250 ANIMAL BIOLOGY 



somes differ qualitatively one from another, but also that the var- 

 ious parts of each chromosome are qualitatively distinct. And 

 further that these qualitative differences are the physical basis of 

 inheritance - - the determiners (genes) of characters which will 

 be realized in the individual or the race to which the cell containing 

 them contributes. Such being the case, the chromosomal complex 

 of each of the nuclei which arises after synapsis — the nuclei of 

 the gametes - - depends on how the various chromosomes happen 

 to be distributed during the two maturation divisions. As a mat- 

 ter of fact, all the chromosomal combinations occur that are 

 mathematically possible with the available number of chromo- 

 somes in a given species, but with one limitation: every cell must 

 receive one member of each synaptic pair of chromosomes, so that 

 each and every gamete receives a complete haploid group of 

 chromosomes, but rarely the same groups (maternal and paternal) 

 which existed before maturation. For example, if the somatic 

 (diploid) number of chromosomes 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 seventeen million possible types of gametes 

 in each sex; and since these combine at random at fertilization, 

 the possible number of 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! (Fig. 193.) 



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

 its influence on the race is concerned, until the maturation of the 

 gametes in the new generation to which it has given rise. We must 

 defer until later the consideration of the significance of these facts 

 in biparental inheritance, and merely emphasize again that the 

 continuity of life implies not only the continuity of cells but also 

 of their nuclear elements, the chromosomes — the genes. 



