METAMORPHIC CHANGES, R. PIPIENS 47 
intestine is moderately filled with ingested material, is close- 
ly applied to the muscularis with but little connective tissue 
intervening. This condition is illustrated microphotographic- 
ally in figure 8 which is taken from a transverse section of the 
small intestine of a tadpole in which approximately 50 per cent 
of the reduction in the length of the digestive tube had taken 
place. The intestinal wall is now appreciably thicker than 
in the preceding stage. The added thickness is made up large- 
ly of the increase in the thickness of the muscularis and the 
submucosa. As the shortening of the digestive tube progress- 
es the intestinal wall becomes thicker, and all the tissues be- 
come more compact. Figure 10 illustrates a portion of a trans- 
verse section of the small intestine of a tadpole in which the 
digestive tube had reached its minimum length during meta- 
morphosis. The small intestine is now practically without 
food content and its caliber is greatly reduced by reason of 
the contraction of the circular muscles. Consequently, the 
mucosa and submucosa are thrown into large longitudinal folds. 
The thickness of the several layers, as indicated by this and 
the preceding figure, may not be compared directly by reason 
of the difference in the caliber of the intestine. Nevertheless, 
figure 10 indicates a relatively enormous increase in the thick- 
ness of all the layers except the mucosa. As the metamorphic 
changes advance to completion and the little frog resumes feed- 
ing the muscularis and the submucosa become relatively some- 
what thinner and the mucosa is thrown into numerous folds. 
As illustrated in figure 11, which is taken from a transverse 
section of the small intestine of a young frog about the time 
feeding is normally resumed, the histological structure is now 
essentially that of the small intestine of the adult frog. 
The most marked changes in the histological structure of the 
stomach and intestine in Rana during metamorphosis, as noted 
above, consist in the increased thickness of the several layers, 
especially the muscularis and submucosa, the more compact ar- 
rangement of the gastric glands and the folding of the intes- 
tinal epithelium. These changes are coincident with the reduc- 
tion in the length and caliber of the digestive tube and are ac- 
complished in a relatively short time. Obviously, the increase 
in the thickness of the several layers in the walls of the sto- 
