THE ACTIVATION OF GENES BY THE CYTOPLASM 365 



probably even after all division has ceased, the genes must manufacture 

 substances w^hich pass into and influence the cytoplasm. We shall discuss 

 the nature of these substances in the next section. The point which is being 

 made here is that the genes are producing substances; and in order to do 

 this they must use some raw materials. It is therefore possible that one of 

 the factors which controls the intensity v^th which the genes operate is 

 the availabiHty of the relevant raw materials. 



One form which the competition for raw materials might take would be 

 that the actual total quantities of certain substances set a limit to the activity 

 of particular genes which specially required them. It is perhaps rather 

 improbable that competition of this kind plays an important part in 

 development. When an egg is cut into fragments the amounts of cyto- 

 plasmic raw materials available to the nucleus will of course be reduced. 

 Nevertheless, if the cut is made in the right direction a normal embryo 

 may be produced. It seems more plausible to suggest that it may be not 

 the total amounts of substances but rather their relative concentrations 

 which influence the activities of the genes. 



There are, of course, other theoretically possible types of control 

 mechanism; for instance some regions of cytoplasm may contain sub- 

 stances which specifically stimulate or inhibit particular genes, somewhat 

 in the manner of co-enzymes or enzyme inhibitors: or the products 

 which the gene passes into the cytoplasm may themselves tend to increase 

 or diminish further gene activity. Our knowledge is so slight that it is 

 hardly profitable to enumerate any more possibilities. It is worth pointing 

 out, however, that it is perhaps at this level that one should look for the 

 explanation of the fact that in any given species of animal there are only a 

 limited number of rather sharply distinct alternative paths of develop- 

 ment. We shall show later that this is an expected consequence of any 

 system in which a number of different chemical processes either compete 

 with another for a limited set of substrates, or interfere by other kinds 

 of mutual stimulation or inliibition. We shall see reason to suppose that 

 the cell contains another set of similarly competing or interfering reactions, 

 namely those which operate between the products which the genes pour 

 out into the cytoplasm. It is not clear to which of these two systems — that 

 involving genes and their raw materials, or that involving gene products 

 and their raw materials — the formation of alternative pathways of 

 development is due, but we shall discuss the matter more thoroughly in 

 connection with the gene-product system, which will be dealt with in 

 the next section. 



Our information about the mechanism of the cytoplasmic control of 

 gene activity is, perhaps, fullest in comiection with the phenomenon of 



