LIFE HISTORY OF DESMOGNATHUS FUSCA. 277 



tail comes the change in its shape owing to the development of 

 the median fin both on the dorsal and ventral side. Dorsally it 

 begins at the level of the posterior margin of the pelvic girdle, 

 where a slight depression in the mid-dorsal line may be seen 

 when the animal is viewed from the lateral aspect. This dorsal 

 portion of the fin, which finally in its widest part comes to 

 contribute from one third to two fifths of the entire dorso- 

 ventral dimension of the tail, extends to the extreme tip of the 

 tail, where its continuity with the ventral portion of the fin is 

 interrupted by the slender tip of the vertebral column and its 

 associated muscles. The ventral portion of the fin is neither so 

 wide nor so extensive as the dorsal, and it narrows anteriorly to 

 end at a point somewhat posterior to the cloaca. The increase 

 in length of the tail is the principal factor in the change in length 

 proportions of the body shown in the second column of Table I., 

 and in Graph II. Thus while, during the terrestrial period, 

 the head actually increases only about 12 per cent, in length, 

 and the trunk 23 per cent., the tail increases 52 per cent, (cf.. 

 Table I., ist column). These changes in the tail are obviously 

 a preparation for the aquatic life which is so soon to follow. 



Not only do the proportionate lengths of the regions of the 

 body thus alter, but the actual growth is, during the first few 

 days after hatching, very rapid, as will be seen from the ist 

 column of Table I., and also by comparison of Plate I, 1-9, all 

 drawn to the same scale. Thus at the end of 16 days the total 

 increase in length (20.17 " !5- 2 5 = 4-9 2 mm.) is 32 per cent, of 

 the length of the body at the time of hatching. 



THE ALIMENTARY CANAL OF THE TERRESTRIAL LARVA. 



Internally no structure exhibits more striking and fundamental 

 changes during the terrestrial period than the alimentary canal. 

 In the walls of the alimentary canal at the time of hatching there 

 is still a considerable supply of yolk material, although several 

 days previous to hatching (i. e., in the embryo of 30 days' develop- 

 ment, 13 mm. long), the yolk material contained in the cells of 

 the other tissues has been already quite consumed. In the newly 

 hatched larva, indeed, the digestive tract has so far progressed 



