752 INHERITANCE 



new combination of chromosomes, half derived from each conjugant, 

 but has the same cytoplasm as before. This gives an opportunity to com- 

 pare the relative roles of chromosomes and cytoplasm in inheritance, 

 since in the ex-con jugants the chromosomal combination is changed, 

 but the cytoplasm is not. 



The only qualification required by the statement that cytoplasm is not 

 changed is the fact that a minute bit of cytoplasm carrying an aster pre- 

 cedes the migratory pronucleus into the opposite conjugant. The results 

 show, as will be seen, that this minute bit of cytoplasm is not effective in 

 determining inheritance. 



The two individuals that conjugate are, in some species at least, physio- 

 logically differentiated into diverse "mating types," which play the same 

 physiological role in bringing about conjugation as do diverse sexes. 

 For an account of these. Chapter XIV is to be consulted. 



Knowledge of biparental inheritance is much less exact and extensive 

 in the ciliates than in the flagellates. The recent discovery of diverse 

 mating types furnishes an opportunity for exact analysis; but the novelty 

 of this discovery has not yet given opportunity for its full investigation. 



The earlier work on inheritance in conjugation before the discovery of 

 mating types, is summarized in the author's Genetics of the Protozoa 

 (1929) . In the early work numerical ratios were not obtained, but quali- 

 tative relations of importance were demonstrated. Most generally ex- 

 pressed, the work showed that conjugation results in the production of 

 many hereditarily diverse biotypes from the two involved in the conju- 

 gation. Production of hereditary diversities at conjugation was demon- 

 strated by the work of Jennings in respect to the following types of 

 characteristics: rate of fission, rate of mortality, presence of abnormali- 

 ties. This demonstration of the production of inherited differences at 

 conjugation was extended in 1930 by the work of Raff el, and in 1932 by 

 Jennings, Raffel, Lynch, and Sonneborn to various other characteristics, 

 including size and form, vigor, resistance, and degeneration. It was fur- 

 ther shown that conjugation causes the descendants of the two members 

 of a pair to become similar in fission rate, in mortality, in the occurrence 

 of abnormalities, and probably in size (see Jennings, 1929, pp. 181-85). 



Later, more exact work on inheritance in ciliates deals with the inherit- 

 ance of mating type (Sonneborn, 1937-39; and Jennings, 1938-39) 



