No. 2.] ORGANIZATION OF THE EGG OF UNIO. 233 



the subsequent history of centrosomes in this o.^^ gives no 

 support to such an hypothesis. 



b. Literature. — There are so many observations of the 

 undoubted origin of the sperm-aster around the middle-piece 

 of the spermatozoon that there is a strong presumption in 

 favor of assuming such an origin even when it cannot be 

 directly observed (Wilson and Mathews, '95; Fick, '93; Foot, 

 '98; Van Der Stricht, '98, and others). But the time at which 

 this aster appears varies greatly in different cases. Foot says 

 ('98, p. 51) : "In studying the literature I have not been able 

 to find any satisfactory evidence of the appearance of a sperm 

 attraction sphere earlier than the anaphase of the first matu- 

 ration spindle." It certainly seems a general rule that the 

 radiations do not appear until about this time, however early 

 the spermatozoon may enter. The case of Unio is a striking 

 exception to this rule, for here the sperm-aster is formed, and 

 divides and vanishes before this time. In Arenicola (Child, '98) 

 the sperm-aster also appears early, just after the first matura- 

 tion spindle has taken a radial position, but division follows 

 much later. 



Disappearance of the sperm-asters and centrosomes has 

 been observed in a number of cases, though the process is not 

 usually described in detail. On account of the importance of 

 this phenomenon I will give rather full citations : 



Van Name ('99) states that in Planocera he has not been 

 able to learn anything of the subsequent fate of the sperm- 

 centrosomes, "nor to demonstrate any connection between 

 them and the cleavage centrosomes." From this I conclude 

 that they disappear before the first cleavage spindle is formed. 



In Arenicola (Child, '97) the sperm-amphiaster is well devel- 

 oped during the last phases of formation of the second polar 

 body. After this is formed "the two centrosomes have dis- 

 appeared, the polar regions of the spindle being occupied by a 

 fine network. The rays are evidently disintegrating, and the 

 spindle is barely indicated in the cytoplasm. These structures 

 present this appearance with regularity at this stage. It is 

 certainly not due to imperfect fixation or errors in technique, 

 for on the same slide are eggs in somewhat earlier stages, with 



