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 
Ect. 
LAS 
SEO o 
© 
Fig. 70. — Median longitudinal section through the hind end of an embryo 
of about 21 s. 
an. pl., 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 yolk-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 
volk-sae into the hind-gut as the posterior intestinal portal (Fig. 
70). At the 27s stage the yvolk-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 ccelome (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 
