METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 19 



The work on differential susceptibility and modification of 

 development carried on in this laboratory by Child and his 

 students supports Dareste's fundamental conclusions but has 

 added one important factor to the interpretation of teratological 

 forms. The statement of Dareste that most abnormalities are 

 the result of retarded or arrested development is correct but 

 needs to be carried one step farther. Inhibition or retardation 

 modifies the development of some parts of the embryo more than of 

 others, so that certain regions frequently exhibit teratological 

 development while other regions seldom do so. Those regions 

 most prone to abnormal development under the action of in- 

 hibiting agents of proper concentration and duration of exposure 

 are those which have been shown by Child and his students to be 

 most susceptible and most quickly killed by lethal concentrations 

 of these agents. In brief differential inhibition of development 

 results from differences in susceptibility of different parts of the 

 embryo. 



On the basis of the susceptibility differences described in this 

 paper the application of external agents of a depressing nature 

 to the chick embryo should yield the following results. Agents 

 applied at very early stages may produce a permanent general 

 diminution of the metabolic rate of the embryo. Such a con- 

 dition will result in the absence or abnormality of those parts 

 of the embryo requiring a high metabolic rate for their normal 

 development, for in such retarded eggs the necessary rate of 

 activity cannot be attained. Sufficiently drastic treatment at 

 an early age will prevent the appearance of the embryo alto- 

 gether, only the blastoderm being present; for as shown in this 

 paper the embryonic is more susceptible than the extra-embryonic 

 portion of the germinal disk. Less drastic treatment of early 

 embryos up through the head process stage (fifteen to eighteen 

 hours of incubation) should cause defects of the anterior end 

 mainly, particularly the brain, since during this period of develop- 

 ment there is but one center of high activity, the anterior end of 

 the embryonic axis. Exposure to depressing agents during later 

 stages of incubation (medullary fold through the two-day chick 

 stage) would be expected primarily to stop or retard the posterior 

 elongation and in addition to evoke abnormalities of the anterior 





