The maturation and fertilization of the egg of Cerebratulus. 461 



there can be no doubt, for they first appear wholly within the sub- 

 stance of the latter, and are widely separated from the general cyto- 

 plasm (Fig. 38). Moreover, their first rays are not unlike the fibres 

 making up the reticulum of the centrosphere. We thus have still 

 further evidence that the centrosphere is merely a differentiated portion 

 of the same general cytoplasmic substance of which the reticulum 

 and aster-fibres are composed; that it forms at the expense both of 

 the general cytoplasm and of aster-fibres; and that, in turn, its sub- 

 stance may be used in the building up of a second generation of 

 aster-fibres. This is equivalent to saying that the cytoplasmic reticulum, 

 the aster-fibres and the centrospheres are merely different mani- 

 festations of one and the same organic structure 



The "primary" centrospheres become less and less evident as the 

 secondary asters continue to grow. This is due to two distinct causes : 

 first, to a gradual obliteration of the boundary between centrosphere 

 and cytoplasm and, second, and more especially, to the growth of the 

 "secondary" aster-fibres throughout the whole body of the centro- 

 sphere (Fig. 40). By this process the "primary" centrosphere very 

 gradually vanishes, and its place is taken by a pair of vigorous young 

 asters which rapidly develop into the cleavage-asters of the second 

 cell-division. Meanwhile the division of the egg into two cells has 

 been completed, and the chromosomal vesicles have fused to form a 

 common nucleus with nuclear reticulum and nucleoli, as ordinarily 

 described. 



As stated above, the plane of cleavage passes nearly through the 

 region where the polar-bodies were formed. In no other direction 

 could the egg be divided into two exactly equal cells ; for, as we have 

 seen, the protoplasm of the cell-body has been gradually collecting 

 itself about the region of the polar bodies, while the yolk-globules 

 have become more closely packed together towards the opposite pole. 

 I cannot determine that the place of entrance of spermatozoon exerts 

 any influence whatever on the plane of cleavage. This is obviously 

 true of eggs which are not fertilized until after the formation of the 

 first polar spindle. Rückert, on the other hand, finds in Cyclops 

 that "die Richtung der ersten Furchungstheilung schon von vornherein 

 durch die Stellung der Spermatocentren am Sperraakern gegeben sei, 

 dass mit andern Worten der Spermakern die Richtung der ersten 

 Furchungstheilung bestimmt, wärend der Eikern sich dieser Richtung 

 accommodirt, gleich viel, an welcher Stelle der Eioberfläche er seine 

 Entstehung aus der zweiten Richtungsspindel genommen hat". 



