3l6 A. W. BELLAMY. 



associated in some way, directly or indirectly, with the rate 

 of certain fundamental metabolic reactions, such, e.g., as oxida- 

 tion-reduction processes, and the protoplasmic conditions asso- 

 ciated with them. Considerable evidence has been accumulated 

 which indicates that in general the susceptibility to lethal or 

 strongly toxic conditions and the capacity for acclimation to 

 less severe conditions and for recovery after temporary exposure 

 all vary more or less directly with, though not necessarily 

 proportionally to, the rate of fundamental metabolic reactions. 

 This does not mean that all the agents employed act directly 

 upon these reactions in every case or that all act in the same way 

 upon protoplasm. It probably means merely that living proto- 

 plasm is a system of more or less closely associated and inter- 

 dependent reactions and conditions so that no essential factor 

 in this system can be altered beyond a certain degree without 

 involving the system as a whole. 



B-ut, however the facts of differential susceptibility may finally 

 be interpreted, concerning the facts themselves there can be no 

 doubt. They have afforded a means not only of demonstrating 

 characteristic differences in physiological conditions along the 

 axes of organisms but also of modifying and controlling develop- 

 ment in definite ways through these differences. This study of 

 the development of the frog was undertaken in the attempt to 

 determine whether, and to what extent, the susceptibility method 

 could be used in modifying and controlling development in a 

 vertebrate, as they have been used in investigations of the physi- 

 ology of development in certain invertebrates. 



I am under obligation to Professor C. M. Child, in whose 

 laboratory this work was done, for essential aid in the way of 

 suggestion and criticism, and to Dr. L. H. Hyman for many 

 thoughtful suggestions. Work on amphibia involving the sus- 

 ceptibility method was begun by Dr. Child in 1913. In the 

 spring of 1916, work was begun by him on the modification and 

 control of development in the frog, on the basis of differential 

 susceptibility; the data and preserved material of these experi- 

 ments were turned over to me late in that year. My own work 

 was done in the spring of 1917 and the spring of 1919 during 

 which time approximately one hundred thousand eggs were 

 handled, of which about forty thousand were preserved. 



