ACCOUNT OF THE GERM-CELL CYCLE 47 



are formed or not, each spermatid containing the 

 haploid number of chromosomes. 



The maturation of the egg differs in no very im- 

 portant respects from the process as it has been 

 described in the male cells. Tetrads may or may 

 not be formed according to the species, and the 

 mature egg and polar bodies each contain the haploid 

 number of chromosomes. Two phases of the matura- 

 tion of the egg may be referred to here : (1) when 

 the nucleus of the primary oocyte prepares for divi- 

 sion a considerable amount of chromatin separates 

 from the chromosomes and is lost in the cytoplasm. 

 The size of the chromosomes is thus diminished, but 

 no entire chromosomes are lost. (2) The cellular 

 divisions are very unequal, the polar bodies being 

 very small as compared with the rest of the egg. 

 The chromatin content of the polar bodies, however, 

 is equal to that of the much larger egg. In the male 

 all of the four spermatids are functional, but in the 

 female only the egg survives, the polar bodies de- 

 generating. As a rule two polar bodies are produced, 

 but in certain cases of parthenogenesis (rotifers, 

 CLADOCERA, OSTRACODA, and aphids) only one is 

 formed. Rarely the first polar body divides into two. 



8. FERTILIZATION. Eggs that develop partheno- 

 genetically are ready to begin a new germ-cell cycle 

 as soon as they become mature ; but the eggs of 

 the majority of species must be fertilized before 

 they are able to develop. Fertilization may be de- 

 fined as the fusion of an egg with a spermatozoon and 

 the resulting processes of rearrangement of the egg 



