METAMORPHIC CHANGES, R. PIPIENS 49 
much less extensive in the Urodela than in the Anura. Conse- 
quently, the changes in the histological structure of the sto- 
mach and the intestine which are correlated and coincident 
with the reduction in the length of the digestive tube, though 
similar in character, are less extensive in Ambystoma tigrinum 
than in Rana pipiens. The walls of the stomach and intestine 
in the larvae of the former are never as thin and fragile as in 
_ those of the latter species at the corresponding stage of devel- 
opment. As the larvae of Ambystoma undergo metamorphosis 
and the digestive tube becomes shorter, the walls of the stomach 
and intestine also increase somewhat in thickness. However, 
the changes in the layers are much less marked than the cor- 
responding changes in the larvae of Rana. Figure 12 illus- 
trates microphotographically a portion of a transverse section 
of the small intestine of a larva of Ambystoma in which the 
reduction in the length of the digestive tube was well advanced. 
Figure 13 illustrates a portion of a transverse section of the 
small intestine of a young adult Ambystoma which had not yet 
resumed feeding. The intestine was contracted to approxi- 
mately the same degree in both cases and the sections were taken 
at corresponding levels. The difference in the thickness of the 
corresponding layers in these two stages is unimportant. Ob- 
viously, the tissues are much more compact in the latter than 
in the former. The submucosa is still very loose and contains 
relatively few fibers in the former, whereas in the latter it is 
more compact and more fibrous. Furthermore, during the 
later stages of metamorphosis the mucosa is thrown into a few 
large folds, while after metamorphosis the folds of the mucosa 
are smaller, more definite and more numerous. The structure 
illustrated in figure 13 is essentially the structure of the intes- 
tine of the adult. Inasmuch as the histological changes in the 
stomach and intestine of A. tigrinwm during metamorphosis, 
though less extensive, are similar in character to the correspond- 
ing changes in R. pipiens a more detailed description of them 
at this time would be superfluous. There is no more evidence 
that the increase in the thickness of the several layers in the 
walls of the stomach and intestine during metamorphosis in- 
volves active cell proliferation in Ambystoma than in Rana. 
On the other hand, all the facts observed indicate that the in- 
