No. I.] 



THE ECHINODERM EGG. 



341 



the chromosomes are farthest separated. Their structure may 

 now be clearly seen. It is an irregular, granular reticulum, 

 the meshes of which are filled with a matter which does not 

 stain, and into the red reticulum the blue rays, spindle and 

 aster, are directly continuous (see Fig. 8, D). 



Fig. 8. Arbacia. A, growth of the archoplasm about the egg-nucleus. Devel- 

 opment of the large rays, and the first appearance of division of archoplasm. 

 Sperm-nucleus vesicular. 



B, two nuclei fused, the sperm appearing as a dark cap at upper part of egg- 

 nucleus. The archoplasm now completely divided. The nucleus has increased 

 very little in size. 



C, pause-stage. Coarse rays have disappeared. Nuclei completely fused. Illus- 

 trates great increase in size of nucleus at this time. 



D, structure of the first cleavage-spindle, showing the reticulum in the archo- 

 plasms. 



Conclusion. 

 The sperm-asters of Asterias and Arbacia have now been 

 followed step by step, from their first beginning, throughout 

 their growth, and directly to the asters of the first cleavage- 

 spindle. There is no centrosome demonstrable about the 



