42 SEXUAL REPRODUCTION. 
primitive germ cell, to form the sperm-mother cell, which 
then divides rapidly by two divisions. ‘The mature sperma- 
tozoon is usually an active organism with a head-portion 
derived from the nucleus and a tail formed from the 
protoplasm of the cell. The nucleus itself is often termed 
the made pronucleus and is evidently one half of the original 
nucleus of the primitive germ cell. In the case of the 
female, the primitive germ cell grows to twice the bulk, to 
form the egg-mother cell, and then divides into two, but they 
are of very unequal size. The lesser is called the first polar 
dédy and consists mainly of half the nucleus of the egg- 
mother cell. Another division of the same kind produces a 
second polar body consisting mainly of one half of the 
original nucleus. These two polar bodies are seen for 
some time resting on the exterior of the remaining portion, 
which is known as the mature ovum or female element, its 
nucleus being the female pronucleus. Eventually the polar 
bodies atrophy. 
The phenomenon of maturation consists in each case of 
the production of the pronucleus, which is a half of the primi- 
tive germ cell nucleus, but in the male the protoplasm is 
also equally divided to form the tails of the male elements, 
whereas in the female practically all the protoplasm is 
aggregated to one of the half nuclei, and the others atrophy. 
The explanation of this curious process will te easier 
after we have taken a review of the following processes— 
2, FERTILISATION.—The essential part of fertilisation 
is the fusion of the male and female elements. The 
spermatozoon embeds itself within the substance of the 
ovum, the tail is absorbed, and the “head” or male pronucleus 
fuses with the female pronucleus to form what is called the 
segmentation-nucleus of the fertilised egg. 
We may note that the one half of the segmentation 
nucleus consists of male and the other half of female nuclear 
material. The life of the new individual dates from the 
formation of the fertilised ovum.* 
* If we suppose that the fertilised ovum is an individual produced 
by sexual reproduction, and that this by asexual reproduction gives rise 
to the fresh individual, the adult metazoan, then there is a complete 
alternation of generations in all metazoa, the sexual individual being 
always a single cell. 
