ferent stages of advancement in the ovary of a mammifer. p, peritoneal covering of the ovary ;
* t, ovarian stroma ; o t>, the two layers of the ovisac ; m g, membrana granulosa, near which is
the discus granulosus, with the ovum imbedded.
[82]
OVUM.
but in some animals, in which the vesicles are
proportionately more expanded, they extend
beyond the general line of the surface in
various places, and even sometimes give the
ovary somewhat of the mammillated or grape-
like appearance more common among ovipa-
rous animals.
The follicle is filled to distension with a
clear albuminous fluid, which escapes with
force when an incision is made through the
Fig. 55*.
Ovarian ovum of the Dog. (From Bischoff.}
a, magnified representation of the ovarian ovum
of the dog, nearly mature, situated in the discus
proligerus and part of the cells of the granular
membrane ; b, several detached granular cells.
d, another ovarian ovum of the same animal
perfectly ripe, immediately previous to the rupture
of the Graafian follicle ; the cells of the proligerous
disc have become of a pediculated shape ; e, some
of these cells detached.
e, the ovum from the same specimen as in fig. a,
freed artificially from the granular cells of the
disc, showing externally the thick clear zona or
external membrane, and internally the opaque
3 r olk substance : in the latter the germinal vesicle
is obscured by the opacity of the substance sur-
rounding it.
/, the same ovum burst by pressure, showing
the contents of the ovum which have escaped,
viz. the finely granular yolk substance and the
germinal vesicle with its macula.
membrane. Close to the inner surface of the
follicle, and surrounding the fluid, is situated
the layer of nucleated granular cells which
has been termed tunica or membrana granulosa,
from the opaque granular appearance of the
cells composing it. These cells form a com-
plete vesicular lining of the follicle; but
throughout the greater part they cohere with
only a moderate degree of firmness, so that the
membrane readily tears when the follicle is
opened. The minute ovum is imbedded in a
thicker portion of this layer, the cumulus or dis-
cus proligerus of Von Baer, and is almost in-
variably situated close to the inner surface of
the most projecting part of the Graafian follicle
(see fig. 54*. A. and c., mg.) where in some
animals, but not in the human ovary, the
ovum may be detected in the undissected fol-
licle through its coats and the ovarian cover-
ings.
The cells of the membrana granulosa are in
general about -^Vu" in diameter. They adhere
with considerable firmness to the surface of
the zona ; so that when the follicle is opened
and its contents are allowed to run upon a
plate of glass for examination, the ovum is
always found placed in its disc in a circum-
scribed attached portion of the membrana
granulosa of about twice its own diameter.
The ovum is itself of a nearly perfect sphe-
rical form when freed from pressure ; but
as it lies thus imbedded in the membrana
granulosa, and moistened on a plate of glass,
it gives rise to a slight rounded elevation in
that membrane, which may easily be detected
with the naked eye when the specimen is
viewed sideways.
When the contents of the Graafian follicle
are discharged naturally during life, a small
aperture occurs nearly in the centre of the
most projecting part of the wall of the
follicle ; and as the ovum lies near this place
in the membrana granulosa, it is liable to be
evacuated first, along with a portion of that
membrane, which is soon torn away from the
rest by the pressure of the fluid behind it,
impelled by the contraction of the walls of
the follicle and surrounding ovarian substance.
During the descent of the ovum through the
Fallopian tube the cells of the proligerous disc
immediately surrounding the ovum alter their
form, and are subsequently detached from the
zona, so as to leave its external surface quite
free and smooth. I have not, any more
than Bischoff, been able to observe the four
retaining straps or retinacula described by
Martin Barry in his first series of Researches,
as portions of the membrana granulosa na-
turally thicker than the rest, and which,
radiating nearly at right angles from the pro-
ligerous disc, serve, as it were, to guide the
ovum and its disc towards the aperture by
which it escapes on the bursting of the fol-
licle. The accompanying figure from Coste
(56*.) gives that author's view of a structure
somewhat similar to the retinacula of Barry.
The size of the mammiferous ovum itself is
much more uniform among the different families
of Mammalia than that of the Graafian follicle,
OVUM.
[83]
Fig. 56*.
Ovum, of the Rabbit in the tunica granulosa.
(.From Coste.)
The middle part of one of M. Coste's figures has
been here copied to show the peculiar arrangement
of the granular cells round the disc and ovum,
which according to him are of the same nature
with those described by Dr. Martin Barry as
retinacula. It may be doubtful whether this
structure is constant.
and bears no regular proportion in different
families of animals to the stature of the whole
body. In the mature state this variation ex-
tends from T o to 2^0 or g-i^ ". The following
are the results of a few measurements made by
myself and others of the external diameter of
the mature ovarian ovum, viz., man T -|^, dog
o, cat yi^, rabbit T ^, rat ^, mouse
Pig ^o> cow ? i n , guinea-pig ^".^
The external tunic, or zona pellucida (a term
founded on the description of Von Baer*, from
its presenting the appearance of a transparent
ring between the opaque granular yolk-mass
within and the granular cells externally), is of
great proportional thickness ; viz., from T-gW
to 2sW /r > or from i to T \ of the whole
diameter of the ovum. When entirely freed
from the granular cells, its external surface
appears smooth ; and the inner surface, which
is exactly parallel to the outer, is also re-
markably smooth. The substance of this
tunic is very tough, and possesses considerable
elasticity ; so that the ovum and its zona may
be flattened by external pressure to a great
extent ; and yet it regains its nearly spherical
form when the pressure is removed. It is of
glassy transparency and homogeneous ; neither
any laminated, nor fibrous, nor other structure
being perceptible under the highest magnify-
ing power. It is easy to obtain evidence that
it is one thick membrane, and not composed
of two layers with intervening fluid, as some
have held, by cleaving it with fine needle-
points, when the cut edge becomes fully ap-
* Epistola de Ovi Mammalium et Hominis genesi,
Lipsiae, 1827.
parent. It has recently been stated by Remak*,
that in the mature ovarian ovum of the rabbit,
when freed from the granular cells, radiated
lines may be perceived running quite through
the zona pellucida ; these linear radiations, he
conceives, indicate a peculiar structure of the
zona, somewhat similar to the perforated con-
dition of the outer membrane of the ovum in
osseous fishes. I have perceived some of this
radiated appearance ; but I am inclined to be-
lieve that it depends not on any structure of
the zona itself, but rather on the marking
produced by the adhesion of parts of the cells
of the tunica granulosa, which become pedi-
culated in very ripe ova, and have then a ra-
diated appearance on the zona under pres-
sure ; as represented by Bischoff in his view
of the ovum of the dog, of which fig. 55 D.
is a copy. I shall have occasion afterwards
to state the nature of the fine canals which
have been observed in the outer tunic of
fishes' eggs.
It has been customary among ovologists,
till very recently, to look upon the zona
as corresponding to the external membrane
of the yolk in the bird's egg. But from
what has been already said in connection
with that subject, the propriety of draw-
ing a distinction between these two enve-
lopes has been fully shown. I prefer, there-
fore, to retain the name of zona pellucida,
though it may not be perhaps the best de-
signation, as it prevents all confusion which
might be introduced by views of its analogies.
It will hereafter be more fully demonstrated
that it not only differs in its mode of origin
from the true vitelline membrane of birds,
but that it also has a different destination in
connection with embryonic development, in-
asmuch as, though formed in the ovary, it
remains and constitutes the basis of, or be-
comes incorporated with, the important struc-
ture which at a later period becomes the
chorion.
In the foregoing description of the mem-
brane of the mammiferous ovum, I have
adopted the view first advocated by Coste-j-,
and by Thomas Wharton Jones J ; and more
fully brought out and established by the re-
searches of Bischoff in his admirable works on
the development of the Rabbit and the Dog.
It may be proper to remark farther, that though
the name of zona pellucida has been employed
to designate the thick single tough membrane
by which, as is now well ascertained, the yolk
of the mammiferous ovum is invariably enclosed
from an early period of its formation in the
ovary, this term is used synonymously with
that of vitelline membrane, and as applied to
* Muller's Archiv. &c. for 1854, p. 252.
f Recherches sur la Ge'ne'ration des Mammiferes,
4to. Paris, 1834, fig. 2.; and Embryoge'nie Com-
pared, torn. i. p. 200, Paris, 1837.
I In a paper read to the Royal Society of London
in 1835 ; printed in the London Medical Gazette for
1838, p. 680.
Entwickelungsgeschichte des Kanincheneies,
Braunschweig, 4to. 1842 ; and Entwickelungsg.
des Hundeies, Braunschweig, 4to. 1845.
[G 2]
[84]
OVUM.
the only covering with which the ovarian ovum
is provided at the time of its leaving the Graa-
fian vesicle, excepting that which it retains
for a time derived from the cells of the tunica
granulosa in the proligerous disc. Von Baer
indeed, from whose description the term pellu-
cid area or zone has been borrowed, was not
fully aware of its consisting only of one thick
membrane ; and more than once, both in the
Epistola and in the Commentary upon it,
expresses himself doubtfully as to whether
this pellucid space or halo* might not be
formed of an external envelope which he
terms the cortical membrane, and of another
situated within it.
It is well known that when the mammi-
ferous ovum has been fecundated, and arrives
in the cavity of the uterus, and begins then
rapidly to expand, it is covered by a mem-
brane which soon undergoes great extension,
and acquires a villous structure on its external
surface, by which it may always be easily re-
cognised. This villous envelope of the ute-
rine ovum, universally now known as the
chorion, is the product no doubt of changes
which only occur in their completeness after
fecundation, and as an accompaniment of
embryonic development ; but still as it ap-
pears probable that the zona pellucida is in-
timately connected with the first condition of
the chorion, and as the first formation of the
latter membrane is by many believed to take
place independently of fecundation or fcetal
development, it is necessary for me to make
some remarks in this place on the relations
of the zona pellucida to the external cover-
ing which the ovum obtains in the first periods
of its residence within the female passages.
This is a subject on which embryological
writers are by no means agreed, and several
of them indeed have themselves changed their
opinions in regard to it in the progress of
their researches.
Von Baer, the correctness of whose general
views on the phenomena of development we
have occasion to admire in almost every part
of the subject of which he has treated, was at
first of opinion that the chorion might arise
in Mammalia from the outer of the two layers
of which he at that time (though also doubt-
ingly) conceived the external covering of the
ovarian ovum to consist, and he looked upon
this as a great difference or departure from
analogy between the bird's egg and the ovum
of the mammifer : but in his work on Deve-
lopment -j-, published eight years later than the
Epistoia and Commentary, he states his convic-
tion from his observations, that in some mam-
miferous animals at least, such as the pig and
sheep, the chorion is formed by external de-
posit on the surface of the ovarian ovum du-
ring its descent into the uterus, and therefore
takes its origin more in analogy with the ex-
* Spatium pellucidum, halo vel peripheria lu-
cida. See Commentary on the Epistola, as trans-
lated by Breschet in his Repertoire, 1828, p. 52.
t Beobachtung. und Reflexion, liber Entwicke-
lungsgescb. &e., Konigsberg, 1837, part ii. p. 185.
ternal covering of the bird's egg. At the
same time he confesses that he was not able
to reconcile this view with what he had seen
in the dog and rabbit.
Valentin, in his Manual of the History of
Development*, regarded it as most probable
that the chorion is formed by the deposition
and consolidation of an albuminous matter
on the surface of the ovum during its descent
through the first part of the Fallopian tubes;
but though taking, as it appears, a perfectly
correct view of this subject, he did not bring
forward observations sufficient to establish
the opinion which he had founded chiefly on
analogical considerations.
We owe to Thomas Wharton Jones the
first direct observation of the actual deposit
of a layer of albuminous matter round the
surface of the zona pellucida. But although
Mr. Jones, in the observation which he made
on the ovum of the rabbit within the Fallopian
tubes on the third day after conception, was
quite assured that a new structure had made
its appearance in considerable thickness on
the surface of the zona, yet he at first sup-
posed that this might proceed from some
change in the remains of the granular tunic
which adhered to that membrane after it had
left the Graafian follicle.f He afterwards,
however, became aware of the source of this
fallacy, and adopted the view that the mam-
miferous ovum receives a superadded struc-
ture, contributing to the formation of the
chorion, in its descent through the tubes.
In the first series of his Embryological Re-
searches, Martin Barry described not only
the zona pellucida as recognised by other ob-
servers, but also a distinct vitelline membrane
within it; and he conceived that the zona
became afterwards the chorion. But in his
second series he became aware, both from the
statements of Wharton Jones and from his
own observations, that a new deposit occurs
in the rabbit's ovurn ; and this new deposit
he now regarded as " the true chorion." He
retained, however, for a time his view of the
separate existence of a vitelline membrane ;
and thus described articulately three mem-
branes as belonging to the mammiferous ovum,
viz., vitelline membrane, zona pellucida, and
chorion. J The first membrane he believed
to disappear previous to the period of full ma-
turity : the two last he regarded as together
the source of the chorion of a later stage.
Bischoff also had not been aware from his
earliest observations that any new deposit
occurred round the ovum of the rabbit in the
tubes; and even after he had become ac-
quainted with this fact, and had observed it
himself, as he did not detect a similar deposit
on the ovum of the dog, he adhered to the
* Handbuch der Entwick. &c. des Menschen,
Berlin, 1835, pp. 38, 39.
t On the First Changes in the Ova of the Mam-
mifera, in consequence of Impregnation, and on the
Mode of Origin of the Chorion, in Phil. Trana.
1837, part. ii. p. 339.
I See Phil. Trans, for 1839, p. 316.
OVUM.
[85]
view which he had previously taken, that the
zona becomes the chorion, or at least that a
new deposit is not in all Mammalia necessary
for the formation of that membrane.
Fig. 57*.
Fig. 58*.
Ova of the Rabbit from the Fallopian tube three days
after impregnation.
A, shows on a dark ground one of these ova, of
which B is an explanatory outline, y, s, are the
yolk segments of which there were eight; z, the
zona ; a, the thick layer of albumen which in this
animal is always deposited on the exterior of the
zona after the granular cells have been removed
from it.
c and D. Other ova from the same animal ; in D,
are shown three projections of the albuminous
covering which have been taken for villi of the
chorion ; but which according to Bischoff are not
so. This ovum was farthest clown in the Fallopian
tube.
In a series of observations made by myself
on the ovum of the dog and rabbit during
their descent from the ovary to the uterus,
in the summer of 1840, I was induced to
adopt the opinion that a new deposit does
really occur on the surface of the ovum in
both of these animals. I repeatedly ob-
served the large gelatinous or firm thick
albuminous covering on the rabbit's ovum
when it had just entered the cavity of the
uterus ; and in several instances 1 thought
I could perceive the first formation of the
villi of the chorion by sprouting or budding
from the surface of the newly deposited sub-
stance, which, as in Wharton Jones' and
Barry's observations, it was quite easy to dis-
tinguish from the membrane of the zona. In
the ovum of the dog I admit, with Bischoff,
the appearance is very different ; but yet my
observations appeared to me to demonstrate
that in that animal also a substance is super-
added to the surface of the zona, for that mem-
brane, which presents at an earlier period a
Ovum of the Dog from the Fallopian tube ten days
after impregnation.
A. The yolk has undergone division into eight
segments, and there is a thin irregular deposit of
albumen on the outer surface of the zona. (This is
represent* d too light in the figure.)
B. Explanatory outline of the same ; y, s, yolk
segments ; z, zona pellucida ; a, layer of albumen,
from which in connection Avith the zona the chorion
takes its origin.
perfectly distinct and smooth outline on its ex-
ternal surface, becomes in the course of the
descent through the Fallopian tubes and by the
time of its first arrival in the uterus, not only
irregularly flocculent on its surface, but also
thickened ; in fact, presents all the appearance
of a granulo-mucous substance having been
deposited upon it.
It may be proper to explain here, that it has
now been fully shown by Bischoff's excellent
observations, that in both the animals men-
tioned, and also in the guinea-pig, the cells
of the tunica granulosa, which adhere to the
surface of the zona when it leaves the Graafian
follicle,are completely separated from it within
the first two or three days of the residence of
the ovum within the tube, so as to leave the
external surface of the zona perfectly smooth.
Bischoff has shown, indeed, as I have also
repeatedly observed, that a change has oc-
curred in the cells of the proligerous disc,
adherent to the ovum while it is still within
the ovary, which indicates its approaching
maturity. This change consists, as already
stated, in these cells becoming somewhat
spindle-shaped or pyriform, their narrow or
pointed ends being attached to and radiating
from the surface of the zona. (See as before
fg. 55. D.) It is quite easy, therefore, after
this separation takes place, to distinguish
any change by addition of new matter or
otherwise which the surface of the zona
may undergo. No one can fail to perceive the
[o 3]
[86]
OVUM.
addition to the ovum of the rabbit, the diame-
ter of which is thus increased between two
and three times, so as to give it somewhat the
aspect of the ovum of a Batrachian in minia-
ture ; and in the dog it has appeared to me
that the increased thickness and more opaque
and flocculent roughness of the surface of the
zona were sufficient proofs of a new deposit
having taken place. This deposit, no doubt,
becomes very completely incorporated with
the substance of the zona, and is not easily to
be distinguished from it ; but in one or two
instances I have thought that I was able to
perceive a line of demarcation between them.
Several of Bischoff 's very faithful figures seem
to me even to represent this deposit as it has
occurred on the ova of the dog. But his
statements in his work on the Development of
the Guinea- Pig * are so precise against the
occurrence of such a deposit, that further ob-
servations will be required fully to determine
the question whether it is of constant oc-
currence or essential to the formation of the
chorion.
Later observations lead me to think that I
may have been in error in supposing that the
villi of the chorion grow directly from the al-
buminous deposit. These villi, which, as I have
said, form a most characteristic feature of the
external covering of the mammiferous ovum in
the course of development, begin to be formed
only when the ovum has reached the cavity of
the uterus. The time of the formation of these
villi, as well as their size, varies, however, con-
siderably in different animals, and probably
also to some extent in the same animal, -j-
They are developed from the external surface,
and their structure is at first nearly homo-
geneous, or at least only slightly granular ;
they afterwards acquire a cellular structure,
and in the course of foetal development be-
come at an early period the seat of a compli-
cated vascular growth, by which the relations
of the maternal parent and offspring are main-
tained through utero-gestation. But the fuller
description of this part of the growth of the
chorion belongs rather to the history of de-
velopment after fecundation. My present
object has been only to show the relation of
this membrane to the zona or ovarian cover-
ings of the ovum.
The contents of the ovum or parts within
the zona consist of the yolk-mass or yolk-
substance, and the germinal vesicle. The first
of these constitutes a spherical mass of varia-
ble consistence, in which granules, or molecules
and globules of various sizes, from the most
minute up to about ^^0 or 7 oW> are sus-
pended in a fluid. The proportion of the gra-
nules to the fluid varies to a considerable ex-
tent in different animals, the ovum being much
more opaque, and usually of a dull-yellow co-
lour when the granules are in large quantity,
as is the case in most Carnivora, and may be
easily seen in the dog or cat. The clear fluid in
* Entwickelungsg. des Meerschweinchens, 4to.
Giessen, 1852.
t Barry and Bischoff.
Fig. 59*.
Ovarian ovum of the Rabbit. (From Coste.}
a, ovarian ovum extracted from a nearly ripe
Graafian follicle, and freed from the adherent
granular cells ; the close set granules of the yolk
substance, among which the germinal vesicle is
perceived with a slightly oval macula or nucleus,
are well represented.
b, the same burst by pressure ; the yolk granules
adhering together by a viscid clearer fluid sub-
stance are seen escaping from the large aperture in
the zona along with the germinal vesicle.
which the granules are suspended varies also in
its consistence, being of a marked viscid qua-
lity in some instances, and thin and limpid in
others; so that in some animals, when the zona
is punctured, the yolk-substance flows freely
out, while in others, and this is the case in
the human ovum, the yolk-substance holds
together as one consistent mass. The yolk-
substance does not adhere in the slightest to
the interior of the the zona, but on the con-
trary is readily detached from it ; and in some
instances, in the entire unimpregnated ovum, a
space is seen between the yolk-substance and
the zona, formed apparently by the imbibition
of water between them. This separation be-
tween the yolk-substance and zona appears
to be, at a later period, a constant and pro-
bably important change in connection with
fecundation and development.
The granules of the yolk-substance are ge-
nerally rather more densely set together and
OVUM.
[87]
more firmly united towards the external surface, mined by observation in the Mammalia, nor
This circumstance has given rise to the belief has any one as yet succeeded in observing a
among some observers in the existence of an canal or pore leading from the surface of the
additional delicate membrane enclosing the yolk-substance towards the germinal vesicle
yolk-mass ; but the most attentive observa- in the mammiferous ovum.
tion by Bischoff, Wharton Jones, myself, and
others has failed to detect such a membrane ;
and there is reason to think that the con-