328 THE FUNDAMENTAL MECHANISMS OF DEVELOPMENT 



and witliin embryonic fields) demonstrates that the initial differentials 

 which guide the development of various parts of the egg into different 

 channels depend in many cases on substances which can pass from cell 

 to cell and which must therefore be extra-nuclear. There is another 

 method of approach which has penetrated very deeply into this field of 

 the activities of cell parts, that is the science of genetics which has operated 

 mainly by the study of heredity. The knowledge it has yielded has in the 

 main concerned the effectiveness of the nucleus and the chromosomes, 

 an aspect of cell behaviour about which the methods of experimental 

 embryology have not told us very much. More recently, moreover, 

 genetics has begun to produce most valuable information about cyto- 

 plasmic particles, which it knows under the name of 'plasmagenes'. 



It would, of course, be inappropriate to attempt to give even a sketch 

 of all aspects of genetics in a book devoted to embryology, but there is 

 great deal of genetical information wliich is not merely relevant but wliich 

 is essential to our purpose. In fact, when we attempt to discuss the funda- 

 mental mechanisms of development we are in a region in which the dis- 

 tinction between the sciences of genetics and embryology breaks down. 

 Genetics using its normal method ot studying heredity has revealed the 

 existence of genes which control to a large extent the character of the 

 animal which will develop from the fertihsed egg ; the way in which the 

 genes operate in doing this belongs by definition to embryology. The 

 union of the two sciences could hardly be more intimate. 



In the first chapter of this part we shall discuss in broad outline what 

 genetics has revealed about the general nature of the epigenetic system and 

 the way genes are involved in it. This supplements the experimental em- 

 bryological material, to complete the picture of development at the bio- 

 logical level. We shall then pass on to consider the basic mechanisms of 

 development in terms of the activities of cell constituents, using facts some 

 of wliich are conventionally considered to belong to genetics, others to 

 embryology. The greater part of this discussion will be concerned with 

 the differentiation of substance which occurs during development; that 

 is to say with the formation of new materials or new types of tissue. The 

 last chapter deals with the other aspect of development, the moulding of 

 tissues into organs of definite shape and form. 



