150 THE BIOSYNTHESIS OF PROTEINS 



are 'epigenetic' or 'epinucleic', to take the same words as Nanney (1958) 

 and Lederberg (1958), i.e. 'metabolic' (De Deken-Grenson, 1960), rather 

 than of the nature of wandering genes or virus-hke elements. That is the 

 justification for considering non-mendehan heredity in the chapter of the 

 present book which deals with the regulation of protein synthesis. 



C. CHANGES IN PROTEIN SYNTHESIS DURING 

 DIFFERENTIATION 



The unique cell out of which a metazoan grows, the fertihzed egg, con- 

 tains all the information for making all the specific proteins of the complete 

 organism. The egg does not grow and multiply like a bacterium which 

 gives rise to an offspring of identical bacteria. The egg first divides a few 

 times without growing. This 'cleavage' results in a group of cells which are 

 not identical to each other and which are disposed in space according to a 

 certain pattern. Some of these cells, or blastomeres, contain more mito- 

 chondria, others contain more storage material ; some are larger, some are 

 smaller; droplets of lipids may be found mostly in one region of the 

 segmented egg, grains of pigment in others. Enzymes also are differently 

 distributed among the blastomeres : certain regions of the blastula reduce 

 oxidized dyes much more actively than others, the oxidation enzymes, 

 sulphhydril rich proteins, ribonucleoproteins, phosphatases, are distributed 

 in the cells according to certain gradients which in some cases coincide 

 with the morphogenetic 'fields' defined by the embryologists to describe 

 and explain development. During the segmentation period, little protein 

 synthesis, if any, takes place. But after the egg material has been frag- 

 mented and distributed into so many cells, with their special position in 

 space, growth begins and with it — as a part of it — protein synthesis. Cer- 

 tain regions of the segmented egg will grow faster than others, sheets of 

 cells will move under or above other groups of cells ; interactions between 

 these will cause visible changes in the morphology and arrangements of the 

 individual cells, and thus the embryo will begin to take shape. Specialized 

 structures, tissues and organs appear; differences in shape and properties 

 of the various regions of the embryo become clearer and clearer as develop- 

 ment progresses. 



Thus one fertilized egg gives rise to a very heterogeneous offspring of cells, 

 the properties of which diverge more and more during development, until 

 they are fully 'differentiated' in the adult organism. It is barely necessary 

 to emphasize the differences in the assortment of enzymes and proteins 

 between liver, brain, muscle and pancreas. Clearly, these cells make a com- 

 pletely different set of proteins. When and how do such differences in the 

 specificity of protein synthesis arise during development? 



