while the natural condition of the contents is
thus preserved.

The shell in most eggs is slightly dimpled
externally, with small depressions visible to
the naked eye ; but these are not the open-
ings of the pores through which evaporation
or exchange gf gases takes place these being
much more minute and numerous but merely
the indication of depressions caused by the
largely villous structure of that part of the
oviduct (uterus) in which the calcareous
shell is deposited.

On removing the earthy matter by means
of a dilute acid, the animal basis remains as a

* Op. cit.

t Dr. Wm. Aitken has, at my request, repeated
these experiments, and has obtained results in ac-
cordance with the above statement. He found the
unboiled yolk to float indifferently in any part of a
saline fluid of specific gravity 1035. By boiling, the
specific gravity was reduced to 1031, and in both
cases the side with the cicatricula floated upper-
most. The upper half, containing the cicatricula,
had a specific gravity of 1030 ; the lower half, 1032.



slightly coherent, cellular, organised structure,
the form of the small compartments in which
corresponds with that of the calcareous par-
ticles of the shell (see^. 45. c). The in-

Fig. 45.





Structure of the shell and shell-membrane in the
Fowl's egg.

A. Lining membrane of the shell ; a, thick matted
or felty portion ; b, tliin shred of the torn margin,
showing the peculiar fibrous tissue of which the
various layers are composed.

B. Outermost layer of the same, which is incor-
porated with the shell ; some of the angular cor-
puscles of the shell lying upon the fibrous substance
and firmly united with it.

c. Small portion of the calcareous shell, which
has been steeped in dilute hydrochloric acid, show-
ing the remains of opaque calcareous substance in
the centre, some portions of it exhibiting a granular
aspect, and round the margins the animal basis or
matrix from which the calcareous matter has been
dissolved, presenting an irregular granular or almost
amorphous aspect. Here and there clear oval cells
are seen, as at a a.

ternal surface is irregular and flocculent, and
adheres very closely~to a different kind of
membrane which lines the shell.

In those instances in which the shell of
eggs is coloured, the pigment substance, of
various hues, is generally deposited in cells,
which are strewed uniformly or in patches
over the external surface of the calcareous
shell. In some other instances, however, the



64



OVUM.



colour seems to be merely a uniform tinge of
the outermost layer of calcareous matter.*

The lining membrane of the shell is a
peculiar fibrous, interwoven structure, depo-
sited in laminae of some thickness and tough-
ness, which is readily divided by tearing into
two layers over the whole surface of the
egg an outer, thicker, and denser, adhering
firmly to the inner surface of the shell j and an
inner, thinner, smoother, and of finer texture,
which may be easily withdrawn from the
outer one, and which naturally separates from
it at the air-space ; but both the outer and
inner layers of this membrane may be torn
into a number of thinner laminae, all agreeing
in their minute structure.

By microscopic examination, this membrane
is found to consist of a closely-interwoven
network of peculiar fibres, which are of va-
rious sizes, generally between 8b * 00 th and
^._u_th of an inch in diameter ; the larger fre-
quently branching into or giving off smaller
fibres at acute angles, the sides rendered un-
even by minute projections or knots upon
them (not represented in the figure) ; the
larger fibres are of a somewhat flattened or
ribband-like form. The external layer of the
membrane contains the largest fibres. These
fibres appear to be analogous in their che-
mical nature to those of the elastic yellow
texture, not being soluble in strong acetic
acid ; but they do not coil up in the manner
of the elastic tissue (see fig. 45. A).

The parchment-like coverings of the eggs
of serpents and lizards, which have no calca-
reous shell, seem to be composed of a greater
number of layers of the fibro-laminar texture
now described.

The albumen, or white of the egg, compre-
hends several layers of glairy, albuminous,
semifluid substance deposited round the yolk,
the chalazae, or grandines, or twisted cords,
and the condensed layer of albumen, forming
a thin membranous investment immediately
over the yolk membrane. In a perfectly-fresh
egg, or in an egg taken from the oviduct pre-
viously to its being laid, the whole albumen
has the consistence of a moderately-firm
jelly ; but very soon the outer part becomes
fluid, and allows of the freer motion of j the
parts within the shell. This solution of the
albumen proceeds to a greater extent after
some hours' incubation, especially over the
cicatricula.

The deeper part of the albumen, or that
next the yolk, is more dense in consistence.
No part of it, when unchanged by reagents,
presents any sensible structure either to the
naked eye or when viewed microscopically.
If, however, the soft contents of a fresh egg,
or one removed from the oviduct, be taken
from within the shell, and thrown into water
either pure or with a little acetic acid mixed
with it, a slight turbidity or coagulation of
the albumen takes place on the surface, which
brings out the appearance of a spiral arrange-

* See the works of Hewetson and others on the
Eggs of Birds. Carus and Otto, Erlauterungstafelu
der Vergleich. Anat. part v.



ment of laminae ; and in a boiled egg these
laminae may be torn in great numbers in suc-
cession from off it, the direction pf the spiral
being from left to right, from the large towards
the small end of the egg. With a little
care, almost the whole of the albumen may
thus be wound off the egg in spiral strips,
the deeper ones enclosing the twisted cha-
lazae (see^g. 46. n).

The coagulated albumen presents, in the
microscope, a minute but indefinite granular
structure.

The chalazcB (grandines) are two irregularly-
twisted cords of albumen, harder than the rest,



Fig. 46.




Manner in which the chalazce, albumen, 8fc., are
deposited round the ovarian ovum of the Fowl.

A. Yolk from the upper part of the oviduct soon
after it has entered it, showing a thin covering of
albumen on the yolk, forming the chalaziferous
membrane, and the twisted chalazae extending from
the opposite poles of the yolk. The twisting in
these is represented more strongly than it can be
seen at this period.

B. Sketch of the fully formed chalazae from
opposite sides of the yolk, stretched to their full
length, and showing the opposite direction of the
spiral in each.

c. Egg from above the middle of the oviduct ;
the first layers of albumen deposited round the yolk
and chalazoe.



OVUM.



65




D. Egg from the lower part of the glandular
oviduct near the isthmus, when the deposit of albu-
men is complete; the spiral arrangement of the
albumen made manifest by slight coagulation.

attached to the opposite ends or poles of the
yolk by means of a membrane which looks ex-
actly like a continuation of the twisted part
of these bodies opening or expanded over the
surface of the vitelline membrane. These
bodies attracted considerable notice from the
earlier observers of the structure of the egg,
and have had various uses attributed to them ;
but, if we may judge from the varieties they
are subject to in the fowl and other birds, and
their absence in the ova of scaly reptiles
(otherwise very similar to those of birds), it
would appear that they are only of secondary
importance.

One of the chalazae is directed towards the
larger, and the other to the smaller end of the
egg, and the latter usually adheres with some
firmness to the inside of the shell-membrane,
while that of the large end floats more freely.
In this manner the yolk moves more freely
at the large than at the small end of the
egg. The spiral twist is in opposite direc-
tions in the two chalazae ; a circumstance
depending on the manner of their production,
by the gradual deposit of albumen and the
spiral motion of the yolk during its descent
in the oviduct. The membrane which pro-
ceeds from the chalazse over the surface of
the yolk has been called chalaziferous ; and
the funnel-shaped dilatation of the chalazae
where they join the membrane, has been sup-
posed to be the opening of a tube passing
through these bodies, and serving as a conduit
from the white to the yolk ; but entirely
without reason. The chalaziferous membrane
and innermost twisted part of the chalazae
are, in fact, nothing more than the first-
deposited and densest parts of the albumen ;
nor is any importance to be attributed to a
curved line or fold of the membrane which is
often seen stretching over the yolk between
the adhering parts of the opposite chalazae.

The fact of the upturning of the side of the
yolk which bears the cicatricula has already
been adverted to, as well as the supposition that

Supp.



the chalaza3 may be the means of securing
this position ; but, although it is well ascer-
tained that these bodies control, in various
directions, the motions of the yolk, they can-
not be the cause of the upturning of the cica-
tricula ; as this is secured by the difference
of specific gravity in the upper and lower
parts of the yolk. The true action of the
chalazae is to limit the motions of the yolk in
the long axis of the egg, and control the rota-
tion during a certain time ; but in incubation
the relations of the chalazae, white, and yolk
are very soon changed ; and, in the progress
of these changes, the remains of the denser
white are collected at the lower part of the
egg. If a fresh egg be turned round on its long
axis, the cicatricula will keep its position up-
wards for one turn or a little more, and then,
by the twisting of the chalazae, the yolk is
carried completely round, and balances itself
again with the cicatricula uppermost in its
new position.

The accessory parts of the egg, now de-
scribed, are formed round the yolk or ovarian
egg during its descent through the oviduct ; and
as they may be regarded as only indirectly
connected with the functions of the true ovu-
lum in their relation to embryonic develop-
ment, it may be best to complete their history
at this place by stating what has been ob-
served as to their formation, referring for this
to the researches of Purkinje, Coste, and
others, which I have confirmed in most parti-
culars by the examination of a considerable
number of fowls during the process.

Formation of the external or accessory parts
of the bird's egg. These parts are produced
with much greater rapidity than those of the
ovulum. Many fowls lay an egg every twenty-
four hours for a part of the season, while
others lay only every second day, or two or
three days in succession, generally at a later
hour on each successive day, and then intermit
for a day ; other fowls lay regularly nearly
every thirty-six hours. There is probably some
difference in the rapidity of the descent of
the egg, or at least in the length of time it
remains in particular parts of the oviduct, in
these various cases ; but in general the whole
passage of the egg, from the time of the re-
ception of the yolk by the infundibulum to its
being laid, occupies about twenty-four hours.

If a fowl which is laying only every second
day, be killed and opened from seventeen to
twenty hours, or if one which is laying daily
be opened from three to six hours after the last
egg was deposited, one of the ovarian capsules
will sometimes be found completely enveloped
by the infundibulum of the oviduct, which is
thus in the act of receiving the ovulum or yolk
about to be discharged by the cleaving of the
capsule along the stigmatic band.* The infun-
dibulum is contracted round the neck or pedi-
cle of the ovarian capsule, so that the whole is
embraced by it with moderate firmness, and
the yolk thus usually passes securely into the

* See a later section for an account of the circum-
stances which influence the discharge of the ovarian
ovula.



66



OVUM.



oviduct; but it occasionally happens that
capsules burst without being so embraced, or
that the process is disturbed, and the sub-
stance of the yolk falling into the abdominal
cavity of the fowl either produces serious
injury by peritoneal inflammation, or may be
gradually removed by absorption.

The yolk enters the infundibulum, with its
long axis corresponding to that of the oviduct,
consequently with the cicatricula on its side,
which we shall find to be its position also in
the completed egg.

The passage of the yolk through the first
two-thirds of the length of the oviduct, in
which part the albumen is deposited, is very
rapid, scarcely occupying more than three
hours, according to Coste*, before it arrives
in the narrow or constricted part of a more
limited extent (isthmus), in which the mem-
brane of the shell is formed. About three hours
more suffice for this process, and the ovum then
enters the dilated portion, which has been
called uterus, in which the substance of the
shell is deposited and gradually consolidated
on its surface.

The albumen begins to be deposited round
the yolk, immediately upon the entrance of
the latter into the oviduct j at first in a thin
layer, immediately investing the yolk, which
subsequently becomes condensed into the
chalaziferous membrane, and in two long
narrow portions extending before and behind
the yolk from its poles, which portions ot
albumen are at first straight and simple, but
afterwards become twisted and form the
chalazas. ( See fig. 46, A.)

In the next part of the albumimferous part
of the oviduct, in which the glandular struc-
ture is most fully developed, the albumen is
deposited in much greater quantity round the
yolk and chalaza3, not following the form ot
the latter, and thus soon gives to the whole
the oval shape which belongs to the egg; and
we then recognise, previous to the formation
of the shell or its lining membrane, that the
narrower end of the oval is placed down-
wards, or advances first in the oviduct.

During the passage of the egg, and the
formation of the albumen, membrane, and
shell, a greatly increased determination of
blood is observed in the vessels of the se-
veral parts of the oviduct. (See j%. 47.)

The formation of the accessory parts ot the
eg( r appears to proceed nearly in the same
manner in the scaly reptiles as in birds. The
accompanying figure, borrowed from the
article Reptilia, is illustrative of the mam
features of the process.

The advancing motion of the egg qf the fowl
is caused by the peristaltic action of the muscu-
lar coat of the oviduct, which may be easily seen
in any laying fowl opened immediately after
death. The egg does not descend, however,
in a straight line, but in a spiral direction,
corresponding with that of the ridges of
glands with which the mucous membrane
of the oviduct is beset. Two peculiarities
in the structure of the albuminous part ^ of
the egg result from this spiral motion viz.,

* Hist. gn. et partic. du De'vel. &c.



fig. 47.




Descent of the egg in the oviduct of the Tortoise

(after Bojanus).

A. Infundibular opening of the oviduct; n, o,p,
canal of the oviduct laid open; s, *, ovum opened,
showing the yolk, albumen and shell; B, allantoid
bladder ; F, oviduct ; c, D, kidney ; E, ureter ; m,
termination of the opposite oviduct,
the spiral laminated form of the outer layers
of albumen, and the marked tortuosity ot t
chalazse. It is easy to understand how t
spiral form is given to the deposit ot the
layers of albumen. The cause of the pecu-
liar manner in which the chalazae are twisted
is not so immediately apparent . it may b
explained as follows. As already remarked,
the spiral twist is in an opposite direction m
the two chalazse ; one end of each ot these
cords must, therefore, have remained in a
state of rest as compared with the other.
Either, it may be supposed, the farther ends
of the two chalazre extending into the ovi-
duct before and behind the descending yolk,
remain comparatively at rest, while that body
with the albumen forming round it being
closely embraced by the oviduct has the ro-
tary motion impressed upon it ; or, as is more
probable, when the chalaza3 become attached
to and involved in the deposited albumen,
their outer ends move with it, while the yolk
within, to which the inner ends of the chalazae
are fixed, does not rotate in the same degree;
a circumstance to which it is possible the dis-
position of the side on which the cicatricula
is placed to remain uppermost may m some
degree contribute; and thus the yolk not turn-
in^ so rapidly, or so often as the white, the
chalazse are twisted upon their roots attached
to the surface of the yolk.*

* It ought to be observed, however, that according
to Coste, the yolk does not at first rotate freely



OVUM.



67



Although it can scarcely be doubted that
the chalazae are produced during the de-
scent of the egg, while the albumen is being
deposited, it is worthy of remark, that the
twisted structure of these bodies is usually
not to be seen till after the shell has begun
to be formed * ; but it is very probable that

Fig. 48.




Position of the egg in the oviduct as it descends.

A portion of the oviduct near the lower end
opened, taken from a fowl killed three and a half
hours after the last egg was laid. The greater part
of the albumen has been deposited, and the egg has
assumed its peculiar form, the small end of the oval
advancing first; the cicatricula placed on the side
of the yolk.

this may depend on their not having pre-
viously acquired sufficient opacity or conden-
sation to render their tortuous structure ob-
vious. Indeed, Von Baer has observed them
to make their appearance by increase of their
opacity from exposure while under actual
observation.

It has been ascertained by experimental
observation that the membrane of the shell
is formed in the narrow part of the oviduct,
termed the isthmus, which intervenes between
the albuminiferous part and the uterine dila-
tation. It consists, no doubt, in the fibrillation
of consolidated albumen, or some analogous
substance, which must take place with great

within the white, and that it is only towards the
end of the period of its passing through the oviduct
that a liquefaction of the albumen, which then occurs,
permits this rotation : but I think it doubtful that
the adhesion between the surface of the yolk and
deeper albumen is so great as to prevent the degree
of rotation above referred to.

* Von Baer, Uber Entwick. p. 31.



rapidity ; but we are not yet sufficiently ac-
quainted with the nature of this process, for
the phenomena of the solidification and fi-
brillar organisation have not been minutely ex-
amined, nor has any difference yet been ascer-
tained between the substance secreted in the
isthmus, which undergoes the fibrillation with-
out calcification, and that of the uterine dila-
tation, which seems to have no such tendency,
remaining amorphous or cellular, and having
very soon a deposit of calcareous matter
formed in it.

By the time the egg arrives in the uterus,
it has acquired its peculiar oval form, the
small end pointing downwards in the oviduct.
The cause of this form, which is already ap-
parent in the white previous to the formation
of the shell, is somewhat obscure, on account
of the complexity of the mechanical condi-
tions influencing the egg in its passage. It
may probably depend on the circumstance
that the soft mass dilates the oviduct more
gradually as it insinuates itself between its
coats, in being propelled onwards, while the
part of the duct through which it has passed
contracts more abruptly and firmly in conse-
quence of the stimulus of distension to which
it has been subjected. But the variety of
forms which occurs in the eggs of different
birds and other animals must not be for-
gotten, as indicating that the peculiarity of a
lesser and greater end is not essential, and
may depend on very slight or transient cir-
cumstances. Perhaps, the greater density of
the albumen, secreted over the end which
advances first in the oviduct, may also have
some effect in giving this part the smaller
volume. It certainly seems remarkable that
the ends of the egg should be moulded into
so smooth and rounded a surface as that of
the membrane and shell by a tubular organ.
In some rare instances, however, I have ob-
served irregularities of form at the extremities
of the egg, indicating an imperfect contraction
of the oviduct during the passage.

The egg remains a much longer time (from
twelve to eighteen or more hours) in the
uterine dilatation of the oviduct during the
formation of the shell. The mucous mem-
brane of this part differs in structure consi-
derably from the rest: it presents over its
whole extent large villous-like processes, or
short folds, of a flattened form, containing small
follicular glands, from which the substance of
the shell is secreted. As soon as the egg
enters this part of the passage a thickish
white fluid is poured out from the membrane,
which speedily coagulates on the surface of
the membrane lining the shell, and very soon
we can perceive with the microscope small
heaps or united groups of particles somewhat
of a crystalline appearance, but in reality cal-
cified blastema studded over the whole surface.
These are the calcareous particles of the shell,
which are deposited in a delicate matrix of
animal tissue of a large cellular structure.
The deposit goes on rapidly increasing: at
first the shell is soft, it remains friable for
a considerable time, and, subsequently, it

F 2



68



OVUM.



gradually acquires the peculiar dry hardness
which characterises it after the egg is laid.*
The view of H. Meckel that the animal basis
of the shell is formed by the separation of
a layer of the mucous membrane of the ute-
rine part of the oviduct does not appear to
be established.

During the time that the shell is forming,
the distinction between the softer and thinner
external albumen, and the more dense and
deeper part, becomes more obvious, and, at
the same time, according to M. Coste, a cer-
tain degree of liquefaction occurs in a layer
of albumen immediately surrounding the yolk,
which allows the latter body to float more
freely within the superincumbent albumen.

The egg remains in the uterine dilatation
till it is about to be laid. The expulsion of
it from this cavity through .the narrow part
of the tube, leading into the cloaca, requires
very strong muscular contraction for its ac-
complishment ; and, although the egg always
descends in the oviduct, and usually lies in
the uterus, with its narrow end downwards,
both Purkinje and Von Baer state that they



have sometimes seen its position inverted
towards the end of the time of its residence
there in consequence of the force of the mus-
cular contractions of the wall of the oviduct.

Ovarian ovum of birds ; ovulum ; yolk and its
contents. The yolk, yelk, or vitellus (Jaime,
Fr. Dotter, Germ.) consists in the newly
laid egg of the external enclosing vitelline
membrane, of the yolk substance, a mass of
vesicular, cellular, and granular matter of va-
rious structure, to which as a whole the
membrane gives a subglobular form, and on
the surface of this mass, below or within the
vitelline membrane, and on that side of the
yolk which naturally turns uppermost in the
complete egg, the cicatricula, or embryo spot,
a thin disc of organised cellular structure, in
which, under the influence of heat and air,
as during ordinary incubation, the embryo,
and its accompanying foetal membranes, &c.,
are first formed.

The cicatricula of the laid egg, as has al-
ready been remarked, however, has, during its
descent through the oviduct, undergone some
part of those changes which belong to the



Fig. 49.




Form of the Fowl's egg and structure of the yolk as exhibited by a section.

A. Sectional view of the fowl's egg ; a, yolk enclosed by its vitelline membrane ; b, //, inner and
outer parts of the albumen : c, c, chalazae ; d, two principal layers of tbe lining membrane of the
shell ; e, calcareous shell ; /, air-space between the two layers of the shell membrane.

B. Outline of the yolk ; , cicatricula ; b, nucleus of the cicatricula ; c, yolk cavity or latebra, and
canal ; d, concentric deposits of yolk substance or halones ; m, vitelline membrane.



fecundated condition, and by which the found-