SITES WITHIN THE CELL 41 



violet absorption and ribonuclease test definitely confirmed that RNA is 

 responsible for both the basophilia and the ultraviolet absorbancy of the 

 cytoplasm (Gersh and Bodian, 1943; Davidson and Waymouth, 1946). 



It had been known for a long time that the intensity of basophilia varies 

 considerably from one type of cell to another. Chemical determination of 

 RNA on various tissues confirmed that basophilia and gross content in 

 RNA go together. Animal tissues can be arranged in the following way 

 according to decreasing basophilia or RNA content : pancreas > intestinal 

 and gastric mucosa > liver > spleen > lymph nodes, testis > kidney, 

 muscle, heart, lung (Brachet, 1941b; Davidson and Waymouth, 1946). 

 Brachet (1941) and Caspersson (1941) both called the attention upon the 

 fact that the cells which contain large amounts of RNA are cells which pro- 

 duce large amounts of proteins. Systematic studies on specialized cells which 

 are making protein, like silk gland or frog oocytes at the time of synthesis 

 of yolk platelets, confirmed this rule which suggested that somehow RNA 

 must be involved in protein synthesis. This hypothesis exerted a very 

 stimulating effect on research in cytochemistry and biochemistry, and much 

 of the development of RNA studies in the past twenty years is due to the 

 recognition of a link between RNA and protein synthesis. An immediate 

 consequence of this idea was to suspect that the ergastoplasm is the main 

 site of protein synthesis. 



B. FRACTIONATION OF CELL ORGANELLES 



1 . Basic Observations 



The work of Claude (1939) on the purification of the agent of Rous 

 papilloma led him to isolate from chick embryos ultramicroscopic particles 

 which contained RNA, lipid and protein (Claude, 1940). A survey of many 

 different organs of vertebrates and invertebrates, as well as plants and yeast 

 showed that particles comparable to those isolated by Claude are present in 

 all kinds of cells (Claude, 1943, 1944, 1946; Brachet and Jeener, 1944). It 

 was further established in these researches that the bulk of cellular RNA is 

 associated with these small particles. When animal tissues are homogenized 

 in dilute salt solution, as was commonly done in these earlier works, a great 

 variety of particles of different sizes are obtained; all of them seem to 

 contain RNA and the smaller the particles, the greater is their RNA con- 

 tent (Chantrenne, 1947). The fractionation procedures used at the time 

 were not very satisfactory. None of the well-known cell constituents, except 

 damaged nuclei, could be recognized in the homogenates and no clear 

 connexions were established between the fractions obtained by centrifuga- 

 tion and cell structures. A great progress was made when it was observed 



