214 bulletin: museum of comparative zoology. 



VI. Summary. 



1. Sex is an attribute of every gamete, whether egg or spermatozoon, 

 and is not subject to control through environment. It is inherited in 

 accordance either witli Mendel's law of heredity or with the principle of 

 mosaic heredity. 



2. Mendel's law includes two principles, (1) the principle of domi- 

 nance in hei'edity of one of two alternative characters over the other, and 

 (2) the principle of segregation of those characters at the formation of 

 the gametes. 



3. Mosaic inheritance is an important exception to both these prin- 

 ciples. In this process alternative characters coexist without domi- 

 nance of either, and pass together (without segregation) into the 

 gametes. 



4. The Mendelian principles of dominance and segregation apply to 

 the heredity of sex among dioecious animals and plants, but among 

 hermaphroditic animals and plants mosaic inheritance of sex takes 

 place. 



5. Latency of one sex in the other, among dioecious animals and 

 plants, is shown by evidence both anatomical and experimental. 



6. Segregation of sex, among the gametes of dioecious animals and 

 plants, is accompixnled l)y morphological differences between the male 

 and female eggs in Dinophilus and certain Lepidoptera, and possibly 

 also by dimorphism among the spermatozoa of Paludina, 



7. Among dioecious animals, a gamete of one sex can unite, in fertili- 

 zation, only with one of the opposite sex ; consequently no individuals 

 are produced from fertilized eggs, which are purely of one sex or the 

 other. 



8. Dominance, in dioecious species, is possessed sometimes by the 

 male character, sometimes by the female. 



9. In parthenogenetic species, the female character invariably domi- 

 nates, when the characters of both sexes are present together. Accord- 

 ingly in such species : (a) All fertilized eggs are female, {h) Unfertilized 

 eggs Avhich are produced without segregation of the sex-characters are 

 female, (c) ]\Iales develop only from unfertilized eggs from which the 

 female character has been eliminated. 



10. The female character, eliminated from the male partlienogenetic 

 egg, passes into the testis ; accordingly the spermatozoa bear the female 

 character, though the individual producing them is in soma purely 

 male. 



