The maturation and fertilizatiou of the egg of Cerebratulus. 467 



which case the centrosomes are found in exactly the same position 

 which they occupied at an earlier stage. 



In nearly all oases, however, the cleavage - centrosonies appear 

 only after the complete fusion of the germ-nuclei. They are then 

 seen at the ends of the elongated cleavage - nucleus , and may attain 

 a considerable size before the total disappearance of the old radi- 

 ations. 



The cleavage-asters have absolutely no relation with the sperm- 

 asters, except it be in the possession of the same centrosomes. 



The centrospheres of the cleavage asters are at first homogeneous. 

 They later increase enormously in size, and acquire a distinct vesicular 

 structure as in the Echinoderms. 



The centrosomes are always very small, though sharply defined 

 bodies and vary but little in size how ever large the centrospheres 

 may become. 



In the early anaphase of the cleavage - spindle the centrosomes 

 move apart a*- right angles to the axis of the spindle, and then wander 

 into that portion of the large reticulated centrosphere which lies 

 farthest from the spindle. 



About each centrosome an exceedingly delicate aster is formed 

 out of the reticulum of the centrosphere, much as Griffin has de- 

 scribed for Thalassema. These delicate little asters separate more 

 widely, and their fibres reach out into the surrounding cytoplasm. 

 The large centrospheres are gradually lost by the growth of the new 

 asters and the obliteration of their boundaries. 



The fibres of the cleavage-aster at this time extend throughout 

 the whole egg, and are much branched peripherally. 



A large proportion of the protoplasm of the cell has become 

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

 have been gradually packed more closely together towards the op- 

 posite pole. 



The plane of cleavage passes through the region where the polar 

 bodies were formed. It seems to be determined by the position of 

 the eccentrically-placed germinal vesicle, and the location of the 

 polar spindles rather than by the place of entrance of the spermato- 

 zoon. 



The egg divides into two cells, and the cleavage-asters begin to 

 degenerate in a manner similar to that of the sperm-asters. 



Meanwhile the new asters, which are to form the asters of the 

 second cleavage, have developed to such an extent that the ends of 



