366 INTRODUCTION TO CYTOLOGY 



oilier hand, 1 lie* presence of X is correlated with femaleness, Y with 

 maleness, and XY with the non-sexual condition of the sporophyte. 

 Here the Y is apparently as important as the X in the transmission of 

 sex-factors. Allen suggests that secondary sexual differences of the 

 gametophytes, such as size, may be connected with the relative amounts 

 of chromatin in the nuclei: the female gametophyte, having the large 

 X-ehromosome and therefore a distinctly greater mass of chromatin, 

 develops more rapidly and becomes much larger than the male gameto- 

 phyte with its small F-chromosome. The primary sexual differences 

 he regards as due to other factors. 



Although Sphoerocarpos affords the only known example of hetero- 

 chromosomes in plants it is not improbable that other cases will be 

 discovered. 



Conclusion. — In all of the organisms included in the foregoing review 

 the individuals of the two sexes differ visibly in their chromosome com- 

 plements. Moreover, in most of them the sexual differences are definitely 

 correlated with special distinguishable chromosomes, which are accord- 

 ingly known as sex-chromosomes. The distribution of these bodies at 

 the time of reduction results in the production of two kinds of male 

 gametes or two kinds of female gametes, and in at least one case two 

 kinds of spores. In all of these the chromosome differentiation in the 

 cells correponds to the sexual differentiation of the organisms into which 

 they develop. The conclusion appears unavoidable that the differentia- 

 tion of the sexes is here determined by a cell mechanism, and that the hetero- 

 chromosomes have a definite causal relationship with sex. How close this 

 relationship may be, and to what degree it is a fixed one, are as yet by no 

 means clear, but it is beyond question that the heterochromosomes are 

 not the sole determining cause of sex, as some workers have hastily 

 concluded. To this question we shall subsequently return. 



Sex -chromosomes and Mendelism. — The heterocbromosome phe- 

 nomena are intimately bound up with the whole matter of Mendelian 

 inheritance. According to the Mendelian interpretation the sexes are 

 due, like other heritable characters, to factors carried by the chromo- 

 somes — by the heterochromosomes where these are present. The ap- 

 proximate 1 : 1 ratio of the sexes in most organisms is accounted for in 

 the following manner. Referring to our typical Mendelian pair of charac- 

 ters in the pea, tall and dwarf, it is found that when a plant heterozygous 

 for tallness (Tt) is crossed with a pure recessive (tt) the resulting off- 

 spring are half tall (Tt) like one parent and half dwarf (tt) like the other, 

 a 1 : 1 ratio. If it is assumed in a similar manner that there is a pair of 

 factors for sex, one sex (the male, Correns; the female, Bateson) being- 

 heterozygous and the other a homozygous recessive, a 1 : 1 ratio of the 

 sexes results. 



Largely because of the observed behavior of sex-chromosomes the 



