DIFFERENTIAL DEVELOPMENTAL MODIFICATION. I 167 



ceptible regions are so injured that they cannot recover, the less suscep- 

 tible may still be able to recover to some extent; it seems desirable to dis- 

 tinguish this, as partial recovery, from the differential recovery of the 

 whole, in which the regions primarily the most susceptible show the great- 

 est recovery. Differential acceleration of development has also been ob- 

 served in some forms with certain agents. As pointed out in chapters iii 

 and iv, the appearance of the same differentials or gradient patterns with 

 many different agents, both physical and chemical, suggests that differ- 

 ential susceptibility depends primarily on factors of rate rather than of 

 kind of activity, the most active regions being primarily most susceptible 

 to the more extreme effects of external factors and most capable of toler- 

 ance or conditioning to, and recovery from, less extreme effects and in 

 some cases most susceptible to accelerating effects. In other words, dif- 

 ferential susceptibility appears to be, to a high degree, nonspecific for 

 different agents and apparently depends on quantitative factors of gradi- 

 ent pattern rather than on specific regional differences in the organism, 

 if such are present, or on the nature of the agent and the particular man- 

 ner in which it acts on living protoplasms. 



The data on experimental modifications of early development resulting 

 from exposure of the entire intact organism to altered environmental 

 conditions indicate that most, if not all, of the modifications thus pro- 

 duced depend on nonspecific differential susceptibility and fall under one 

 of the above heads (Child, 1924a, igiSd). Moreover, it appears that phys- 

 iological inhibiting factors of various sorts act differentially on develop- 

 ment in the same way as external factors. The present chapter and the 

 two following are chiefly concerned with some of the differential modifica- 

 tions of development and their bearing on the problem of pattern. 



DIFFERENTIAL MODIFICATION OF HYDROID DEVELOPMENT 



Differential inhibition of embryonic development of the hydromedusa 

 Phialidium gregarium has been effected by KCN, LiCl, phenyl urethane, 

 dilute neutral red, and CO^ with essentially similar modification by all 

 agents (Child, 1925&). In normal development cells from the basal re- 

 gion of the blastula immigrate singly into the blastocoel to form ento- 

 derm (Fig. 57, A-C). Under the inhibiting conditions the number of im- 

 migrating cells increases, the region of immigration extends farther api- 

 cally (Fig. 57, D), and, instead of forming a single cell layer of entoderm, 

 the immigrating cells may form a solid mass, more or less completely 

 obliterating the blastocoel (Fig. 57, E-G). Cells may also be given off 



