MODIFICATION OF DEVELOPMENT IN THE FROG A499 
THE INTERPRETATION OF ABNORMAL DEVELOPMENT 
Quite apart from any interpretation that may be given to the 
facts, the egg and embryo of the frog exhibit a differential sus- 
ceptibility to various factors of the environment. ‘That is, those 
regions of the egg or embryo that are functionally most active 
are most susceptible to agents or conditions that at all seriously 
disturb developmental processes. This is only another way 
of saying that these differences in susceptibility bear a direct 
relation to the principal axes of symmetry of the egg and embryo. 
The different methods of demonstrating differential susceptibility 
have already been discussed. 
Given what may be called gradients in susceptibility which 
is demonstrated, it follows as a necessary consequence that any 
considerable disturbance of development, experimentally or 
otherwise induced, save certain mechanical disturbances of course, 
will appear as a differential. Hence, in the absence of specific 
effects of particular agents or conditions on particular regions of 
the egg or embryo, inhibition, acceleration, acclimation, or re- 
covery in development must be differential. As a matter of 
fact, inhibition that is at all marked is differential, regardless of 
how it is induced. The same is true to a considerable extent of 
accelerated development and in those cases where the egg or em- 
bryo acclimate to or recover from some disturbing condition. 
If additional evidence is needed to convince one of the dif- 
ferential nature of inhibited development in the frog, it can be 
had by examining the data of previous workers in this field— 
Gurwitsch, 96; Hertwig, ’92, 95; Bataillon, 01; Morgan, ’03; 
Jenkinson, ’06; these are a few of the representative papers. 
The great majority of modifications in the development of the 
frog obtained by these other workers in the field are so obviously 
differential inhibition forms that a detailed discussion of them 
seems quite unnecessary. 
It is significant that practically all, certainly the most common 
types of abnormalities and especially those experimentally in- 
duced not only in the frog, but also in organisms generally, fall 
naturally into the categories mentioned earlier in the paper, viz., 
differential inhibition forms, or more rarely, since the conditions 
