10 C. M. CHILD 



of the prototroch which is the last portion of the ectoderm to die. 

 As the larval parapodia develop, they become regions of high 

 susceptibility and the developing tentacles and cirri also show a 

 relatively high susceptibility. 



As regards the different body layers the mes-entodermal 

 region is certainly less susceptible than the ectoderm before 

 gastrulation is completed, for instance, while the cells of this 

 region are directly exposed to the action of the agent. In some 

 experiments on temporary inhibition in the earlier stages of 

 development, followed by return to water, some degree of differ- 

 ential recovery occurs if the concentration is not too high and 

 time of exposure too long, and in many such cases the entoblast, 

 mesoblast and sometimes the first somatoblast fail to recover 

 and die and disintegrate some time, perhaps even a day or two 

 after return to water, while the apical and antero-ventral ecto- 

 derm recover and continue development. As I have shown 

 (Child, '16 c), differential recovery resembles differential acclima- 

 tion in that the regions of low susceptibility to the direct action 

 of the agent show less capacity for recovery than those of high 

 susceptibility when the inhibiting action has not been too severe. 

 These cases of partial recovery in Nereis are, I believe, cases of 

 this sort and constitute further evidence for the lower metabolic 

 activity of the mesodermal and entodermal regions as compared 

 with the ectoderm and particularly the apical and antero-ventral 

 ectoderm. These experiments indicate, further, that the somato- 

 blast which gives rise to the ectoderm of the trunk region has a 

 lower metabolic rate in the earlier stages than the ectoderm of 

 the more strictly larval regions. 



Although the general susceptibihty gradients are the same in 

 Nel-eis as in Chaetopterus, the behavior of the two protoplasm.^ 

 in the killing agents differs in certain respects. In Nereis there 

 is little or none of the swelling and separation of cells so char- 

 acteristic of all stages of Chaetopterus, death occurs wdth com- 

 paratively . little change in appearance, and the death point is 

 most easily determined by the decoloration after staining with 

 neutral red. 



In KNC and various other agents, however, the Nereis pro- 

 toplasm, like that of Chaetopterus, undoubtedly does become 



