DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 41 



in this region of the body the primary gradient, decreasing in 

 rate posteriorly, may always be present and the developmental 

 gradient characteristic of the earlier stages of annehd seg- 

 mentation may never appear, although it does appear in the 

 tail bud. Such differences are, however, incidental rather than 

 fundamental. 



These facts are mentioned merely by way of calling attention 

 to the possibility of a general physiological conception of the 

 process of segmentation in animals and it remains for the future 

 to determine whether the course of events in the annehds has 

 any general significance. 



SUMIMARY 



1. The results of the susceptibility method, both in the killing 

 experiments and in modification of development, indicate that 

 at the beginning of development the apical region is the most 

 active region metaboUcally and that metabohc rate decreases 

 toward the basal pole. 



2. As development proceeds the posterior portion of the 

 somatic plate shows a relatively rapid increase in susceptibility 

 and, as elongation begins, or soon after, the posterior growing 

 region becomes the most active region of the larva. 



3. The general modifications of larval form and proportions 

 by differential inhibition are of two sorts. Inhibiting agents, 

 acting from the beginning of development, produce forms more 

 or less microcephahc, with relatively large anterior segments 

 and more or less completel}^ inhibited posterior region. In- 

 hibition beginning at the stage of the young trochophore, for 

 instance, after the increased activity of the posterior region has 

 occurred, produces forms more or less megacephahc with more 

 or less completel}^ inhibited posterior gromng region. 



4. Particular organs such as the prototroch, the seta-sacs, 

 the tentacles and cirri can be differentialh^ inhibited in the early 

 stages of their development, an indication that they are the 

 seat of high metabolic acti\ity at that time. 



5. The facts indicate that, in general, the more apical or ante- 

 rior and probably the antero-ventral regions of the egg and embryo 



