TOWARD A GENETICS OF STENTOR 323 



oxygen tension (see p. 265). Presence of 3, or of 2 rows of cilia in 

 the membranelles (p. 30). Acclimatization, or not, to alcohols 

 (p. 248). Suitability, or not, of certain culture media, like Benecke's 

 solution ior polymorphus (p. 268). Negative, or indifferent, response 

 to light, as shown by coeruleus (p. 22). Presence, or absence of 

 visible nucleoli; for de Terra (1959) remarked a race of coeruleus 

 without nucleoli, but Schwartz (1935) demonstrated their presence 

 in his strain. And average number of macronuclear nodes (MoUer, 

 unpublished). 



3. Conjugation 



Sexual reproduction by temporary fusion of partner animals 

 with cross-fertiHzation and complete renewal of the nuclear 

 apparatus occurs in Stentor as in other ciliates. Breeding experi- 

 ments towards an analysis of inheritance and the roles of nucleus 

 and cytoplasm in the development of racial differences, which 

 may well include mating types, should therefore eventually be 

 possible in this genus. Yet conjugation is rarely observed and 

 seems to be quite adventitious in its appearance, for the means 

 have yet to be discovered by which stentors can be induced to 

 conjugate as we desire. There is available, nevertheless, a com- 

 prehensive cytological study of sexual reproduction in Stentor 

 which has been generally neglected in reviews of this subject. 



The general occurrence of conjugation in the genus Stentor is 

 attested by the observation of pairs in niger (Stein, 1867), coeruleus 

 (Moxon, 1869; Balbiani, 1891c), igneus (Johnson, 1893), poly- 

 morphus (Mulsow, 1913) and in roeseli (Balbiani). I also observed 

 fusion pairs in a colorless race oi polymorphus in which symbiotic 

 chlorellae were almost completely lacking. In seven stocks of 

 coeruleus, conjugation in five was observed at least once during a 

 period of 10 years. Schwartz (1935) remarked that he found no 

 evidence of autogamy in his extensive studies of coeruleus; and I, 

 too, have never seen any indication of nuclear renewal in unpaired 

 animals. 



Exploratory studies of conjugation were included within the 

 compass of those works by Balbiani and Johnson to which we have 

 so often referred. Finding that the old macronucleus could be 

 distinguished by its no longer being clear and refractive in living 

 animals, Balbiani (1891c, 1893) studied the behavior of fragments 



