Intestinal Epithelium of Bufo Lentiginosus. 277 



in respiration and circulation take place. Likewise they are initiated 

 before there is any change in the feeding habits. It appears probable 

 that the muscular contraction is in some way dependent upon the 

 epithelial condition, perhaps both chemical and mechanical changes, 

 which accompany degeneration. 



Why the degeneration of all but the giant cells of the larval 

 epithelium should take place seems to me to be a question of life 

 cycles, which cannot be answered until we know more of the laws 

 of growth and senescence. My observations of the histological con- 

 ditions of the Bufo larva lead me to believe that there is an early 

 differentiation of the two sets of epithelial cells, one set, the principal 

 and round cells, destined to function through the larval existence, to 

 run its whole life cycle within six or eight weeks, the other, the giant 

 cells, becoming active only after a latent period of four or five weeks, 

 and functional as absorbing and secreting cells only after meta- 

 morphosis is completed. 



Summary. 



I. Between the time of hatching and the end of metamorphosis 

 the alimentary tract of Bufo undergoes striking changes, both macro- 

 scopic and microscopic. 



II. The macroscopic changes consist of : 



1. The differentiation, from the posterior part of the yolk mass, 

 of a huge intestinal coil, which crowds the stomach and duodenum 

 to the right side of the body. 



2. This intestinal coil reaches its maximum size at a time when 

 the hind legs are well developed, but not yet drawn up on the body. 



3. A gradual shortening and narrowing of the intestine then takes 

 place, by means of a slow, even contraction of the longitudinal and 

 circular muscles. This contraction occurs during the week or ten 

 days previous to the breaking through of the fore legs and is usually 

 completed in the following 24 hours. 



4. With the completion of the intestinal reduction the stomach 

 resumes its original position on the left, and the intestine moves, ap- 

 parently by means of contraction of the mesentery, to its adult posi- 

 tion on the right. 



