The maturation and fertilization of the egg of Cerebratulus. 4:S1 



in size, and move towards the center of the egg in company with the 

 sperm-nucleus. 



After the sperm-nucleus has reached a point near the centre, 

 the egg-nucleus, which has remained nearly passive up to this time, 

 likewise begins to move towards the centre of the egg or, more 

 accurately, towards the sperm-nucleus. As the two germ-nuclei come 

 in contact the sperm-asters separate to give them an opportunity to 

 fuse completely. Both nuclei have increased greatly in size, and, as 

 has been described for other eggs, both send out slight amoeboid 

 processes from time to time. At the moment of union a large pro- 

 cess is often sent out from each germ-nucleus, and the fusion of 

 these is quickly followed by the fusion of the whole nuclei. It sometimes 

 happens, as is shown in Fig. 50, that the pro-nuclei do not actually 

 fuse, but merely lie close together without the absorption of the 

 nuclear membrane between them. 



When complete fusion occurs the process occupies some six or 

 eight minutes after the nuclei lie side by side. It takes place about 

 55 minutes after the fertilization of the egg. 



The behavior of the asters is the most noteworthy feature of the 

 whole process of fertilization. At about 38 minutes after fertilization 

 the germ-nuclei, as described above, lie near together with a pair of 

 well developed asters (derived from the spermatozoon) lying between 

 them in such a position that a line joining the asters would lie at 

 right angles to a line joining the germ-nuclei. These asters increase 

 still further in size until their radiations extend through a large part 

 of the egg. As the germ-nuclei are about to fuse, however, these 

 radiations suddenly begin to fade away , and by the time the 

 fusion of the nuclei is completed, scarcely a trace of 

 any aster is to be seen anywhere in the egg. The fusion 

 of the nuclei usually takes place at a point between the polar bodies 

 and the center of the egg, so that the egg-nucleus is required to 

 move a very short distance only. After their fusion the cleavage- 

 nucleus sometimes moves nearer the centre of the egg. This cleavage- 

 nucleus, which usually has a perfectly smooth outline, elongates 

 slightly in a direction at right angles to the axes of the former polar 

 spindles, and in 5 to 8 minutes after the union of the germ-nuclei, 

 or about one hour after the fertilization of the egg, a pair of delicate 

 asters appear at its elongated extremities. The membrane of the 

 cleavage-nucleus rapidly disappears with a corresponding increase in 

 the size of the asters. In those cases in which the germ-nuclei do 



