348 



EMBRYOLOGY 



follows that the most anterior segments of the trunk are 

 formed first, and therefore are the most differentiated ones 

 in the developing- larva, while behind follow younger and 

 younger ones. The growth of the Annelid body therefore 

 does not depend upon growth of the body in all directions, 

 but upon a partial (terminal) growth, since new segments 

 are always being supplied from a zone of proliferation lying 

 near the posterior end of the body (in front of the terminal 

 segment). This pi'oductiveness of a restricted portion of 

 the body strongly recalls certain kinds of non-sexual re- 

 production, and therefore the process has been called, even 

 in this case, a " budding of the segments." That, however, 

 is an inaccurate mode of expression. The most natural 

 comparisons are those with the tapeworm chain and Avith 

 the strobila of the Scyphomedusse. The point of comparison 

 m all these cases lies in the production, from a certain zone 

 ot proliferation, of homodynamic portions of the body, which 

 become to a certain extent independent. For this reason 

 the view has been expressed that in the segments of the 

 Annelid body we have before us single individuals (which 

 do not arrive at complete separation), and accordingly in 

 the entire body of the Annelid a stock or corm. It seems 

 scarcely favorable to this theory that the degree of in- 

 dependence which the individual segments present is com- 

 parativelj^ slight. The most important organs (nervous 

 system, body musculature, blood-vascular system) show 

 themselves to be single fundaments of the entire body, and 

 are also developed as such even though they also exhibit 

 evidences of metamerism. Even the excretory canals may 

 give up their segmental isolation and become united to one 

 another by means of longitudinal canals. The comparison 

 with the single fundaments of the other systems of organs 

 inclines us to the opinion that the development of the 

 nephridia from separate fundaments (Bkkgh) represents a 

 coenogenetically altered condition, and that the nephridial 

 system was originally developed by sepax'ation from a com- 

 mon cord (Hatschkk). By such an assumption the com- 

 parison of the nephridial system of the Annelida as a whole 

 with the excretoi-y oi'gaiis of the riatyhelniiuihus would 



