No. I.] POLYCHOERUS CAUDATUS. 95 



pronuclei, which they penetrate in a manner very different 

 from that in which the aster rays of the first maturation 

 centrosphere attach the egg nucleus, as will be explained 

 presently. 



Formation of tlie First Cleavage Spindle. 



The formation of this amphiaster is from its very inception so 

 different from that of the first maturation spindle that the two 

 can never be mistaken the one for the other. When in the 

 first maturation spindle the two centrospheres begin to act on 

 the nucleus they are at the opposite poles of the nucleus, while 

 in the first segmentation spindle the centrospheres lie between 

 the pronuclei (Fig. 15). I have never seen a complete union 

 of the pronuclei until both have been deeply penetrated by the 

 rays from the centrospheres. The relation of the rays to the 

 chromatin granules is different in the two cases, as a comparison 

 of Figs. 13 and 35 will show. In the former the rays which 

 emanate from the centrosphere do not reach to the surface of 

 the nucleus, while in the latter they pierce deeply into the 

 chromatic substance, the granules of which seem attracted by 

 the rays. In the first maturation spindle the centrospheres 

 never draw widely apart so as to form a very large structure, 

 while in the segmentation spindle the whole structure grows very 

 rapidly and soon occupies the whole interior of the ovum. In 

 the former the clumps and rods of chromatin are formed before 

 the spindle breaks away from the nucleus, while in the latter 

 these rods and clumps are first formed when the spindle has 

 almost attained its full size (Figs. 36 and 37). It is evident, 

 however, in both cases that the bulk of the chromatin con- 

 tained in these clumps far exceeds the amount which at a 

 later stage is found in the chromosomes. It is clear, there- 

 fore, that either a condensation of particles takes place or else 

 that some of the material in the clumps is removed. That 

 this latter is the case is very clearly shown in a later stage 

 before the chromosomes are completely formed. But before 

 the description of this is given it will be well to account for 

 that portion of the pronuclei not drawn into the clumps. 



