232 GENERAL ZOOLOGY 



male is 21. During oogenesis the number is reduced to 11 in 

 one of the maturation divisions, with the result that the ootid 

 remains with 1 1 chromosomes, the gametic number of the female. 

 The zygotic chromosomes of the female therefore consist of 11 

 synaptic pairs which undergo disjunction in one of the maturation 

 divisions. In spermatogenesis, the prophases of the first 

 maturation division show 11 chromosomes, 10 of which are 

 synaptic pairs and the eleventh a single chromosome, known 

 as the odd chromosome or the sex chromosome. It should be 

 noted that the homologue of this chromosome is paired in the 

 female zygotic series (Fig. 142). In the first maturation division, 

 in the male, the 10 paired chromosomes undergo disjunction 

 and the odd chromosome divides quantitatively, with the result 

 that each secondary spermatocyte receives 11 chromosomes, one 

 of which is an odd chromosome. In the second maturation 

 division, the odd chromosome passes undivided to one pole of the 

 spindle, while the remaining 10 chromosomes divide quantita- 

 tively. This results in the production of two kinds of spermatids 

 (and later two kinds of spermatozoa), one containing 10 chromo- 

 somes and the other 11. Since the ootids all contain 11 chro- 

 mosomes, a male is produced when a sperm containing 10 

 chromosomes fertilizes an egg (10 + 11 = 21), and a female 

 when a sperm containing 11 chromosomes fertilizes an egg 

 (11 + 11 = 22). In Anasa tristis, the presence or absence of 

 the odd chromosome apparently determines the zygotic sex, and 

 it is for this reason called the sex chromosome. It is also known 

 as the X chromosome or as the hetero chromosome. The remaining 

 chromosomes, which are equally represented in the two sexes are 

 known as autosomes. 



An inspection of the spermatogonial chromosomes (Fig. 142) 

 shows that the diploid complex of the male can be arranged into 

 a series of 10 synaptic pairs of autosomes plus one X chromosome, 

 which lacks a synaptic mate. The oogonia of the female consists 

 of 10 synaptic pairs of autosomes, corresponding to those of the 

 male, plus two X chromosomes. The difference in the zygotic 

 chromosomes of the two sexes is the difference in the number of 

 X chromosomes, of which the male has one and the female two. 



X and Y Chromosomes. — Inequality of some sort in the chro- 

 mosomes of the male and of the female seems to prevail in 

 practically all animals. Frequently, as in Anasa tristis, the 



