44 IOWA STUDIES IN NATURAL HISTORY 
the adults of Ambystoma tigrinum is much more strictly earniv- 
orous. Therefore, the reduction in the length of the digestive 
tube during metamorphosis is probably an essential part of the 
adjustment of the animals to the more restricted carnivorous 
diet. The relatively smaller size of the liver in Ambystoma 
than in Rana during larval life is probably correlated with the 
lower carbohydrate content in the diet. Inasmuch as the liver 
does not assume larger proportions in the larvae of this spe- 
cies, an actual reduction in the size of this organ need not take 
place during metamorphosis. 
HISTOLOGICAL CHANGES IN THE STOMACH AND IN- 
TESTINE DURING METAMORPHOSIS 
The histological structure of the digestive tube of certain 
types of Amphibia, especially the frog, is adequately described. 
The histological changes in the stomach and intestine of Rana 
temporaria which are correlated with the reduction in the 
length of the digestive tube during metamorphosis, were set 
forth by Ratner (’91). The purpose of the present histolog- 
ical study is to extend our knowledge of the structural changes 
in the stomach and intestine which are correlated with the re- 
duction in the length of the digestive tube during metamor- 
phosis in Rana, and to compare with these changes the corres- 
ponding structural changes in the stomach and intestine of an 
Urodelan species in which the reduction in the length of the 
digestive tube during metamorphosis is less extensive than in 
the Anura. 
The accompanying microphotographs illustrating the histo- 
logical structure of the stomach and intestine were all taken un- 
der the same magnification, viz., 165 diameters. If the diges- 
tive tube had been equally distended in every ease the thickness 
of the corresponding layers in the successive stages could be 
compared directly. Inasmuch as the digestive tube was still 
well filled with ingested material during the two earlier stages 
of Rana pipiens illustrated (figs. 3, 4, 8, 9), it may be assumed 
that in these cases the corresponding portions of the digestive 
tubes were distended to approximately the same degree. On 
the other hand, the digestive tube was practically empty during 
