28 



BIOLOGICAL LECTURES. 



draw the nuclei from one place to another in the cell, as well 

 as in the pulling asunder of the chromatic elements of the 

 nucleus, the expansion in the division of the asters and the 

 subsequent divergence of the resulting daughter asters, as well 

 as in the formation of the karyokinetic spindle and the pushing 

 together of the chromatic elements into the equatorial plate. 



After the asters have reached the outer sides of the two pro- 

 nuclei, each divides into two half-asters, which diverge from 

 each other until they come to lie at opposite poles of the 

 nuclei and in the plane of contact between the two. At this 

 stage, therefore, there is found at each pole of the two pro- 

 nuclei one-half of the sperm aster and one-half of the egg aster, 

 Fig. 8. Each of these couples soon fuses into a single aster, 

 and the two asters thus formed lie at opposite poles of the 

 first cleavage-spindle-, Fig. 10, and from them all the other 

 asters of the developing ovum are derived. 



As an interesting variation of this more usual behavior of 

 the asters, it should be mentioned that in cases where the 

 sperm nucleus and aster meet the corresponding parts of the 



egg cell, while there is yet 

 considerable disparity in size 

 between the two, the egg 

 aster after having reached its 

 full size divides, and the two 

 half-asters pass to the two 

 poles of the nuclei, while the 

 sperm aster, during all this 

 time, remains undivided until 

 it has reached its full size, 

 when it divides, and its halves 

 move around to meet the 

 waiting halves of the egg 

 aster, Fig. 9. 



With the fusion of the half- 

 asters, one part of the fecunda- 

 tion, and that a very important one, is completed, although the 

 two pronuclei may still be recognized as such. In fact, the 

 boundary line between the egg and the sperm nucleus can be 



FIG. 10. Formation of the first 

 cleavage-spindle. The half-asters have 

 fused, but a trace of the nuclear mem- 

 brane remains between the two pro- 

 nuclei. 



