Experimental Studies on Germinal Localization. 55 



definite order of events predetermined in a particular cytoplasmic 

 area and wholly independent of the immediate action of nucleus 

 or centrosome. An additional point of great embryological in- 

 terest is the fact, shown by a comparison of Fig, 6 with Fig. 94, 

 that in these fragments the polar lobe is, at least in some cases, 

 nearly or quite as large absolutely as in one entire egg; whereas 

 in the lower fragment of an unfertilized egg it is typically re- 

 duced to the correct proportional volume of the lobe in a whole 

 egg. This is however not invariable, for in some cases, an ex- 

 ample of which is shown in Fig. 96, the lobe is reduced to its 

 proper proportional size. I have not accurately studied this mat- 

 ter in a sufficient number of cases to speak very positively; yet I 

 feel confident that the contrast in this respect between the lower 

 fragments from unfertilized and fertilized eggs is a general, 

 though not an invariable rule. The interest of this fact is 

 pointed out in the sequel. 



VII. 



COMMENT. 



Without undertaking at this time a complete discussion of the 

 foregoing observations, I may briefly indicate their bearing on 

 the general questions referred to at the beginning.^ My observa- 

 tions demonstrate conclusively, I think, both the mosaic character 

 of cleavage in these eggs, and the definite prelocalization of some 

 of the most important morphogenic factors in the unsegmented 

 egg. The Dentalium egg shows, even before it breaks loose from 

 its attachment in the ovary,and long before even the initial changes 

 of maturation, a visible definite topographical grouping of the 

 cytoplasmic materials. This is proved by the experiments to 

 stand in definite causal relation to the subsequent differentiation 

 of the embryo in such wise that the removal of a particular cyto- 

 plasmic area of the unsegmented egg results in definite defects 

 in the resulting embryo that are not restored by regenerative or 

 other regulative processes within the time-limits of the experi- 

 ment. Since both the egg-fragments and the isolated blastomeres 



1 A more general discussion of the mosaic-theory of development, with a 

 fuller review of the literature, will be given in a following paper. 



