26 C. M. CHILD 



DISCUSSION 



Metabolic relations during early development 



The results of the two modifications of the susceptibility 

 method, differential killing and differential inhibition of de- 

 velopment, agree in that in the early stages of development in 

 the polychetes susceptibility is highest in the apical region and 

 decreases basally, while, as development proceeds, the somatic 

 plate, or more particularly the posterior growing region, shows 

 a more rapid increase in susceptibility, until, after elongation 

 begins, it is the most susceptible region of the body. In a recent 

 study of the oligochetes Hyman ('16) has found that the posterior 

 growing region is, in most forms, the region of highest suscepti- 

 bility, from an early stage on and in the adult. 



The method also shows, as far as observations go, that the 

 primary gradient becomes more complicated as development 

 goes on, by the appearance of particular organs or parts as regions 

 of relatively high susceptibility as they begin rapid growth. In 

 the prototroch cells, on the other hand, we have apparently a 

 case of rapid decrease in susceptibility, a senescence following 

 the completion of growth and differentiation. The general 

 relation between susceptibility, metabolic rate, physiological 

 condition, vitality, protoplasmic activity or whatever we prefer 

 to call it, has been discussed elsewhere (Child '13; '15 a, Chap- 

 ters III, IX; '15b, Chapter III; '16 b; '16 c) and need not be 

 further considered here. 



Developmental modifications 



The general modifications of development resulting from dif- 

 ferential inhibition are of two kinds: differential inhibition be- 

 ginning with the beginning of development at first inhibits the 

 apical region more than other parts, but later, as the activity 

 of the posterior growing region increases, it also is inhibited, 

 and the result is a more or less microcephalic form with a second- 

 ary inhibition of the posterior growing region. If, however, 

 the inhibiting action begin later (with the early trochophore 



