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 odcyte 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. Asarule two polar bodies are produced, 
but in certain cases of parthenogenesis (rotifers, 
Cuiapocera, Ostracopa, and aphids) only one is 
formed. Rarely the first polar body divides into two. 
8. Frertinization. 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 spermatozoén and 
the resulting processes of rearrangement of the egg 
