The maturation and fertilization of the egg of Cerebratulus. 445 



The second polar body is often formed directly beneath the first, 

 and soDQetimes pushes the latter before it. 



The sixteen rod-Hke chromosomes remaining in the egg now 

 break up into an equal number of groups of small granules, and 

 each group swells out into an elongated vesicle (Fig. 12). These 

 vesicles fuse into several larger ones (Figs. 24, 25), and these, finally, 

 into the single female pro-nucleus. While the chromosomes are uniting 

 to form the nucleus they move deeper into the substance of the egg, 

 and occupy the position formerly held by the centrosome, so that the 

 aster-fibres at this time radiate in all directions from the clusters of 

 chromosomal vesicles. The centrosome (sometimes divided) has mean- 

 while been lost among these vesicles. The radiations become much 

 less regular and distinct as the egg-nucleus forms, although they may 

 still be detected up to a short time before the fusion of the germ- 

 nuclei. Occasionally they show a tendency to form a sort of spiral 

 figure (Fig. 12) as Mark (19) found in Limax, and MacFarland (18) 

 in Fleur ophyllidia. 



The Sperm-nucleus and its Asters. 



As soon as the slender spermatozoon has made its way inside 

 the membrane of the egg, it contracts greatly and assumes a more 

 or less regularly oval or spherical form. If the egg be still in the 

 germinal vesicle stage, the spermatozoon usually remains near the 

 periphery of the egg, and near its point of entrance, until after the 

 formation of the maturation spindle. As soon as the formation of the 

 polar bodies is under way, however, the spermatozoon (after a previous 

 rotation as Wilson has so fully .described for Toxopneustes) begins 

 to move towards the center of the egg. As the spermatozoon usually 

 enters the egg on the side farthest from the polar spindle its path 

 is likewise towards the point where the egg-nucleus is to form later. 

 A few delicate radiations with a minute centrosome at their focus 

 appear at one side of the sperm-nucleus (Fig. 42). During the growth 

 of this aster the larger yolk-globules are pushed outwards towards 

 the ends of the fibres, while nearer the centrosome we find only 

 granular protoplasm as is shown in Fig. 14. About the minute centro- 

 some we find a gradually increasing layer of very homogeneous sub- 

 stance usually distinctly marked off from the surrounding protoplasm. 

 The aster-fibres pass directly into this sheath, or centrosphere, although 

 I have never seen them actually attached to the centrosome as is 

 described by Kostanecki & Wierzejski (15). On reaching the centro- 



