THE FERTILIZATION-PROCESS 



appears in front of the sperm-head (now designated the 

 sperm-nucleus). This minute granule arises from within 

 the sperm-nucleus^ (Fig. 26). As the sperm-nucleus 

 advances toward the centre of the egg, the aster grows in 

 size and it and the centrosome or centriole then divide; 

 parallel with the growth of the aster the sperm-nucleus 



t''-:M^^^J^<i'^':K ^ -I'^^P^-fc:^ 







'■.■»:: ■■•f\—.: ls',«' ■- 



•^^■.■•.a'.:-.V-> ~' * ■'-"■ 





a b 



Fig. 26. — Sperm-nucleus within the egg of Nereis (after Lillie); a, with cone 

 attached and h, separated from cone. 



enlarges, losing its staining power. In the meantime, the 

 entrance-cone has disappeared. 



The egg-nucleus during the progress of these events has 

 undergone changes. First, as germinal vesicle, it breaks 

 down. The first maturation spindle forms near the centre 

 of the egg and moves to the animal pole where the first 

 polar body and, after the formation of the second matura- 

 tion spindle, also the second is given off. The chromosomes 

 remaining in the egg constitute the egg-nucleus. The union 

 of egg- and sperm-nucleus follows and thus arises the first 



^ Just, 1933b. But cf. Lillie, igi2. 



163 



