386 PRINCIPLES OF EMBRYOLOGY 



are relatively long structures, rather than 'point-genes'; and electron 

 microscope studies on cytoplasmic structure (Fig. 17.2) suggest that we 

 may have tended also to under-estimate the size and complexity of the 

 active agents in protoplasm. We may, perhaps, find that it is necessary to 

 think of gene action in terms not of enzymes or other protein molecules 

 as we know them in solution, but of extended protein sheets and fibres, 

 which may have properties which largely escape our present biochemical 

 methods. Possibly it is in this way that we shall fmd an explanation both 

 for the very large number of genes which affect the development of an 

 organ and for the extreme precision with which they control it. 



SUGGESTED READING 



Bracket 1952^, b, Haldane 1954, Chapter 2, Goldschmidt 195 1, Muller 1947, Pontc- 

 corvo I952fl, b, Spiegelman 1948, Schultz 1952, Wright 1945. 



