3IO EVOLUTION, GENETICS, AND EUGENICS 



be sure of the identity of each individual chromosome, but on the 

 other hand, there are some organisms known in which the chromo- 

 somes of a single nucleus are not of the same size and form (Fig. 46). 

 These latter cases enable us to determine some very significant facts. 

 Where such differences of shape and proportion occur they are constant 

 in each succeeding division so that similar chromosomes may be iden- 

 tified each time. Moreover, in all ordinary mitotic divisions where 

 the conditions are accurately known, these chromosomes of different 

 types are found to be present as pairs of similar elements; that is, 

 there are two of each form or size. 



Pairs of similar chromosomes in the nucleus because one chromo- 

 some comes from each parent. — ^When we recall that the original 

 fertilized egg from which the individual develops is really formed by 

 the union of two gametes, ovum and spermatozoon, and that each 



% 



A B 



Fig. 46. — A, chromosomes of the mosquito (Culex). (After Stevens.) B, 

 chromosomes of the fruit fly (Drosophila). (After Metz.) Both of these forms 

 have an unusually small number of chromosomes. (From Guyer.) 



gamete, being a true cell, must carry its own set of chromosomes, the 

 significance of the pairs of similar chromosomes becomes evident; one 

 of each kind has probably been contributed by each gamete. This 

 means that the zygote or fertile ovum contains double the number of 

 chromosomes possessed by either gamete, and that, moreover, each 

 tissue-cell of the new individual will contain this dual number. For, 

 as we have seen, the number of chromosomes is, with possibly a few 

 exceptions, constant in the tissue-cells and early germ-cells in suc- 

 cessive generations of individuals. For this to be true it is obvious 

 that in some way the nuclei of the conjugating gametes have come to 

 contain only half the usual number. Technically the tissue-cells are 

 said to contain the diploid number of chromosomes, the gametes the 

 reduced or haploid number. 



In maturation the number of chromosomes is reduced by one- 

 half. — -This halving, or as it is known, reduction in the number of 

 chromosomes is the essential feature of the process of maturation. It 



