THE MITOTIC CYCLE 



mixtures of amino acids, vitamins, etc. (Fischer et alii^^^ ^2') or with 

 extracts of kidney, yeast or malt (Fischer ^-^ a/i^^^e i28-i31) restored the 

 deficient medium to an extent that growth was again possible. Although 

 it has been demonstrated (White and Lasfargues132) ^h^t dialysis does 

 not achieve a clear cut distinction between accessory factors and others, 

 inasmuch as some activity can be restored to the dialysed mixture by 

 dilution with Tyrode solution containing no additional supplements, 

 Fischer's experiments are of great importance in demonstrating the re- 

 quirements of cells for certain low molecular nutrients, and particularly 



0-2 2'V^ V-S B-8 



'2-2V 2't-ZS 26-28 2fi- 30 



6-10 10-12 12-n 11-16 16-18 18-20 20-2:1 

 Time in 2-\v periods 



Figure 62 Mean curves showing the effect on the mitotic index of applying 1 5 per 



cent embryo juice to cultures of chick periosteal fibroblasts for different lengths of 



time, from 1-30 hours. From Jacoby et aliP^^ {By courtesy, J. exp. Biol.). 



for amino acids. Other evidence, especially from the work with fully 

 synthetic media (White ;^^ ^° Morgan et alii^^), demonstrates that the 

 low molecular 'accessory growth factors' with which Fischer supple- 

 ments his medium are precisely those of primary importance for main- 

 tenance. It underlines again the fact that, for cell division to be able to 

 take place, the nutritional requirements of the cells must first of all be 

 supplied. In a medium completely optimal except for the omission of a 

 single component, restoration of that one limiting factor would raise it 

 to the status of 'the' chemical stimulus to cell division. 



176 



