48 IOWA STUDIES IN NATURAL HISTORY 
mach and intestine is not accomplished by prolification of the 
tissue elements, but by rearrangement of those already present. 
As pointed out by Ratner, mitotic figures occur only rarely in 
these tissues during the period of metamorphosis. Further- 
more, the increase in the volume of the tissue per unit length 
of the tube occurs so rapidly that it could not be accounted 
for by the prolification of cellular elements alone. Neither is 
there any evidence of degeneration or destruction of tissue in 
the walls of the digestive tube while the process of shortening is 
going on, as would necessarily be the case if the tissue elements 
did not become rearranged and aggregated. By what mechani- 
eal process the shortening of the tube and the rearrangement of 
the tissue elements is accomplished is not clear. As suggested 
by Ratner, the aggregation of the muscle tissue is the impor- 
tant factor. The subserosa and submucosa necessarily become 
thicker and more compact in response to the changes in the 
muscularis. Likewise the epithelium becomes more compact 
and, with the submucosa, is thrown into numerous folds as the 
tube is reduced in length and caliber. 
The subserous and submucous connective tissue shows more 
evidence of normal growth changes during the metamorphosis 
than any of the other tissues in the walls of the stomach and 
intestine. At the beginning of metamorphosis the connective 
tissue in these layers is not only very meager but also contains 
very few fibers. As metamorphosis advances, this connective 
tissue becomes more fibrous and the number of cellular elements 
is materially increased. After the metamorphic changes are 
completed and the digestive tube resumes growth this connec- 
tive tissue soon assumes the character of the connective tissue 
in the digestive tube of the adult frog. The difference in the 
submucosa of the stomach during the later stages of metamor- 
phosis and in the recently emerged frog is well illustrated in 
figures 5 and 6. 
The histological changes in the stomach and intestine of R. 
pipiens during metamorphosis, as described in this paper, con- 
form in general to the corresponding changes in R. temporaria 
as described by Ratner. 
As indicated in an earlier section of this paper the reduction 
' in the length of the digestive tube during metamorphosis is 
