THE SYNTHESIS OF NEW SUBSTANCES 383 



cytoplasm, and that it is at these particles that the main synthesis of 

 cytoplasmic proteins occurs. 



There is as yet, perhaps, no evidence which finally settles the question 

 of whether cytoplasmic synthesis is directly under the control of the 

 nucleus or whether the microsomes are essentially involved. The import- 

 ance of the nucleus, either at first or second hand, can of course not be 



FiGunE 17.5 



Protein synthesis according to Caspersson. A, the 'nucleolus-associated 

 chromatin', containing ribo-desoxy-nucleotides (RDN), proteins (P), di- 

 amino-rich proteins (DP) and perhaps ribose-nucleotides (RN). B, the 

 nucleolus, containing DP and smaller amounts of RN. C, the nucleus, in 

 which there is a gradient of P and DP towards the nuclear membrane D. 

 In the cytoplasm E there is a gradient of P and RN from the nuclear mem- 

 brane outwards. (After Caspersson 1950.) 



denied. For example, Weiss and Hiscoe (1945) have shown that in a 

 neuron growth does not take place throughout the enormously elongated 

 axon, but synthesis occurs only in the cell body in the neighbourhood of 

 the nucleus; the new cytoplasm flows from this region towards the tip of 

 the fibre, and if the axon is constricted, the flow becomes dammed up 

 and a swelling appears proximal to the constriction (Fig. 17.6). The cell 

 body is, however, also the region in which the cytoplasm is most baso- 

 philic and probably the site of the main concentration of microsomes, so 



