352 A. W. BELLAMY. 



agents that have been used to modify development. The litera- 

 ture of this subject has been reviewed incidentally so many times 

 (Korschelt and Heider, 1902; Hertwig, 1906; Jenkinson, 1909) 

 that only certain essential facts and conclusions need be men- 

 tioned here. Gurwitsch (1896) attributed the various abnormali- 

 ties of the frog produced by him in sodium bromide, lithium 

 chloride, and weak solutions of strychnine, caffein, and nicotine, 

 to an " einigermaassen specifische" action of individual chemicals 

 upon certain regions of the egg. Stockard (1909) at one time 

 believed that cyclopia in Fiindulns was due to a specific cyclopic 

 producing property of MgClo and went so far as to suggest that 

 cyclopia in man may be due to an unusually high amount of Mg 

 in the amniotic fluid or in the blood of the pregnant mother. 

 When, however, it had been shown that a large number of dif- 

 ferent substances produce the same anomaly, Stockard (1914) 

 abandoned his earlier view that there are "specific responses to 

 the given chemical substances employed" and suggested that 

 since "a certain definite response on the part of the developing 

 organism may be consistently obtained after carefully adjusted 

 treatments with a large number of different substances ... in 

 certain cases they may serve simply to lower the developmental 

 metabolism and thus prevent or arrest the formation of particular 

 structures." No reason is offered to explain how lowering the 

 developmental metabolism may cause the prevention or arrest 

 of the formation of particular structures or why, under properly 

 controlled conditions, particular regions of the embryo are affected 

 more than other parts and in a definite and sequential relation 

 to one another, though he notes that the nervous system and 

 special sense organs are most affected by the use of chemical 

 agents that inhibit development. 



To digress for a moment, in the light of present knowledge 

 we believe it possible to give a rational interpretation of these 

 facts. The reason that a lowering of the developmental meta- 

 bolism under conditions where acclimation does not occur, can 

 prevent or arrest the formation of such structures as nervous 

 system and special sense organs, is the fact that there is a 

 gradient in the rate at which the processes described by the 

 collective term "metabolism" are taking place along physio- 



