THE SCIENCE OF EMBRYOLOGY 23 



ledge are comparatively trivial ones; we do not have such information 

 for anything as complex as a protein, let alone for any particular type of 

 cell or tissue. 



APPENDIX 

 THE CONCEPT OF EMBRYONIC FIELDS 



The field concept has been widely used in some recent discussions of 

 development, notably by authors such as Huxley and de Beer (i934),Weiss 

 (1939) and Lehmann (1945). It is, however, rather difficult to make clear 

 exactly what is meant by it and unless the term is given a fairly precise 

 meaning it is only too easy to use it as a sort of joker' by which almost 

 anything can be explained (see review of Huxley's and de Beer's book 

 by Waddington 1934^, and Needham's discussion 1942, p. 127). 



The first confusion arises from a tendency to use the work 'field' when 

 all that is meant is a reference to the geographical location in which some- 

 thing is happening, while not implying anything about the nature of the 

 events going on there. In such circumstances it is better to use a more 

 neutral and clearly geographical term. For instance, in the neurula of an 

 amphibian embryo the right forelimb will arise from a quite definite 

 place. This should be referred to as the hmb area, not as the limb field. At 

 an earlier stage the localisation of the limb is not so precise. Experimentally 

 it may be caused to appear anywhere within a somewhat larger region 

 of the embryo. Needham has suggested that these larger regions may be 

 referred to as 'limb districts'. We may thus speak of the 'limb district' in 

 an earher embryo, meaning the whole region out of which a limb could 

 be caused to appear, and in a later stage in which the position of the limb 

 had been more precisely fixed we could begin to speak of limb 'area'. 



The word 'field' should be used only when we wish to refer to the 

 character of the processes which go on in an area or district. By using the 

 word we mean to imply that there are a number of processes which inter- 

 act with one another in such a way that they take up definite relations to 

 one another in space. It is easier to show what this means in a concrete 

 example than by abstract defmitions. Unfortunately there is no actual 

 case in which the causal mechanisms of an embryological field are truly 

 understood. It will therefore be necessary to give an imaginary example, 

 which however will serve to show the general nature of the ideas which 

 should be at the back of one's mind when one uses the concept of fields. 



Imagine, then, a flat expanse of tissue, as it might be ectoderm on the 



