TOWARD A GENETICS OF STENTOR 331 



In the polymorphus studied by Mulsow, the micronuclei showed 

 larger and fewer chromosomes than his coeriileus. The latter 

 seemed to have about 80 in the vegetative stage, while the com- 

 parable number in polymorphus was close to 56. Therefore, in 

 polymorphus, Mulsow could demonstrate reduction of chromosomes 

 during maturation divisions as he could not in coeruleus. During 

 maturation the micronucleus is not homogeneous but has a central 

 chromatic body and it is surrounded by a halo of clear cytoplasm 

 without symbiotic chlorellae. The number of chromosomes 

 seemed to be at first halved to about 28 and then, when the final 

 division of the last remaining micronucleus into the two pronuclei 

 occurred, the chromosomes appeared to be further reduced to 14. 

 Such double reduction would be entirely anomalous, as Mulsow 

 noted, and would even call into question whether the bodies 

 observed were in fact true chromosomes. I suppose that this 

 paradox can be resolved on the basis that in ciliates chromosomes 

 are sometimes dumbbell-shaped and only apparently double. If the 

 chromosome number in coeruleus is reduced from 80 to 20, as 

 Mulsow indicates, one wonders why he did not also consider this 

 a case of apparent double reduction. 



Conjugation accomplishes at least, and probably mainly, a 

 recombination of chromosomes with their varying genetic deter- 

 minants, both by the selection of one set of chromosomes from 

 two for the pronucleus of one animal, and the combination of these 

 with a pronucleus from a different individual. This is accomplished 

 through the micronuclei which alone seem to retain the capacity 

 to form typical chromosomes, but any recombinant difference 

 resulting has to be transmitted to the macronucleus which alone 



k: Anlage enlarges. 



/: Spireme fragments into pieces — like chromosomes — of 



different length but equal thickness. 

 m: These chromosomes split longitudinally into 56 pairs. 

 n: Some chromosomes disappear (?). 

 o: Chromatic bodies now with nucleoli, "plastin" body with 



chromatic center develops and later disappears. 

 p: Anlagen nodulate and attach; micronuclei multipled by 



mitosis. Normal vegetative stentor produced before 



first fission following separation of conjugants. 



