674 



SEXUALITY 



copulates with one or more sexes that the other does not copulate with. 

 Thus the primary differences among the sexes lie in these breeding rela- 

 tions. There are at least two further kinds of sex differences that throw 

 much light on the nature of the sexes in Chlamydomonas. 



The first of these involves the intensity of the mating reaction. It 

 is known that algal gametes of certain species (including Chlamydo- 

 monas) form groups as a preliminary to copulation (Fig. 165). When 

 ripe cultures of gametes that can copulate with each other are mixed 

 together, the gametes at once form clusters of as many as 100 or more 



r. *. 



^ 





WMM 



Figure 165. Group for- 

 mation in Chlamydomonas , 

 showing the groups formed 

 in a mixture of cells dif- 

 fering in sex. (From 

 Moewus, 1933.) 



gametes. Within the clusters the gametes pair off, with the result that the 

 cluster disintegrates into copulating pairs. The size of the initial clusters 

 is partly determined by the number of gametes per unit of volume. When 

 this concentration is uniform (e.g., 2 X ^^^ gametes per cc. ) , the size of 

 the clusters depends upon which two sexes are present in the mixture. 

 Certain combinations of sexes yield groups of 100 or more gametes, 

 others give groups of but 10 to 20 gametes, others give only pairs, and of 

 course some do not even give pairs. These four grades of reaction have 

 been designated as 3, 2, 1, and respectively, and the intensities of the 

 reaction of the sex mixtures shown in Table 8 are given in Table 9. As 

 shown in Table 9, the same sex may give different grades of reaction 

 in mixtures with different sexes: thus G reacts to L, M, N, and O with 



