THE SCIENCE OF EMBRYOLOGY 2$ 



thickness of the material, so that the formation of A is reduced ; while 

 outside the ring a higher concentration of yl will be able to build up. 

 Thus we shall have a system consisting of a central knob of tissue sur- 

 rounded by a groove outside which is a thickened ring of lower elevation 

 than the knob. This is already a structure which could be considered as 

 an organ, with a defmite characteristic shape. We may refer to it as the 

 organ X. If the substance A interacts with another substance B which is 

 present in a graded concentration in the tissue, the shape of the organ 

 will not be radially symmetrical round the point of origin, but will be 

 bilaterally symmetrical. 



Now it is in situations such as this that embryologists have often used 

 the expression 'the X field'. They have meant two rather different things 

 by it. The most valid use of the term refers to the situation within the 

 region around the point at which A is being formed. Here we have a 

 series of processes — of the appearance of^, its diffusion, its reaction with 

 oxygen and with substance B, the heaping up of the tissue in one place 

 and its thinning nearby — all of which interact on each other in a way 

 which results in the region developing through a defmite series of steps 

 into a well-defined end-result, the organ X. The term 'field' is used to 

 emphasise the co-ordinated and integrated character of the whole com- 

 plex of processes. When it is used in connection with the formation of a 

 defmite organ with a characteristic individual shape, the term can be made 

 more precise by qualifying it as an 'individuation field' (Waddington 

 and Schmidt 1933). 



The word 'field* is also sometimes used, in a rather less legitimate 

 manner, to refer to the conditions within such a region of tissue before 

 the point at which A will be formed is precisely localised. For instance, in 

 the flank of an early amphibian embryo, the formation of a limb can be 

 induced by implanting various substances into the mesoderm (p. 273). 

 One may come across such a phrase as 'the forelimb field extends from 

 about the second segment to about the tenth, reaching a maximum intensity 

 in segments three to six'. Here we are dealing, not with the individuation 

 field, which is confmed to the area in which a limb is actually developing 

 and the immediate neighbourhood of this, but with the large region 

 in which there are the preconditions necessary for the appearance on the 

 individuation field. Such a region could better be referred to as the 

 'region of competence' for the organ (Waddington 19346) or the organ 

 'district' (Needham 1942). But the fact that its properties are not usually 

 equal throughout, but are graded from a high value in the centre to low 

 values at the periphery, has frequently tempted people to go on using 

 the word 'field' for it; and they probably will continue to do so. Not 



