THE SCIENCE OF EMBRYOLOGY 21 



remove part of the tissue from which it would normally develop; or when 

 we notice that embryonic cells usually develop either into one definite 

 tissue (say liver) or into another (such as kidney) but not often into inter- 

 mediates. The situation has been described by saying that development is 

 'canalised' (Waddington 1940^), that is, that there are only a certain num- 

 ber of defined channels along which the developmental processes can go ; 

 and it must be remembered that each course of development involves 

 complex processes in which many different genes are concerned. 



(5) It is obviously not the case that all genes are being equally effective 

 in all cells of the organism; if this were so, there could be no regional 

 differentiation. We must suppose that a group of cells follows one par- 

 ticular canalised process of development because one of the possible com- 

 binations of gene-controlled processes is set going, while in another group 

 a different set of activities occurs. It is in the investigation of how this 

 differential activation of sets of genes is brought about that the genetical 

 and embryological viewpoints are coming closest together at the present 

 time. We have seen that experimental embryology has developed one 

 set of ideas about such matters ; there may be a segregation of ooplasms 

 which can react differently with the nuclei which move into them, or 

 there may be interactions between neighbouring tissues which are sharply 

 distinct in character (in evocation) or only quantitatively different (in field 

 action). It is seldom, in these embryological investigations, that the 

 genes enter explicitly into the picture, but we are dealing with the activa- 

 tion of different pathways of development, and these we know, on general 

 grounds, to be ultimately under genetical control. It is, then, only a differ- 

 ence in the nature of the material being studied, and the techniques 

 available, which distinguishes such work from genetical investigation 

 into the way in which alterations in the cytoplasm or the presence or 

 absence of certain substances in the external medium may stimulate or 

 inhibit the operation of particular genes. This problem, is perhaps, worthy 

 of being called the focus of present-day analytical embryology; it is 

 discussed at some length in Chapter XVI. 



There are, of course, many other principles of more or less restricted 

 validity, which have emerged from developmental studies, but those 

 listed above form the main body of theory which can be generally applied 

 throughout the whole field of embryology. When one reflects on the 

 character of these principles one realises that experimental embryology 

 has as yet hardly reached the stage of being able to investigate the actual 

 causal mechanisms which bring about developmental changes. For the 

 most part, it is still concerned to discover and describe the general nature 

 of the system which is in operation on any particular embryo. If one says 



