THE ALIMENTARY CANAL 125 



confined to the ventral portion of the body, region, not extend- 

 ing into either the head or the tail. 



The hypoblast, which is a definite layer of cells, at first con- 

 fined to the roof of the mesenteron, gradually spreads round its 

 sides until it encloses the whole of the food-yolk, and the 

 alimentary canal is completed as a tube, which from the first is 

 slightly convoluted. When the tadpole begins to feed, the 

 alimentary canal lengthens rapidly, and becomes coiled in a 

 spiral manner. Except at the anterior end, in the gill-bearing 

 region, it is of approximately uniform diameter throughout. 

 DuTLQg the metamorphosis, the alimentary canal shortens con- 

 siderably, and the distinction between stomach, small intestine, 

 and large intestine, is definitely established. 



The liver is recognisable at a very early stage (Pig. 27) as a 

 ventral and backwardly directed diverticulum of the anterior 

 part of the mesenteron, forming the anterior boundary of the 

 mass of food-yolls. In the later stages the walls of the diver- 

 ticulum thicken, and become thrown into folds between which 

 the vascular mesoblast makes its way : the diverticulum itself 

 persists as the bile duct, and the gall bladder arises as an out- 

 growth from this. 



The pancreas is developed as a pair of hollow outgrowths 

 from the mesenteron, behind the liver : in the later stages the 

 ducts shift so as to open into the bile duct instead of dnectly 

 into the intestine. 



The bladder arises shortly before the metamorphosis as a 

 ventral outgrowth from the hinder end of the mesenteron, which 

 soon becomes bifid at its distal blind end. 



The post-anal gut is an extension of the hinder end of the 

 mesenteron into the base of the taU, which appears as this latter 

 is developed : it becomes solid after a short time, and later on 

 disappears altogether. It is perhaps to be regarded as formed 

 by a mechanical drawing out of the intestine by the outgrowing 

 tail. 



The lungs. Immediately behind the gill-bearing region or 

 pharynx, the alimentary canal narrows very considerably ; its 

 sides become folded inwards, and the two folds meeting each 

 other divide the canal into a dorsal tube or oesophagus, and a 

 ventral one which forms the laryngeal chamber : from this 

 latter the lungs arise as thin-walled lateral outgrowths. They 

 appear first in young tadpoles of about 8 mm. length, i.e., some 



