50 



THE CELL AND PROTOPLASM 



it would not disappear after many genera- 

 tions in an altered environment. But a 

 change in the cytoplasm would in the 

 course of time be overcome and dominated 

 by the unchanged nucleus, bringing about 

 a return to the original characteristics. 

 The environmental modifications — which 

 Jollos calls Dauermodifikationen — accord- 

 ing to this view are essentially transitory 

 conditions, forming no part of the system 

 of genuinely inherited characters, which 

 are dependent on diversities of chromo- 

 somal materials. A school of followers 

 has accepted and expounded the views of 

 Jollos on these phenomena (see the sum- 

 mary and exposition by Hammerling 1929). 

 The usual disappearance of the inherited 

 modification after conjugation is by this 

 school seemingly attributed to the profound 

 working over of the cytoplasm that has 

 been believed to accompany conjugation and 

 to produce rejuvenescense. The long con- 

 tinued inheritance of these modifications 

 during vegetative reproduction is held to be 

 an example of cytoplasmic inheritance. 



Does cytoplasmic inheritance indeed oc- 

 cur in these organisms? And if so, what 

 are its conditions and manifestations? 

 What light do the phenomena in Protozoa 

 throw on the role of the cytoplasm in 

 genetic processes? The conditions in the 

 normal sexual reproduction of the ciliate 

 infusoria are such as to give a clear an- 

 swer to these questions, when appropriate 

 crosses of stocks with diverse characteristics 

 are made. To bring this out I must remind 

 you of conditions which may be familiar 

 to you. In the ciliate infusoria sexual re- 

 production takes the form of conjugation. 

 In conjugation two individuals place them- 

 selves side by side, and through small open- 

 ings in the body wall they exchange parts 

 of their nuclei; then they separate, and 

 each continues to reproduce by division as 

 before. 



The essential fact here, for our present 

 purposes, is that each of the two individuals 

 retains its own cytoplasm unmixed, but 

 each receives a half nucleus from the other 

 individual. After conjugation, when the 

 two individuals separate, therefore, each 



has a nucleus that is half from one of the 

 two individuals, half from the other; but 

 each has cytoplasm that is unmixed. The 

 relations of cytoplasm and nucleus in the 

 two may be expressed as follows : The two 

 individuals that conjugate commonly be- 

 long to two different races, differing in 

 characteristics. Call one race A, the other 

 B, A differing from B in both nucleus and 

 cytoplasm. Before conjugation one indi- 

 vidual has nucleus and cytoplasm of race 

 A; the other has nucleus and cytoplasm of 

 race B. After conjugation both individuals 

 have nuclei that are constituted half from 

 race A, half from race B. But one of the 

 two individuals still has the pure cytoplasm 

 of race A, the other the pure cytoplasm of 

 race B. The two are now alike in the con- 

 stitution of their nuclei, but they differ in 

 their cytoplasm. "Will the difference in 

 their cytoplasm make differences in the 

 inherited characters of the two individu- 

 als and their descendants? We have here, 

 placed squarely before us in comparative 

 and experimental form, the question 

 whether differences in cytoplasm result 

 in diversities of inherited characters. No- 

 where else, I believe, is there so favorable 

 an opportunity for testing the relative 

 genetic roles of nucleus and cytoplasm. 



The results of such breeding experi- 

 ments demonstrate that the difference in 

 cytoplasm does indeed make a difference 

 in the inherited characters of the descen- 

 dants. And it shows the conditions and 

 limitations of this cytoplasmic inheritance 

 and its relation to nuclear inheritance. 

 Such results have already been reached 

 with relation to a number of different types 

 of characteristics. They are shown most 

 vividly in crosses between large and small 

 races of these animals. Such crosses be- 

 tween large and small races of Paramecium 

 have been extensively made by De Garis 

 (1935) working at Johns Hopkins Univer- 

 sity. The nature of the phenomena will 

 best be seen from typical examples. 



In a certain race, which we may call A, 

 the individuals are large; the mean length 

 is 198 microns. Individuals of this large 

 race were induced to conjugate with indi- 



