No. 2.] ORGANIZATION OF THE EGG OF UNIO. 25 1 



certainly no prejudice against Boveri's theory. The reasons 

 why it is impossible to take this view of the origin of the 

 cleavage centers in Uiiio are: i. The sperm-amphiaster dis- 

 appears utterly before the metaphase of the first maturation 

 spindle, and the sperm-centrosomes become indistinguishable 

 from cytomicrosomes. 2. There is no evidence of the persist- 

 ence of the sperm-centrosomes during the maturation as any- 

 thing more than cytomicrosomes, unless we identify with them 

 the centrosomes of the accessory aster, which, however, contains 

 at first only a single centrosome. 3. The cleavage centers 

 always arise in a definite position, viz., on each side of the 

 plane separating the two germ-nuclei, one in contact with each 

 nucleus. 



If the cleavage centrosomes really are the persistent sperm- 

 centers, it becomes necessary to assume: i. That the sperm- 

 centrosomes, which are no larger than cytomicrosomes, maintain 

 a persistent identity among these, though the chief distinguish- 

 ing mark of a centrosome is its position in the center of an 

 aster or centroplasm. 2. That one of the sperm-centrosomes 

 becomes active during the second maturation division (acces- 

 sory amphiaster) and divides again, the resulting amphiaster 

 then vanishing. 3. That two of the three sperm-centrosomes 

 migrate to the perfectly definite position, in which the cleavage 

 centers arise, and there again resume activity. If any theory 

 of fertilization, otherwise tenable, depended on such assump- 

 tions, I should feel justified in making them. But it seems to 

 me that Boveri's justly celebrated theory must be given up, in 

 large part at least, for other reasons to be spoken of below. 



As already mentioned, disappearance of the sperm-centro- 

 somes has been observed by a number of authors (pp. 233,234). 

 Of these authors, viz.. Child, Van Name, Foot, MacFarland, 

 and Klinckowstrom, only MacFarland positively maintains that 

 the cleavage centers arise from persistent sperm-centers, and 

 his argument appears to me most inadequate. The only reason 

 that he gives for his positive identification is, that the cleavage 

 centers may arise in almost any position relative to one another 

 and the germ-nuclei, though always well above the vegetative 

 pole ; and the sperm-centrosomes exhibit a similar relationship, 



