KINETOSOME AS A VISIBLE CYTOIM.ASMIC UNIT 3 



already been described. Many of tlicm deal with ihv prop- 

 erties of chl()ro]:)lasts, absent in animals. The i)hisnia^enes, 

 theoretically necessary for dev(dopni(Mit. are, so far, exceed- 

 ingly rare in heredity. Their j)reseii('e lias not Ixhmi demon- 

 strated directly; they have not been seen; and they remain 

 in the most irritating form of a logical hypothesis. It seems 

 therefore of utmost importance for the progress of biology, 

 and also of biochemistry, to develop this particulate con- 

 ception of cytoplasm. 



While discussing the problems of modulation and differ- 

 entiation, Paul Weiss (1947) has tried to transcribe what 

 he calls ''the symbolic concepts of cells and protoplasm" in 

 terms of molecular phenomena and has introduced the con- 

 cept of "molecular ecology," according to which a cell is to 

 be viewed as an organized mixed population of molecules 

 and molecular groups. 



Let us quote some of his propositions: 



1. "Each population is made up of molecular species of 

 very different composition, sizes, densities, rank, and stabil- 

 ity, from trivial inorganic compounds to the huge and highly 

 organized protein systems. Some segments of these popu- 

 lations occur in relatively constant 'symbiotic' groupings, 

 often of a limited size range ; these form the various particu- 

 lates of the cell content. 



2. 'Tt is one of the fundamental characteristics of cellu- 

 lar organization that the various species constituting the 

 population are not self-sufficient, but depend in various de- 

 grees upon other members of the population as well as upon 

 the physical conditions prevailing in the space they occupy. 

 Survival and orderly function of the total population are 

 predicated on the presence of all essential members in defi- 

 nite concentrations, combinations, and distributions. 



3. "In view of this intricate interdependence, given mo- 

 lecular species can exist, and given interactions between 

 species can occur, only within a certain limited range of 

 conditions specific for each kind. We might call these con- 



