150 THE ORIGIN OF THE NERVOUS SYSTEM 



Even in Planaria it has not yet been possible to deter- 

 mine the primary symmetry gradients with certainty 

 because of the lack of early embryonic stages, but in all 

 the Turbellaria examined the ventral region is, at least 

 primarily, a region of higher activity than the dorsal, 

 and while the lateral margins of the body may 

 develop high activity in the ectoderm in later stages 

 the growth of new tissue from the cut ends of pieces 

 and various other facts suggest that in the parenchyma 

 the median ventral region is more active than the 

 lateral. 



There are some reasons for believing that dorsiven- 

 trality is the first step in the development of bilateral 

 symmetry in some of these forms and that the distinction 

 between median and lateral is of later origin. If this is 

 true, the localization of the nerve cords in these lower 

 vertebrate groups may represent the high ends of the 

 symmetry gradients, i.e., the cord may arise where 

 the high activity of the ventral region begins to decrease 

 laterally and dorsally. Where the whole ventral region 

 is alike and the gradient begins laterally the cords may 

 be lateral, and as the width of the ventral active zone 

 decreases they may arise nearer the median line. This 

 interpretation is offered merely as a suggestion, although 

 there is some evidence in support of it. Until we know 

 more concerning the early embryonic stages in these 

 forms, definite conclusions are not possible. But what- 

 ever the actual conditions in these lower forms may 

 prove to be, it appears to be true in general for the 

 higher invertebrates and the vertebrates that differ- 

 entiation of nervous structure is primarily and pre- 

 dominantly localized at the higher levels of the chief 



