i 3 2 THE ORIGIN OF THE NERVOUS SYSTEM 



adequate physiological basis for the theory of the 

 mesodermal origin of segmentation. Even if reduplica- 

 tion of mesodermal organs did occur, it is difficult to 

 understand how or why it should have led to the segmen- 

 tation of ectodermal structures and even of the nervous 

 system. Moreover, many of the lower invertebrates, 

 e.g., various Turbellaria, show reduplication of certain 

 mesodermal and of entodermal parts without any indica- 

 tion of actual segmentation in muscles, nervous system, 

 or ectoderm in general. The theory of mesodermal 

 segmentation regards such forms as the precursors of 

 segmented animals, but what evidence we have seems to 

 me to point to a different conclusion. 



In the polychaete annelids, so far as they have been 

 studied in this way, the earliest stages of development 

 show a single apico-basal gradient in susceptibility, but 

 at a stage much earlier than the beginnings of segmen- 

 tation in the mesoderm an ectodermal region of high 

 susceptibility, indicating high physiological activity, 

 appears in the posterior part of the developing larvae 

 (Fig. 37) and from this region the ectoderm of the trunk 

 arises (Child, 191 7 d). The appearance of this region 

 resembles physiologically the appearance of a second 

 zooid in the posterior region of Planaria (Child, 191 id, 

 19136), and it seems probable that it is determined by 

 some degree of physiological isolation in much the same 

 way as in Planaria, except, of course, that it is much 

 earlier in development. This region is already estab- 

 lished as a growing region when the mesoderm consists 

 of only a few cells and the formation of successive seg- 

 ments from it is evidently a repeated reproductive 

 process. 



