280 THEORIES ON THE ORIGIN OE VERTEBRATES 



dorsal muscle-plates forming a tube above the notochord, 

 the "Ventral plates a tube below the notochord, whereas 

 Articulates do not possess this axis, and form only one tube, 

 namely, that round the " vegetative " organs (cvolntio gcinina}. 

 Semper is at pains to prove that evolutio bigeinina is charac- 

 teristic also of Annelidan development. 



n 



FlG. 14. Transverse Section (Inverted) of the Worm .\',tis. (After Semper.) 



a.c. Alimentary canal. 



n.c. Nervi; conl. 



sp.tj. Sniiuil ganglion. 

 n. Notochord. 



d.p. Neural inii^rlr. plate. 

 ..'. 1 1 ai'inal muscle-plate. 



He gets his facts from an elaborate study of the process 

 of budding in the Nnirfu-, making the somewhat risky 

 assumption that regeneration takes essentially the same 

 course as embryonic development. 



He succeeds in showing to his own satisfaction at least 



-that in the formation of new segments in Xais and 



Chictogaslcr a strand of cells appears between the alimentary 



canal and the nerve-cord, and that from this axial strand the 



