GAMETOGENESIS AND FERTILISATION IN NEMATUS EJBESU. 103 



egg-nucleus itself. It is never possible^ therefore, to say 

 with certainty that a given egg is fertilised or not. 



Bat after much time spent in vainly trying to follow the 

 entrance of the spermatozoon and its conversion into the 

 male pronucleus^ I at last was able to observe the conjugation 

 of the sperm-nucleus with that of the egg, and so to prove 

 that true fertilisation does take place (fig. 1). It occurs 

 immediately after the maturation divisions ; the three polar 

 nuclei lie near the edge of the egg (two of them in the same 

 section as the egg and sperm nuclei), and the fusion of the 

 two inner polar nuclei has not yet taken place. The male 

 and female pronuclei are in contact, the male being distinctly 

 smaller than the female, but in another egg in which the 

 same stage is seen the two are of about equal size. The 

 subsequent stages of the coujugatiou and division of the 

 zygote nucleus have not been observed, but the section 

 represented in fig. 1 leaves no reasonable doubt that normal 

 conjugation takes place. It therefore became necessary to 

 reconsider my previous conclusions with regard to the number 

 of chromosomes, since never more than eight have been 

 found in either fertilised or virgin eggs. I was thus led to 

 work out the spermatogenesis, and to the fresh work on the 

 maturation divisions to be described later. 



In my previous paper I mentioned that the behaviour of 

 the polar nuclei appeared to be slightly different in fertilised 

 and in virgin eggs, and subsequent work has confirmed this. 



In the virgin egg of N. ribesii the two inner polar nuclei 

 fuse and give rise to a group of chromosomes, which is 

 generally clearly double, with eight in each half. The two 

 halves of the group do not lie far apart, and commonly 

 remain without much change for some time. But in the 

 majority of eggs from impregnated females the chromosome 

 groups derived from the two inner polar nuclei lie completely 

 and sometimes widely separated, as if the conjugation 

 between the nuclei had been much less complete than in 

 virgin eggs (figs. 2, 3, and 4). Further, in virgin eggs the 

 polar chromosomes usually do not divide, at least for some 



