METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 6l 



sary. It is of itself the explanation. The defects are due not to 

 the agents used, except in a general way, but to the metabolic 

 conditions in the egg and embryo. 



The work of Child and his students upon the susceptibility 

 gradients of organisms has shown that in fact some parts of the 

 organism are more susceptible to external agents than others. 

 The differential susceptibility required to explain teratological 

 development is then no longer an assumption but a demonstrated 

 fact. In the sea-urchin ('16) and in annelids ('17) Child showed 

 that the development could be controlled and modified on the 

 basis of the susceptibility gradients and predictable types of 

 terata experimentally produced. A similar demonstration of the 

 relation between the susceptibility gradients and the teratological 

 development was made by Bellamy on the frog. 



The relation between the susceptibility gradients and the pro- 

 duction of terata is the following: Those parts of the egg or 

 embryo having the highest susceptibility and metabolic rate are 

 the most strongly affected by altered conditions of a depressing 

 nature and the most greatly inhibited by them, providing that the 

 circumstances do not permit of recovery or acclimation. On the 

 other hand if the circumstances do permit of such recovery and 

 acclimation than those same parts which under more severe condi- 

 tions succumb are able to recover and continue to develop while 

 parts of lower metabolic rate cannot. 



In order to apply these conceptions to any particular organism 

 it is first necessary to study the metabolic gradients in that organ- 

 ism. This I have done in the case of the teleost fishes and I have 

 shown that the most susceptible parts are the forebrain, the eyes 

 (particularly in Fundulus}, the heart, the posterior end, and to a 

 less extent the other sense organs. 1 It will be obvious without 



1 No observations were made on the susceptibility of the olfactory pits but 

 in the frog Bellamy noted that they are regions of high susceptibility. In gen- 

 eral it may be said of the sense organs of the head, that the eye is the most 

 susceptible, the ear vesicles next, and the olfactory pits last. It is therefore 

 possible to obtain defective eyes in embryos otherwise fairly normal but de- 

 fective ear vesicles and approximated olfactory pits occur only in embryos 

 otherwise considerably abnormal. As the matter is not discussed in the text 

 a word may be said here about the cerebellum. The high susceptibility of the 



