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, 



mini 



Kale garaetophyte 



IIIHH' 



X 



Fertillzad Sporophyte Sporocyte 



egg 



Maturation divislono 



5PMAEROCARP05 



Female gametophyte 



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 

 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 



X Y with maleness (male heterOZygOUS FIG. 143. Chromosome groups from 



for sex), while in other forms XX ma j e and female gametophyte cells of 



, 



Sphcerocarpos Donnelhi. 



Allen, 1919.) 



(After C. E. 



is correlated with maleness and A 



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. 



2 See in this connection Castle 1921. 



