62 LIBBIE H. HYMAN. 



further discussion that these parts of the embryo, shown by me 

 to be the most susceptible to toxic agents, are also the ones show- 

 ing the most defective development in the experiments which have 

 been quoted. In all of these experiments it is evident that the 

 agents used are inhibiting or depressing agents because as stated 

 by the authors the development of the eggs subjected to them is 

 slower than that of the control. Under such depressing condi- 

 tions the parts with the highest susceptibility or, in other words, 

 highest metabolic rate, will be inhibited while other parts develop ; 

 and this is actually the fact. On the other hand, if the circum- 

 stances permit, such parts can recover more readily than others, 

 and these same parts may be found developed while other parts 

 have succumbed. This explains the development of small parts 

 of the embryo described by Kellicott, Werber and others iso- 

 lated eyes, hearts, fragments of brain, etc. 



The susceptibility gradients therefore furnish a basis for the 

 explanation of teratological development. No other conception 

 which has been advanced does so serve to account for all of the 

 facts. In particular it seems to me impossible on any other basis 

 to explain the production of the same terata in eggs fertilized by 

 injured sperm or by foreign sperm or in cases where the egg is 

 treated before fertilization as in Gee's experiments. In such 

 cases a general lowering of the metabolic rate of the egg as shown 

 by its slower development has occurred and this could produce 

 specific terata only in case certain parts of the embryo require a 

 higher metabolic rate for their expression than others. 



The application of the metabolic gradient conception to verte- 

 brate teratology has already been made by several investigators. 

 Werber ('16) recognized its bearing on the teratological Fundulus 

 embryos which he produced but failed to grasp the full signifi- 

 cance of the conception and failed to see that it rendered his own 

 conception of differential blastolysis superfluous. Newman ('17) 



cerebellum is interesting in view of the fact recognized by neurologists that 

 the cerebellum is a supra-segmental structure added on to the brain stem in 

 the course of evolution ; and the further fact, discovered by MacArthur and 

 Jones (':/), that the cerebellum respires about as rapidly as the cerebral 

 hemispheres, both respiring more rapidly than other parts of the central nerv- 

 ous system. 



