SEX 363 



as other material is a question which only future investigation can answer. 

 It is not improbable, however, that a segmentation of the chromosomes at 

 points of constriction may be mainly responsible for this condition. 



In all of the cases reviewed above the male produces two sorts of 

 spermatozoa differing visibly in chromatin content: in the language of 

 Mendelism, the male is "heterozygous for sex." The female produces 

 but one kind of egg; she is "homozygous for sex." The sex of the off- 

 spring is clearly dependent on the kind of spermatozoon which functions, 

 and is therefore definitely correlated with the chromosome mechanism. 



Female Heterogametic. There are also on record a number of cases, 

 chiefly among moths and birds, in which the female produces two kinds 

 of eggs differing in chromosome content, while the male produces but one 

 kind of spermatozoon: the female is heterozygous for sex, and therefore 

 heterogametic, while the male is homozygous. Certain cases of this type 

 will now be reviewed. 



In the moth, Phragmatobia, Seiler (1913) has described the following 

 condition. In the male the somatic number of chromosomes is 56, 

 including 54 autosomes and two Z-chromosomes 1 (Figs. 139 F; 140). 

 Each sperm receives 28 (27 + Z). In the female the somatic number 

 is likewise 56, but includes a ZW pair instead of the ZZ pair of the 

 male. Half the eggs receive 27 + Z and the other half 27 + Ww (the 

 TT-chromosome breaks temporarily into two parts during maturation). 

 An egg with 28 (27 + Z) chromosomes fertilized by a sperm with 

 28 (27 + Z) develops into a male moth with 56 (54 + ZZ). An egg 

 with 29 (27 + Ww} chromosomes fertilized by a sperm with 28 (27 + 

 Z) develops into a female moth with 57 (54 + ZWw). The W and 

 w subsequently reunite to form a single W, both sexes then having 

 the same number, 56. Since some embryos show more than the normal 

 number of chromosomes Seiler thinks it probable that the Z-chromo- 

 some is compound and may under certain conditions subdivide into 

 smaller elements. The same investigator has recently (1919) reported 

 a digametic condition in the female in two other moths, Talceporia 

 tubulosa and Fumea casta. In the former the eggs have 29 and 30 

 chromosomes, and in the latter 30 and 31. 



In the moth, Abraxas grossulariata (Doncaster 1914), sex inheritance 

 is apparently of the WZ type, though there is often an aberrant behavior 

 on the part of the chromosomes which has not been entirely explained. 



In the common fowls Guyer (1909, 1916) has made observations which 

 he interprets as follows (Fig. 140). In the male there are 18 chromo- 

 somes : 16 autosomes and two accessories. Both of the latter go to one 

 pole in the first maturation mitosis, and in the second mitosis they sepa- 



1 It is customary to refer to the sex-chromosomes in species with sexually hetero- 

 zygous females as W and Z, instead of Y and X as in the more common sexually 

 heterozygous males. 



