THE SCIENCE OF EMBRYOLOGY 19 



Very important advances are, and will undoubtedly continue to be, 

 made by these methods. The more strictly biochemical approach, how- 

 ever, encounters the difficulty that many of the happenings in a developing 

 cell are probably related more closely to its maintenance as a living con- 

 cern than to its determination and differentiation. It seems unhkely that we 

 can hope to obtain anything like a satisfactory understanding of develop- 

 ment in biochemical terms until we can comprehend the whole working 

 of the cell, as regards maintenance as well as change. This consideration 

 leads me personally to the opinion, which is by no means the most fashion- 

 able one, that it is premature to look to biochemistry to provide the main 

 framework of ideas for embryology. 



There is another approach which still requires discussion: that derived 

 from the genetical fact that the character of differentiated organs and 

 tissues is controlled by genes. Most people are willing to admit the rele- 

 vance of this to embryology, but a study of recent books and discussion- 

 symposia will show that in practice the contribution of genetics to embryo- 

 logical thought is still rather tenuous. This is in the main due to the fact 

 that developmental genetics has been studied chiefly by people whose 

 interests were primarily genetical, and who have posed the question: 

 How does a given gene operate, what is the connection between a certain 

 nucleo-protein constituent of a chromosome and some event in the cell 

 containing it? This is obviously a fundamental question in its own right. 

 But the progress towards an answer to it has arrived so far at httle more 

 than the statement that a change in a gene often affects the activity of a 

 cellular enzyme or other complex molecule. From an embryological 

 point of view, such a conclusion is somewhat trite. For embryology the 

 question should be turned upside down; not, how does a gene operate, 

 but how is a developing tissue affected by the genotype of the cells ? We 

 already have an answer to this which goes far enough beyond the com- 

 monplace to make a considerable difference to our whole outlook on 

 embryological problems. 



Let us therefore turn to sketch, in equally bold outline, the kind of 

 information which has been acquired by the genetical methods of 

 analysing development. This can be summarised as follows : 



(i) There is no reason to suppose that there is any category of develop- 

 mental processes which is not ultimately controlled by genes. Many of 

 the older authors suggested that genes affect only the details of an animal's 

 structure while the broad outlines of it were dependent on something 

 else. This idea contained a certain germ of truth in so far as the basic plan 

 of the animal body is laid down in the ooplasmic segregations in the 

 fertihsed egg ; but we now know of cases which show that the pattern of 



