24 C. M. CHILD 



and therefore of high susceptibihty and so are very readily 

 inhibited. There is no evidence of any specificity of action of the 

 agents in these inhibitions, for they show all gradations, they 

 occur in essentially the same way with different agents, and the 

 killing experiments show that these are regions of high suscepti- 

 bility when the organs begin to grow. Evidently the inhibiting 

 agents either prevent the establishment of the high metabolic 

 activity necessary for the formation of these organs or, because 

 of their high metabolic rate, inhibit their development after 

 it has begun. 



The trochal cells, as noted above, show a high susceptibility 

 in their earlier stages and the development of trochal cilia may 

 be completely inhibited. The formation of the trochal pigment 

 is also more or less completely inhibited by the agents used, the 

 prototroch being entirely without visible pigment in many cases 

 where the inhibiting agent acts on the earlier stages. In some 

 cases, also, the later divisions of the trochoblasts are inhibited 

 and if the young larvae are returned to sea-water and recover, 

 the prototroch may consist of four separate groups of cells with 

 distinct gaps between them. 



To what extent the inhibiting agents may induce variations in 

 cleavage or changes in cell lineage has not been determined, but 

 the indications of regulation which I have observed incidentally, 

 suggest that interesting facts remain for future investigation in 

 this field. 



Arenicola 



The earliest stages of Arenicola available for this work were 

 stages in which the prototroch was already formed and elongation 

 of the embryo just beginning. Development in KNC and HCl 

 from these stages on gives megacephalic forms with more or less 

 inhibited posterior segmental region. Figures 48 to 50 show 

 three stages in the later normal development and figures 51 to 

 55 show forms which have developed in KNC from the beginning 

 of elongation. Arenicola is less susceptible than Chaetopterus 

 and Nereis, development in KNC m/ 100000 being practically 

 normal and only slightly retarded in m/50000. In m/25000 



