CHROMOSOMES AND CYTOPLASM IN PROTOZOA 



53 



and cytoplasm in producing these results. 

 The final size is in each case determined by 

 the nuclear constitution, since in every pair 

 the final sizes are the same for the de- 

 scendants of the two ex-conjugants. The 

 nuclear constitution, so far as it affects 

 size, is the same in the two ex-conjugants 

 of any pair — a fact which reveals to us 

 at what point in maturation the reduction 

 division for size genes occurs. It shows 

 that reduction does not occur at the third 

 maturation division; otherwise in half the 

 cases the ultimate sizes would be different 

 in the descendants of the two ex-conju- 

 gants. 



But for a long time — for 22 generations 

 in the case illustrated in Fig. 1 and for 

 some 36 generations in the extreme case 

 observed — the cytoplasm affects the size of 

 the individuals. The two individuals be- 

 gan life after conjugation with cytoplasm 

 of different constitution, and the difference 

 in cytoplasm results in a difference in size. 



A similar differential effect of the cyto- 

 plasm is seen in the inheritance of other 

 characters. Sonneborn and Lynch (1934) 

 in the laboratory at the Johns Hopkins 

 University found such a cytoplasmic effect 

 in crosses of two races that differed much 

 in rate of multiplication. This work was 

 done before the work of De Garis; I em- 

 ployed the results of De Garis in intro- 

 ducing the matter because of the advan- 

 tages in presentation of such a character 

 as size diversity. In the cases described 

 by Sonneborn and Lynch, one of the two 

 races reproduced by fission frequently and 

 rapidly, while in the other reproduction 

 was slow, occurring only at long intervals. 

 After two such races have united in con- 

 jugation and exchanged halves of their 

 nuclei, the one that retains the cytoplasm 

 of the slow race continues to divide slowly, 

 while the other, having the cytoplasm of 

 the rapid race, continues to divide rapidly. 

 This effect of the cytoplasm continues for 

 about ten generations. But during that 

 time the difference in fission rate for the 

 two sets becomes gradually less, until after 

 a certain number of generations the rate 

 of reproduction is the same in the two — a 



rate determined by the nuclear constitu- 

 tion. 



A similar effect of the cytoplasm in de- 

 laying the change of characters induced 

 by the nuclear constitution is at times seen 

 in the inheritance of differing sex types 

 (Kimball 1939). This matter is complex 

 and I shall not present it. The effect of 

 the cytoplasm in delaying the assumption 

 of the final characteristics resulting from 

 the nuclear constitution has come to be 

 spoken of by the workers in this field as 

 the ** cytoplasmic lag." 



Analysis of these phenomena yields some 

 insight into the method of action of the 

 cytoplasm, and the relative role of nucleus 

 and cytoplasm in inheritance. The influ- 

 ence of the cytoplasm on inheritance must 

 be considered in connection with what 

 happens to the cytoplasm at vegetative 

 reproduction. At every fission the volume 

 of cytoplasm present is reduced one half; 

 then the original volume is restored by 

 growth, that is, by production of an equal 

 volume of new cytoplasm. In the case of 

 the large race A which is gradually dimin- 

 ished in size after conjugation (Fig. 1) the 

 size is reduced at the first division after 

 conjugation to one half the original size, 

 so that if growth did not occur, the body 

 would in one or two generations be reduced 

 to the final size. But owing to the proper- 

 ties of the cytoplasm of that race A, growth 

 occurs after the first division to practically 

 the original racial size. At every succeed- 

 ing fission the original cytoplasm is diluted 

 to one half, so that after ten generations it 

 is diluted to less than 1/1000 part, the re- 

 mainder being new cytoplasm produced by 

 growth. Yet after ten generations the 

 nature of the original cytoplasm still has a 

 marked effect on size. The original cyto- 

 plasm seemingly must therefore have to 

 some extent the power of reproducing itself 

 in its distinctive nature, at the time that 

 growth occurs. In this respect it partakes 

 of the character of a gene or genetic mate- 

 rial, in that it affects the characteristics of 

 the individuals and reproduces itself in 

 some degree true to type. But in time it is 

 made over by the nucleus. 



