POLARITY AND BILATERALITY OF THE ANNELID EGG. 65 



and the remainder were substantially normal in the relative sizes of 

 the two cells, although the smallest piece was about one eighth 

 the volume of the entire egg, and the others ranged in size up 

 to about one third the bulk of the normal egg. The pieces in 

 which the small cell was too large relatively were the largest 

 pieces and contained some yolk. 



Now if bilateral symmetry existed prior to fertilization, one 

 would expect parts of the bilateral egg to vary greatly in the pro- 

 portions of the first two cells, because a fractional bilaterality and 

 not a whole bilaterality would exist in each part. I therefore 

 conclude that bilaterality has developed subsequent to fertiliza- 

 tion. And this conclusion must be true for the parts even if it 

 were only a question of regulation of a preexisting bilaterality 

 (of which there is no evidence). But if bilaterality can develop 

 in the parts there is no reason for assuming its prior existence in 

 the whole ; and its origin must be regarded as a truly epigenetic 

 process. Brachet makes a similar argument for the egg of the 

 frog (Roux's Archiv, XXII., 3, 1906). 



III. THE ORGANIZATION IN THE GROUND SUBSTANCE. EX- 

 AMINATION OF THE CONCEPTIONS OF POLARITY 

 AND BILATERALITY. 



The hypothesis that the nature and direction of polarity and 

 bilaterality are unaltered by centrifuging involves the assumption 

 that the arrangement of the ground substance is substantially 

 unaltered. If it could be shown, for instance, that the segrega- 

 tion produced by centrifuging is a mass movement of the areas 

 and zones of the protoplasm, and not simply a movement of 

 granules through the ground substance, the entire organization 

 of the egg would be altered, and there would be no reason why 

 polarity should appear in the original direction rather than in any 

 other direction. 



I propose therefore to examine the thesis that the movements 

 produced by centrifuging are purely granule movements, with 

 the single exception of the nucleus or spindle. To do this it is 

 necessary to compare minutely the structure of the egg both be- 

 fore and after centrifuging, using a fixed and determinate stage 

 for such examination. Fortunately, the latter condition is a rela- 



