448 SOME GENERAL PROBLEMS OF DEVELOPMENT 



mitosis and maintains its original condition as it descends to all 

 the body cells and to all the germ cells until the time of matura- 

 tion. ^Vllen synapsis occurs the autosomes unite in pairs, while 

 the sex chromosome has no synaptic mate. At the first matura- 

 tion division the synaptic pairs of autosomes divide, and also the 

 sex chromosome, and there is no change in the relationship; but 

 in the second maturation, when the synaptic pairs separate instead 

 of dividing lengthwise as in all other mitotic divisions of body and 

 germ cells, the sex chromosome passes undivided to one member of 

 each of the two pairs of cells shown in the figure. There are, 

 therefore, two kinds of spermatozoa produced in equal numbers, 

 one with and one without the sex or X-chromosome. 



In the female, on the other hand, there are two sex chromosomes 

 in the original zygote and in all of its descendants, both body and 

 germ cells (Fig. 114, p. 228). In contrast with the spermatozoa, 

 the mature ova are all alike, since every one contains a single sex 

 chromosome and the haploid number of autosomes. 



As shown in Fig. 114, the male combination is produced when 

 an egg is fertihzed by a spermatozoon without an X-chromosome; 

 the female, when the spermatozoon possesses an X-chromosome. 

 The chromosome combination that happens to be present at the 

 time of fertilization is supposed to descend from the zygote, by 

 mitosis, to all the cells of the individual, both body and germ cells. 

 The sex chromosome would, therefore, function by chance com- 

 bination at every period of maturation and fertilization as the 

 determiner, or at least the indicator, of sex. According to the 

 laws of chance, the number of males and that of females would 

 be approximately equal wherever large numbers of individuals 

 were involved. This seems to be the fact in most instances where 

 large counts have been made to determine the sex ratio, although 

 there are some exceptional cases. 



Such combinations of chromosomes in relation to sex have been 

 discovered in many insects and in other animals. A modification 

 of the relationship described is seen in some insects and in other 

 species, where the male has an X-chromosome and another smaller 

 one, the Y-chromosome, which is its mate, while the female has 

 two X-chromosomes. The diploid number of the male in such 

 cases is, therefore, 2n + xy, and that of the female 2n + xx. 

 These appear as In -\- x and In + y, respectively, in the two types 

 of spermatozoa, and as In -\- x in all the ova. The substitution of 



