No. 2.] ORGANIZATION OF THE EGG OF UNIO. 229 



have found unfertilized eggs with the first maturation spindle 

 fully formed. Moreover, I have never found an ^g^, into 

 which the spermatozoon had entered, in which, at the same 

 time, the first maturation spindle was not forming. 



It seems probable, from some observations I have made on 

 the breeding habits of Unio, that the presence of spermatozoa 

 in the water may act as a stimulus to the female, causing the 

 extrusion of eggs. 



The general course of events after the entrance of the 

 spermatozoon is this : a sperm-amphiaster is formed very soon 

 after the penetration of the sperm-head, which moves up 

 towards a definite zone above the equator of the Qgg and there 

 comes to rest. The sperm-amphiaster gradually disappears, 

 and the sperm-nucleus, which has enlarged considerably during 

 its penetration, contracts in volume nearly to its original bulk. 

 By the time that this is accomplished the first maturation 

 spindle is entering on its metaphase. The sperm-nucleus 

 undergoes no further changes until after the completion of the 

 second maturation division, at which time it grows in size 

 synchronously with the egg-nucleus and begins to describe its 

 copulation path. The copulation path of the germ-nuclei is 

 by no means simple, but they ultimately come into contact 

 at or near the center of the Qgg. The first cleavage spindle is 

 then formed, and moves axially to one end of the Q.gg ; thus 

 the first cleavage is very unequal. 



I. The Behavior of the Spermatozoon in the Egg up 



TO the Time of Extrusion of the Second 



Polar Globule. 



I. The Spcrm-Nucleics. 



The spermatozoon enters through the micropyle shortly 

 after the breaking down of the germinal vesicle. In mounts 

 of entire eggs at this stage one can see a number of sperma- 

 tozoa clustered around the region of the micropyle, and may 

 even see one within it. But it very rarely happens that more 

 than one enters and fuses with the tgg, for I have not seen 



