192 PRINCIPLES OF EMBRYOLOGY 



evocation, somewhat similar to that by which mesoderm calls forth the 

 production of neural tissue. But the whole complex of such interactions, 

 together with the geometrical aspects of the process, clearly form an organ- 

 ised system which results in the development of an organ with a recognis- 

 able structure. For this reason, individuation can be considered as a typical 

 example of a 'field' phenomenon. 



Nearly all developing masses of tissue exhibit some degree of individua- 

 tion, which may be a self-individuation (i.e. arising autonomously within 

 the mass) or be partly or wholly imposed on it by an inducer. Individua- 

 tion is least in evidence in fragments of presumptive ectoderm isolated in 

 salt solution; they form quite disordered epidermal tissues. If they produce 

 neural tissue as a result of the action of a structureless evocator (such as a 

 chemical in solution or a fragment of dead tissue), this may also be almost 

 completely without defmite form or arrangement, although even in the 

 most disordered cases there is usually a tendency for the neural cells to 

 arrange themselves into tubules and cysts (Fig. lo.pA). Self-individuation 

 may, however, go very much further in such cases, so that definitely 

 recognisable parts of the neural system are formed (e.g. brain, trunk 

 neural tube, etc.) ; and when the individuation of the earlier stages is 

 fairly well achieved, that of later organs such as the eye, ear, nasal placodes, 

 etc. is often very much better. Isolated fragments of gastrula mesoderm 

 seem always to possess a considerable power of self-individuation, and 

 develop into tissue complexes containing notochord, somites, prone- 

 phros etc. with some fairly definite arrangement. It seems likely that 

 the greater tendency to self-individuation in the mesoderm depends on 

 the fact that it develops into several different types of tissue, which can 

 mutually influence one another, whereas the ectoderm tends to form 

 more homogeneous masses. It is noteworthy that the individuation of 

 isolated pieces of mesoderm is better the larger the mass involved, which 

 again suggests that the process depends on interactions between the 

 different parts. 



The phenomenon to which the name self-individuation has been 

 applied here has been particularly emphasised by Lehmann (1945). He 

 suggests that when we are dealing with a small lump of tissue which is 

 starting on a course of development, for instance a fragment of presump- 

 tive mesoderm or a region of ectoderm which has responded to an 

 inductor, we should always regard this not as a mere conglomeration of 

 cells, but as a 'blastema ; and this name, which is the Greek word for a bud, 

 is intended to imply a degree of organisation and an interplay of recipro- 

 cal influences between its parts. 



Rose (1952a) has recently suggested that the appearance of different 



