SEXUALITY 677 



above). These observations by Moewus are perhaps subject to a very 

 different interpretation. The agglutinative reactions observed between 

 gametes of the same sex are weak and transient. Failure to copulate might 

 well be due to this, rather than to the absence of an additional factor 

 such as a sex difference. Similar weak mating reactions between cells of 

 the same mating type were observed by Sonneborn (1937) in Para- 

 mecium aurelia after the cells had been in contact with animals of an- 

 other mating type. As in Chlamydomonas, the mating reaction was tran- 

 sient and did not lead to copulation. Similar behavior was also observed 

 by Sonneborn (1938a) when cultures of mating types II and V, belong- 

 ing to non-interbreeding varieties, were mixed together. Here the mating 

 reaction occurs between animals of different mating types, and yet they 

 fail to conjugate. Further, cultures known to belong to two mating 

 types that will interbreed under favorable conditions will, under other 

 conditions, give a weak and brief mating reaction without proceeding 

 to conjugate. In view of these observations, it appears to be still an 

 open question whether the failure of copulation to take place between 

 cells of the same sex in Chlamydomonas that have given a weak sex 

 reaction with each other is due to the weakness of the reaction or to 

 some other aspect of sex, different from the production of diverse sex 

 stuffs. 



In later publications, Moewus (1938b, 1939a) reported the discovery 

 of the chemical nature of the sex stuffs in the group of sexes G to O. 

 The active stuffs for these eight sexes are all diverse percentage combi- 

 nations of the cis and trans forms of the dimethyl ester of crocetin. The 

 proportions are as follows : 



^^xGHJKLMNO 



Percentage cis 95 85 75 65 35 25 15 5 



Percentage trans 5 15 25 35 65 75 85 95 



The chemical nature of the sex stuffs aids greatly in understanding 

 the breeding relations summarized in Tables 8 and 9. The order of 

 sexes from G to O in the table is in the order of decreasing percentages 

 of cis and increasing percentages of trans dimethyl crocetin. The dif- 

 ference in percentage of either cis or trans between any two successive 

 sexes in the table is always 10 percent, except between sexes K and L 

 which show a difference of 30 percent. Copulation occurs between any 



