24 
THE ORGANIC CELL 
there is a period of rejuvenescence, in which the functions of 
cell- division and growth are fully restored. 
In parthenogenesis, however, the egg develops without 
fertilisation, and from this fact it is extremely difficult to 
decide whether a tendency to senile decay, and the necessity 
for fertilisation, are necessary properties of living matter. 
The other teaching may be termed the Variation Theory. 
According to this view fertilisation is necessary to the pro- 
duction of variations, on which the process of natural selection 
can operate. 
Both theories are in unison with the work of practical 
breeders, which shows that crossing results in greater vigour 
and variability. 
As the time of fertilisation approaches, the nucleus of each 
cell contains its full supply of chromosomes ; it is therefore 
evident that if some of these bodies are not got rid of the 
number of chromosomes would be doubled during each genera- 
tion. As a matter of fact, however, it has been proved beyond 
a doubt that during the period of maturation there is a 
reduction of the number of chromosomes to one half. It is 
also certain that this reduction of chromosomes in the male 
and female germ-cells is a process preparatory to their sub- 
sequent union. Thus, when the male and female cells unite, 
the normal number of chromosomes for the species is restored. 
It will now be necessary to examine very briefly this complicated 
process. 
Reduction in the Female. 
Each primordial germ-cell, by the usual mitotic type of 
division, gives rise to a number of cells called oogonia. These 
divide for a certain number of times, and then cease. Each 
develops into an ovarian ovum, the nucleus increasing very 
considerably in size to form the Germinal Vesicle, the cytoplasm 
becoming loaded with food material. The egg-cell remains in 
this state until the time of fertilisation approaches, when 
the process of chromatin reduction occurs. Two minute cells 
develop near the upper pole of the ovum, and as a rule one of 
these further divides into two. Thus a group of four cells 
arises, the mature egg and three small cells which are called 
polar bodies ; the polar bodies take no further part in develop- 
