ovule, differs in different animals. Haller asserts, that in sheep, whose term of gestation is five months, he could observe nothing. more than a homogeneous mucus for the first sixteen days; but, at this time, membranes seemed to envelope the ovule and to give it shape; and on the twenty-fifth day, an opaque point indicated the fœtus. Haighton, in experimenting on rabbits, could detect no change before the sixth day, and the foetus was not perceptible till the tenth. In the case, related by Sir Everard Home to which we have so frequently referred, the embryo was perceptible, under the microscope of Mr. Bauer, and although its weight did not probably exceed a grain, the future situation of the brain and spinal marrow was apparent. From this period, and especially after the fifteenth day, the ovule can be separated into two distinct sets of parts, the dependencies of the foetus, and the foetus itself. These, in the course of pregnancy, become more and more readily separable. Each will require some consideration.

Prior to this, however, it may be well to refer to the changes that the egg undergoes during incubation; where we have an opportunity of observing the transmutations at all periods of fatal formation, independently of all connexion with either parent. The subject has engaged the attention of physiologists of all ages; but it is chiefly to those of more modern times—as Hunter, Cuvier, Dutrochet, Parker, Rolando, Sir Everard Home, MM. Prevost and DuMAS, &c. that we are indebted for more precise information on the subject; although, unfortunately, they are by no means of accordance on many points. The investigations of Sir Everard Home, aided by those of the excellent microscopic observer, Mr. Bauer, are peculiarly interesting from the engravings that accompany them, some of which we shall borrow in elucidation of the following brief description.

The egg of a bird, of a hen for example, consists of two descriptions of parts; those which are but little concerned in the development of the new being, and which remain after the chick is hatched, as the shell and the membrane lining it,—and such as undergo changes along with those of the chick and co-operate in its formation, as the white, the yolk, and the cicatricula or molecule. The shell is porous, to allow of the absorption of air through it; and of the evaporation of a part of the albumen or white. In the ovarium it is albuminous, but in the cloaca becomes calcareous. The membrane, membrana albuminis, that lines the shell, is of a white colour, and consists of two layers, which separate from each other at the greater end of the egg, and leave a space filled with air, owing to the evaporation of the white and the absorption of air. This space is larger the older the egg. The white does not exist, whilst the egg is attached to the ovary. It is deposited between the yolk and the shell as the egg passes through the oviduct. Of the white there are two distinct kinds;—the outermost, thin and fluid, which evaporates in part, and is less abundant in the old than in the fresh laid egg,

and another, situated within the last, which is much denser, and only touches the shell at the smaller extremity of the egg by a prolongation of its substance, which has been called the ligament of the white. The yolk seems to be, at first sight, a semifluid mass without organization; but by examining it, it is found to consist of a yolk-bag, two epidermic membranes, which envelope it as well as the cicatricula or molecule. Two prolongations of these membranes, knotty, and terminating in a flocculent extremity in the albumen, called chalazes, or poles are attached to the two ends of the egg and thus suspend it. It is also surrounded by a proper membrane; and lastly, under the epidermic coats of the yolk, and upon its proper coat lies the cicatricula, macula, tread of the cock, or gelatinous molecule from which the future embryo is to be formed. It is found before the yolk leaves the ovarium.

The external membrane of the yolk, when it quits the yolk-bag, is very thin and deliFig. 147(1)

cate; its surface is studded over with red dots, which disappear in its passage along the oviduct. When this membrane is removed, there is a natural aperture in the thick, spongy covering under it, through which is seen the cicatricula or molecule, surrounded by an areola, halo or circulus. On examination, this areola proves to be nothing more than that part of the surface of the yolk, which is circumscribed by the margin of the aper

ture.

The molecule or cicatricula itself, Fig. 1472), has a granulated appearance. and according to Sir Everard Home is made up, in the centre, of globules 2800th part of an inch in diameter, surrounded by circles of a mixed substance; about two-thirds consisting of the same small globules, and one-third of larger oval globules, about 7th part of an inch

The ova at different stages of increment. Ovarium of a laying hen, natural size.

1

in diameter; the last resembling in shape the oval red globules of

Fig. 147(2),

the blood in the bird, excepting in colour. Besides the globules, there is some fine oil, which appears in drops, when the parts are immersed in water. Oval globules and oil are also met with in the yolk itself, but in small proportion and devoid of colour.

When the egg leaves the ovarium, Fig.147,the ovarial yolk-bag gives way at the median line, and the yolk drops into the commencement of the oviduct. The yolk-bags are exceedingly vascular, the outer membrane of the yolk being connected to them by vessels and fasciculi of fibres, but being readily separable from them. During the first hours of incubation no change A new-laid egg, with its molecule, &c. is perceptible in the egg, but about the seventh, the molecule is evidently enlarged, and a membrane, containing a fluid substance, is observable. This membrane is the amnion. At this time a white line is perceptible in the molecule which is the rudimental foetus; and even at this early period,

Fig. 147 (3).

according to Sir Everard Home, the brain and spinal marrow can be detected. The areola has extended itself, and the surface, beyond the line which formed its boundary, has acquired the consistence of a membrane, and has also a distinct line by which it is circumscribed. This Sir Everard calls the outer areola. In the space betweeen these two areolæ are distinct dots of an oily matter.

In twelve hours, the rudiments of the brain are more distinct, as well as those of the spinal marrow. In thirty-six hours, the head is turned to the left side. The cerebrum and cerebellum appear to be distinct bodies. The iris is perceptible through the pupil. The intervertebral nerves are nearly formed; those, nearest the head, being the most distinct. A portion of the heart is seen.

At

this period, under the inner areola, apparently at the termination of the spinal marrow, a vesicle begins to protrude, which is seen earlier in some eggs than in others. The white of egg is found to be successively absorbed by the yolk, so that the latter is rendered more fluid and its mass augmented. The first appearance of red blood is discerned on the surface of the yolk-bag towards the end of the second day. A series of points is observed, which form grooves; and these closing constitute vessels, the trunks of which become connected with the chick. The vascular surface itself is called figura venosa or arca vasculosa; and the vessel, by which its margin is defined, vena terminalis. The trunk of all the veins joins the vena portæ, whilst the arteries, that ramify on the yolkbag, arise from the mesenteric artery of the chick and have hence been called omphalo-mesenteric.

In two days and a half, the spinal marrow has its posterior part inclosed: the auricles and ventricles of the heart are perceptible, and the auricles are filled with red blood. An arterial trunk from the left ventricle gives off two large vessels,—one to the right side of the embryo, the other to the left;—sending branches over the whole of the areolar membrane, which is bounded on each side by a large trunk carrying red blood; but the branches of the two trunks do not unite, there being a small space on one side, which renders the circle incomplete. This Sir Everard Home calls the areolar circulation.

Fig. 147(4),

In three days, the outer areola has extended itself over one-third of the circumference of the yolk, carrying the marginal arteries along with it to the outer edge but diminished in size. The brain is much enlarged; the cerebellum being still the larger of the two. The spinal marrow and its nerves are most distinctly formed; and the eye appears to want only the pigmentum nigrum. The right ventricle of the heart contains red blood: the arteries can be traced to the head: the rudiments of the wings and legs are formed, and the vesicle is farther enlarged, but its vessels do not carry red blood. It has forced its way through the external covering of the yolk, and opened a communication through this slit, by which a part of the

albumen is admitted to mix itself with the yolk, and gives it a more oval form. At this period, the embryo is generally found to have changed its position and to be wholly turned on the left side.

In four days, the vesicle is more enlarged, and more vascular, its

vessels containing red blood. grum of the eye are visible. the yolk, with which a larger In five days the vesicle has acquired a great size, and become exceedingly vascular; the yolk too has become thinner, in consequence of its admixture with more of the albumen.

The optic nerve and pigmentum niThe outer areola extends half over portion of the white is now mixed.

Egg, five days after incubation.

vessels are distinctly seen. Fig. 1470).

In six days, the vascular membrane of the areola has extended farther over the yolk. The vesicle, at this time, has suddenly expanded itself in the form of a double night-cap over the yolk, and its coverings are beginning to inclose the embryo, the outermost layer being termed the chorion, the innermost the middle membrane. The amnion contains a fluid, in which the embryo is suspended by the vessels of the vesicular membrane. The brain has become enlarged so as to equal in size the body of the embryo. Its The two eyes equal the whole brain in size. The parietes of the thorax and abdomen have begun to form; and the wings and legs are nearly completed as well as the bill. At this period muscular action has been noticed.

In seven days, the vesicle,—having extended over the embryo,has begun to inclose the areolar covering of the yolk, and a pulsation is distinctly seen in the trunk that supplies the vesicular bag with blood. The pulsations were, in one case, seventy-nine in a minute, whilst the embryo was kept in a temperature of 105°; but when the temperature was diminished, they ceased, and when again raised to the same point, the pulsation was reproduced. The muscles of the limbs now move with vigour. In eight days, the anastomosing branches of the vesicular circulation have strong pulsation in them.

In nine days the vesicle has nearly inclosed the yolk.