46 



THE CELL. AND PROTOPLASM 



different cells. What chromosomal mate- 

 rials shall come into action in any cell must 

 depend on what cytoplasmic materials are 

 already in the cell. Thus the cytoplasm is 

 not merely passive; it actively determines 

 what shall happen in the chromosomes, as 

 was demonstrated in a striking case long 

 ago by Boveri. Every cell retains all the 

 chromosomal materials, but which of these 

 materials comes into action in each cell 

 depends on what cytoplasm is there present, 

 as well as upon the conditions under which 

 the reactions occur, a matter with which we 

 are not here concerned. 



This process of diversification of parts 

 continues, by the methods already set forth. 

 It is not my province to follow in detail the 

 infinitely varied phenomena of develop- 

 ment. But the fundamental phenomena, 

 for our purposes, are those just indicated: 

 interaction of chromosomes and cytoplasm 

 to produce new materials that are incor- 

 porated into the cytoplasm; localization of 

 these diverse materials in definite patterns 

 and in different cells; repeated interaction 

 of these diverse materials with each other 

 and with the chromosomes, yielding addi- 

 tional differentiations, until at the end the 

 many diverse tissues and organs of the 

 developed body have been formed. Every 

 cell, at least for the greater part of develop- 

 ment, contains the whole set of chromosomes 

 with their genes. The differentiations of 

 the cells are not in their chromosomes, but 

 in their cytoplasm, produced through reac- 

 tion with the chromosomes. 



Such, then, appears the nature of chromo- 

 somal and cytoplasmic interaction in pro- 

 ducing one kind of differentiation — the dif- 

 ferentiation of the single individual into 

 diverse tissues, organs, and functions, dur- 

 ing ontogenetic development. 



But there is another type of differentia- 

 tion, quite diverse from that so far sketched. 

 A species is not uniform ; its individuals are 

 diverse. They develop differently, yielding 

 diversities of form, structure, and function 

 — diversities in what we call their heredi- 

 tary characteristics. It is this diversity 

 among individuals, along with the comple- 

 mentary similarity of some individuals, 



that presents the great problem of genetics. 

 The differentiations within the single body 

 form the problem of experimental embryol- 

 ogy. It is the province of genetics to in- 

 quire as to the origin of the differences and 

 similarities among different individuals. 

 Ontogenetic differentiation and genetic dif- 

 ferentiation are two diverse problems. 



The studies of genetics show that differ- 

 ences among the chromosomes play a very 

 great role in the production of diversities 

 among individuals. Individuals are diverse 

 in their hereditary characters because they 

 begin life with diverse chromosomal mate- 

 rials. The chromosomes present in the 

 collective members of a given species are 

 highly diversified ; they differ greatly in the 

 different individuals. When these individ- 

 uals unite in pairs for reproduction, great 

 numbers of diverse combinations of chromo- 

 somal materials are produced; and these 

 diverse combinations yield individuals dif- 

 fering in their characteristics — in form, 

 colors, structure, chemical properties, and 

 functions. The system according to which 

 these diversities are produced through the 

 distribution of chromosomal materials con- 

 stitutes Mendelian heredity. 



Has the cytoplasm a role in producing 

 these hereditary differences? Are some of 

 the hereditary differences between indi- 

 viduals due to the fact that the different 

 individuals begin life with different types 

 of cytoplasm? May differences in cyto- 

 plasm at the beginning of development pro- 

 duce diverse characteristics in different 

 individuals, just as differences in chromo- 

 somal materials may do? And may such 

 cytoplasmic diversities be transmitted from 

 generation to generation, as chromosomal 

 diversities are transmitted? These are the 

 fundamental questions as to the role of 

 cytoplasm in genetics. 



In a series of articles a few years ago, 

 East (1934) summarized the evidence on 

 these questions drawn from the study of 

 genetics in multicellular organisms. I will 

 not attempt to summarize the summary of 

 East, but will present certain phenomena 

 in the genetics of some of the higher Pro- 

 tozoa that appear to illuminate some of 



