RNA AND CONTROL OF CELLULAR PROCESSES 125 



iris. Induction is mediated by a substance excreted by the retina 

 (Wachs, 1914; Zalokar, 1944), and the presence of tlie inducer is 

 required during the early phases of lens growth. The nature of this 

 substance is still entirely unknown. The important thing in this 

 developmental process is that as an effect of induction, cells, besides 

 their morphological changes, start to produce a new set of proteins, 

 specific for lens tissues. Preceding this protein formation, an in- 

 crease in cellular basophilia could be observed. 



There are other systems in the development of organisms, where 

 cells differentiate into new types destined to produce specific pro- 

 teins. Thus, mesenchyme cells differentiate either into cartilage pro- 

 ducing chondroblasts or muscle-fiber producing myoblasts. Either 

 new tvpe of cell is specialized in the production of proteins relatively 

 well known chemically and structurally. Holtzer ( 1961 ) discusses 

 this problem in a recent review. In the formation of red blood cells, 

 erythroblasts differentiate into cells specialized in the production of 

 one main protein, haemoglobin. Haemoglobin is one of the better 

 known proteins today, with even its tertiary structure determined 

 ( Perutz et al., 1960 ) . The formation of haemoglobin has been stud- 

 ied extensively and the process can be obtained in vitro by whole 

 cells (Borsook, 1956) or by homogenized preparations (Schweet 

 et al., 1958b). Cytochemical studies indicated the importance of 

 RNA in erythroblast growth and production of haemoglobin (Tho- 

 rell, 1947)'. 



Both lens formation and mammalian red blood cells offer another 

 interesting feature of importance in the study of gene action and 

 ENA function. After the specific protein formation is initiated, the 

 cells lose their nuclei. Protein formation can continue, but no new 

 RNA formation is observable. Cellular RNA decreases with time 

 and protein synthesis becomes reduced also. No new protein syn- 

 thesis can be found after total RNA disappearance. 



In the silkworm {Bombijx mori) silk-gland cells are synthesizing 

 proteins at a rate surpassing all other known protein producing cells. 

 The gland consists of two functionally different parts, the first pro- 

 ducing fibroin, the main silk protein (Lucas et al., 1958), the second 

 producing sericin, the second component of silk. The character- 

 istic of gland cells is an enormously increased nucleus and chroma- 

 tin (Gilson, 1890), as if genes had to multiply in order to provide 

 enough sites for the high synthetic activity. Needless to say, the 

 rate of production of RNA in those cells is very high. It appears 



