THE ALIMENTARY CANAL. 133 



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. During the 

 metamorphosis, the alimentary canal shortens considerably, and 

 the distinction between stomach, small intestine, and large 

 intestine, is definitely established. 



The liver is recognisable at a very early stage — Fig. 28 — as a 

 ventral and backwardly directed diverticulum of the anterior 

 part of the mesenteron, forming the anterior boundary of the 

 mass of food-yolk. 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 directly 

 into the intestine. 



The bladder arises shortly before the metamorphosis as a 

 ventral outgrowth from the hinder end of the mesenteron, which 

 soon becomes bind at its distal blind end. 



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

 mesenteron into the base of the tail, 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 

 time after hatching, but shortly before the opening of the 

 mouth. About the time that the lungs first appear, in tadpoles 

 of about 8 mm. length, the oesophagus, which up to this time 



