140 THE ORIGIN OF THE NERVOUS SYSTEM 



or near the apical pole of the egg. That is to say, they 

 are wholly anterior to the secondary region of growth 

 from which the postcephalic segments arise, and it has 

 been suggested above that the appearance of this 

 secondary growing region is the result of physiological 

 isolation. The presence of the cephalic neuromeres 

 indicates the occurrence of a rhythmic, repetitive 

 process in the nervous primordium even in this anterior 

 region. If we regard the cephalic neuromeres as true 

 segments, we account in evolutionary terms for their 

 appearance by assuming that they represent the remain- 

 ing traces of an ancestral segmented condition of the 

 cephalic region. So far as I am aware, however, the 

 problem of the physiological process involved has not 

 been considered. Obviously it is a true repetitive or 

 reproductive process in the cephalic primordium. 

 There is little prospect of obtaining actual experimental 

 knowledge concerning this process until physiological 

 technique is much more highly developed than at 

 present, but it may be suggested that the increasing 

 physiological activity and its progress in the posterior 

 direction in the embryonic primordium are the chief 

 factors in the repetitive process of neuromere forma- 

 tion. As each level of the primordium attains a certain 

 degree of activity it becomes to some slight extent 

 physiologically isolated from more anterior levels and 

 some slight amount of localized growth occurs, but the 

 degree of isolation is so slight that it does not proceed 

 very far. 



According to this point of view the origin of the 

 secondary posterior growing region in the embryos of 

 segmented animals and the development of the trunk 



