iv THE SEX CHROMOSOMES in 



(6) Cases where the Differential Sex Chromosome is present in the Female 



In all the cases described so far it is the spermatozoa which carry the 

 determining factor for sex ; the male produces two kinds of gametes, 

 male - producing and female - producing, and is said therefore to be 

 heterozygous for sex. The eggs, on the other hand, are all similar, or 

 indifferent, and so the female is said to be homozygous. So far there is 

 comparatively little cytological evidence of the condition of the sexes 

 being reversed in this respect. In 1909 Baltzer described such a case 

 in Echinoderms, but in 1913 he withdrew the statement, and meantime 

 Tennant (1911) found that the sea-urchin Hipponoe conforms to the 

 usual rule, that the male produces the two kinds of sex-determining 

 gametes, while the female gametes are all alike. 



The Lepidoptera and Birds present specially interesting features in 

 this connection, since experiments on sex-linked inheritance (see p. 180) 

 require that the female should be heterozygous and the male homozygous. 



Unfortunately neither of these groups are favourable objects for 

 cytological study. In the Lepidoptera the number of chromosomes is 

 usually very high, and in Birds the conditions for observation seem 

 especially difficult, chiefly owing to a pronounced tendency of the 

 chromosomes to become agglutinated together. 



In the latter group Guyer (1909) described an X chromosome in the 

 male fowl and guinea-fowl, resulting in the formation of the usual two 

 classes of spermatozoa, and thus apparently indicating male heterozygosity 

 in birds as in insects. These observations have been questioned by 

 Boring and Pearl (1914), but Guyer's subsequent observations (1916) 

 confirm his original description and at the same time show that it may 

 not be incompatible with homozygosity of the male and heterozygosity 

 of the female. It appears that in the somatic tissues of the male fowl 

 there are eighteen chromosomes, of which sixteen are rod-shaped and 

 two are U's. The latter are the sex chromosomes, the chromosome 

 formula being therefore 2n r = 16 + XX. In the female only one of the 

 U's is present, the formula being 2n' = i6 + X. In the male meiotic 

 prophase all the chromosomes pair, giving nine bivalents, one of which 

 is larger than the others and curved ; this is the X bivalent. At 

 metaphase I. this fails to dissociate, passing undivided to one pole, so 

 that the secondary spermatocytes contain either eight univalents or 

 eight univalents and the X bivalent. The latter dissociates in metaphase 

 II., so that two kinds of spermatids are produced with the chromosome 

 formulae 8 and 8+X respectively. Guyer gives reasons for believing 

 that the spermatids without the X chromosome degenerate without 

 metamorphosing into spermatozoa, though this could not be actually 

 demonstrated. The meiosis of the female was not worked out, but the 



