ation is laid of that structure in which the
future embryo is more immediately developed ;
for it has now lost its germinal vesicle, and
from being formed, as at first, of mere granules
or simple spherules, it has acquired a true
organised cellular structure. It now consists,
in fact, of the delicate discoid collection of
cells, which has been called blastoderma. It
may be proper, therefore, to consider the mass
of the yolk and the germ, in their unfecundated
state, while still within the ovarian capsule,
* It is to be remarked that the animal basis of
the calcareous shell is of quite a different structure
from the fibrous lining membrane of the shell ; and
the calcareous deposit is not to be regarded as
taking place in that fibrous membrane. The outer-
most layer of the lining membrane adheres very
firmly to the shell, which may have misled some
on this point, who describe the animal basis of the
calcareous shell as of the same structure with the
fibrous lining membrane.
next, after the ovulum has entered the ovi-
duct, and, subsequently, when it is laid ;
reserving, however, for a latter part of the
article the account of the process by which
the change in the cicatricula referred to takes
place.
In the newly laid egg the yolk forms an
ellipsoidal mass, somewhat flattened on the
upper or cicatricular surface, and with its
long axis corresponding to that of the egg.
Its largest diameter is about one inch and a
quarter, its shortest about an inch : it floats
within the white, capable of a certain degree
of motion, which is controlled, as before ex-
plained, by its own specific gravity, and by
the attachment of the chalazae.
The yolk substance is not of the same
nature throughout, there being a part of a
lighter colour in the centre, about one fourth
of the diameter of the whole; from this, a
narrower prolongation extends upwards
OVUM.
69
towards the cicatricula, near which it again
widens and spreads out like a shallow cone.
This whiter internal substance constitutes
what has been called the central cavity (or
latebra) of the yolk : the whole of this inner
part has something of the shape of a flask,
with a narrowing neck and a wider mouth
ut the top, which is, as it were, surmounted
or closed in by the cicatricula. (See^g. 49.)
The shape of the yolk, I have said, is not
that of a regular ellipsoid ; the less density
of the upper part, which is towards the cica-
tricula, giving rise to a widening of the yolk
on that side, as may be seen in fig. 44, A, which
represents a vertical section of an egg boiled
while lying on its side. This does not depend
simply on the rising of oil globules in greater
quantity to the upper side of the yolk, but,
as has already been noticed, on the fixed
predominance of globules containing oil in
the neighbourhood of the cicatricula.
Neither is the outer deeper-coloured por-
tion of the yolk altogether uniform in structure
or appearance ; for it will be seen, both in the
raw and boiled egg, but most easily in the
latter, that several concentric layers surround
the central cavity and canal of the yolk, as
well as the funnel-shaped dilatation which
lies below the cicatricula. These layers are
marked by a slight variation in colour, and
are attended by a difference in the minute
structure of the corpuscles composing the
alternate layers. They probably depend upon
the growth of the coloured part of the yolk
being more or less rapid at different successive
periods.
The cicatricula of the newly laid egg is a
spot of an opaque yellowish white, easily dis-
tinguished by its difference of colour from the
rest of the yolk, about one sixth of an inch
in diameter, and King immediately within the
vitelline membrane, in connection at its mar-
gins with the most superficial layer of the
yolk substance. Examined in a favourable
light* it will be found, that in the laid egg,
when fecundated, the cicatricula consists "of
a central clearer and thinner part, and of an
external more opaque annular portion. The
central part is about one third the diameter
of the whole, and seems as if it perforated
the remainder of the disc with a circular
aperture, something after the manner of the
pupil of the iris. There is not, however, any
perforation in reality, but only a greater thin-
ness and transparency of the central part of
the disc. Neither is this central part entirely
clear; for there is placed below its middle a
round heap of whitish granules, described
by Pander as the nucleus cicatriculce (see the
figure in section), which gives greater opacity
to that part when viewed directly from above.
The central part of the cicatricula, already
* It may be here mentioned, that by far the best
mode of examining the natural appearances of the
parts as they lie in the opened egg, is to allow a
ray of strong or of direct sunlight to fall upon the
pa'rt which it is wished to investigate, through an
aperture in a screen, which places the rest of the
egg and the observer in comparative darkness.
obvious when the egg is first laid, is the same
which, after some hours of incubation, ex-
pands, changes its figure, and becoming still
Fig. 50.
Structure of the cicatricula in a laid FowPs egg.
A. Diagrammatic section of the yolk near the
cicatricula, enlarged; a, vitelline membrane; b,
cicatricula; c, nucleus; d, canal leading to the
cavity ; e, e, large yolk corpuscles of the coloured
part: the corpuscles are not represented of their
real proportional sizes, but more with a view to
show their general difference.
B. Enlarged view of the cicatricula, as seen from
above on the surface of the yolk in an impregnated
egg : the dark central space or transparent area
surrounded by an opaque zone and one or two
delicate haloes.
c. Cicatricula of an unfecundated laid egg : instead
of the central transparent area a number of rather
irregular transparent spots are seen.
more clear, receives the name of transparent
area, in the centre of which the embryo be-
gins to be formed; while the outer more opaque
part retains its greater thickness, and is con-
verted afterwards into the vascular and peri-
pheral part of the germinal membrane which
spreads over the yolk. Round the margin of
the cicatricula the deeper-coloured yolk sub-
stance is seen even in a perfectly fresh or
newly laid egg to be intersected by one or more
fine circles of a lighter colour. These seem
to be the same which afterwards, expanding
and widening, constitute the haloes which pre-
cede and accompany the extension of the ger-
minal membrane over the yolk. These circles
are the terminations at the surface of the
concentric layers of lighter substance, which,
as already mentioned, may be seen surround-
ing the central cavity and canal of the yolk
(see fig. 49). It seems not improbable that,
the difference in the structure of the central
and peripheral parts of the cicatricula just
stated proceeds from, or is connected with,
the peculiar process of fissuring or segmen-
tation which follows the disappearance of the
germinal vesicle from its central part in the
fecundated egg ; but the description of this
process belongs to a later section of the,
present chapter.
F 3
70
OVUM.
The cicatricula of the unfecundated egg,
such as is laid by fowls secluded from the
cock, differs from that now described princi-
pally in the absence of the marked dis-
tinction between the central clear and the
peripheral opaque part. The germinal ve-
sicle, which to all appearance remains the
same in the ovary till the yolk leaves the
ovarian capsule, is now no longer to be seen;
and the cicatricula is often marked irregularly
throughout, but more especially towards the
circumference, with clearer intervals, or small
irregular circular or^oval spaces, mingled with
the opaque substance of the disc. I have,
not, however, had the means in more recent
times of making a sufficiently careful exami-
nation of the cicatricula in this condition to
enable me to state more minutely in what
respects it differs from that of the fecundated
egg-
In the ovarian yolk, while still within its
capsule, a white spot corresponding to the
cicatricula also exists, and occupies the same
place in relation to the yolk cavity and canal.
Its structure and appearance, however, are
somewhat different from that of the true cica-
Fig.5l.
Cicatricula, and its contents, in the ovarian egg of
the Fowl.
A. A square portion of the surface of the ripe
ovarian yolk, showing the vitelline disc or cicatricula,
with the germinal vesicle in the centre, magnified
about six diameters.
B. Lateral view of the same, to show the con-
vexity produced by the thickness of the disc round
the germinal vesicle.
c. Vertical diagrammatic section of the same ; m,
vitelline membrane; d, granular disc ; g, germinal
vesicle.
D, E, P. Germinal vesicles more highly magnified;
D, from a yolk of about one tenth of an inch dia-
meter, showing scattered globules or germinal
spots; E, from a nearly ripe yolk, quite clear; F,
from another of the same period, exhibiting a turbid
or minutely granular mass from the action of water.
tricula of the egg which has passed through
the oviduct ; it is covered by a layer of closely
set nucleated cells which lie below the vitel-
line membrane ; it contains the germinal vesicle
in its centre, and, instead of being thinnest
towards the middle, the mass of its granular
substance is accumulated in greater quantity
in that part round and below the germinal
vesicle, and thins gradually off towards the
margin. Nevertheless, its much lighter colour
than the surrounding part of the yolk makes
it always easy to distinguish it. Its margin,
however, is not so well marked as that of the
true cicatricula ; for the opaque whitish sub-
stance seems there gradually to pass into or
be continuous with the most superficial layer
of cells covering the yolk. To this ovarian
representative of the cicatricula, Von Baer
has given the name of stratum proligerum. It
is also somewhat smaller than that of the laid
egg. It is usually to be found on that part
of the yolk which is next the ovary, which,
as the yolk hangs within its capsule in the
usual attitude of the bird, will be upper-
most, and for the most part is situated close
to the pedicle of the ovarian capsule. This
position is not, however, a constant one ; for
sometimes the cicatricula is seen on the sides
of the yolk, or towards the stigmatic band
of the capsule, but rarely, it would appear,
towards the ends or poles of the yolk.
The cicatricula may generally be perceived
on the surface of the yolk when the outer-
most layers of the capsule have been re-
moved, and the germinal vesicle can be distin-
guished in it shining through the inner layer of
the capsule and the vitelline membrane. It is
placed close below the nucleated cells which
line the latter, and adheres along with them
somewhat to its inner surface; so that in gene-
ral, it is easiest to remove this disc along with
a portion of the vitelline membrane, when it is
desired to obtain it for separate and more mi-
nute observation by transmitted light. The
vitelline membrane being cut round with scis-
sors at a short distance from ,the margin of
the disc, the parts are floated off in water or
serum, and then may readily be separated with
a little careful manipulation.
The germinal vesicle, or vesicle of Pur-
kinje, may always be seen with the unassisted
eye, with a good light, in the centre of the
ovarian cicatricula, or proligerous disc, in all
ripe ovula, and in most of those which are
above a tenth of an inch in diameter. It
constitutes there a welNdefined shaded cir-
cular spot, from tfo to ^ of an inch in diameter.
When the proligerous disc alone has been
removed [for observation and laid on a flat
surface, and viewed somewhat from the side,
or when the granules are torn asunder with
needles, so as to make a partial section
of it without removing or bursting the ger-
minal vesicle, it is easy to perceive that the
middle part, containing the vesicle, is more
elevated than the rest ; and that, although
the substance of the disc seems to pass quite
smoothly or evenly from the sides over the
germinal vesicle, the granules of the disc en-
velope the vesicle only slightly, and none
cover its middle part: the vesicle, there-
fore, is set, as it were, in a depression of the
disc, which fits round and overlaps its
margins, and a considerable thickness of gra-
nular substance is continued in the disc below
the vesicle. (See./zg. 51, in section).
If we select for examination the most ad-
vanced yolk of the ovary, which, in a hen laying
daily, or almost daily, would probably have
OVUM.
71
been discharged from the capsule in a few
hours, we may find some difficulty in isolating
the vesicle of Purkinje from the granular disc ;
for, by this time, the vesicle has become
flaccid, weak, and flattened down, and has
begun to be softened and dissolved, prepara-
tory to its complete disappearance, which
generally occurs about the time when the
stigma of the capsule opens to allow of the
escape of the yolk into the infundibulum
which embraces it. But, in all the other
yolks down to those of -J^ of an inch in dia-
meter, it is quite easy to break up the granular
disc with needle points, and to preserve the
vesicle uninjured. We may then free it entirely
from adhering granules, and cause it to roll
along in the fluid in which it is immersed, or
on a plate of glass ; and we may perceive that
it is a simple membranous vesicle filled with
fluid, and without any very obvious granules or
nuclei. In the perfectly fresh state,the contents
of the vesicleare almost limpid, exhibiting only
a slight turbidity scarcely amounting to a
granular deposit, provided it has been placed
in a medium which does not change its ap-
pearance ; but, if it is allowed to remain a
short time in water, and still more if it is im-
mersed in fluids which coagulate albumen, its
interior speedily assumes a minutely granular
aspect. The external wall of the vesicle then
separates somewhat from the spherical gra-
nular mass within ; and I have sometimes seen
(as represented in fg. 51, F) a considerable
condensation of the granular mass, so as to
leave a large clear space between it and the
external vesicle, and give it very much the ap-
pearance of the yolk mass in the ova of some
small animals within the vitelline membrane.
This change seems to be a combined effect of
the condensation of the granular mass and
the imbibition of fluid by the external vesicle.
In the earlier ovula this rounded molecular
mass is of proportionately smaller size ; and
although it differs very much from the smaller
nucleus or macula contained in the germinal
vesicle of the ova of many other animals, there
can be little doubt that it is derived from this
structure, as will appear from what is hereafter
said of the progress of its development.
When the yolk has passed into the ovi-
duct, and, in most instances, probably even
sooner, or when it has entered the infundibulum,
the germinal vesicle has entirely disappeared.
Sometimes it is already gone before the open-
ing of the ovarian capsule. The cicatricula
then presents an irregularly broken appear-
ance in consequence of the want of support
from the wall of the vesicle, and the dif-
fusion of the contents of the vesicle over
the surface of the proligerous disc. The
solution of the wall of the vesicle is probably
a gradual process connected with the state of
complete maturation of the ovule. It occurs,
as is well known, in the unfecundated as well
as in the fecundated egg, and cannot, there-
fore, in itself, be dependent on the action of
the spermatozoa; neither is it altogether
caused by the mechanical pressure to which
the yolk is subjected in issuing from the
ovarian capsule, nor by the pressure of the
oviduct itself; for it usually begins, and is
sometimes completed before these causes can
operate.
The diffusion of the germinal substance from
the vesicle (which in the fowl must have
already received the spermatic influence in
the ovary) has the effect thus of mingling
with the remainder of the cicatricula, a ma-
terial which, it can scarcely be doubted, ex-
erts some immediate influence in inducing the
change of segmentation and subsequent pro-
cess of organisation by which the blastoderm
is produced.
Microscopic structure of the ovum. The in-
vestigation of the microscopic structure of the
yolk is attended with considerable difficulty,
in consequence both of the variety and the deli-
cacy of the organised elements of which it
consists. The following parts require our
separate attention viz., 1st. The yellow or
external yolk substance ; 2nd, the substance
of the cavity and canal ; 3rd, that of the cica-
tricula and cumulus ; 4-th, the vitelline mem-
brane. We shall consider these both in the
laid egg and in the ovarian capsule.
1. From the effect of boiling the yolk,
every one is familiar with the fact that its
yellow substance is coarsely granular ; but the
exact nature of the small bodies giving this
granular structure has not been equally well
understood. The examination of this sub-
stance with a microscope of moderate magni-
fying power in a newly laid egg, shows that
almost all of the deeply coloured part of the
yolk consists of spherical corpuscles of con-
siderable size, so closely set together that
sthey are mutually compressed ; and thus,
when the yolk has been hardened by boil-
ing, the substance of the corpuscles being
coagulated by heat, they present polyhedral
forms ; but when diffused in fluid in the un-
boiled state, they are all nearly or quite sphe-
rical. The size of these corpuscles varies
between T i^- and -g^ of an inch ; but the
greater number of them are more near -^^ or
2 ^Q. Some have described the yolk cor-
puscles as floating in a fluid ; and no doubt
in the earlier condition of the yolk, a consi-
derable quantity of fluid exists, but in the more
advanced condition the amount of mutual com-
pression they exert when coagulated is suf-
ficient to show that its quantity must be
very small indeed. Those who have de-
scribed the yolk substance as mainly consist-
ing of a fluid holding in suspension a quan-
tity of extremely minute granules or molecu-
les, together'with some larger corpuscles, have
probably been misled, by making an examina-
tion of the yolk when not perfectly fresh, and
when the larger corpuscles have been in part
broken up, and thus resolved into the granular
fluid of which they consist. There is no
doubt that in birds, and in all the large-yolked
animals, the deeply coloured vitelline sub-
stance, which, in fact, forms the great mass af
these ova, consists almost entirely of the large
and usually spherical corpuscles just now
noticed. In some animals the form is not
OVUM.
Fig. 52.
Microscopic structure of the elements of the yolk
and ovarian ovum of the Fowl.
A. Large granular corpuscles of the yellow part
of the vitellus ; one of them quite spherical, as they
are seen when free ; two others angular from mutual
compression, from a boiled yolk.
B. Various corpuscles found on the confines of the
yellow yolk and the cavity and canal, showing
transition forms to the next set.
c. Clear vesicles containing oil globules and de-
tached oil globules of various sizes from the cavity
and canal.
D. From the cicatricula ; a, various-sized granules
and globules forming the vitelline disc of the yolk
before its discharge from the ovary ; b, the organised
nucleated cells forming the upper layer of the
cicatricula in a laid egg ; c, larger cells of the lower
layer; d, cells of the cicatricula from an egg in its
descent through the oviduct in process of formation.
A scale with divisions of T J gs of an inch is appended.
spherical ; as, for example, in the cartilaginous
fishes, in which a remarkable variety occurs
of a cubical form, and sometimes these mixed
with tetrahedral forms, as in the skate.*
When free, these corpuscles in the yolk of
the bird's egg roll easily on the surface of a
plate of glass as perfectly distinct spherical bo-
dies. They present (see Jig. 52, A) a minutely
molecular or granular aspect, but with quite>
smooth outline or margin to the whole cor-
puscle. If subjected to pressure, or cautiously
ruptured with needle points, they break
readily at one or more places, and allow the
escape from their interior of the thick granular
fluid, which flows slowly out of them in a
stream. The granules are in large quantity,
as compared with the fluid in which they are
suspended, and are most of them of an ex-
tremely minute size, probably below
an inch in diameter.
* See Mutter's Physiology, vol. ii.
Although the yolk corpuscles present the
distinct external margin now mentioned, and
thus constitute capsules containing the gra-
nular fluid, yet we cannot, in most instances,
detect any vesicular membranous envelope
surrounding them. One may sometimes ob-
serve a delicate limiting line ; but it has been
impossible for me to determine whether it
consisted really of a membrane or of a thin
condensed layer of the granular substance or
plasma containing it. At an earlier period it
is probable that these corpuscles have mem-
branous envelopes, but when fully formed the
greater number are certainly destitute of
them ; for occasionally a larger corpuscle may
be observed to divide into smaller ones, the
outlines of which are nearly as distinct as
that of the larger corpuscles.
Nor is any nucleus in general to be per-
ceived in these corpuscles. I have occasion-
ally seen in those from which the granular
matter was escaping, and which had thus be-
come less opaque than usual, a slight ap-
pearance of a clear hyaline circular space, but
it scarcely deserved the name of nucleus ; and
if these spherical bodies are to be regarded as
cells, which 1 think they ought, it must be in
a somewhat different acceptation from that in
which the term cell has hitherto been gene-
rally applied to vesicular organised structures.
But recent researches on the early condition
of cells seem to have rendered it necessary
that we should include under this denomina-
tion several simple spherical minute forms of
organised or organising matter, which were
not at first regarded as true cells by the
authors of the cellular view of organic struc-
ture ; and when we consider the mode of their
formation, it is more than probable that the
vitelline corpuscles now under consideration
may be included among the number.*
They probably constitute, at all events, as
Schwann has first shown, one stage of deve-
lopment of a cellular structure ; and, in the
meantime, they may with propriety be styled
the larger granular yolk corpuscles.
There is considerable uniformity in the ap-
pearance and structure of these corpuscles in
nearly the whole of the deeper-coloured por-
tion of the yolk ; but immediately below the
vitelline membrane, several layers of them
are of a smaller size, and the outermost layer
of all consists of cells which are much smaller
and more compressed, distinctly nucleated and
of a short cylindrical or prismatic shape. In
some places also, corresponding to the con-
centric lighter lines which run through the
yellow yolk, some approach is seen to the
next kind of yolk cells or corpuscles, which I
shall have to describe viz., those of the cavity.
The substance of the yolk-cavity and canal,
which in the unboiled egg may be distin-
guished from the other part by its lighter
* The above observation has a general application
to such minute spherical masses of matter as are
destitute of external envelope or nucleus: but in
reference to the corpuscles of the yolk, I ought to
observe that Schwann regarded them as cells in
various stages of growth.
OVUM.
73
colour, and in the boiled egg by its softer
consistence and less granular appearance, is
found by microscopic examination to consist
of organised corpuscles floating in a larger
portion of fluid, and different (from those of
the external part of the yolk. The transition
from the one kind of corpuscles to the other
in these two portions of the yolk, is not
sudden ; but many gradations of intermediate
forms are to be met with on the confines of
the two regions.
In the central part of the cavity or latebra,
which, when boiled, appears like a thick milky
fluid, corpuscles very different from those of the
external part are to be found (see Jig. 52, c).
They are almost all of a very regular spherical