No. I.] 



THE ECHINODERM EGG. 



339 



tion, and the segmentation-spindle is soon formed. The archo- 

 plasms of this spindle are of the same reticular structure 

 described for Toxopneiistes and Arbacia. 



C. Fertilization in Arbacia punctulata. 

 The fertilization in Arbacia presents a striking similarity to 

 that in Toxopnetistes, but on account of differences in texture, 



Fig. 7. Arbacia. A, sperm of Arbacia before fertUization, showing middle- 



piece. 



^, sperm after entrance, showing middle-piece. Entrance-cone. 



C, Entrance of sperm before rotation, showing entrance-cone. No middle- 

 piece demonstrable. 



D, sperm-star developing from base of sperm before rotation. 



E, sperm rotated and advancing to egg-nucleus. Shows slight bulging of egg- 

 nucleus toward sperm. 



/^, meeting of sperm and egg-nucleus. Sperm still conical; sperm-star stUl 

 small ; and projection of egg-nucleus to meet sperm. 



