H 



Edmund B. Wilson. 



in his interesting experimental paper on Ilyanassa, published in 

 1896. In order to understand the significance of the experiments 

 to be described it will be necessary to trace briefly the subsequent 

 development. The second cleavage is ushered in by the reap- 

 pearance of the polar lobe at the vegetative pole of the larger 

 cell, CD, of the same size and form as before, and again consist- 

 ing entirely of white material (Figs. 8, 9). The cleavage in 

 this cell, whether separated from its fellow or remaining united 



Fig. III. 

 Cleavage, from living Eggs. 

 14, Four-cell stage, from lower pole; 15, beginning of third cleavage, from 

 lower pole, third polar lobe; 16, eight-cell stage, from lower pole; 17, beginning 

 of fourth cleavage , first somatoblast in formation ; 18, sixteen-cell stage, from 

 lower pole; 19, view from lower pole, after the formation of the third quartet; 

 19a, D (pigmented) and 4d, immediately after division ; surface view. 



with it, follows the same general course as in the first cleavage of 

 the entire egg, the polar lobe finally fusing with one of the cells, 

 namely, D, the left posterior quadrant, where it again forms a 

 very definite lower polar white area.^ The anterior cell, AB, 



1 Cf. Wheeler's Fig. 49, Driesch's ('96 ) Fig. 12, of Mysostoma. 



