156 THE FKOG. 



of large size are developed. The wall now becomes thrown into 

 folds (Fig. 64, w) ; the blood-vessels following in between the 

 folds. By a continuation of this process, accompanied by the 

 formation of outgrowths from the hypoblast cells, and ingrowth 

 of the blood-vessels, the liver rapidly increases in size and acquires 

 the structure shown in Fig. 71, w; consisting of a trabecular 

 framework of solid rods of hypoblast cells, the meshes of the 

 framework being occupied by the hepatic blood-vessels. As the 

 liver attains definite shape and increased size, it separates more 

 distinctly from the intestine, remaining, however, connected 

 with this by the bile-duct, which is formed by lengthening out 

 of the original diverticulum from the mesenteron. The gall- 

 bladder is a lateral outgrowth from the bile-duct ; it develops 

 at an early period (Fig. 64, WG), and is of large size during the 

 whole of tadpole life (Fig. 71, WG). 



9. The Pancreas. 



The pancreas develops as a pair of hollow outgrowths from 

 the mesenteron, behind the liver. In the later stages (Fig. 71. 

 PA), the ducts shift so as to open into the bile-duct instead of, as 

 at first, directly into the intestine. 



The secreting cells of the pancreas, like those of the liver, are 

 of hypoblastic origin. 



10. The Bladder. 



The bladder is absent during the greater part of the tadpole 

 period; but shortly before the metamorphosis it arises as a 

 median ventral outgrowth from the hinder end of the mesen- 

 teron, which soon becomes bifid distally (Fig. 89, TB). 



11. The Post-anal Gut. 



Post-anal gut is the name given to an extension of the hinder 

 end of the mesenteron into the base of the tail, which appears 

 as this latter is developed. 



The mode of formation of the neurenteric canal as a tubular 

 communication between the hinder end of the neural canal and 

 the mesenteron has already been described (cf. Fig. 61, NT). As 

 the tail lengthens, the notochord and spinal cord grow back- 

 wards with it, and the neurenteric canal becomes drawn out into 

 the post-anal gut. This is an evanescent structure, disappearing 

 completely at a very early stage : at the time of hatching of 



