EXPLANATION OF PLATE XII. 



Entrance of the spermatozoon into the egg and its conjugation with the egg- 

 nucleus in the Sea-urchin Toxopneustes, after WILSON and LEAMING. 

 The fertilisation membrane is not represented. 



A. Sperm-head before entrance; N, nucleus; M, middle-piece. 



B. Immediately after entrance, showing reception-cone. 



C F. Rotation of sperm-head and formation of sperm aster in connection 

 with middle-piece. (F was drawn 3* minutes after the first entrance.) 



G, H. Stages corresponding with D and E, drawn in greater detail (C = re- 

 ception-cone). 



I, J. Approach of the two nuclei to each other ( = egg nucleus, S, 5 = sperm 

 heads outside the egg). 



K, L. Conjugation of the two nuclei. 



The magnification of figs. / L is about one-quarter of that of A H. 



The figures on Plate II form a continuation of the series shown on Plate XII. 



