768 INHERITANCE 



the power of reproducing itself in its distinctive nature, at the time that 

 growth occurs. In this respect it partakes of the character of genetic ma- 

 terial, since it shows the two distinctive features of that material: it affects 

 the characteristics of the individuals, and it reproduces itself in some 

 degree true to type. But in time it is made over by the new nucleus. 



These facts as to the differential effect of the cytoplasm on inherited 

 characteristics in crosses furnish a basis in normal genetics for the idea 

 of Jollos, set forth in a previous section, that inherited environmental 

 modifications may have their seat in the cytoplasm. These, like the size 

 due to the nature of the cytoplasm in the crosses made by De Garis, are 

 inherited for a number of generations, but finally fade away. But there 

 is so great a difference in the time, the number of generations, during 

 which the inheritance continues, in the two cases, that it raises doubts 

 as to the fundamental similarity of the two. In the crosses, the inherited 

 cytoplasmic effect continues in different cases for ten, twenty, thirty 

 generations, the extreme limit observed being thirty-six generations. By 

 the end of such a period, the cytoplasm has been made over by the 

 nucleus, so that it is thereafter the constitution of the nucleus that de- 

 termines the characteristics. But such experimental modifications as 

 acclimatization are inherited for hundreds of generations. If they are 

 merely modifications of the cytoplasm, it might be anticipated that long 

 before so many generations had passed the cytoplasm would have been 

 made over by the nucleus, so that its modifications would have disap- 

 peared. Yet of course it is not certain that the time required for the 

 nucleus to dominate the cytoplasm would be subject to similar limits in 

 all cases. Here much remains to be discovered. The question whether 

 inherited environmental modifications are exclusively cytoplasmic, or 

 whether they affect the chromosomal materials of the nucleus, must be 

 left open for the present. Decision of this question appears practicable 

 by experimental breeding. What is required is to induce environmental 

 modifications in a clone, then to cross this clone with another which 

 lacks the modification. In conjugation only nuclei with their chromosomal 

 materials are transferred from one clone to the other. If the modifications 

 have affected the nuclei, they should be transferred by conjugation from 

 the modified clone to the unmodified one. But if they affect only the 

 cytoplasm, they will not thus be transferred. The prospects for success- 

 fully carrying through such experiments have been greatly increased by 



