itself, we
must refrain from any attempt to establish its
homology till we shall be more fully ac-