IV. i INITIAL STRUCTURE OF THE GERM 



249 



nucleus at all, but by the position of the excentric segmentation 

 nucleus. Both male and female pronuclei move through the egg 

 for a longer or shorter distance till they meet, and the combined 

 nucleus then takes up its definitive excentric position (Fig. 151). 

 The plane of the first furrow is given by the egg-axis and the 

 point of entrance, approximately, of the spermatozoon, which may 

 thus be said to determine the symmetry of segmentation, and so 

 of the embryo, since this plane becomes, it is said (Selenka, 



C D 



FIG. 151. Diagrams from successive camera drawings of the living 

 eggs of Toxopneustes, to show the determination of symmetry during 

 fertilization. 



The original position of the egg nucleus is shown by the position of 

 the dotted circle marked $ ; its path by the succession of such circles. 



E, the entrance-point of the spermatozoon, and cone of entrance. The 

 male pronucleus is rendered in black, its path marked by the line uniting 

 the black dots. 



M is the meeting-point of the pronuclei. The cleavage nucleus, c, is 

 shaded ; its successive positions are indicated. The axis of the fertilization 

 spindle is shown by the line with a small circle at each end passing 

 through the cleavage nucleus ; the arrow passing through this is the 

 definitive egg-axis, its point the animal pole. The cleavage nucleus is 

 slightly excentric, and nearer the animal than the vegetative pole. 



F is the meridian of the first furrow. 



A and B are opposed in polarity, similar in other respects, c and D 

 differ in polarity, and otherwise as well. (After Wilson and Mathews.) 



