54 



THE CELL AND PROTOPLASM 



These facts as to the differential effect of 

 the cytoplasm on inherited characteristics 

 in crosses furnish a basis from normal 

 genetics for JoUos' idea that inherited 

 environmental modifications may have their 

 seat in the cytoplasm. These environmental 

 modifications, like the size due to the nature 

 of the cytoplasm in De Garis' crosses, are in- 

 herited for a number of generations, but 

 finally fade away. But there is so great a 

 difference in the time (in the number of 

 generations) that the inheritance continues, 

 in the two cases, as to raise doubts as to 

 the fundamental similarity of the two. In 

 the established cases which I have described, 

 the inherited cytoplasmic effect continues 

 in different cases 10, 20, 30 generations ; the 

 extreme limit observed was 36 generations. 

 By the end of such a period the cytoplasm 

 has been made over by the nucleus, and it 

 is thereafter the constitution of the nucleus 

 that determines the characteristics. But 

 such environmental modifications as ac- 

 climatization are inherited for hundreds of 

 generations — in some of Jollos ' experiments 

 for as much as 800 generations. If they 

 are merely modifications of the cytoplasm 

 one might anticipate that long before so 

 many generations had passed the cytoplasm 

 would have been made over by the nucleus, 

 and its modifications would have disap- 

 peared. Yet we do not know that the time 

 required for the nucleus to dominate the 

 cytoplasm would be subject to the same 

 limits in all cases. The question whether 

 inherited environmental modifications are 

 exclusively cytoplasmic, or whether they 

 affect the nucleus, the chromosomes, must 

 be left open for the present. Decision of 

 this question now appears practicable by 

 the methods of experimental breeding. 

 What is required is to induce environmental 

 modifications — acclimatization or the like — 

 in a certain race, then to cross this race with 

 another which lacks the modification. In 

 the conjugation of the two races only nuclei 

 with their chromosomes are transferred 

 from one race to the other. If the modifi- 

 cations have affected the nuclei they should 

 be transferred by conjugation from one 

 race to the other. But if they affect only 



the cytoplasm they will not be thus trans- 

 ferred. The prospects for successfully car- 

 rying through such experiments have been 

 greatly increased by the recent discovery of 

 diverse mating types in these organisms: 

 types which play the role of different sexes. 

 This discovery makes it possible to make 

 any desired crosses as readily in these or- 

 ganisms as in fruit-flies or in rats. 



Returning to the relative role of chromo- 

 somes and cytoplasm in determining in- 

 herited characters in these organisms, we 

 have seen that different individuals may 

 begin life with diverse cytoplasm, though 

 with identical nuclei; and that the differ- 

 ence in cytoplasm produces differences in 

 inherited characters which are inherited for 

 20 to 40 generations or possibly more. We 

 have also seen that the length of time that 

 this differential cytoplasmic inheritance 

 continues makes it probable — perhaps cer- 

 tain — that the different kinds of cytoplasm 

 reproduce to some extent true to type. 



But further, we have seen that while this 

 is occurring the nucleus is slowly influenc- 

 ing the cytoplasm, so that the character- 

 istics influenced by the cytoplasmic consti- 

 tution are slowly changing toward the 

 condition that corresponds to the nuclear 

 constitution. After some time the cyto- 

 plasm is fully dominated by the nuclear 

 constitution, as is shown by the fact that 

 the two ex-conjugants, differing greatly in 

 their original cytoplasmic characters, finally 

 come to identical characteristics. The char- 

 acters induced by the cytoplasm are hence- 

 forth the nuclear characters. 



These phenomena appear to be typical 

 for the relations of nucleus and cytoplasm 

 in genetic differentiations — the differentia- 

 tions of diverse individuals. The primary 

 source of such differentiations is the 

 nucleus. But the nucleus impresses its 

 constitution on the cytoplasm, doubtless 

 through the material interchanges described 

 in our earlier paragraphs. The cytoplasm 

 retains the constitution so impressed for a 

 considerable time, during which it assimi- 

 lates and multiplies true to its impressed 

 character. It may do this after removal 

 from actual contact with the nucleus to 



