THE DIGESTIVE CANAL. 61 



lage, Lu, to the hepatic, Li, extends a short tube which comprises the future 

 oesophagus, stomach, and part of the duodenum. The liver, Li, which arose as 

 an outgrowth of the entoderm at the fovea cardiaca, has enlarged and become 

 distinctly an appendage. Between the liver and the yolk-sac, Yk.s, is a short 

 broad tube, the beginning of part of the small intestine. In the 7.0 mm. embryo, 

 the fore-gut is much longer, and the differentiation of the oesophagus, oe, stomach, 

 St, and duodenum, from which the anlage of the dorsal pancreas, P, has developed, 

 is established. The liver is connected with the duodenum only by the narrow 

 hepatic duct, Li.d, between which and the yolk -stalk, Yk.s, there is a consider- 

 able stretch of small intestine. In the 13.8 mm. embryo, the relations have been 

 greatly altered by the growth and migration of the stomach (Fig. 27, C, Si) which 

 has descended from its original position into the abdomen, so that it is caudad of 

 the diaphragm, and lies asymmetrically placed on the left side of the embryo. The 

 stomach also turns so that its cesophageal end is toward the left, its duodenal 

 end toward the right, and further revolves so that its left surface faces ventrally. 

 In the 13.8 mm. embryo, the migration and revolution of the stomach has not , 

 been completed. The descent of the stomach involves the elongation of the oeso- 

 phagus (Fig. 27, C, oe) and the twisting of the duodenum. 



The hind-gut has a simpler history. In the 4.2 mm. embryo it has elongated 

 and terminates blindly in the tail. Its caudal end is somewhat enlarged to form 

 the cloaca, into which open also the Wolffian ducts and allantois (Fig. 27, A, W 

 and Al). Between the cloaca and the yolk-sac, Yk.s, extends the cephalad por- 

 tion of the hind-gut, nearly uniform in diameter. In the 7.0 mm. embryo the 

 conditions are similar, but the intestinal portion has lengthened and bent ventral- 

 ward. The insertion of the yolk-stalk, Yk.s, marks the apex of the primitive 

 intestinal loop. In the 13.8 mm. embryo, the loop has greatly lengthened and 

 projects into the cavity of the umbilical cord (extra-embryonic ccelom), and a blind 

 pouch, Coe, has appeared, the anlage of both the caecum and the appendix. It 

 marks the boundary between the large and small intestines, which as yet differ 

 very little in diameter. 



For some time a portion of the intestine lies in the umbilical cord, and may 

 form several coils there, but gradually it is withdrawn so as to lie wholly within 

 the abdomen proper. 



The pharynx undergoes many modifications in form, and also produces an im- 

 portant series of accessory organs, including the thyroid gland, the tonsils, and the 

 thymus. It comprises the cephalic portion of the fore-gut and originally overlies 

 the heart (Figs. 25 and 132). The stretch of the fore-gut, which extends from the 

 pharynx to the fovea cardiaca, remains at first short and narrow, most of the fore- 

 gut being absorbed in the pharynx, which is produced by the expansion of the 

 entodermal tube toward both sides of the neck; but the dorso- ventral diameter 

 remains small. The expansion is greatest a short distance behind the mouth, and 

 thence diminishes gradually toward the oesophagus, so that the pharynx of the em- 



