Lilian V. Sampson 



481 



the archenteric cavity is very large. In III the mouth cavity and the 

 pharynx are already formed; posterior to the pharynx the walls of the 

 archenteron, covered by splanchnopleure, are folding under from the sides. 

 Thus the oesophagus is formed, and in stage III is ventrally entirely sep- 

 arated from the yolk-mass (cf. stage V, Fig. G2), but leads posteriorly 

 into the archenteric cavity of the middle region. After stage V, the 

 oesophagus being full of yolk, is for a time nearly solid (stages VI and 

 VII), after the mesentery is first completed. The distinction between 



Fig. F. Sagittal section of stag-c VI. ao, aorta : au, auricle : cl, cloaca ; da, dorsal aorta 

 g, gut; gb, gall bladder; 1, liver; ma, mesenterial artery; oe, oesophagus; ph, pharynx 

 pvc, posterior vena cava; rlu, rudiment of lung; r'p, 1st rudiment of pancreas; sv, sinus 

 venosus; vv, vitelline vein ; Wd, WolfHati duct; y, yolk-mass. 



stomach and intestine is not apparent, except that the lumen is greater in 

 the definitive stomach; the intestinal part, near its connection with the 

 yolk-mass in thrown into a loop (Fig. HS, g). 



Liver. — In stage III, when the lateral walls of the archenteron are fold- 

 ing under to form the gut, the archenteron is extended into a diver- 

 ticulum, from a portion of which the liver is subsequently formed. 

 Owing to the large amount of yolk in the egg, the yolk-mass projects be- 

 yond the head of the embryo, and so it happens that the diverticulum ex- 

 tends from the gut portion of the archenteron in an anterior as well as a 

 ventral direction, following the curve of the yolk (see Fig. I). In stage 



