DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 29 



The fact remains, however, that the modifications produced 

 by differential inhibition are much less extreme than those 

 produced in the sea urchin (Child '16 c). As noted above, one 

 reason for this is the short time available for the action of ex- 

 perimental conditions in consequence of the very small amount 

 of growth and the extremely early complete differentiation of 

 the strictly larval structures. As regards the trunk and the 

 posterior growing region, where growth and development con- 

 tinue, it is evident that extensive experimental modification is 

 possible. Visible differentiation of segments may be completely 

 inhibited, the size relations of successive segments may be altered, 

 the growing region itself may be obliterated as a growing region, 

 the setae, the cirri and the paratroch may fail to develop, without 

 the actual killing or removal of any portion of the larva. But 

 even in the head-region some differential modifications have 

 been produced, for instance in Nereis the development of the 

 prototroch as four isolated groups of cells and the absence of 

 ciliation of the prototroch. Moreover, I have some evidence that 

 the cephalic ganglia may be differentially delayed in develop- 

 ment and therefore smaller than in corresponding normal stages, 

 though further investigation concerning this point is necessary. 

 The possibility that the distension of the coelom by fluid in 

 Chaetopterus is also a result of differential inhibition has already 

 been mentioned (p. 17). In short, in those parts where any 

 considerable degree of growth occurs, extensive modification is 

 possible by altering the metabolic relations through differential 

 inhibition and, even in the more strictly larval parts, size and 

 form relations may be altered to a very appreciable degree. 



Regional specialization 



The facts indicate that the more apical, for instance, more 

 strictly larval regions of the egg are in some way more highly 

 specialized and determined than in the sea urchin egg before 

 embryonic development begins. I believe, however, that this 

 difference is of the same sort as that between earlier and later 

 stages of development in any species. The egg of the polychete 

 has passed beyond the stage of the simple axial gradient in me- 



