414 PRINCIPLES OF EMBRYOLOGY 



This is the only case in which the location of an evocating substance 

 within the cell has been verified; and the trail appears to lead to cyto- 

 plasmic particles and eventually to the microsomes. Brachet (1944, 1947, 

 1952) has advanced a number of lines of argument suggesting that these 

 particles are intimately involved in evocation (p. 212). It may be that all 

 or most of the many substances capable of evocating neural tissue act in 

 the first place on these granules. We saw, however (p. 222), that the 

 microsomes are only one element, though an important one, in the com- 

 plex flux of cellular metabolism, and that one cannot consider the 

 mechanism of evocation without taking account of gene action, protein 

 synthesis, respiration, etc. The biochemical processes symbolised by the 

 two cycles of reactions in Fig. 16.1, from the cytoplasm to the genes, 

 and from the cytoplasm to the gene products, must actually involve all the 

 basic metabolism of the cell. It is fruitless to envisage evocation as a special 

 reaction, carried out by some particular part of the cell which can be, in 

 theory, isolated from the general body of the living system. We must be 

 prepared, therefore, to^md that several different types of influence, im- 

 pinging on the double cycle of cellular metabolism, may result in the 

 same effect of swinging these cycles into one or other of the alternative 

 modes open to them. Similarly, when we think of particular groups of 

 genes being activated in different tissues, the cytoplasmic conditions 

 responsible for this probably cannot be reduced to the mere concentration 

 of certain inactive raw materials which lie quietly there for the genes to 

 utilise. The 'substrates', for which we have envisaged the genes competing, 

 will themselves in many cases be involved in active chemical processes of 

 respiration, energy transfer, etc. The genes and gene products must be 

 thought of as focal points in a continuously active and dynamic system. 



SUGGESTED READING 

 Horowitz 195 1, Lehmann 1950, Schultz 1952. 



