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 [Pg 292] 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. Fig. 189.—Hen’s Egg after 3 days’ incubation. The amnion has been removed. References as in Fig. 188. (× 1.) Fig. 190.—Chick Embryo after 4 days’ incubation. The amnion has been removed. all, allantois; am, cut edge of amnion; f.l., fore-limb; h.l., hind-limb; ht, heart; t, tail. (After Duval.) (× 2.) On the fourth day the embryo turns to lie entirely on its left side. It becomes more completely folded off from the yolk, and the connecting yolk-stalk is a narrower tube than before. The previously formed organs increase in size and complexity. The disproportionate size of the head, owing to the great development of the brain, is still more marked than before, and the head is so strongly bent upon itself that the snout almost touches the tail. One of the most noteworthy features of the fourth day is the appearance of the limbs. As yet these are merely a pair of small buds (fl., hl., Fig. 190), and no joints can be detected in them. [Pg 293] Fig. 191.—Hen’s Egg after 5 days’ incubation. a, air-chamber; all, allantois; am, amnion; ar. vasc., area of yolk blood-vessels; emb, embryo; yk, yolk. (After Duval.) (× 1.) 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 [Pg 294] 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.)