SEGMENTATION 135 



case the segment resembles a headless individual of 

 Planaria (see Figs. 11, 12). The process of growth and 

 differentiation is repeated for each segment, but instead 

 of becoming free and developing heads, like the zooids 

 of Planaria, the segments are integrated into a new 

 whole, probably chiefly through the development and 

 increase in conductivity in the nervous system. 



The early appearance of a posterior growing region 

 involving primarily the ectoderm and giving rise to the 

 segments has been demonstrated not only in the poly- 

 chaetes but in the oligochaetes (Hyman, 191 6), various 

 Crustacea and fishes, and Bellamy (1919) has shown 

 that it appears in the frog at an early stage (Figs. 38, 

 39) and that through the action of inhibiting agents its 

 localization can be altered so that it arises nearer the 

 apical pole of the egg than normally, much as the 

 anterior end of a new zooid can under inhibiting condi- 

 tions be localized nearer the head in Planaria and 

 various other forms (Child, 191 id). In unsegmented 

 animals a definite posterior growing region of this sort 

 is usually not present, except in cases of agamic repro- 

 duction, though even in these forms more or less growth 

 may occur at the posterior end as they increase in length. 

 These facts show at least that the posterior growing 

 region is determined in the ectoderm earlier than in the 

 mesoderm and, I believe, increase the probability that 

 segmentation as a physiological process begins in the 

 most active layer, the ectoderm, and that it is physi- 

 ologically a process resembling various processes of 

 agamic reproduction. 



That the central nervous system of segmented 

 animals shows more or less definite indications of being 



