460 
.GEOFFREY SMITH AN1) EDGAR SCHUSTER. 
\ 
authors. In the first place, it is not possible to make reliable 
observations on the condition of the thumb-pad in the living 
frog by the mere inspection or feel of the pad without 
examining it carefully under the microscope. By mere 
inspection or feel of the pad an observer may consider the 
pad to be nearly smooth, when in reality papillae of a fair 
size may be present. Then, later, at the close, perhaps, of 
an experiment, the pad may be examined under a lens or 
microscope, and the presence of the papillae will probably be 
put down to the experimental treatment. Every grada- 
tion exists in the thumb-pads of male frogs at various 
periods, between the quite smooth and the exceedingly rough 
papillated condition of the breeding season, and the deter- 
mination as to whether there has been an increase in papillation 
or pigment during the course of an experiment depends upon 
quantitative comparisons, which are difficult to make, and 
unless the state of the pad at the beginning of the experiment 
is very carefully determined, the end result of the experiment 
may be interpreted in a misleading manner. That the experi- 
ments of Nussbaumand Meisenheimer are unfortunately open 
to these objections is forced upon us by two principal facts : 
firstly, the changes which take place in the thumbs of the 
normal males have apparently not been quite correctly 
observed by these authors; and secondly, their belief that 
castration has the effect of causing a rapid reduction aud 
disappearance of the papillae is contradicted by our results. 
In the case of normal frogs (see Table, p. 468) we find that 
there is considerable variation in the development of the 
papillae in different frogs at the same time of year, but a still 
more important fact is that in spring, after the breeding 
season is over and the external layers of the pad are thrown 
off, the thumb-pad does not at once return to a perfectly 
smooth condition, but remains papillated to about the same 
extent as a September or October frog. The attainment of 
the smooth condition is a gradual process, and is not com- 
pleted until the late summer in the great majority of frogs. 
In the case of frogs castrated in autumn, when the papillae 
