UNION OF THE GERM-CELLS 



203 



the egg at any point, the sperm-nucleus first moves rapidly inward 

 along an entrance-path that shows no constant relation to the position 

 of the egg-nucleus and is approximately but never exactly radial, 

 i.e. toward a point near the centre of the ^gg. After penetrating a 



Fig. 103. — Diagrams showing the paths of the germ-nuclei in four different eggs of the sea- 

 urchin, Toxopneustes. From camera drawings of the transparent living eggs. 



In all the figures the original 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 tlie meeting-point (yl/) the egg-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 &gg 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. 



certain distance its direction changes slightly to that of the copula- 

 tion-path, which, again, is directed not precisely toward the egg- 

 nucleus, but toward a meeting-point where it comes in contact with 

 the egg-nucleus. The latter does not begin to move until the 



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