THE DIGESTIVE SYSTEM 



317 



means of anterior and posterior intestinal portals. From the fore-gut 

 develop pharynx, esophagus, stomach, and the anterior part of the small 

 intestine; from the hind-gut the remainder of the intestine. Early in 

 development, an allantois arises as a ventral outpocketing of the hind- 



NEURAL PLATE ^.,.,.,_— =>-;,/NOTOCHORD 



NEURAb 

 GROCVE 



NOTDCHORD 



DORSAL ROOT 

 GANGUON 



MATIC MESODERM 



DERMATOME 



\\_MUSCLE 

 \ BUNDLE 

 MYOTOME 



VISCERAL OR 



.SPLANCHNIC 

 MESODERM 



KIDNEY 

 GENITAL RIDGE 

 MESENCHYME- 



DIGESTIVE TRACT- 

 COELOI 



PERITONEUM 



SPINAL CORD 



NOTOCHORD 



NEURENTERIC 

 "CANAL 



ANAL PIT 



^ , "'-DIGESTIVE TRACT 



MOUTH PIT — ^yy ^ 



GILL CLEFT&^\S;lo>:=^5^,g;3fc^^,,;:;,j^^ p 



'^ •^UVER OUTGROWTH 



Fig. 266. — Stereograms of stages in the ontogenesis of the chordate showing early 

 stages in the development of the digestive system. A, B, and C are based upon early 

 stages in the development of Amphioxus; D and E upon elasmobranch embryos; F upon 

 an amphibian embryo. F represents the right half of an embryo cut in a median longi- 

 tudinal section. The blastopore has been converted into the neurenteric canal. 

 (From "Being Well Born," by M. F. Guyer, Copyright 1916, 1917, used by special 

 permission of the publishers, The Bobbs- Merrill Company.) 



gut, with which it retains connexion by an allantoic stalk. The cloaca is 

 the posterior portion of the hind-gut into which allantois and intestine 

 open, and which is closed to the exterior by the cloacal membrane. 



The later development of the intestine involves its elongation and 

 twisting. The opening into the yolk-sac becomes reduced to a slender 



