Experimental Studies on Germinal Localization. 3 



ones on the nemertlne egg (Wilson, '03) bear upon both these 

 questions. In that paper I approached especially the second ques- 

 tion in an experimental study of the egg of Cerehratulus, which 

 has since been extended by the work of Yatsu ('04). My 

 results clearly showed that in this egg the cleavage-factors are 

 not definitely localized until after the completion of the ma- 

 turation of the egg, but they gave no definite evidence regarding 

 the localization of the morphogenic factors (as distinguished 

 from those of cleavage) at this period; it was, however, shown 

 that in the comparatively young blastula, before the formation of 

 the mesoblast, morphogenic localization, as shown in the pre- 

 determination of the gut and apical organ, has become much 

 more definite than in the unsegmented egg. Yatsu subsequently 

 obtained evidence, in the same species, that the localization of the 

 morphogenic factors is a progressive process even in the stages 

 preceding cleavage, since the percentage of normal larvae ob- 

 tained from egg-fragments at successive periods steadily dimin- 

 ishes from the first discharge of the eggs (when maturation be- 

 gins) up to the period immediately preceding the first cleavage; 

 and the nature of the defective larvae, correlated with the plane 

 of section, pointed to a increasingly definite localization, in the 

 later stages preceding cleavage, of the bases of several important 

 organs, such as the apical organ, gut, and ciliated lobes of the 

 pilidium. I am now able to offer an experimental analysis along 

 the same lines — perhaps I should say the beginning of such an 

 analysis — of the molluscan egg, in which pure observation of 

 the cell-lineage has produced such convincing evidence of mosaic 

 development, sustained by Crampton's initial experimental exam- 

 ination of the gasteropod egg ('96), and by the interesting cyto- 

 logical work of Lillie ('01) and Conklin ('02) on the cyto- 

 plasmic regions of the unsegmented and segmenting egg. The 

 cytological and experimental results coincide in demonstrating 

 in this egg (specifically in Dentalium) the existence of a very 

 definite prelocalization of some of the most important factors 

 both of cleavage and morphogenesis, which here closely coincide. 

 They show conclusively also, contrary to what the nemertine 

 experiments had led me to expect, that in its main features this 



