i6 CELL GENETICS 



and Cannabis sativa) the kind of gamete is determined by the 

 period of its development, and an individual, which at one time 

 produces exclusively male gametes, later produces exclusively 

 female. 



(c) Genotype. In Sphcerocarpus Donnellii (Allen, 1919 ; Lorbeer, 

 1927) the diploid sporophyte produces two kinds of haploid spores. 

 One develops into prothallia with only female gametes, the other 

 into prothallia with only male gametes independently of any 

 variation in environmental or developmental conditions. 



Thus it will be seen that in the second type, differentiation of 

 cells arises directly within the individual. It is a primary differentia- 

 tion. In the first and third types it arises from a differentiation 

 of the individuals which bear the cells. It is a secondary 

 differentiation. The differentiations within and between individuals 

 may equally be genetically determined, and this introduces a 

 source of confusion. The genetic determination of sex within an 

 individual depends on genetically determined differentiation within 

 that individual, and does not differ in this from any other such 

 differentiation. The genetic determination of sex between indi- 

 viduals depends on a special mechanism, the segregation of dis- 

 similar genotypes in a heterozygous parent at meiosis. The 

 investigation of this mechanism is an important part of chromosome 

 studies. But it must be remembered that the chromosome 

 mechanism of segregation is merely the switch which sets the 

 organism on one of two alternative paths of development. 



This classification is sufficient for a large number of cases, but for 

 others a further analysis is convenient. Thus in the Metazoa, not 

 the haploid but the diploid is genetically differentiated in regard to 

 the sex of the gamete produced. And since the two sexes are 

 equally produced by the fusion of the opposite kinds of gametes one 

 of these must be composed genetically of two types. The kind of 

 gamete formed is not therefore determined b}^ its own genetic 

 constitution, but by that of the diploid parent which bears it. The 

 same principle applies to the flowering plants where the sexual 

 properties of the whole haploid generation are similarly determined 

 by the constitution of the parent, whether male, female, or herma- 

 phrodite, on which it is, in effect, parasitic. Thus the immediate 



