Fig. 192.—Diagrams illustrating the method of development of the allantois. al, allantois; am, amnion fold; am.c., amniotic cavity; em, embryo; yk, yolk. (After Foster and Balfour.)
By the end of the fifth day ([Fig. 191]) the head is enormous, and the limbs now show signs of being divided into definite segments. A thin bladder-like structure—the allantois (all.), which appeared on the fourth day—has grown out from the lower part of the body, behind the yolk-stalk. Its mode of origin is well shown in [Fig. 192]. It rapidly increases in size, and soon extends over the embryo (Figs. [ 192 B], and [ 191]) and becomes closely applied to the shell-membrane. Air passes through the pores of the shell, and its oxygen is taken up by the blood which circulates in the vessels of the allantois. At the same time, waste carbon dioxide is able to escape from the blood to the outer air. The allantois is therefore the breathing organ of the developing chick. If the egg is varnished and the shell thus rendered air-tight, the embryo dies of suffocation at an early stage.
Fig. 193.—Chick after
9 days’ incubation.
(After Duval.) (× 1.)
The chief organs of the bird are now established, and the later development may be sketched more briefly. By the end of the ninth day ([Fig. 193]) the white of the egg is almost used up; the yolk, however, is still large, and is connected with the chick’s body by the narrow yolk-stalk. It thus appears that the white is not directly absorbed by the chick, but is first taken up by the yolk and afterwards passed on by the yolk blood-vessels which run to the heart. By this time, too, the allantois has spread at least halfway round the inside of the shell, that a supply of oxygen adequate to the increased needs of the animal may be obtained from the air. The chick has now a characteristic bird-like appearance; the beak has appeared; feathers have begun to sprout; the neck is long and slender; and the segments of the limbs, including the fingers and toes, are well defined.
About the fourteenth day the chick turns so as to lie lengthwise in the shell, with its head near the broad end. The yolk-sac dwindles in size, and at last, about the twentieth day, it is drawn into the interior of the body. Now the chick becomes restless, and—usually on the twenty-first day—thrusts its beak through the inner shell-membrane into the air-chamber at the broad end of the egg. For the first time it draws refreshing air into its lungs, and is stimulated to break the shell by a knob on its beak, and to creep out into the world.