66 Edmund B. Wilson. 



exert so great an effect on the morphogenic process is a most 

 convincing piece of evidence in favor of the theory of specific 

 formative stuffs in development. The only intelligible view of 

 the polar lobe seems to me to be that it is, so to say, a reservoir 

 of such stuffs destined for allotment to particular cells which 

 thereby become definitely specified, irrespective of their subsequent 

 relation to the embryo as a whole. This is a very different result 

 from the oft-quoted one of O. Hertwig that the lineage of par- 

 ticular structures from particular blastomeres is nothing more 

 than an incidental result of the continuity of development. It is 

 equally opposed to the conclusions of other writers who have too 

 hastily rejected the principle of mosaic development for which 

 Roux and others have contended. 



Lastly I may point out that in so far as these observations show 

 the course of differentiation, and the correlation of parts, to be 

 determined by a preexisting topographical grouping of specific 

 egg-materials they sustain an essentially mechanistic (as opposed 

 to a vitalistic) interpretation of development. To conclude how- 

 ever that these eggs are devoid of regulative capacity would be 

 to overlook some of the most striking of the phenomena I have 

 described. The experiments give clear evidence that a power of 

 regulation exists in the unsegmented egg that is no less striking 

 in form, if more limited in degree, than in the nemertine or echlno- 

 derm. As in the case of the nemertine, the typical spiral cleavage, 

 alternately dexiotropic and lelotropic, is not affected by section in 

 any plane. Far more striking is the fact that in the cleavage of an 

 egg-fragment the size of the polar lobe, on which the proportions 

 of the trochophore largely depend, is proportional to the size of 

 the piece. Since this Is true even after horizontal section, when the 

 whole of the lower polar area is included in the piece, it follows that 

 the predetermination of this area is qualitative, but not quantita- 

 tive, or only quantitative in so far as it is subject to regulative 

 control by other factors. This conclusion receives further sup- 

 port from the one reached above that the material of the lower 

 polar area Is as such specifically concerned not merely with the 

 formation of the structures that arise from it but with the form 

 of growth that results In the metamorphosis. But if this par- 



