Localization of the Nemertine Egg. 329 



tion there are profound changes in the distribution of materials 

 in the egg, and these changes seem to be accompanied by an in- 

 creased apical-basal differentiation. The first two cleavages being 

 vertical and equal merely effect a quantitative and not a qualita- 

 tive separation of materials, but the third plane of division, a 

 horizontal one, bringing about an unequal division, separates the 

 egg into two qualitatively different parts. That such is the case 

 is absolutely demonstrated by my experiments on the eight-cell 

 stage in which I obtained complete larvae from lateral four-cell 

 groups, larvse with an apical organ but without an enteron from 

 the upper quartet and larvae with an enteron but without an apical 

 organ from the lower quartet. 



Hull Zoological Laboratory, University of Chicago. 

 February 29, 1904. 



