jQQ PRINCIPLES OF EMBRYOLOGY 



Whereas evocation may be, and probably is, a straightforward bio- 

 chemical process, individuation must always involve a biophysical 

 element, since the organisation of an embryonic rudiment is a matter ot 

 geometry as much as of the chemical or histological nature of the tissues. 

 Ldividuation must also usually involve a number of different bio- 

 chemical interactions, by which the various tissues compnsmg the organ 

 are brousht into being. For instance, in an induction such as that shown 

 in Fi2 lo.i the combined mass of the graft and the induced tissues have 

 developed into neural plate, notochord, somites and nephric mesoderm 

 The induced neural tissue is immediately in contact with the graft neural 

 tissue, and the same is true of the induced and graft somitic mesoderm, 

 etc It seems fairly clear that each tissue developing in the graft must have 

 evocated the formation of tissue similar to itself. Mangold (1932) spoke 

 of such phenomena as 'assimilative induction'; more recently some 

 authors (e.g. Medawar 1947) have named them mfective transformations , 

 and drawn a parallel with the processes of virus infection from cell to 

 ceU. We shall discuss later (p. 401) the grounds which exist for such a 



suffsestion. , , r 1 • 1 • 1 • 



^dividuation certainly also involves other kinds of biochemical in- 

 duction besides assimilative evocations. For instance, a weU-mdividuated 

 embryonic axis may be induced by mesoderm wHch itself forms no 

 neural tissue; mdeed this is what happens in normal development. Withm 

 the mesoderm itself, inductive phenomena, not of an assimilative kind, 

 can be shown to be involved in its individuation. For mstance pre- 

 sumptive lateral mesoderm, if isolated, develops only mto mesenchyme 

 and not into somites or nephros, but if some presumptive notochord is 

 put together with it, the lateral mesoderm is caused to differentiate into 

 one or other or both of these tissues (Yamada 1940). It js as though the 

 notochord were at a high point in a gradient of some kmd, the lateral 

 mesoderm at a low one, and in combinations some mfluence diffuses 

 from the notochord and raises part of the lateral mesoderm to the mter- 

 mediate level corresponding to somites or nephros (Fig. 10.10) 



A considerable amount of study has been devoted to the modification 

 which can be made to the mesoderm gradient field by chemical agents. 

 Lehmami (Review : 1945) has shown that lithium ions actmg on the gastru- 

 la tend to suppress the development of the notochord; the presumptive 

 chorda cehs differentiate into a somitic mesoderm mstead of into their 

 usual fate; a similar result can be produced by Trypan Blue (Waddington 

 and Perry, 1955). Ranzi (1951) found that thiocyanate has the opposite 

 effect of causing a hypertrophy of the chorda. These facts are exactly 

 parallel to those which have been discovered m echmoderm development : 



