152 



FERTILIZATION OF THE OVUM 



mined by at least two different factors, one of which is an attraction 

 or other dynamical relation between the nuclei and the cytoplasm, 

 the other an attraction between the nuclei. The former determines 



Fig. 77. Diagrams showing the paths of the germ-nuclei in four different eggs of the sea- 

 urchin Toxopncustcs. From camera drawings of the transparent living eggs. 



In all the figures the origin;il position of the egg-nucleus (reticulated) is shown at 9 ; the point 

 at which the spermatozoon enters at E (entrance-cone). Arrows indicate the paths traversed by 

 the nuclei. At the meeting-point (M) the esg-nucleus is dotted. The cleavage-nucleus in its 

 final position is ruled in parallel lines, and through it is drawn the axis of the resulting cleavage- 

 figure. The axis of the egg is indicated by an arrow, the point of which is turned away from the 

 micromere-pole. Plane of first cleavage, passing near the entrance-point, shown by the curved 

 dotted line. 



the entrance-path of the sperm-nucleus, while both factors probably 

 operate in the determination of the copulation-path along which it 

 travels to meet the egg-nucleus. The real nature of neither factor 

 is known. 



