DETERMINATION OF SEX 193 



One male is capable of fertilising a large number of females, 

 and so the number of individuals produced in the next 

 generation would be very large indeed. On the other hand, 

 if the food supply were scanty and a large number of males 

 were produced, the fertilisation of the comparatively small 

 number of females would be absolutely ensured without 

 throwing too great a strain on the already scanty food 

 supply. Thus the environment would play an important 

 part in regulating the number of individuals in succeeding 

 generations, and is readily explicable as the result of natural 

 selection. 



In some cases the sex of the individual which is to be 

 produced from the fertilised ovum may possibly remain 

 undetermined for some time after development has com- 

 menced. In many cases, on the other hand, it appears to 

 be determined from the very beginning. In some organisms 

 we find that the eggs destined to produce females are 

 different, and can be recognised before segmentation has 

 commenced. 1 In other cases, as among bees and ants, the 

 unfertilised eggs produce one sex, the fertilised eggs the other. 



A new theory with regard to the determination of sex 

 has been recently advocated. In order to grasp the full 

 meaning of this theory it is necessary to glance back at 

 the sequence of events occurring in the production of the 

 gametes, and upon fertilisation. 



The chromosomes are distributed to the gametes in such 

 a manner that each contains only half the number of indi- 

 vidual chromosomes contained in the somatic or body cells. 

 In most organisms that have been specially investigated, 

 the number of somatic chromosomes has been found to be 

 an even number. The gametes, therefore, contain exactly 

 half the number contained in the somatic cells. When 

 fertilisation takes place, as each gamete contains half the 

 somatic number, the full number of chromosomes is restored 

 in the fertilised ovum. 



1 e.g. in Phylloxera Hydatina, and Dinophilus, two kinds of eggs are pro- 

 duced, large and small. The large produce females, the small males. 



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