398 INTRODUCTION TO CYTOLOGY 



evident that all nuclei, monoploid and otherwise, however greatly they 

 may differ in the proportions of their "male-producing" and "female- 

 producing" genes, regularly carry both kinds; and it is probable that the 

 tissues in which they lie might be made to develop either sex if we but 

 knew how to alter the other factors concerned. 



In the evolution of sex-chromosomes organisms have established a 

 mechanism insuring a degree of sex separation which is sufficient to 

 confer upon them the benefits of such separation, but the condition has 

 apparently become rigid and unalterable only in a minority of the cases, 

 at least among plants. As emphasized by Correns (1926a), the essence 

 of dicecism is the presence of two genotypes correlated with sex and not 

 the uniformly sharp separation of the sexes. The role of genetic factors 

 in sex differentiation is strikingly illustrated in the production of a 

 dioecious strain of maize by the genetic manipulation of genes previously 

 known to be associated with male or female sterility in the ordinary 

 monoecious strains.^* 



Only a beginning has been made on the problem of the ontogenetic 

 development of the sexual states. The influences of genetic and environ- 

 mental factors are exerted through a long and complicated series of 

 intracellular and intercellular reactions. The correlation of sexual differ- 

 ences with metabolic rate (Riddle et at.), chemical composition (Manoilov 

 et al.),^^ and endocrine secretions (F. R. Lillie, Witschi, et al.) are of 

 special interest in this connection. Goldschmidt, as a result of his 

 extensive researches on intersexuality in diploid moths, ^^ has suggested 

 that the final expression of sexual characters is dependent upon the 

 relative velocities of the male-producing and female-producing reactions 

 in the early stages of ontogenetic differentiation. Moreover, the veloci- 

 ties vary with the amounts of gene substance present. As an embryo 

 with the male genetic complex begins development, the male-producing 

 reactions proceed at a faster rate than the female-producing ones, and 

 this continues throughout development, giving a normal male moth. 

 When, however, the quantity of the male-producing gene substance 

 is less, the rate of the male-producing reactions may sooner or later 

 fall below that of the female-producing ones; if this occurs before the 

 critical stage of differentiation is reached, female characters develop and 

 the moth is intersexual. The earlier the turning point with reference to 



" D. F. Jones (1931). See Emerson (1924, 1932). For discussions of the factorial 

 interpretation of sex determination in the life cycles of plant groups, see Correns 

 (1928a) and Sharp (1925). 



" Manoilov (1922 et seq.), Griinberg (1922, 1923), Minenkov (1924), Satina and 

 Demerec (1925), Satina and Blakeslee (1925, 1926a6, 1927), Joyet-Lavergne (1927 

 et seq.), Camp (1929), Dahlgren (1929). For general accounts, see Schratz (1928), 

 Joyet-Lavergne (1928c, 1932), and Abromavich and Lynn (1930). 



^^ Sec Goldschmidt's recent review (1931). For a brief summary of work on 

 amphibians, see Witschi (1932). 



