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.
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.)