446 WESLEY R. COE, 



sphere the fibres become much finer, and gradually disappear entirely 

 before reaching the centrosome. This is better seen in the cleavage 

 asters. The zone which lies between the protoplasm about the centro- 

 sphere and the ordinary yolk-globules peripherally is filled with yolk- 

 globules of smaller size. This agrees perfectly with the observations 

 of RüCKERT (24) on Cyclops and with those of Kostanecki & 

 WiERZEjsKi on Physa, and with others. 



As the sperm-aster increases in size the minute centrosome en- 

 larges somewhat, and then divides (Fig. 43). The two daughter 

 centrosomes move to opposite ends of the elongated centrosphere with 

 the formation of a delicate central spindle (Fig. 44). The centro- 

 somes almost always separate in a line at right angles to the path 

 of the sperm-nucleus. The centrosphere disappears, and the central 

 spindle between the centrosomes becomes very distinct and beautiful 

 (Fig. 45), much as MacFarland has described (18) in the second 

 polar spindle of Pleurophyllidia. The behavior of the structures about 

 the spindle, however, are quite different, for a large number of the 

 aster-fibres, instead of degenerating as they do about the polar spindle 

 in Pleurophyllidia, in all cases pass directly into the daughter-asters. 

 By the further elongation of the central spindle the fibres near its 

 extremities increase rapidly in size while those nearer its central 

 portions become far removed from either of the two centrosomes and 

 consequently degenerate. By the continuation of this process two 

 asters arise in place of the original one. Fibres grow out obliquely 

 towards opposite asters and form a characteristic decussation above 

 and below. This is shown in Fig. 45, where the old radiations may 

 still be seen passing out a right angles to the body of the spindle. 

 This spindle may remain intact until the sperm-nucleus is well ad- 

 vanced towards the center of the egg, but it usually begins to dege- 

 nerate soon after it has reached its full development. The asters 

 apparently separate more rapidly than the central spindle can elongate 

 so that it seems to be torn assunder, and its fibres converted into 

 aster-fibres (Fig. 46). As the asters separate more widely they 

 become entirely independent and the central-spindle disappears (Fig. 47). 



Now the two sperm-asters may assume almost any position in 

 relation to the sperm-nucleus. Most commonly they remain in the 

 immediate vicinity of the nucleus and precede it, one on either side, 

 as shown in Fig. 12, in its migration towards the egg-nucleus. At 

 other times the sperm-nucleus lies near one of the asters only (Fig. 46), 

 or the asters may leave the nucleus far behind (Fig. 11). In still 



