100 CYTOLOGY chap. 



is the odd sex chromosome, and on account of its large size it is easily 

 identified at all stages. 



B is a late prophase I., showing six bivalents and the sex chromosome, 

 which, being without a homologue, must remain univalent. Correspond- 

 ing with the difference in valency, the behaviour of the sex chromosome 

 in the two meiotic divisions is remarkably unlike that of the other 

 chromosomes. In metaphase I. the bivalents separate into their 

 constituents in the usual way. The unpaired sex chromosome, on the 

 other hand, divides longitudinally as if the mitosis were somatic, thus 

 anticipating the normal division of the chromosomes in metaphase II. 

 Each spermatocyte II. therefore contains a similar chromosome group 

 of six ordinary chromosomes and one sex chromosome. In metaphase II. 

 the ordinary chromosomes divide longitudinally in the usual way, but 

 the sex chromosome, which has already undergone this division in 

 metaphase I., does not divide again, but passes intact to one or other 

 pole of the mitotic figure (E). Hence the two spermatids formed by 

 each secondary spermatocyte differ in their chromosome equipment, 

 one (G, and upper cell in H) containing the sex chromosome, and the 

 other (F, and lower cell in H) lacking it. 



The female Protenor has fourteen chromosomes instead of thirteen, 

 there being two of the large sex chromosomes instead of only one. 

 Hence all the mature eggs will have 6, or 5 + X (X standing for the sex 

 chromosome). If now an egg is fertiUzed by a spermatozoon of the 

 composition shown in F — i.e. without the X chromosome — the resulting 

 zygote will have 13 chromosomes thus : 



(6 + X)+6 = i2 + X [S. Fig. J). 

 If fertiUzed by the spermatozoon shown in Fig. G the result will be : 



(6 + X) + (6 + X) = i2 + XX ($, Fig. K). 



Protenor is an example of the simplest case known, in which the male 

 differs from the female in possessing one chromosome fewer. Several 

 more comphcated conditions than this are known, however. 



For instance, in Lygaens, another Hemipteran, the male has the same 

 number of chromosomes as the female, each having a pair of sex 

 chromosomes, but while in the female these are equal, like any other pair 

 of homologous chromosomes, in the male they are unequal, one being 

 much smaller than its mate ; this latter is of the same size as those of 

 the female. The relation between the male chromosome groups in 

 Lygaens and Protenor may therefore be expressed by the statement 

 that in Lygaens one of the pair of sex chromosomes is reduced, while 

 in Protenor one is absent. Or, using the convenient notation now in 



