THE EPIGENETICS OF THE EMBRYONIC AXIS 189 



the appearance of a well-organised embryo or part of an embryo ; but 

 if it does, this embryo must have organised or individuated itself, and its 

 structure v^ill have no connection with any corresponding structure in 

 the evocating material (Waddington 1933b). Evocation is an essentially 

 unitary process, in which one single stimulus calls forth some response; 

 whether the response is simple or complex, organised or disorganised, is 

 another matter, hidividuation on the other hand is the process by which a 

 structurally organised entity is built up, and is essentially complex, to a 

 degree which corresponds with the number of elements involved in the 

 organisation. It may occur within a piece of isolated tissue (Fig. 10.9c) 

 or a graft wliich fails to induce (perhaps because its surroundings are too 

 old) ; or within the rudiment evocated by a graft which does not take 

 part in the individuation (e.g. a dead graft, Fig. 10.9B) ; or within the graft 



Figure 10.9 



Phenomena of individuation. A, gastrula ectoderm, isolated in an evocating 

 solution, has in part differentiated into chaotic neural vesicles, probably 

 representing parts of the brain, which have induced placodes in the epi- 

 dermis. B, a fragment of adult liver, grafted into an isolated region of gastru- 

 la ectoderm, may induce a structure which self-individuates into a well- 

 formed axis. C, an isolated fragment of somite mesoderm from the early 

 gastrula tends to develop into a relatively well-organised axis, with central 

 notochord, accompanied by a neural tube which may swell into a brain-like 

 vesicle at one end; there are muscle cells on each side of the axis, and cepha- 

 lic neural crest cells at the 'anterior'. (From Holtfreter, 1951). 



and the induced tissues together (Fig. lo.i, p. 175). Holtfreter gives a clear 

 account of these facts; his criticism (1951) of the concepts of evocation and 

 individuation seems to be mainly about the words to be used rather than 

 about the phenomena themselves. 



