168 



PROTOZOOLOGY 



of hybrid clones obtained by crossing diverse clones manifested, 

 on the average, characteristics intermediate between those of the 

 l)arent clones or intermediate between those of the two groups of 

 clones obtained by inbreeding the parent clones. Hybrid clones 

 are of two types differing in the origin of their cytoplasm and 



Fig. 83. Hybridization of Chlamydomonas (Strehlow). a, C. paradoxa; 

 h, C. botryodes; c-f, heterozygotes between them. 



macronuclear fragments; one type derives these from one parent 

 clone, the other type from the other parent clone entering into the 

 cross. One hybrid clone of each type arises from each pair of 

 hybrid exconjugants. Sets of each of the two types of hybrid 

 clones, as well as groups including both types of sets, were inter- 

 mediate in characteristics. When hybrids of either type of cyto- 

 plasmic descent were inbred, the resulting F2 generation included 

 some clones resembling one parent, some clones resembling the 

 other parent, and some clones with intermediate characteristics. 

 When such F2 segregates were further inbred, the resulting F3 

 generation showed that some F2 segregates were pure and others 

 still mixed in genetic constitution." 



Sonneborn and Lynch point out further that "there is no longer 

 ground for doubting that the nucleus carries the determiners of 

 hereditary characters, and there is considerable evidence that the 

 nucleus carries these determiners arranged in separable pairs like 

 the chromosomes or genes of higher organisms. If the cytology 

 of the chromosomes in Paramecium were better known, the exact 

 strength of the latter point could be more precisely estimated. 

 For P. aurelia, Hertwig (1889) described a small number of 

 chromosomes (8-10) undergoing conventional reduction during 

 conjugation; but other investigators of this species have not 

 hazarded chromosome counts and have given the impression that 

 little progress in Paramecium chromosome cytology can be ex- 



