INHERITANCE 767 



1.6 and 1.8. After an interval of twelve days the rates were taken again. 

 They were now the same in the two ex-con jugant clones; during seven- 

 teen days they stood for both at 2.0 to 2.5 fissions daily. Other crosses 

 between fast and slow clones showed similar conditions. 



The effect of the cytoplasmic constitution in thus delaying the as- 

 sumption of the final characteristics resulting from the nuclear consti- 

 tution is commonly known as the "cytoplasmic lag." 



A cytoplasmic lag of a similar sort is at times seen in the inheritance 

 of mating type at endomixis in exceptional clones of P. aurelia, as de- 

 scribed by Kimball (1939). Normally in both P. aurelia and P. bursaria 

 no such lag is evident, unless it be in the fact that there is a period of 

 immaturity during which no mating reaction occurs. The cases described 

 by Kimball are in certain clones of variety 1 of P. aurelia. These clones 

 were of the mating type II. At endomixis some of these are transformed 

 to type I, as before set forth. But for a few generations after the forma- 

 tion of the new nucleus, they remain of type II, and will still conjugate 

 with individuals of type I. After these few fissions, however, the new 

 nucleus asserts itself and the members of the clone transform to type I; 

 they now conjugate only with type II. The animals thus, after the forma- 

 tion of the new nucleus, remain for a time of the mating type appropriate 

 to the cytoplasmic constitution only; then transform to correspond to the 

 new nuclear constitution. 



These phenomena illuminate the role of the cytoplasm in inheritance. 

 At every fission the volume of the cytoplasm present in any individual 

 is reduced one half; then the original volume is restored by new growth. 

 Thus, in the case of the ex-con jugant of the large race A of Figure 176, 

 the descendants of which are gradually diminished in size after conjuga- 

 tion, the size of the individual is reduced at the first fission after conju- 

 gation to one half the original size, so that if growth did not occur the 

 body would in two or three generations be reduced to the final size. 

 But owing to the properties of the cytoplasm of that race A, growth oc- 

 curs after the first fission to practically the original racial size. At every 

 succeeding fission the original cytoplasm is again diluted one half, so 

 that after ten generations, it has been diluted to less than one-thousandth 

 part, the remainder 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 cytoplasm must therefore have to some extent 



