From the fourth day, when the chicken has attained the length of four lines, and its most important abdominal viscera, as the stomach, intestines, and liver, are visi ble, (the gall bladder, however, does not appear till the sixth day,) a vascular mem brane (chorion, or membrana umbilicalis) begins to form about the navel, and in creases in the following days with such ra pidity, that it covers nearly the whole in ner surface of the shell within the mem brana albuminis, during the latter half of incubation. This seems to supply the place of the lungs, and to carry on the respiratory process instead of those or gans. The lungs themselves begin in deed to be formed on the fifth day ; but, as in the foetus of the mammalia, they must be quite incapable of performing their functions while the chick is contain ed in the amnion.
Voluntary motion is first observed on the sixth day ; when the chick is about seven lines in length.
Ossification commences on the ninth day, when the ossific juice is first secret ed, and hardened into bony points (punc ta ossificationis.) These form the rudiments of the bony ring of the sclerotica, which resembles at that time a circular row of the most deli cate pearls.
At the same period, the marks of the elegant yellow vessels (vasa vitelli lutes) on the yolk-bag, begin to be visible.
On the fourteenth day, the feathers appear ; and the animal is now able to open its mouth for air, if taken out of the egg.
On the nineteenth day it is able to utter sounds ; and on the twenty-first to break through its prison, and commence a se cond life.
We shall conclude with one or two re marks on those very singular membranes, the yolk-bag and chorion, which are so essential to the life and preservation of the animal.
The chorion, that most simple yet most perfect temporary substitute for the lungs, if examined in the latter half of incubation in an egg very cautiously opened, pre sents, without any artificial injection, one of the most splendid spectacles that oc curs in the whole organic creation. It ex
hibits a surface covered with numberless ramifications of arterial and venous ves sels. The latter are of the bright scarlet colour, as they are carrying oxygenated blood to the chick ; the arteries, on the contrary, are of the deep or livid red, and bring the carbonated blood from the body of the animal. Their trunks are connected with the iliac vessels ; and, on account of the thinness of their coats, they afford the best microscopical object for demonstrating the circulation in a warm.blooded animal.
The other membrane, the membrana vitelli, is also connected to the body of the chick, but by a twofold union, and in a very different manner from the former. It is joined to the small intes tine, by means of the ductusvitello-intes tinalis (pedunculus, apophysis ;) and also by the blood •vessels, which have been already mentioned, with the mesenteric artery and vena portx.
In the course of the incubation the yolk becomes constantly thinner and paler, by the admixture of the inner white. At the same time innumerable fringe-like vessels, with flocculent extremities of a most sin gular and unexampled structure, form on the inner surface of the yolk-bag, oppo site to the yellow ramified marks above mentioned, and hang into the yolk. There can be no doubt that they have the office of absorbing the yolk, and conveying it into the veins of the yolk-bag, where it is assimilated to the blood, and applied to the nutrition of the chick. Thus, in the chicken which has just quitted the egg, there is only a remainder of the yolk and its bag to be discovered in the abdomen. These are completely removed in the fol lowing weeks, so that the only remaining trace is a kind of cicatrix on the surface of the intestine.