VARIATION AND HEREDITY 233 



producing male gametes and the other two female gametes; but a 

 small number of zygotes gives rise to four clones which contain both 

 gametes. It is considered that this is due to crossing-over that 

 brought the two sex factors (P and M) together into one chromo- 

 some, and hence the "mixed" condition, while the other chromosome 

 which is devoid of the sex factors gives rise to individuals that soon 

 perish. 



In crosses between Chlamydo?no?ias eugametos which possesses a 

 stigma and 10 haploid chromosomes and C. paupera which lacks a 

 stigma and 10 haploid chromosomes, 12 pairs of factors including 

 sex factor are distinguishable. Consequently at least two chromo- 

 somes must have two factors in them. Thus adaptation to acid or 

 alkaline culture media was found to be linked with differences in 

 the number of divisions in zygote. That there occurs a sex-linked in- 

 heritance in Chlamydomonas was demonstrated by crossing stigma- 

 bearing C. eugametos of one sex with stigma-lacking C. paupera of 

 the opposite sex. The progeny that were of the same sex as C. euga- 

 metos parent possessed stigma, while those that were of the same sex 

 as C. paupera parent lacked stigma. Thus it is seen that the sex factor 

 and stigma factor are located in the same chromosome. 



The genetics of conjugation which takes place between two diploid 

 conjugants has been studied by various investigators. Pure-line 

 cultures of exconjugants show that conjugation brings about diverse 

 inherited constitutions in the clones characterized by difference in 

 size, form, division-rate, mortality-rate, vigor, resistance, etc. The 

 discovery of mating types in Paramecium and in Euplotes, and in- 

 tensive studies of conjugation and related phenomena, are bringing 

 to light hitherto unknown information on some of the fundamental 

 problems in genetics. 



Sonneborn (1939) has made extended studies of variety 1 of 

 Paramecium aurelia (p. 194) and found that genetically diverse ma- 

 terials show different types of inheritance, as follows: 



(1) Stocks containing two mating types. When types I and II 

 conjugate, among a set of exconjugants some produce all of one 

 mating type, others all of the other mating type and still others 

 both types (one of one type and the other of the other type). In the 

 last mentioned exconjugants, the types segregate usually at the 

 first division, since of the two individuals produced by the first divi- 

 sion, one and all its progeny, are of one mating type, and the other 

 and all its progeny are of the other mating type. A similar change 

 was also found to take place at autogamy. Sonneborn therefore con- 

 siders that the mating types are determined by macronuclei, as 



