40 BASIS OF THE PEOBLEM 



forms the essential feature of sexual reproduction is the same ; 

 it consists, that is to say, in the fusion of two cells. It no longer 

 consists in the fusion of two individuals, but in the fusion of two 

 cells, one given off by one individual, and one by the other. 



The manner in which these cells, known as gametes, are given 

 off by the male and the female and the manner in which they 

 fuse require some further explanation. What happens in plants 

 is in all important features similar to that which happens among 

 animals, and we may confine our attention to the latter. In 

 every normal member of every species there is a generative 

 orgau.^ It consists of a surrounding wall within which is a mass 

 of developing germ-cells. In the female the fully developed 

 gamete, called the egg or ovum, is, relative to the male gamete, of 

 large size. It is typically a motionless cell containing a varying 

 quantity of food substance which may, as in the case of the 

 bird's egg, reach a large amount. The male gamete is a much 

 smaller cell. With a few exceptions it consists of a small oval- 

 shaped head, to which is attached, by means of an intervening 

 neck or middle-piece, a long vibratile tail. The head is the 

 nucleus of the cell ; if there is any cytoplasm, it is reduced to 

 a very small amount and its presence cannot be readily demon- 

 strated.^ In strong contrast to the ovum the spermatozoon is 

 typically motile and can swim in those fluids in which fertihzation 

 usually takes place. 



Fertihzation consists in the penetration of an ovum by a 

 spermatozoon. The tail does not always enter the egg; as soon 

 as the head has entered, a change takes place in the egg which 

 sometimes prevents the tail, and usually other spermatozoa, from 

 entering the egg. The head or nucleus of the spermatozoon 

 approaches the nucleus of the egg and fuses with it. At this 

 point, therefore, we have a single cell, known as the zygote, formed 

 by the fusion of two cells, one derived from the male and one 



1 Among many plants and not infrequently among lower animals the sexes 

 are not separate — both male and female generative organs being found on the 

 same individual. This condition occurs in some fish, though examples of herma- 

 phroditism are rare in the Vertebrates. As a general rule in such cases the two 

 generative organs found in the same individual ripen at different times and cross- 

 fertilization is effected. In such cases the description given above applies. Where 

 self-fertilization occurs, the above account does not apply in detail ; the occurrence 

 of self-fertilization, however, in no way invalidates the general conclusions that 

 will be drawn in what follows. 



' ' The nucleus [of the spermatozoon] is probably enclosed within sheaths of 

 cytoplasm, although this is not usually readily visible ' (Doncaster, Cytology, p. 92). 



