IN the introductory chapter a Hst was given of the basic concepts wHch 

 experimental embryology has developed-those of determination and 

 differentiation, brought about by the mechanisms of ooplasmic segrega- 

 tion, evocation and field action. The accounts of the development of the 

 various classes of animals given in Part One have exhibited the type of 

 facts from which these concepts have been derived. The facts have, for 

 the most part, been obtained by typically biological modes of experiment; 

 for instance, by the transplantation from one place to another of whole 

 cells, or even large masses of cells. The theories to which they have given 

 rise have in consequence also been framed in terms of concepts which 

 belong to the biological realm and cannot be immediately brought into 

 Ime with the ideas of the more fuUy developed sciences of physics and 

 chemistry. As Weiss (1947, 1950^) in particular has emphasised, we cannot 

 be permanently satisfied with this situation, but must attempt to push our 

 analyses down towards the level at which we are discussing the inter- 

 actions, combinations and synthetic activities of particular substances. In 

 this connection he has coined the expressive phrase molecular ecology' 

 and there is no doubt that a fuUy developed embryology ought to be 

 able to expound the processes of development in terms of the changes in 

 the populations of molecules making up the ceUs of the different tissues 

 and should not have to rely on concepts such as evocation and competence, 

 which are special to it alone. 



The programme which Weiss proposes is, however, a very difficult 

 one and we are still far from the goal which he envisages. Indeed the 

 distance is so great that there is probably some danger in attempting to 

 cover It m a single step. There is an intermediate level, between that of the 

 organisers, fields, etc. usuaUy considered by embryologists and the ulti- 

 mate molecular constituents of living matter, which requires to be 

 thoroughly explored before we can feel the ground firm enough under 

 out feet to have any confidence in attempting to frame theories in 

 chemical or physical terms. This is the level which deals with the activities 

 of the different categories of ceU constituents. As we have pointed out 

 experimental embryology has already approached it in several diff'erent 

 contexts; for instance, in relation to the cell granules of mosaic eggs 

 (p. loi) the mitochondria of sea-urchins (p. 90) and the microsomes of 

 Amphibia (p. 212). These are all constituents of die cell cytoplasm, not of 

 the nucleus. The crucial role of inductive processes (both between tissues 



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