440 WESLEY R. COE, 



equatorial plate of the spindle (Fig. 8). It not rarely happens that 

 the asters are so widely separated that the spindle is formed directly 

 through the body of the germinal vesicle leaving its membrane intact 

 both above and below (Fig. 6). Kostanecki figures a similar con- 

 dition in Myzostoma. At this stage the centrospheres are very distinct, 

 and stain a very bright red in the Bordeaux red-iron-haematoxylin 

 preparations. The nuclear membrane, after having been broken through 

 in the regions of the asters, becomes much fainter and soon disappears 

 entirely (Fig. 7). This dissolution of the nuclear membrane must 

 depend upon the action of the protoplasm itself, and not upon the 

 influence of the centrosomes or asters, for the greater portion mem- 

 brane is far removed from any of the aster fibres. 



The First Polar Body. 



When fully formed, the maturation spindle usually lies at right 

 angles to a radius of the egg, with both asters at about equal distances 

 from the cell-periphery. The whole spindle must now rotate through 

 about 90", so that one of its poles shall come to lie on the surface 

 of the egg. In doing this the spindle contracts greatly in size, so 

 that after reaching the surface it is no more than three-fourths as 

 large as formerly (Fig. 9). This seems to be a feature common to 

 many eggs. It is figured by Korschelt for Ophryotrocha (13), by 

 Kostanecki & Wierzejski for Fhysa (15) and by others. The cen- 

 trosomes usually divide even before the spindle has completely formed 

 (Fig, 7). Sometimes, however, the division is deferred in one or both 

 of the asters until the spindle has reached its definite position. The 

 division ordinarily takes place at right angles to the axis of the 

 spindle, but where the division of the centrosome of the inner pole 

 of the spindle does not occur until the spindle is in position, this division 

 is apt to take place along the axis of the spindle. 



During the rotation of the spindle the egg has acquired a more 

 distinct polarity by the gradual transferrence of the yolk-globules 

 towards the opposite pole, while a constantly increasing amount of 

 protoplasm free from yolk has collected about the spindle. 



In an earlier stage not the slightest difference could be detected 

 in the two asters, but at the rotation of the spindle, the aster which 

 passes towards the periphery continually diminishes in size by a 

 gradual decrease in the size, number and length of its radiations, 

 while the one that is to remain in the egg becomes somewhat larger 

 than before. 



