SEXUALITY 671 



others are apparent. The only cases about which a reasonable doubt may 

 still be entertained are those in which copulation occurs within a clone. 

 This matter-has been intensively studied by Pascher (1931) in C. paupera 

 and by Pringsheim and Ondracek (1939) mainly in Polytoma. Their 

 observations are in fundamental disagreement with those of Moewus, 

 leading them to conclude that these forms show copulation without any 

 physiological sex differentiation. Further, Pringsheim and Ondracek 

 could not confirm Moewus's observation that the cells left over after 

 copulation were unable to copulate with each other because they were 

 all of one physiological type. They attribute the cessation of copulation 

 to a change in the chemical conditions in the culture, rendering it un- 

 suitable for copulation. Appropriate modification of the conditions leads 

 to resumed copulation. They therefore deny the validity of the left-over 

 technique for the analysis of the question at issue. The reader is referred 

 to their article for a detailed criticism of numerous points in Moewus's 

 work. Moewus (1940) has replied to these criticisms in an article that 

 appeared too late for inclusion in this review. 



THE NATURE OF THE PHYSIOLOGICAL DIFFERENCES BETWEEN GAMETES 



IN Chlamydonionas 



As the union of gametes in Chlaniydomonas is obviously a sex act, 

 the physiological differences that usually, if not always, characterize the 

 gametes may be considered sex differences. This section will set forth the 

 number and interrelations of these sexes, their chemical characteristics, 

 and their possible relation to male and female. 



The number of sexes and their interrelations. — The system of breed- 

 ing relations in Chlamydomonas was discovered by mixing together, in 

 combinations of two, cultures of the sexes isolated from the species and 

 varieties of Chla?nydomonas examined. The two species C. par ado xa 

 and C. pseudoparadoxa constitute one interbreeding system, and the four 

 species C. eugametos, C. paupera, C. hraunii, and C. dresdensis consti- 

 tute another interbreeding system; but these two systems of species will 

 not breed with each other. 



Among the first group of species, Moewus (Hartmann, 1934) found 

 two sexes in each of two races (from Giessen and from Coimbra) of C. 

 pseudoparadoxa and in C. paradoxa. In order to discover whether the 

 two sexes were alike in the three races, they were matched up in all 



