CHROMOSOMES AND SEX 399 



the critical stage, the greater the amount of sex reversal. The same 

 interpretation applies, mutatis mutandis, to intersexes beginning develop- 

 ment as females. In triploid Drosophila, also, the degree of intersexuality 

 varies with the time of sex reversal, which is influenced by both genetic 

 factors and temperature (Dobzhansky, 1930c). 



These suggestions are of special importance in that they direct atten- 

 tion to the quantitative and ontogenetic aspects of genie action, a fuller 

 knowledge of which is prerequisite to a true understanding of the physical 

 basis of heredity. 



The direct control of sex in a given individual is not to be confused 

 with the control of sex in a population. Nutritive conditions sometimes 

 influence the form of reproduction and therefore the sex of the animals 

 resulting; but here the sex of no individual, once decided, is altered." 

 Abnormal temperatures may prevent the laying of the eggs from which 

 one sex develops, without interfering with those giving rise to the other 

 sex.^^ In such cases the environmental influences operate "by affecting 

 the production and survival of sexually predestined germ-cells" (Thom- 

 son, 1913). 



Conclusion. — In bisexual organisms the gene complexes in all indi- 

 viduals are nearly enough alike (with respect to genes affecting sex) for 

 these to show the same sex manifestation under similar environmental 

 conditions, and the constitution of this gene complex is such as to result 

 in the production of both sexes as responses to the different special con- 

 ditions in tissues in different regions or at different stages of development. 

 Artificial modifiability here depends on the extent to which those partic- 

 ular special conditions, which are in this case the differential factors, can 

 be controlled or altered. 



In unisexual (dioecious) organisms the gene complexes of some indi- 

 viduals differ from those of others in such a way that some develop one 

 sex, and others the other sex, under the same general environmental 

 conditions and in the presence of the range of special internal conditions 

 normally prevailing. Under ordinary conditions, therefore, it is the 

 genetic factors that are differential. In the heterothallic bryophytes the 

 segregation of genes at meiosis results directly in spores and gametophyte 

 individuals of the two sexes; in heterophytic seed plants it results in two 

 sorts of microspores and hence male gametophytes whose respective 

 gametes cause a difference in the sex expression of the succeeding sporo- 

 phytes; in animals it results directly in two kinds of gametes of one sex. 

 Artificial modifiability here depends on the kind and degree of difference 

 between the male and female gene complexes, and on the extent to which 

 their differential effect can be overcome by altering controllable extrinsic 



5' Whitney (1914, 1916, 1919) on Hydatina senta. 

 ^ Malsen (1906; on Dinovhilus) . 



