MATING TYPE DETERMINATION 269 



The fate of the fragments of the old niacronucleus is the same 

 in all three fertilization processes. They are passively distributed to 

 tlie products of successive iissions. Since the old niacronucleus breaks 

 down ordinarily into about 30 to 40 fragments, the cells contain on 

 the average 15 to 20 after one fission, 7 to 10 after two fissions, and 

 so on. Meanwhile the fragments slowly distintegrate, but their disin- 

 tegration usually does not begin until after the first or second cell 

 division. 



In 1942 Sonneborn reported a variation in the behavior of these 

 fragments and of the new macronuclear anlagen which occurs in both 

 groups of varieties and which provides a useful tool in genetic analy- 

 sis. The new macronuclear anlagen may either fail to arise from 

 products of the syncaryon or their development may be temporarily 

 inhibited so that, although segregated at the first cell division, they 

 may fail to divide at the second or third cell division. In either case, 

 cells arise that lack macronuclear anlagen but contain fragments of 

 the old macronucleus. In these the fragments fail to disintegrate. 

 Instead they grow and continue to be segregated until only one is 

 present in each cell. By this time each fragment has reached the 

 size of the normal macronucleus and thereafter divides at each cell 

 division. The whole process of development of a new macronucleus 

 from a single fragment of the old macronucleus is known as macro- 

 nuclear regeneration. 



In so far as present information goes, regenerated fragments con- 

 trol the same hereditary traits as the macronucleus from which they 

 were derived. Presumably, therefore, each fragment contains at least 

 one complete set of nuclear genes. These results would be expected 

 on either of two very different hypotheses. Sonneborn (1942) origi- 

 nally interpreted macronuclear regeneration as indicating a compound 

 macronuclear structure, i.e., a macronucleus composed of discrete 

 genetically balanced subnuclei. Kimball (1943) proposed as an alter- 

 native explanation that the macronucleus contains a great many sets 

 of chromosomes randomly arranged and assorting more or less at 

 random when the macronucleus divides or breaks down into frag- 

 ments. It is apparent that either of the postulated nuclear structures 

 could result in macronuclear regeneration as found, but that the nec- 

 essary number of chromosome sets would be much greater in a mac- 

 ronucleus constructed according to Kimball's hypothesis. 



Macronuclear regeneration occurs under ordinary conditions 



