138 THE ORIGIN OF THE NERVOUS SYSTEM 



segmentation of the cephalic, and in fact of the whole 

 central nervous system of the vertebrate, appears in the 

 adult in the arrangement of cranial and spinal nerves 

 and spinal ganglia and to some extent in certain parts 

 in the arrangement of centers or nuclei of the nerves. 

 Concerning the embryonic development of this segmen- 

 tation and its relation to the segmentation of other parts 

 there has been much difference of opinion, centering 

 chiefly about the question whether the segmentation of 

 the nervous system is primary or secondary, i.e., whether 

 segmentation is primarily an ectodermal or general 

 reproductive process or a mesodermal process. Like the 

 evidence on segmentation in general the evidence con- 

 cerning the segmentation of the vertebrate nervous 

 system seems to me to support the former alternative. 

 The discovery by Locy (1895) of distinct neuromeres 

 in very early stages of the embryonic development of 

 vertebrates, and the later work along this line by Hill 

 (1900) indicate very clearly the occurrence of some sort 

 of repetitive process in the primordium of the central 

 nervous system as it arises (Figs. 40-44). Neal (1898, 

 1918) among others has criticized this evidence and has 

 attempted to show that the neuromeres of early stages 

 do not correspond to the final segmentation, but in 

 spite of these and other criticisms the physiological 

 significance of Locy's neuromeres appears beyond ques- 

 tion as indicating a rhythmical repetitive process of 

 some sort along the anteroposterior axis in very early 

 stages. 



The neuromeres of the head region appear of course 

 in that portion of the embryo developing from the 

 primary region of growth which has its anterior end at 



