DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 33 



independent of regions anterior to it and, as in various flatworms 

 (Child, '11), the development of a new indi\'idual begins, but is 

 later inhibited by a new physiological integration through the 

 nervous system. If segmentation be a process of asexual re- 

 production of this kind, it must begin in the ectoderm, since this 

 is the most active body-layer and precedes the others in develop- 

 ment, and the fundamental feature of the segment is a new 

 dominant region, which becomes a new nervous center, a gan- 

 glion. Other processes, for example those in the mesoderm, must 

 be secondary to the ectodermal processes. 



If, on the other hand, segmentation be primarily a mesodermal 

 or mesoblastic process, it may be conceived as a sort of repro- 

 ductive process in the mesoblast. As the mesoblast bands grow, 

 groups of cells become successively separated, either by a process 

 of physiological isolation within the mesoblast bands or, per- 

 haps, by a physical separation of the cells into groups which 

 form sacs by the secretion of fluid from their internal surfaces. 

 If the process of segmentation is of this character the ectodermal 

 reduplications must be secondary features. 



Experimental evidence is not yet available for a final choice 

 between these two alternatives, and the morphological evidence 

 points toward one alternative in some cases and toward the other 

 in others and cannot be regarded as conclusive. In embryonic 

 development the ectoderm arises from the most active regions 

 of the egg and morphogenesis begins in it, and we know that 

 physiological differences and specialization may exist before 

 visible morphological differences arise. In the growing region 

 of the annelid body the ectoderm is certainly the most active 

 body-layer, and there is no reason to doubt that there, as in the 

 embryo, it is physiologically in advance of 'the other layers. 

 Moreover, if segmentation be fundamentally a mesoblastic 

 process, it is difficult to account for the reduplication in th3 

 nervous system. Reduplication of intestinal branches and 

 testes occurs in the flatworms, without any corresponding re- 

 duplication of nerve centers, and it is not clear why or how the 

 reduplication of coelom sacs in the annelids can bring about 

 reduplication in the nervous system. Moreover, the synchron- 



JOUHNAL OF MOHPHOLOGY, VOL. 30, NO. 1 



