. The development of the chick; an introduction to embryology. Birds -- Embryology. 132 THE DEVELOPMENT OF THE CHICK The fore-gut is thus being continually lengthened backwards by fusion of the lateral limbs of the splanchnopleure. At the 31s stage this has proceeded about to the fourteenth somite. At about the 26 s stage the tail-fold appears in the splanchno- pleure, thus establishing the hind-gut (Fig. 70) which gradually ' So'pl. Sp'pl. Fig. 70. — Median longitudinal section through the hind end of an embryo of about 21 s. an. pi., Anal plate, an. t., Anal tube. p. i. p., Posteri
. The development of the chick; an introduction to embryology. Birds -- Embryology. 132 THE DEVELOPMENT OF THE CHICK The fore-gut is thus being continually lengthened backwards by fusion of the lateral limbs of the splanchnopleure. At the 31s stage this has proceeded about to the fourteenth somite. At about the 26 s stage the tail-fold appears in the splanchno- pleure, thus establishing the hind-gut (Fig. 70) which gradually ' So'pl. Sp'pl. Fig. 70. — Median longitudinal section through the hind end of an embryo of about 21 s. an. pi., Anal plate, an. t., Anal tube. p. i. p., Posterior intestinal portal. T. B., Tail-bud. t. f. So'pl., Tail fold in the Somatopleure. t. f. Sp'pL, Tail fold in the splanchnopleure. Other abbreviations as before. elongates forwards. There remains then an open portion of the alimentary tract, where its walls are continuous with the extra- embryonic splanchnopleure or 3'olk-sac. This is known as the yolk-stalk. The entrance from the yolk-sac into the fore-gut is known as the anterior intestinal portal, and that from the yolk-sac into the hind-gut as the posterior intestinal portal (Fig. 70). At the 27 s stage the yolk-stalk is long and narrow (Fig. 106); the stems of the splanchnic (omphalo-mesenteric) veins run to the heart in its anterior portion, and the omphalo-mesenteric arteries pass out about its center. As it gradually closes, the stems of the omphalo-mesenteric arteries and veins are brought closer together. At about five days it becomes a tubular, thick- walled stalk, connecting intestine and yolk-sac, and so remains throughout embryonic life. The limiting sulci in the somatopleure lead to the formation of the body-wall. In the trunk the somatopleure is separated from the splanchnopleure by the coelome (Fig. 69), and the folds in the somatopleure take the same general direction as those in the splanchnopleure; they thus lead to the formation of a tube (body-wall) outside of a tube (alimentary canal), the intervening.
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