MODIFICATION OF DEVELOPMENT IN THE FROG 501 
inhibitions, differential accelerations, differential acclimations, 
or differential recoveries. 
3. These differential modifications, are obvious and necessary 
consequences of the differential susceptibility of the organism 
to the agents used in experimental teratology. 
4. The modifications produced are perfectly characteristic, not 
of a particular agent or condition, but of a particular concentra- 
tion or ‘intensity’ of action of that agent. In other words, the 
developmental response is primarily quantitative and non- 
specific. 
5. Differences in susceptibility parallel the axes of symmetry. 
Those regions which usually differentiate earliest and grow most 
rapidly are most distorted, the degree and direction of the dis- 
tortion depending largely upon the severity of the treatment and 
physiological condition of the organism. There are, in short, 
gradients in susceptibility paralleling the axes of symmetry, both 
primary and secondary. 
6. The existence of susceptibility gradients—whose existence 
is demonstrated in a great variety of organisms—must mean an 
underlying gradient or gradients in the structural and functional 
aspects of protoplasm generally. 
7. It isin terms of disturbances in this underlying graded order 
in the dynamic relations and activities of protoplasm—physiolog- 
ical gradients—that we believe teratological development is most 
rationally accounted for. 
LITERATURE CITED 
BartarLton, E. 1901 Etudes experimentales sur l’evolution des Amphibiens. 
Arch. Entw.-Mech., Bd. 12, 8. 610-655. 
Betiamy, A.W. 1919 Differential susceptibility as a basis for modification and 
control of early development in the frog. Biol. Bull., vol. 37, pp. 
312-361. 
Cuitp, C. M. 1916 Experimental control and modification of development in 
the sea-urchin in relation to the axial gradient. Jour. Morph., vol. 28, 
pp. 65-133. 
1920 Some considerations concerning the nature and origin of physio- 
logical gradients. Biol. Bull., vol. 39, pp. 147-187. 
1921 The origin and development of the nervous system. Chaps. 1-5. 
The University of Chicago Press. 
