Fig. 186.—Hen’s Egg after 1 day’s incubation. a, air-chamber; hd, head of embryo; v.a., area in which yolk blood-vessels will appear later; yk, yolk. (× 1.)
The early development of the chick.—When the hen’s egg is exposed to a temperature of about 40°C., the germinal disc—the circular patch of living matter which lies, somewhat like a small inverted watch-glass, upon the upper surface of the yolk—grows larger, and its parts are gradually modified to form the various regions of the chick’s body. The white and yolk of the egg are used up during the process, and by the time they are absorbed, the young bird is in a condition to break the shell and take up the activities of an independent life. To watch the orderly appearance and gradual development of the different systems of organs is most fascinating, and fortunately the observation of the main features presents no great difficulty if the foregoing instructions are followed.
The first signs of the chick are to be seen ([Fig. 186]), towards the end of the first day of incubation, in a streak which crosses the germinal disc in a direction at right angles to the long axis of the egg. One end of the streak is distinctly rounded, and this is the head end (hd.). In almost all cases, if the egg be placed so that its broad end is to the observer’s left, the head of the embryo will be directed away from him.
A day later, the embryo is markedly larger, and is partly covered by a double membrane called the amnion.[17] Surrounding the embryo is now ([Fig. 188]) a network of fine blood-vessels which ramify over the yolk and carry small particles of yolk to feed the growing organs. At the end of the second day the network could be about covered by a sixpenny piece. On each side, its veins join to form a single small tube running to the heart (ht.), a tiny red dot which is situated behind the head and can be seen, by the help of a lens, to be beating rapidly. The head has grown more than the rest of the embryo, owing to the rapid development of the primitive brain, and is bent forwards. The head has turned over a little, so that its right side is directed towards the egg-shell; the rest of the embryo still lies “face-down.” The right eye (e.) can easily be seen as a dark spot on the side of the head.
Fig. 188.—Hen’s Egg after 2 days’ incubation. The amnion has been removed. a, air-chamber; au, beginning of right ear; e, right eye; ht, heart; v.a., network of yolk blood-vessels; yk, yolk. (× 1.)
By the end of the third day ([Fig. 189]) the head and neck are raised distinctly above the yolk, and the tail is also more plainly marked off, so that the body is connected with the yolk only by a short wide tube, the yolk-stalk (yk.st., [Fig. 187], B). The neck as well as the head has now turned so as to lie with the right side directed towards the shell. The commencement of the ear is to be seen on each side as a small pit (au.) a little above and behind the large eye.
