THE SCIENCE OF EMBRYOLOGY I3 



by the operation of forces, and thus requires discussion in terms of 

 physics. 



It is only in recent years, as our understanding has increased, that the 

 distinction between these types of phenomena has become important for 

 experimental embryology. The greater part of the subject has been 

 developed in terms of more loosely defmed notions, which have in 

 practice been closer to the idea of differentiation than to the other two 

 concepts. For instance, experimentalists have attempted to discover the 

 factors which bring about the development of the gut from the lower 

 end of an ecliinoderm egg, or that of a neural plate from a certain region 

 of a frog's egg. Both these developments actually involve some regionalis- 

 ation and individuation ; but in the main the experiments have not been 

 concerned with finding out what forces, arising from what sources, push 

 the gut into the interior or fold the neural plate into its characteristic 

 shape. Far more, the point has been to discover how the gut-developing 

 region comes to differ from the parts which develop into something else, 

 or from tissues which cannot develop at all. It is only after we have got 

 at least some inkling of an answer to this problem of differentiation that 

 we can proceed to tackle the other aspects of the processes which go on 

 under our eyes. It will be more appropriate to postpone a discussion 

 of the mechanisms of regionalisation and individuation until more of the 

 facts have been presented (see Chapter XX), but it may be helpful to give 

 here some indication of the general nature of the ideas wliich have 

 developed concerning differentiation in the rather broad sense of that 

 term which has just been mentioned. 



Differentiation itself can be regarded as occurring in two phases. At an 

 early stage during the development of any given region of the egg, its 

 future fate becomes more or less fixed, so that it can only be altered within 

 a narrow range by any knov^oi experimental means ; thereafter, that region 

 will always develop into one fairly definite end-product, provided of 

 course that the conditions are such that it can develop at all. The process 

 by which this frxity of end-result is brought about is spoken of as the 

 process of determination. After it has occurred there follows a long series 

 of events which gradually transform the cells into this adult form. These 

 are the changes wliich are most usually referred to when the word 

 differentiation is used in a rather restricted sense. During determination 

 something occurs which decides which, out of a number of possible types 

 of development, will actually be reahsed ; during the later phases of differ- 

 entiation, this reahsation comes to pass. The most important agents con- 

 trolling development are, we shall argue in detail later, the genes in the 

 nucleus. Determination is the process of bringing into operation one or 



