MODIFICATION OF DEVELOPMENT IN THE FROG 475 
specific protoplasmic constitution of each particular type or 
species, represents in at least certain fundamental respects a 
non-specific or quantitative factor in organization characteristic 
of at least all axiate organisms and transcending the differences 
in details which constitute specific or type differences in 
organisms. ‘To this fundamental feature of organismic pat- 
tern, Child (20, p. 152) has applied the descriptive term, ‘physio- 
logical gradient.’ Differential susceptibility or, in other words, 
a gradient in susceptibility, then, is merely one expression of 
underlying graded differences in the rate of fundamental activities 
and dynamic relations in protoplasm. A summary and discus- 
sion of the evidence demonstrating the existence of physiological 
eradients in organisms is given in the above citation so that 
further discussion of this principle is unnecessary. I wish merely 
here to indicate the usefulness of the conception in the interpre- 
tation of teratological development. Indeed, since the various 
types of experimentally produced terata in the frog have been 
produced time and again through the action of almost every 
agent and device known to experimental teratology, there would 
be little excuse for going over the ground again except for two 
facts which now appear very clearly. First, the degree and di- 
rection of teratological modification of development evidently 
depend primarily upon quantitative rather than specific factors 
in the action of the agents used; and, second, on this basis, the 
data attain considerable added significance and can be so satis- 
factorily accounted for in terms of the conception mentioned 
above. 
In 1919 I presented data to show: 1) that the frog egg and 
embryo exhibit a differential susceptibility to such agents as 
KNC, CH.O, KMnO,, LiCl, HCl, NaOH, and C.H;OH. Those 
regions of the egg which differentiate earliest and in which growth 
is most rapid (apical and dorsal regions—in the early stages of 
development; in later stages any rapidly proliferating or physio- 
logically active region) die soonest in concentrations of external 
agents that are lethal within a relatively short time; are most 
inhibited by somewhat lower concentrations, and acclimate or 
recover first in concentrations or treatments permitting acclima- 
