SEXUALITY 699 



Are any of these observations of conjugation within a caryonide evi- 

 dence of conjugation betvv'een animals identical in mating type? The 

 direct test has not been made in most cases; but, in the few examples in 

 which it was made, it was demonstrated that conjugants were always of 

 different types, in spite of the fact that they were members of the same 

 caryonide. The evidence is therefore strongly against the occurrence of 

 conjugation between animals of the same mating type, though final 

 judgment must await further analysis. 



MATING TYPES IN RELATION TO THE MAUPASIAN LIFE CYCLE 



According to the well-known theory of Maupas (1889), the ciliate 

 exconjugant is conceived as being a young individual producing by re- 

 peated fissions immature cells unable to mate; the cells produced after 

 many youthful fissions become sexually mature and are capable of con- 

 jugating; after many more fissions, the cells grow old, losing their power 

 of conjugating and showing other signs of senescence; and they finally 

 die. If conjugation occurs during the period of maturity, the conjugants 

 are rejuvenated and the cycle is renewed. In some ciliates, such as Uyo- 

 leptus mohilh, investigated by Calkins (1920), this Maupasian life 

 cycle is clearly shown. In 'Paramecium, however, there are striking spe- 

 cific and racial differences in presumably so fundamental a matter as the 

 life cycle. 



Many races of P. aurelia (Sonneborn 1937, 1938a) show a definite 

 period of immaturity: during the first week or two after conjugation, 

 cultures do not give the mating reaction and cannot conjugate. In a few 

 more days, the power of conjugating rapidly develops to full strength, 

 inaugurating a period of maturity. But the organisms remain mature 

 indefinitely; no period of senescence appears. Only for a day or so during 

 the periodically recurring processes of nuclear reorganization, are they 

 unable to conjugate. As soon as reorganization is completed, the mating 

 reaction reappears in full strength. Why the reorganized cells fail to 

 begin again with a period of immaturity, as they do after conjugation, 

 is at present a puzzling and probably a significant fact. 



Other races of P. aurelia (Sonneborn, 1938a) not only lack a period 

 of senescence, but also a period of immaturity: they are able to conjugate 

 immediately after conjugation. Eight successive conjugations have been 

 obtained in a period of seventeen days (Sonneborn, 1936). As the 



