264 COMPARATIVE ANATOMY 



such points will be enlarged upon as are of special importance 

 in the study of comparative anatomy. 



The inner germinal layer or hypoblast gives rise to the 

 epithelium lining the mid -gut, or enteron, to the liver, 

 pancreas, cloacal bladder, lungs, and gill-slits. 



The enteron, in the stage represented in fig. 63, has no 

 anterior opening or mouth. Its anterior end is wide, but 

 its posterior end is narrow, owing to the large mass of yolk- 

 cells forming its floor. For some time after hatching the 

 tadpole is incapable of feeding, and subsists .on the reserve 

 material stored up in the yolk -cells. By the time it has 

 reached a length of 8 mm., a deep depression of the ectoderm 

 is formed on the lower side of the fore-part of the head, and 

 this meets and fuses with the anterior end of the enteron. 

 Soon afterwards, the wall dividing stomodaeum from enteron 

 is perforated, and the mouth is thus established. The 

 epiblastic invagination is the stomodseum ; it forms the lining 

 epithelium of the buccal cavity, and an outgrowth from its 

 roof forms the pituitary body. At the hind end of the body 

 the anus was formed at an earlier stage by a smaller epiblastic 

 invagination, the proctodaeum. 



The liver is formed at an early stage as a hollow outgrowth 

 of the ventral wall of the enteron, reaching back into fhe 

 mass of yolk-cells (fig. 63, //). The walls of the outgrowth 

 are thickened and thrown into numerous folds, the whole 

 organ is surrounded and its folds penetrated by a vascular 

 connective tissue, and is converted into the glandular sub- 

 stance of the liver ; the proximal end of the hollow outgrowth 

 persists as the bile-duct. 



The pancreas is formed as a pair of diverticula of the gut 

 close behind the liver. At first its duct opens directly into 

 the gut, but later it is shifted to open into the bile-duct. 



The cloacal bladder is formed, late in larval life, as a 

 ventral outgrowth from the hind end of the enteron. 



The gill-slits are indicated, just before the larva is hatched, 

 by a series of paired lateral outgrowths of the hypoblastic 

 epithelium lining the anterior part of the enteron, or, as 

 we may now call this region, the pharynx. Three anterior 

 pairs are first formed, and two posterior pairs appear succes- 

 sively after short intervals of time, making five pairs in all. 

 These outgrowths are shown in fig. 64. Each outgrowth is 



