The maturation and fertilization of the egg of Cerebratulus. 465 



spindle than normally. The aster-fibres have been collected into a 

 close bundle as would actually be the case either if the spindle had 

 been revolved mechanically, or if the spindle had been fixed and the 

 peripheral portions of the cell-substance had revolved. The centro- 

 somes of each pair remain near together, and are as usual imbedded 

 in a delicate layer of protoplasm. They are not at the foci of the 

 aster-fibres ; no more fibres radiate from the region where the centro- 

 somes have been mechanically (?) placed than elsewhere in the bundle. 

 The centrospheres, being of nearly the same nature as the ordinary 

 cytoplasm, have mixed with the cell-substance and disappeared ; while 

 the centrosomes, having a constitution peculiar to themselves , have 

 retained their identity. 



The foregoing account shows that the general course of maturation 

 and fertilization in the egg of Cerebratulus marginatus is closely 

 similar to that of Echinoderms, Annelids, MoUusks etc. Its most 

 noteworthy features may be briefly summarized as follows: 



By the eccentric position of the germinal vesicle, on which the 

 location of the polar spindles is dependent, the main axis of the 

 egg is roughly predetermined while the nuclear membrane is still 

 intact. 



The asters of the first polar spindle are first seen when widely 

 apart and situated in two slight indentations of the nuclear membrane. 



A minute portion only of the chromatic network of the germinal 

 vesicle enters into the formation of the sixteen ring-shaped chromo- 

 somes found in the first polar spindle; the rest is absorbed into the 

 cytoplasm. 



The first polar spindle acquires its definite position, its chromo- 

 somes divide, and a slight depression appears on the surface of the 

 egg near its outer pole whether the egg be fertihzed or not. 



The aster-fibres at the poles of the second polar spindle are 

 formed in part de novo, and in part are identical with those at the 

 inner pole of the first polar spindle which have been transferred to 

 the new centres. 



The centrosomes (or, sometimes, pair of centrosomes) remaining 

 in the egg after the formation of the polar bodies become lost among 

 the chromosomal vesicles which fuse to form the egg-nucleus, and are 



