ORIGIN OF THE INDIVIDUAL 237 



groups (maternal and paternal) which existed before matura- 

 tion. For example, if the somatic (diploid) number of chro- 

 mosomes is eight, sixteen different types of gametes are 

 possible. In Man with 48 somatic chromosomes, and 

 after synapsis 24 pairs of paternal and maternal chromosomes, 

 there are 2 24 , or about twenty million possible types of 

 gametes in each sex, and since these combine at random at 

 fertilization, the number of possible different types of zygotes 

 from one parental pair mounts far up in the trillions. No 

 wonder the children of a family differ there is variation! 



In a way, therefore, fertilization is not consummated, so 

 far as its influence on the race is concerned, until the matura- 

 tion of the gametes in the new generation. We must defer 

 until later the consideration of the significance of these facts 

 in biparental inheritance, and take up now some necessary 

 details of the gametes themselves and of how they unite to 

 form the zygote. 



6. Fertilization 



The gametes, while exhibiting in certain cases peculiar 

 adaptations to special conditions, are remarkably similar in 

 general structure throughout the animal series. It is possible 

 in animals, just as in plants, to arrange a series of lower forms 

 which shows various stages in sex differentiation. Beginning 

 with animals in which both gametes are structurally similar, 

 we pass by slow gradations to others in which the egg is a 

 relatively large, passive, food-laden cell and the sperm a 

 minute, active, flagellated cell. As a matter of fact the egg 

 is subject to somewhat more variation in size and general 

 appearance than the sperm, for after fertilization it must 

 be adapted to meet the special conditions of development 

 peculiar to the species. Thus, for instance, the actual size 

 of the egg in both plants and animals is determined chiefly by 



