THE ALIMENTARY CANAL. 



545 



It consists of three parts, fore-gut, mid-gut, and hind-gut, which 



are approximately equal in length. 



The fore-gut is widened transversely at its anterior end to 



form the pharynx, tp, which is separated in front by a thin, 

 , obliquely placed septum, du, from the bottom of the stomatodasal, 

 I or mouth invagination, ds. Behind the pharynx, the fore-gut 



narrows to form a short tubular portion, the oesophagus, which 

 i lies immediately above the heart. Behind the oesophagus is a 

 I fusiform dilatation, the stomach, ts, beyond which the fore-gut 



TR W TS TP 



BM 



Fig. 232. — Human Embryo, lettered by Professor His, Lg, and estimated as 

 fifteen days old (cf. Fig. 197). The brain and heart are exposed from the 

 right side ; the alimentary canal and the yolk-stalk are represented in 

 median sagittal section. (From His.) x 30. 



AA, allantoic artery. B3P, thalamencephalop. BL, cerebellum. BM, mid-brain. 

 BO, optic vesicle. DS, stoma todasuin. DTJ, septum between stomatodasum and pharynx. 

 EI, auditory pit. GH, hind-gut. GT, mid-gut and yolk-stalk. ET, truncus 

 arteriosus. B,V, ventricular portion of heart. TA, allantoic diverticulum. TP, 

 pharyngeal region of fore-gut. TR, cloacal dilatation of hind-gut. TS, stomach. TZ, 

 allantoic stalk. VA, allantoic vein. "W, liver. 



passes into the mid-gut, GT, which latter opens through the wide 

 yolk-stalk into the yolk-sac. The hind-gut, GH, is at first narrow 

 and tubular ; but at its hinder end it dilates to form the large 

 cloacal chamber, TR, from the ventral surface of which the 

 allantois, ta, arises as a narrow tubular diverticulum. There is 

 as yet no trace of a proctodaeal, or anal invagination. 



In embryos about a day older than the one represented 

 in Fig. 232, i.e. of about the sixteenth day, the stomatodaeal 

 septum is perforated, and the mouth opening established 



N N 



