39^ 'Journal of Comparative Neurology and Psychology. 



primitive and dominates the nervous system of lower animals, but 

 becomes relatively less important as we pass up the phyletic series, 

 whose higher types come into progressively more varied relation 

 with the environment. Nevertheless the influence of metamerism 

 is always apparent, even in the human brain; and the fact that two 

 tendencies, independent in origin and often exerting antagonistic 

 influences on the course of the differentiation, have operated in 

 the architectonic of the vertebrate nervous system is a cause of 

 great perplexity and confusion in attempting the analysis of cere- 

 bral structure in higher animals. A purely metameric scheme can 

 in the nature of the case be no more satisfactory than one based on 

 adult or embryonic human structure, even though it is based on a 

 correctly interpreted phylogeny. 



The matter is still further complicated by the fact that, either 

 primarily in chordate evolution or secondarily, there appeared a 

 difference between the metameres in the rostral part of the body 

 and those in the more caudal part; viz: the presence of gills in the 

 rostral portion. The structural unit here is the branchiomere. 

 The delimitation of the branchial region from the rest of the body 

 gives the most fundamental plane of differentiation which crosses 

 the mid-line of the body. 



Our problem, then, is first to analyze the two elementary factors 

 in the phylogenesis, metamerism and longitudinal functional dif- 

 ferentiation, and then to endeavor to trace the influence exerted by 

 each and to construct a scheme of cerebral structure which shall 

 hold good both in lower and in higher vertebrates and take due 

 account of both of these directive influences. 



In the spinal cord region, where the primitive relations seem to 

 have suffered the least modification in the course of phylogenesis, 

 the two factors referred to above can be quite readily distinguished. 

 The primitive metamerism is highly modified in the peripheral 

 distribution of the spinal nerves, but it is preserved almost un- 

 changed at the surface of the spinal cord, as shown by the serial 

 arrangement of the spinal roots and their ganglia. Again, inter- 

 nally the demand for correlation between the different levels has 

 produced longitudinal arrangements which largely obscure the 

 transverse segmentation. Only in the early embryo is the internal 

 structure of the neural tube evidently segmental. There is a 

 stage when neuromeres are clearly defined in the neural tube as 

 a transient beading of its contour. The internal longitudinal 



