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 male 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 

 body 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 be 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. 



