Manchester Memoirs, Vol. l (1906), No. 3. 85 



\ox \ blastomere is partial, but its ultimate development 

 total. Further, complete Pilidia may be reared from 

 either animal or vegetative fragments.*'''" 



These forms lead us naturally to the Ctenophora and 

 Mollusca, the extreme term at the other end of our series. 

 Here the segmentation of the isolated blastomeres is, 

 as in the Echinoderms, partial ; the resulting embryo is, 

 however, partial too. In the Ctenophora"^ the product of 

 a \ blastomere is a little more than a half-larva, and 

 eventually regenerates the missing half ; but in the 

 Mollusca*'- — one of the groups we may notice with a 

 ' mosaic ' type of segmentation — not only does each blasto- 

 mere segment as though the remaining blastomeres were 

 there, but no regeneration of the remaining parts ever 

 takes place. 



If we had before us these results alone, it would be 

 easy to draw from them a strong confirmation of the 

 evolutionist theory. When, however, we remember 

 Wilson's experiments on Nereis — ^just another of these 

 'mosaic' forms — and when further we take into con- 

 sideration the great mass of evidence which the behaviour 

 of the isolated blastomeres of the other groups presents 

 to us, it is impossible, to say the least, to accord to the 

 preformationist hypothesis a universal validity. On the 

 contrary, this hypothesis finds in this evidence, as Driesch 

 puts it, its formal contradiction. 



"^ Arch. Eiit.-viech., vol. i6, p. 411 — 458, 1903. 



*® "According to Zeleny, larvae from \ animal blastomeres are devoid of 

 an archenteron, from g vegetative blastomeres of an apical organ {Joiirn. 

 Exp. ZooL, vol. I, p. 293 — 329, 1904). 



«'^ Chun, C, " Festchr. f. Leuckart," Leipzig, 1892, p. 77 — 108. Driesch, 

 H. and Morgan, T. H., Arch. Ent.-mech. vol. 2, p. 204 — 224, 1896. 



^^ Crampton, H. E., Arch. Ent.-mech., vol. 3, p. 1 — 16, 1896. The 

 results recently obtained by Wilson on Dentalhmi and Patella are similar 

 i/oin-/!. Exp. ZooL, vol. i, p. i — 70, 197 — 266, 1904). 



