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 stomatodseal, 

 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 



TR 



W 



BL 



VA 



3M 



FlG. 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. BF.thalamencephalon. BL, cerebellum. BM, mid-brain. 

 BO, optic vesicle. DS, stomatoda?um. DTT, septum between stomatodaeum and pharynx. 

 El, auditory pit. GH, hind-gut. Q-T, mid-gut and yolk-stalk. B.T, truncns 

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

 pharyngealregion 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, Til, 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 stomatodagal 

 septum is perforated, and the mouth opening established 



N N 



