METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. IJ 



embryo has expressed the opinion that it is necessary to distin- 

 guish between differentiation metabolism and proliferation me- 

 tabolism. The former has probably a greater integrative action 

 than the latter. This idea may be very important in the in- 

 terpretation of developmental processes but cannot be evaluated 

 at present. 



The question of the origin of metabolic differences in embryonic 

 development is naturally of prime importance but little evidence 

 is as yet available upon this matter. In the case of the hen's 

 egg it seems definitely established that the axis of the embryo 

 is determined by the position of the ovocyte in the ovary, i.e., by 

 an environmental factor acting in a definite direction. Similarly 

 it may be supposed that the metabolic conditions present at each 

 developmental stage are determined by factors operative at 

 preceding stages. In the last analysis environment must be the 

 cause of metabolic differences. 



III. APPLICATION TO TERATOLOGICAL DEVELOPMENT. 



Abnormal chick embryos accidentally or experimentally pro- 

 duced have long been of interest to zoologists. Many articles 

 have been published dealing with the experimental production of 

 abnormalities in the chick but most of them are rather old and I 

 have not been able to obtain access to a number of them. Others 

 give such inadequate accounts of the abnormalities found that 

 they are of little value from the present point of view. 



The interpretation of experimentally produced abnormalities 

 in the chick on the basis of differential susceptibility will naturally 

 follow the same lines as stated in previous publications by 

 Child and his students on developmental modifications in other 

 animals. Those portions of the embryo which are the most 

 susceptible to lethal concentrations of toxic agents will be most 

 affected and modified by non-lethal exposures of proper duration 

 and concentration in which acclimation does not occur. These 

 same portions will recover most readily if the concentration and 

 duration of exposure employed permit recovery and acclimation. 

 If the metabolic rate of the egg or early embryo is permanently 

 depressed and slowed down by the action of external agents, 

 parts requiring for their normal development a high metabolic 



