chalazae, membrane, and shell are more rapidly
formed and are added during the passage of the
egg through the oviduct. It is by an entirely
similar process that these accessory parts are
formed in the scaly reptiles, the eggs of which
agree with those of birds in the most essential
points. The albumen, however, is generally
in less quantity and softer, and the twisted
chalazae have not been observed. The mem-
brane which immediately covers the albumen
has the same structure'as that of the bird's
egg; and the calcareous shell, when it exists,
as in turtles and crocodiles, is more porous
and thinner.

In cartilaginous fishes there is also a glairy
albumen investing the yolk, and secreted from
the oviduct. In most animals of the second



group, or with the large-yolked ova, the
vitelline substance consists almost entirely of
oily and albuminous matter enclosed in or-
ganised cells, the nature of which differs, as
previously explained, in the vicinity of the
germ and in the other parts of the yolk ; this
substance contains, besides, the peculiar co-
louring matter which has given the name to
this part of the egg. In all of them a cicatri-
cula exists, which is the seat of the germinal
vesicle, and of the first formation of the
rudiments of the embryo.

The ovum of the frog, when newly expelled
from the oviduct of the parent, consists of
the yolk-ball, closely surrounded by a tough
layer of peculiar albuminous matter deposited
on it in the course of its passage through the
oviduct. This substance has the property
of imbibing a large, but yet a limited, quantity
of water whenever it is immersed in it ; and
thus, within a short time after the expulsion
of the egg from the female, the external sub-
stance has assumed a gelatinous consistence,
ana has enlarged to such an extent as to be
on every side equal in thickness to the dia-
meter of the dark-coloured yolk within. I
shall have occasion afterwards to state more
particularly the important relation which
subsists between this process of imbibition
and the action of the spermatic substance in
fecundation. In the common frog, the ova
are thus united in large masses, floating in the
water of stagnant pools or rivulets : in the
common toad, they are united in long cords,
which become entangled among aquatic plants.

In the newts, the external covering of the
ovum is membranous, homogeneous and
transparent, and of an elongated oval shape,
and there is merely fluid intervening between
it and the spherical yolk and its membrane ;
but when the ova are deposited by the parent
in the folded leaves of water-plants or other
situations, a small quantity of a peculiar glu-
tinous matter, not readily acted on by water,
is excreted along with the ova, which serves
to fix the ova in a suitable place during the
development of the young.* Various ex-
amples of a similar kind occur among the
oviparous animals of the invertebrata, more
especially among insects and mollusca, when
the ova are destined to remain exposed, and
require protection during a considerable time
before development takes place.

In batrachia the yolk is variously coloured
in different species : thus, in the common frog,
toad, and some others, the surface or ger-
minal part of the yolk is of a black or dark-
brown colour, owing to a deposit of pigment
granules in the cells of the germinal layer,
while the remainder of the yolk internally is
grey. In some other batrachia the colour
is light brown. In the larger water-newt, or
triton, the yolk is of a brilliant light yellow ;
while in the smaller one, or lissotriton, it is

* See the interesting description of this process
by Rusconi, in Amours des Salamandres Aquatiques ;
Milan, 4to. 1821. I have often confirmed his ob-
servations on this process in ponds, and with animalg
kept in vessels in the house.

E 2



OVUM.



ash-coloured. In the land-newt, which is oyo-
viviparous, the yolk is of considerable size,
and of a dark yellow, approaching to orange.

In osseous fishes, which are almost all
oviparous, the ovule receives, apparently in
the ovarian capsule itself, before leaving that
cavity, an external covering (or chorion) of
considerable firmness. This membrane ap-
pears to consist of a substance deposited on
the external surface of the vitelline membrane,
and becomes coagulated under the action of
water ; so that its density increases greatly
after the ova are deposited, while it is sepa-
rated at the same time from the yolk by
the imbibition of water. The ova are in
spawning either deposited separately, or are
united in chains or bundles, and in some
less common examples* in peculiar nida-
mental structures, more after the manner
of some of the mollusca. The structure of
the ovarian ovule, or yolk, and its relation
to the germ, differs somewhat from that of
the batrachia ; for while in the latter animals
the yolk substance consists of granules and
cells of nearly uniform size, and the germinal
layer covers the greater part of the surface,
in osseous fishes this layer is more circum-
scribed, riot extending at first over more than
a third, or, at most, a half of the yolk, and
the remainder of the yolk, which contains a
much greater quantity of transparent fluid than
in most other vertebrate animals, presents al-
most invariably a peculiar heap or mass of
large oil globules, which float to the upper
part of the fluid below the germinal layer.f

The minute ovula of mammalia, when they
have reached maturity in the Graafian capsules
of the ovary, are nearly spherical bodies, of
from T-LJ. to -jphr of an inch in diameter, and
consist of a mass of finely granular yolk sub-
stance, more loose in the interior and more
dense towards the surface, and enclosed in a
thick firm and transparent vesicular envelope,
the vitelline membrane, or so-called zona
pellucida. While still within the Graafian
capsules, they occupy a situation near the
most projecting part of the capsule, or
towards the external surface of the ovary,
being there imbedded in a layer of granular
cells, the discus proligerus of Von Baer, which
lines the ovicapsule, and lies on the exterior
of the clear coagulable fluid with which this
capsule is filled. A portion of this lining
membrane of granular cells, remains adherent
to the ovum after it leaves theGraafian capsule,
and has passed into the Fallopian tube; but as
it descends towards the uterus, these cells gra-
dually loosen and fall away from the surface
of the ovum, the zona pellucida or vitelline
membrane of which is thus finally left free.
In the farther progress of its descent, there
is formed, in some mammalia at least (rabbit),

* As Gobius. See Prof. Owen's Lectures on the
Compar. Anat. and Physiol. of Vertebrated Animals,
part i. p. 304. A. Hancock on the Nidification of
the Gasterosteus aculeatus, &c., in Annals of Natu-
ral History, Oct. 1852.

t See a "paper by Dr. Davy on the chemical pro-
perties of the vitellus of Osseous fishes in Trans,
tfoy. Soc. for 1851.



on the surface of the zona by a new deposit,
in others, perhaps, by conversion of the zona
itself, the external membrane of the ovum,
which at a later stage constitutes the chorion.
But, in accordance with the destination of the
ovum in this tribe of animals for true utero-
gestation, this external membrane has then no
longer the character of mere inactive limita-
tion of the exterior of the ovum, or defence
from injury, which belongs to it in the lower
animals ; but it becomes an organised and
growing texture of active functions, which is
the more immediate means of uniting organi-
cally the blood-vessels of the mother and
foetus, in such a manner as to allow of the
transmission of nourishment from the one
to the other.

Varieties of form of the ova among the
invertebrata are too numerous to allow of
their being described in this place. In the
greater number, an external envelope, besides
the vitelline membrane, exists ; but it must
be admitted, that there are some in which
these two coverings cannot be distinguished.
In some, as in insects, arachnida, polypes, &c.,
the chorion, or outer surface, presents pecu-
liar markings, ridges, tubercles, or long spines,
and is strong and opaque ; in others, it is

Fig. 34.




Ovum of Cristatella mucedo.
(From Turpin, Annal. des Scien. Nat. 1837. torn,
vii.) Showing peculiar spinous projections from the
outer shell.

smooth, delicate, and transparent, so as to
allow the whole internal structure of the
ovum to be seen through it, and thus to
afford most favourable opportunities of wit-
nessing the early changes of development.
In most of the invertebrata the germinal
part of the yolk covers the whole, or a con-
siderable part, of its surface ; they present,
however, great varieties of colour and struc-
ture, and may, probably, belong to various
modifications of the second and third groups
before distinguished.

It does not appear that any essential dif-
ference has yet been observed in the structure
of the ova of those animals which are subject
to alternate generation, and those of animals
in which the adult form is directly developed
from the ovum.

3. Of the ovary in general as the forma-
tive organ for the ova of animals.



OVUM.



53



The name of ovary is in all animals applied cle and enclosing membrane that is formed

to the organ, however varied in its structure in the ovary, while the external or cortical

and relations, in which the ova are formed, parts of the ovum are added to these in their

As already indicated, however, it is to be descent through the female passages after

observed, that in the higher animals, it is leaving the ovary. There are some examples

only the ovule, or yolk, with its germinal vesi- in which it would appear that the whole

Fig. 35.




Relation of the ovaries, ovum, oviduct and uterus in Mammalia.

A. Gravid uterus, &c. of the rabbit, ten days advanced in pregnancy ; a' a, right and left ovaries,
four corpora lutea in the right, two in the left; b' b, fimbriated openings of tf c, the Fallopian
tubes; did, the right and left cornua of the uterus; d', with four dilatations from contained ova, d,
with two dilatations, one of which is opened to show the ovum ; e, the body of the uterus ; /, the
vagina.

B. A diagrammatic transverse section of the human uterus, at twelve or fourteen days after conception,
somewhat less than the natural size ; e, the uterine cavity, near which the ovum with its villous
chorion is involved in the substance of the decidua indicated by the dotted shading ; c' c, the Fallo-
pian tubes cut short, by one of which the ovum had previously descended while still of small size.

c. Enlarged view of the exterior of the human ovum, of twelve or fourteen days after conception,
showing the villi of the chorion projecting from its surface.

E 3



OVUM.



formative process of the ovum, including
the addition of the external coverings, is
completed within the ovary; and, on the
other hand, there are a few instances in
which, as in the trematoua and cestoid en-
tozoa, the germinal vesicle and yolk sub-
stance of the ovule are formed in separate
organs, instead of in the usual manner entirely
in the ovary.

The varieties of the ovaries in different
animals may be considered under two heads
viz., 1st. Their relation to the passages or
outlets as influencing the mode of discharge
of the ova from them ; and 2nd, their internal
structure as related to the form of the ovum
produced.

Fig. 36




Ovary and oviduct of a laying Fowl, killed twelve
hours after laying the last egg.

a. Left ovary ; b, opening of the infundibulum of
the oviduct ; c, d, glandular portion of the oviduct ;
at d, the isthmus ; e, an egg in the uterine portion
of the oviduct, in -which the shell is begun to be
deposited ; /, the rectum, ending in the cloaca ; g,
the undeveloped right oviduct occasionally met
with.



a. Relations of the form of the ovaries to the
discharge of ova. In the majority of verte-
brated animals the ovary or ovaries are quite
detached from the conducting tube or ovi-
duct ; the ovules are formed in close capsules
of the ovary, by the bursting or fissure of the
wall of which they escape ; the oviduct opens
at its upper end into the abdominal cavity,
and there receives the ovum which has been
discharged from the ovary. This is the general
arrangement in mammalia, birds, reptiles, am-
phibia, and cartilaginous fishes. There is some
difference in the form of the ovary in the
higher and lower of these animals. In mam-
malia and birds, in chelonia and the crocodiles
among the reptiles, and in cartilaginous fishes
the ovary is more or less solid, and the ovules
are developed in capsules which project
towards the external surface ; but in the lizards

Fig 37.




(.From Cams and Otto.)

Female of the Falco buteo opened, showing the
left larger oviduct and ovary, and the smaller right
oviduct and ovary, of a, the right and left ovaries ;
b, the left infundibulum ; c, d, the left oviduct ; /,
the rectum, ending in the cloaca, which has been
opened, showing at h> h the openings of the right
and left oviducts, and at i' i those of the ureters ; g,
the vestige of the right oviduct.



OVUM.



55



and serpents, and in the batrachia, this organ
is hollow, and the capsules in which the
ovules are formed burst in dehiscence into
an internal cavity, whence the ovules escape
into the abdomen by the rupture or open-
ing of the sack of the oviduct, generally
at one, but sometimes, as in the frog, at a
greater number of places. In the higher
animals, in which the ovules escape from
the external surface of the ovary, their en-
trance into the oviduct is in general secured
by the temporary apposition of the dilated
upper end or infundibulum of the oviduct to
the ovary, or the capsule containing a ripe
ovule ; in the other animals, in which the
ova come from the interior of the hollow ovary,
the apposition of the oviduct does not ap-

Fig. 38.




Common adder, in which the ova have descended
to occupy both oviducts, five in the right, and three
in the left : the infundibulum is shown in each ovi-
duct ; a' a, the right and left ovaries, each forming
a sac, opening anteriorly near the infundibulum for
the discharge of the ova, which, when ripe, fall
into the interior of the sac, and thence pass into the
oviduct.



pear to be so direct, and there are various
other means by which the ova, when they have
escaped into the abdominal cavity, reach the
open extremity of the oviduct.

It is in the class of fishes that the transi-
tion occurs from the higher to the lower type
of organisation of the ovaries and oviducts.
In all of them the ovules are formed in ova-
rian follicles, and escape by dehiscence from
these follicles ; but there are several modifi-
cations of the relation between the oviduct
and ovary among them. 1st. In the sharks
and rays, as already stated, the arrangement
is nearly similar to that existing in higher ani-
mals. The ova, which are of large size, come to
maturity singly, or in small numbers at once :
on being discharged externally from the ovarian
capsules, they pass into the oviduct, and there
receive a considerable addition from this
organ. The majority of them, as previously
stated, are oviparous, and in them a hard
covering is formed by a peculiar glandular
organ connected with the oviduct ; in a few
which are ovo viviparous, as the common dog-
fish, torpedo, &c., the external covering of
the ovum is membranous and soft. 2nd. In
the sturgeon and in the lamprey the oviduct
is very short ; still, as it opens superiorly into
the abdominal cavity, the relation may be
considered the same as in the previous ex-
amples. 3rd. In the genus salmo and in

Fig. 39.




Ovaries and oviduct of an osseous Fish.

A. Sketch of the two largely developed saccu-
lated ovaries of an osseous fish, with 'the short ovi-
ducts proceeding from near their posterior ex-
tremities.

B. Diagrammatic section of a portion of the ova-
rian sac, showing two of the ovarian plates, from
which the developed ova hang in small pediculated
vesicles or ovisacs.

E 4-



50



OVUM.



the eel among the osseous fishes, the oviduct
is entirely wanting, and the numerous ova,
which are discharged by external dehiscence
from the ovary into the cavity of the
abdomen, escape from that cavity by an
orifice (porus abdominalis) situated on each
side close to the anus. 4th. In other osseous
fishes, the ovary and oviduct are united,
or the ovary forms a saccular organ, in
the interior of the wall of which the ovi-
capsules are situated, occupying a variable
extent of it in different genera ; and the wall
of the oviduct, usually very short, is continued
from that of the ovary to the outlet from the
animal's body. The ova, therefore, which
drop by internal dehiscence into the cavity of
the ovary, pass directly out by the short ovi-
duct in the laying of the spawn. Most osseous
fishes are oviparous; but in a few, as the
viviparous blenny, the anableps, paecilia, and
some siluroids, the ova, on escaping from their
capsules into the cavity of the ovary, remain
there during the development of the embryo.
In the invertebrate animals there are very
many varieties in the form and relations of
the productive and conducting parts of the
female generative organs. Three principal

Fig. 40.




Oviduct and ovary in a continuous tube in Insects
and Entozoa.

A. (From R. Wagner). Upper part of the ovi-
duct or ovary of the Acheta campestris.

B. (From ~H. Nelson.) Upper part of the oviduct
or ovary of the Ascaris mystax. In both of these
figures the germ-cells and germinal vesicles, with
their nuclei, are seen surrounded by the granular
matter which afterwards collects round them as
vitelline or yolk substance.



varieties may be distinguished among them
1st. A form similar to that just now described
as generally prevalent among osseous fishes,
in which the ovary and oviduct are con-
tinuous, but in which the ova, being formed
in ovarian capsules, are dropped by dehiscence
into the upper part of the oviducts. Such is
still the structure in cephalopoda and some
other mollusca. 2nd. A form in which the
oviduct may be said to be, as in the last, con-
tinuous with the ovary, but in which there
is no true dehiscence of the ovules from
ovarian capsules, as they are formed at once
in the internal cavities of the ovary, which
directly open into, or are mere prolongations
of, the oviducal tubes. In this form the
oviducts may be considered to stand in the
relation of excretory ducts to the ovarian
glands. In many of this class the ovaries
present very various forms ; in some the
continuity of the ovarian and oviducal tubes
is very obvious and simple, as in the ne-
matoid entozoa, insects, &c. ; while in others,
the ovary is more complex and race-
mose, and the oviducal tubes comparatively
simple. 3rd. That form in which the ovaries
are variously disseminated over the body of
the animals, and in which there are no true
oviducts, but the ova escape on various parts
of the internal or external surface of the
body.*

b. Structure of the ovaries themselves, as
related to the production of the cvula. In
mammalia these organs consist of a pair of
solid oval flattened bodies, attached by inter-
vening fibrous tissue to the posterior surface
of the broad ligaments of the uterus, and are
covered completely, excepting at this attached
part, by peritoneum. Below this serous co-
vering there is also a layer of firm fibrous
tissue, or tunica albuginea. The internal sub-
stance, or parenchyma, or stroma, as it has
been called, consists of a firm basis of fibro-
cellular texture, of considerable vascularity.
The fibres, as well as the blood-vessels of this
substance, radiate principally from the at-
tached border of the organ towards the oppo-
site, or free side, and the rest of the surface.
The ovicapsules, or so-called vesicles or fol-
licles of De Graaf, in the human ovary, are
situated in this stroma; and at or after the
period of puberty are found of some size ;
a variable number, from twelve to thirty, or
more, being of from V to i f an inch, and
a few even a little larger. These mem-
branous vesicles, filled with fluid, are situated
chiefly towards the surface of the free side of
the ovary. A larger number of undeveloped
capsules, of minute size, also exist in the

* See Von Baer's Entwickelungsgesch. der Thiere ;
Owen's Lectures on Invertebrate Animals, 1843, and
on Fishes, 1846 ; Rathke (on Development of
Fishes, &c.), in Geschjchte der Thierwelt, Th. 3. ;
J. Miiller (on Sharks), in Mem. of Berlin Acad.
1842; John Davy (on the Torpedo), in Philos.
Trans, for 1834; and the works of Von Siebold and
Stannius, R. Wagner, Carus, and others on Compar.
Anat. See also in this Cyclopaedia, the articles
Monotremata, Pisces, Reptifia, and Organs of Ge-
neration.




57



Relation of the ova and ovaries in Mammalia.

A. (From Coste, as reduced by Longet.) Human ovary, enlarged four diameters, partially dissected at
ooo, to show the Graafian follicles in the ovarian stroma : one of these, more advanced^ has had its
double tunic, o r, cut into and reflected ; the granular membrane, m g, has also been partially opened,
showing the thickened portion or granular disc, d g, in which the ovum is imbedded near the most
projecting part. At o r', another Graafian follicle has been burst, and the ovum in its granular disc
is seen expelled from it.

B. Transverse section of human ovary, to show the general arrangement of the developed Graafian
follicles towards the surface ; twice the natural size.

c. Diagrammatic representation, in section, of two.Graafian follicles, in different stages of advance-
ment in the ovary of a mammifer, enlarged about ten diameters, p. Peritoneal covering of the ovary,
st, ovarian stroma; ov, the two layers of the ovisac; mg, membrana granulosa, near which is the
discus granulosus, with the ovum imbedded.



stroma ; and it has been observed, that these
are present from a very early period in the
ovaries, as first noticed by Cams, and since
by myself and others in the child at birth.

The more developed of these ovi-capsules
are enclosed by a strong theca or membrane,
consisting of two layers ; the external thinner
and firmer, of a fibrous and vascular struc-
ture, the internal thicker and softer, of a
fibro-cellular structure and also of consider-
able vascularity. The capsules are filled with
a fluid nearly transparent, which coagulates
under the action of heat ; and inside the
theca, or lining it and covering the fluid, there
is a layer of nucleated cells united together in
the form of a soft, easily-lacerated membrane,
somewhat like an epithelial lining of the cap-
sule. It is in this cellular layer (tunica gra-
nulosa of Von Baer) that the ovum is placed,
being situated in the thicker portion of it



(cumulus proligerus), directed towards the
surface of the ovary.

When one of the ovicapsules and its con-
tained ovule has reached maturity, which
takes place in one or more of them at regu-
larly recurrent periods, besides the swelling
of the ovicapsule itself from the increase of
its fluid and other causes, the stroma of the
ovary between the capsule and the surface
undergoes considerable thinning, and the ovi-
capsule comes thus to project more imme-
diately from the surface of the ovary. An
increased vascularity is also apparent in the
same situation ; and finally a small circum-
scribed fissure near the middle of the most
projecting part occurs, allowing the escape of
the ovule and the granular layer and fluid
from the ovicapsule.

The ovule, surrounded by a portion of the
cellular layer in which it was embedded, is



58



OVUM.



received by the open fimbriated extremity of
the Fallopian tube.

The empty ovicapsule now undergoes a
remarkable change by the deposit in its inte-
rior of the substance termed corpus luteum,
the quantity and nature of which vary greatly
according as the escape of the ovule is fol-
lowed or not by pregnancy. Of this change
more will be said hereafter. The result in both
cases is the ultimate closure and obliteration
of the ovicapsules.

In birds, scaly reptiles, and cartilaginous
fishes, the greater size of the ovules when in
a state of maturity is connected with a modi-
Fis. 42.




Relation of the ova to the ovary in Birds.

A. Ovary of a fowl, showing at a a a the most
developed ova hanging from the ovary in their pedi-
culated capsules ; the non- vascular bands are seen
on their most projecting sides; at bb, the empty