148 THE FROG. 



of approximately uniform diameter along its whole length (Fig. 

 65). Owing to this rapid elongation, and the convolutions into 

 which it necessarily becomes thrown, the intestine, which at 

 first is closely attached to the dorsal wall of the body cavity, 

 immediately beneath the notochord, shifts ventralwards, re- 

 maining, however, suspended from the mid-dorsal wall of the 

 body cavity by the mesentery. 



At the time of the metamorphosis the alimentary canal 

 shortens rapidly and very considerably ; and the distinction in 

 diameter between the stomach, small intestine, and large intes- 

 tine becomes much more pronounced. During these changes 

 the entire alimentary canal is in a condition of active inflam- 

 mation, and no food is taken, nutrition being effected by the 

 gradual absorption of the tadpole's tail. 



///I 

 The stomatodaeum. At the time of hatching (Fig. 09, DS), 



the stomatodaBum is a well marked though shadow pit on the 

 under surface of the head ; its floor is in close re^tion with the 

 anterior wall of the pharynx, the epiblast of the stomatodasal pit 

 and the hypoblast of the pharyngeal wall being in contact with 

 each other, without any intervening rnesoblast. From the 

 dorsal border of the stomatodaeum, the pituitary body (Fig. 69, 

 PT) projects inwards between the brain and the pharynx. 



The stomatodagal pit rapidly deepens, not by depression of 

 its floor, but by uprising of its walls (Fig. 64), the margins of 

 which give rise to the lips. The septum between the stomato- 

 daBum and the pharynx gradually becomes thinner, and in tad- 

 poles of from 9 to 10 mm. length is perforated ; the mouth 

 opening is thus established, and the pharynx placed in direct 

 communication with the exterior. 



In the later stages the limits of the original stomatodaeal 

 invagination can be fairly accurately determined. In the section 

 of a 12 mm. tadpole given in Fig. 65 the boundary is indicated 

 by a difference in the mode of shading employed ; the epiblastic 

 lining of the stomatodasurn is represented by a thick black line, 

 while the hypoblastic wall of the pharynx is shown by a double, 

 cross-hatched line. The posterior nares mark the boundary 

 between the two regions exactly ; they open (cf. Fig. 76, zi) 

 into the pharynx immediately behind the septum, so that a line 

 drawn across the roof of the mouth, through the anterior borders 



