THE ALIMENTARY CANAX. 



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 stomatodaeal, 

 or mouth invagination, Ds. Behind the pharynx, the fore-gut 

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

 lies immediately above the heart. Behind the oesophagus is a 

 fusiform dilatation, the stomach, TS, beyond which the fore-gut 



IS TP 



h'iG. 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. BP,tlialainenceplialon. BL, cerebelluin. BM, mid-brain. 

 BO, optic vesicle. DS, stomatodEemn. DTJ, septum between stomatodaeum and pharynx. 

 EL auditory pit. G-H, hind-gut. GrT, mid-gut and yolk-stalk. B.T, trnncus 

 arteriosus. EV, ventricular portion of heart. TA, allantoic diverticulum. TP, 

 pharyngealregion of fore-gut. TB, 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, TE, from the ventral surface of which the 

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

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



In embryos about a day older than the one represented 

 in Pig. 232, i.e. of about the sixteenth day, the stomatodasal 

 septum is perforated, and the mouth opening established 



N N 



