SEX 



365 



the y in place of the X and develop into male gametophytes. The same 

 condition has been reported for Sphcerocarpos texanus (Miss Schacke 

 1919). 



Although the situation in Sphcerocarpos suggests the XY type of sex 

 inheritance in Drosophila and other forms, it differs- in several respects, 



FIG. 142. The history of the chromosomes in the life cycle of Sphcerocarpos. (After 

 data of C. E. Allen, 1917, 1919.) 



as Allen (1919) points out. In Sphcerocarpos the separation of the XY 



pair results in the production of two kinds of spores which develop directly 



into haploid organisms (gametophytes) of two sexes, whereas in Droso- 



phila the corresponding separation results in two sorts of male gametes 



which determine the sexes of the 



diploid organisms developing from + % O 



the eggs they fertilize. Furthermore, 



in animals with sex-chromosomes some 



forms show the presence of XX to be 



correlated with femaleness and X or 



XY with maleness (male heterozygous 



for sex), while in other forms XX 



is correlated with maleness and X Allen, 1919.) 



or XY with femaleness (female 



heterozygous for sex 1 ). There is evidence to show that the F-chromo- 



some carries no sex-factors in these cases, 2 though its absence may result 



in sterility (Bridges). (See Chapter XVII.) In Sphcerocarpos, on the 



1 Sex-chromosomes referred to in this case as Z and W rather than X and Y. 

 3 See in this connection Castle 1921. 



FIG. 143. chromosome groups from 



