56 Edmund B. fVilson. 



become perfectly spherical before development proceeds, the re- 

 sulting defects cannot be due to a failure of regulation traceable 

 to the shape of the fragment, as was formerly assumed by- several 

 writers. Neither are they due to insufficient mass; for perfect 

 dwarfs may arise from fragments much smaller than those that 

 show the characteristic defects. Further, these facts, like those 

 earlier determined by Crampton ('96) in the gasteropod egg, and 

 by Driesch and Morgan ('95) and more recently by Fischel 

 ('98) in the ctenophore egg, are fatal to the view that embryonic 

 differentiation is brought about through quahtative nuclear divi- 

 sion during the cleavage. The conclusion is therefore unavoidable 

 that the specification of the blastomeres in these eggs is due to 

 their reception, not of a particular kind of chromatin, but of a par- 

 ticular kind of cytoplasm; and that the unsegmented egg con- 

 tains such different kinds of cytoplasm in a definite topographical 

 arrangement. How many such specific stuffs exist in the unseg- 

 mented egg of Dentalhim and what is their arrangement it is 

 impossible at present to say; for the pigment-band and the two 

 polar areas can only be considered as an outward sign of an or- 

 ganization that for the most part doubtless escapes the eye. My 

 experiments have only positively determined the cytoplasmic pre- 

 localization in the lower polar area of material essential for the 

 development of that complex of structures that I have included 

 in the term "post-trochal region," and of one other structure, the 

 apical organ. The first of these includes material that is essential 

 to the development of the typical larval form, including the foot, 

 to certain characteristic ectoblastic structures of the post-trochal 

 region, such as the shell-gland, mantle-fold, and probably also the 

 pedal ganglia ; it also appears probable that it includes material 

 essential for the formation of the coelomesoblast. I do not doubt 

 that further experiments on this egg will show a still more definite 

 and detailed prelocalization ; though, as already stated, it is 

 not easy to determine this, owing to the difficulty of distinguishing 

 between defects in the partial larvae that result directly from the 

 plane of section and those that are due to other causes. 



Two additional facts clearly appear from the experiments, on 

 which I would lay stress. First, the amount of material removed 



