16 CM. CHILD 



still less prospect of producing modifications by differential 

 acclimation, since this requires a considerably longer time than 

 differential inhibition. 



In spite of this situation, however, I have found it possible to 

 bring about some degree of modification in these forms. Two 

 chief lines of modification appear. If exposure to the inhibiting 

 agent begin very soon after fertilization, when a simple apical- 

 basal susceptibility gradient is present, with the highest suscepti- 

 bility in the apical region, the larvae are more or less micro- 

 cephalic and the posterior growing region is of course also more 

 or less inhibited. If, on the other hand, exposure to the in- 

 hibiting agent begin at a later stage of development, say sixteen 

 to twenty-four hours (for instance after the larval head region 

 has undergone a large part of its development and the posterior 

 growing region has attained a high susceptibility, then the larvae 

 will be more or less megacephalic and the inhibition will be 

 chiefly posterior. Of course special organs, such as tentacles, 

 cirri, paropodia, may be partially or completely inhibited ac- 

 cording to the experimental conditions. The characteristic 

 forms produced in my experiments are described below. In 

 these experiments only KNC and HCl have been used thus far 

 as inhibiting agents. Work with various other agents is de- 

 sirable and certain unpublished experiments on .the sea urchin 

 suggest the possibility that some other agents may have a greater 

 differential effect and so determine a greater degree of modifi- 

 cation than the two used, but it has not yet been possible to 

 test them. 



The results presented below of this first attempt to use differ- 

 ential susceptibility as a means of modifying polychete develop- 

 ment are by no means exhaustive. Attention has been directed 

 chiefly to the general change in form and proportion resulting 

 from the chief regional differences in susceptibility, and many 

 details remain to be investigated. The figures are semi-dia- 

 grammatic and are intended, primarily, to show as simply as 

 possible the general modifications in size and proportions of body- 

 regions. 



