384 PRINCIPLES OF EMBRYOLOGY 



that it remains quite possible that the influence of the nucleus is indirect 

 and mediated through them. Again, Brachet and Chantreime have shown 

 by radioactive tracer studies that if one removes the nucleus from the large 

 unicellular alga Acetabularia (cf. p. 377), protein synthesis eventually de- 

 creases in the non-nucleated fragment (cf. Brachet 1954). But it is remark- 

 able that the activity continues unaltered for almost a fortnight and re- 

 mains very considerable for much longer; from this they draw the 



^■■■H 



Figure 17.6 



Diagram of nerve regeneration. Rows ^ to jE show the normal process when 

 the nerve is simply cut; regeneration takes place from the proximal portion. 

 Rows F, G, H show that if, after the stage of row D, the fibre is constricted, 

 the new cytoplasm being synthesised in the proximal (nuclear) end of the 

 cell forms a swelling. When the constriction is removed (/) this cytoplasm 

 gradually spreads distally. (After Weiss and Hiscoe 1948.) 



conclusion that the nuclear control of protein synthesis is indirect. The 

 final proof that the microsomes take part in the process, and are the actual 

 site of the new production of protein, would be given if they could be 

 transplanted to some unusual location, say to another cell of different 

 developmental fate, and it was found that the protein characteristic of the 

 transplanted material was produced in the new position. Attempts to do 

 this with isolated microsomes have so far been unsuccessful. The results 

 of high-speed centrifugation of ascidian eggs (p. J14) or of echinoderm 

 eggs (p. 90) may, however, probably be interpreted in this way, since 



