132 HOW ANIMALS DEVELOP 



simple story of how one gene influences its correspond- 

 ing protein, and this is one of the most active fields of 

 present-day research. However, there is still another 

 problem, which is more difficult and in which we have 

 as yet made less progress. The developing cell is not 

 simply a bag in which a lot of quite separate processes 

 of protein synthesis are proceeding. On the contrary, 

 it is clear that the processes going on in it must be 

 co-ordinated in some way. A muscle cell produces, in 

 the right quantities, the various proteins that have to 

 be joined together to build up a functioning muscle 

 fibre. Similarly, in every other type of cell, there is a 

 fairly definite quantitative relation between the 

 amounts of the different proteins it contains. We have 

 very little idea how this integration of the processes 

 of synthesis is brought about. One hypothesis— though 

 perhaps at present it is little more than a guess— is 

 that the complex structural architecture which the 

 electron microscope is revealing has something to do 

 with this matter. It is certainly remarkable that now 

 that we can examine the interior of cells in much 

 greater detail than before, we keep on finding that 

 they possess elaborate systems of membranes and other 

 structures. These do not seem to be called for by the 

 simple story of protein synthesis described above, but 

 it is very difficult to believe that they have no function 

 at all. If we suppose that they operate in some way to 

 co-ordinate the processes of development, that does at 

 least provide them with a raison d'etre. 



