REPRODUCTION 191 



and two micronuclei. The original macronucleus in the conjugant 

 becomes transformed into a skein which breaks up into 20 to 40 

 small masses. These are resorbed in the cytoplasm as in other species. 

 As to when these nuclear fragments are absorbed, depends upon the 

 nutritive condition of the organism (Sonneborn); namely, under a 

 poor nutritional condition the resorption begins and is completed 

 early, but under a better condition this resorption takes place after 

 several divisions. 



During conjugation reciprocal migration of a pronucleus thus oc- 

 curs in all cases. During biassociation and even in autogamy (p. 203), 

 there develops a conical elevation ("paroral cone") and the nuclear 

 migration takes place through this region. Although there is ordi- 

 narily no cytoplasmic exchange between the conjugants, this may 

 occur in some cases as observed by Sonneborn (1943a, 1944). P. 

 aurelia of variety 4, according to Sonneborn, do occasionally not 

 separate after fertilization, but remain united by a thin strand in the 

 region of the paroral cones. In some pairs, the strand enlarges into a 

 broad band through which cytoplasm flows from one individual to 

 the other. The first division gives off a normal single animal from 

 each of the "parabiotic twins" and the two clones derived from the 

 two individuals belong to the same mating type (p. 192). 



Conjugation between different species of Paramecium has been 

 attempted by several workers. Muller (1932) succeeded in producing 

 a few pairings between normal P. caudatum and exconjugant P. 

 multimicronucleatum. The nuclear process ran normally in cauda- 

 tum, which led Muller to believe that crossing might be possible, but 

 without success. De Garis (1935) mixed "double animals" (p. 228) of 

 P. caudatum and conjugating population of P. aurelia. Pairing be- 

 tween them occurred readily, in which the aurelia mates remained 

 attached to caudatum for five to 12 hours. Four pairs remained to- 

 gether, but aurelia underwent cytolysis on the second day. The 

 separated aurelia from other pairs died after showing "cloudy swell- 

 ing" on the second or third day after biassociation. The caudatum 

 double-animals on the other hand lived for two to 12 (average six) 

 days during which there was neither growth nor division and finally 

 perished after "hyaline degeneration." No information on nuclear 

 behavior in these animals is available. Apparently, the different spe- 

 cies of Paramecium are incompatible with one another. 



In 1937, Sonneborn discovered that in certain races of P. aurelia, 

 there are two classes of individuals with respect to "sexual" differ- 

 entiation and that the members of different classes conjugate with 

 each other, while the members of each class do not. The members of 



