£22 GENETICS 



the sexes on separate plants. Among such dimorphic 

 plants, Bryonia in particular has been investigated by 

 Correns and Lychnis by Shull. Without describing 

 the crosses made in their experiments in detail, it 

 may be stated that when dioecious types are recipro- 

 cally crossed with hermaphroditic forms, the result- 

 ing progeny indicate plainly that one sex is homozy- 

 gous while the other is heterozygous with respect to 

 the sex character. This confirmatory evidence is 

 quite in line with that already brought forward that 

 sex is a Mendelian character the determiners of which 

 are carried in the germplasm. 



7. Conclusion 



The evidence thus far obtainable from all sources 

 points to the conclusion that sex is unalterably fixed 

 at the time the egg is fertilized, by definite deter- 

 miners which act in the same way as other Mendelian 

 determiners. Dr. Shull, whose exhaustive studies in 

 sex determination place him in the front rank as an 

 authority on the subject, makes this conservative 

 statement: "Nearly all the recent investigations 

 indicate that sex is at least predominantly dependent 

 upon the genotypic nature of the individual." 



If this is so, while it furnishes the best of confirma- 

 tory evidence in support of Mendel's law, it shows 

 that it is not possible for man to predetermine the 

 sex of his offspring, which he has long hoped to be 

 able to do. The following quotation from Castle 

 may suitably close this chapter: "Negative as are 

 the results of our study of sex control, they are perhaps 



