METAMORPHIC CHANGES, R. PIPIENS 45 
the two later stages of Rana illustrated (figs. 5, 6, 10, 11). Con- 
sequently, the corresponding portions were contracted to ap- 
proximately the same degree. Therefore, in comparing the 
thickness of the several layers of the wall of the digestive tube 
in the earlier and later stages, allowance must be made for the 
difference in the state of contraction. The sections of the in- 
testine of the larvae of Ambystoma tigrinum illustrated (figs. 
12, 13) were taken from portions of the intestine which were 
contracted in approximately the same degree. Consequently, 
the corresponding layers in the two stages illustrated may be 
compared directly. 
The reduction in the length and capacity of the stomach 
during metamorphosis in Rana is less extensive than that of the 
intestine. Nevertheless, the structural changes in the stomach 
are well marked. In tadpoles which are still growing and 
whose digestive tube has not yet reached its maximum length, 
the walls of the stomach and intestine remain relatively thin 
and delicate. As pointed out in an earlier section of this 
paper the reduction in length occurs somewhat earlier in the 
stomach than in the intestine. The gastric glands arise ear- 
liest at the pyloric end and gradually advance toward the ecar- 
diac end of the stomach. Glands are not present in the most 
anterior portion of the stomach until shortly before this organ 
has attained its maximum larval length, which oceurs in tad- 
poles with hind limbs 5 to 15 mm. in length. Figure 3 is a 
microphotographic reproduction of a portion of a transverse 
section of the stomach of a tadpole in which the process of re- 
duction in the length of the stomach was already initiated. The 
serosa and muscularis are very thin. Longitudinal muscle 
fibers are not apparent. The muscularis is, therefore, repre- 
sented by relatively few circular fibers. The deep portions 
of the gastric glands which are still loosely aggregated come 
into very close proximity with the muscularis. The submucosa 
is represented by a small amount of loose connective tissue 
which lies between the glands. As the process of reduction 
advances the serosa and muscularis of the stomach become ap- 
preciably thicker. A subserous layer of connective tissue makes 
its appearance, but longitudinal muscle fibers are still absent. 
The existence of longitudinal muscle in the stomach of the frog 
