224 



SCIENCE. 



[N. S. Vol. VII. No. 164. 



Upon deposition, a series of changes is 

 inaugurated leading to the formation, en- 

 tirely from the germinal vesicle, of a barrel- 

 shaped maturation spindle. This spindle is 

 devoid, as far as can be ascertained, of centro- 

 somes, asters, centrospheres, etc., at both ends. 

 The spindle moves as a whole to the periph- 

 ery, the sixteen chromosomes divide, the 

 daughter chromosomes diverge to the head 

 of the barrel, and the first polar body is ex- 

 truded. The spindle fibers withdraw from 

 the chromosomes and condense at the middle 

 of the extent, forming a dense Zwischenkor- 

 per. A second maturation spindle is formed, 

 a counterpart of the first, except that eight 

 chromosomes pass into the second polar 

 body, while eight remain in the egg. A 

 vesicular nucleus is formed by these latter. 

 The polar bodies arise at the area destined 

 to be the vegetative pole. 



The sperm enters at or near the future 

 animal pole. The sperm-head is preceded 

 by a double centrosome, surrounded by a 

 distinct aster. The centrosomes diverge, 

 as they progress inwards, each surrounded 

 by an aster, but without any fibers passing 

 between them comparable to the ' central 

 spindle ' of the annelid, mollusk and other 

 types. After the asters have taken up their 

 positions for the future cleavage-figure the 

 now vesicular sperm-nucleus and the egg- 

 nucleus take up their positions side by side 

 midway between the asters. 



A barrel-sJiaped spindle, precisely similar 

 to that of the maturation stages, is 

 formed entirely from the segmentation nucleus. 

 The presence of an aster and a dou- 

 ble centrosome at either end of the figure 

 gives the appearance of a continuous 

 spindle passing from center to center. 

 Such, however, is not the case. After di- 

 vision of the chromosomes the daughter 

 products diverge only to the heads of the 

 barrel, not one-half the distance to the cen- 

 trosome. There they become vesicular and 

 ultimately fuse, while the spindle-fibers 



withdraw from them to form a ' Zwischen' 

 korper,' as in the maturation stages. Only 

 then does the cell divide. And only after 

 the formation of the vesicular daughter- 

 nucleus do the two centrosomes in each cell 

 move apart. When they do, the daughter- 

 nucleus moves up between them, and the 

 series is repeated. A compai'ative inde- 

 pendence, then, of the processes undergone 

 by the nucleus on the one hand, and the 

 centrosomes and asters on the other, is in- 

 dicated. 



The Asters in Fertilization and Cleavage. E. 



G. CONKLIN. 



In Crejndula and several other genera of 

 marine gasteropods there is a well-marked 

 centrosome and sphere in both polar spin- 

 dles. In the metaphase this centrosome is 

 a single densely-staining body ; in the ana- 

 phase it greatly enlarges, and the center of 

 the body does not stain ; in the telephase it 

 becomes a large sphere with an extremely 

 thin surface layer, containing a large num- 

 ber of coarse granules. During the meta- 

 morphosis the centrosome has changed its 

 staining reactions ; in the prophase and 

 metaphase it takes only nuclear stains ; in 

 the telephase it takes only plasma stains, 

 while in the anaphase it takes both. 



Though the spermatozoon frequently en- 

 ters before the first polar body is formed no 

 sperm aster appears until the metaphase of 

 the second polar spindle. This aster is 

 large and conspicuous, though not as large 

 as the aster of the second polar spindle 

 which remains in the egg ; it frequently 

 contains several dark-staining granules. 

 At the same time one or more accessory as- 

 ters appear in the egg ; these are much 

 smaller than either the egg or sperm aster, 

 and no centrosome could be found in 

 them. The sperm and egg asters become 

 very large and haA'e the same structure and 

 staining reactions, the radiatio nsfrom 

 them proceeding for some distance through 



