2l8 PRINCIPLES OF EMBRYOLOGY 



the individuation of the induced axis (p. 462) that there is evidence that 

 the mesodermal inductor (or trunk inductor) acts after the archencephahc 

 or neural inductor and probably antagonistically to it. There is not yet 

 any corresponding evidence as to the time of action of the adult evocators, 

 but in general it seems not improbable that Toivonen's adult evocators 

 are at least very similar to, if not identical w^ith, substances which are 

 active during gastrulation. 



Kuusi (195 1, 1953) has studied the evocatory pow^ers of various frac- 

 tions (nuclei, microsomes, plasma, etc.) of the guinea-pig liver and kidney 

 tissue. The results are not very easy to interpret but she comes to the 

 tentative conclusion that the spinal (mesodermal) inducer is probably a 

 protein while the archencephahc (neural) one may be represented by the 

 microsomes. 



A point which still requires considerably further study is the exact 

 mode of action of the two classes of evocators. The gastrula ectoderm is 

 normally two-layered, with an outer 'epidermal' layer and an inner 

 'sensory' one. In the normal induction of the neural plate, both layers 

 become converted into neural tissue, but when abnormal evocators are 

 used, their effect is sometimes confined to the inner sensory layer. Fujii 

 (1944), comparing inductions produced by the coloured dorsal skin of 

 the adult frog (which tends to evocate neural tissue) with those by the 

 white ventral skin (which tends to induce mesoderm), raised the question 

 of whether the two evocators act differentially on the two components of 

 the ectoderm. Fujii's inductions were rather feeble, and his material not 

 very convincing, but the problem would seem likely to repay further 

 study. 



7. Competence 



The fact that it has been necessary to discuss at some length whether 

 there is or is not any degree of specificity in the evocator(s) is sufficient 

 to emphasise the great part played in development by the competence 

 of the reacting tissues. What does this competence consist of, and what 

 can be learnt about its behaviour? (Reviews: Waddington 1940(1, Holt- 

 freter 195 1). 



Competence, as was said above (p. 179), is a state of instability between 

 certain alternatives. The gastrula ectoderm is competent with respect to 

 the broad alternatives of epidermal, neural and mesodermal differentia- 

 tion. Within each of these main categories there are certain subdivisions : 

 for the first, true epidermis, lens placode, ear placode, etc. ; for the second, 

 the various regions of the brain, the spinal cord, neural crest, etc.; for the 

 third, chorda, somites, nephros, lateral plate, etc. The state of instabihty 



