462 PRINCIPLES OF EMBRYOLOGY 



various times after the presumptive neural plate had been underlain by 

 mesoderm, they removed small fragments of the plate, cleared them of 

 adhering mesoderm cells and transplanted them to the ventral side of 

 another embryo. They found that ectoderm which had been acted on 

 for only a short time tended to differentiate into neural crest rather than 

 into neural material proper. There seems little doubt then that neural 

 crest presents a weaker grade of induction than neural tissue. These 

 experiments do not, however, provide direct evidence that differences 

 along the anterior-posterior axis (e.g. differences between midbrain and 

 hindbrain, trunk, tail, etc.) also represent a series of grades of the strength 

 of the inducing action. 



A somewhat different account of the action of the prechordal plate 

 and the chorda-mesoderm has been given by Nieuwkoop (1952). He had 

 the ingenious idea of joining elongated flaps of ectoderm to the gastrula 

 in such a way that the base was near the archenteron roof, but there was 

 quite a long extent of ectoderm into which the inducing material could 

 diffuse. He found that in general the regional character of the structures 

 induced at the base of the ectoderm flap was the same as that of the region 

 to which it was joined, and that the induced structures extended from 

 there towards the forebrain, which might be located at the free end of 

 the process of ectoderm, although sometimes the inductions were in- 

 complete and lacked the most anterior regions. Nieuwkoop interpreted 

 his results by the hypothesis that all parts of the archenteron roof (chorda- 

 mesoderm as well as prechordal plate) exert a first inducing action which 

 stimulates the ectoderm to develop into forebrain-like organs; after this 

 has occurred the chorda-mesoderm, if it reaches the ectoderm in question, 

 exerts a second 'transforming' activity which changes the development 

 of the induced material towards the production of some more posterior 

 level of the axis (Fig. 20.26). 



Such a hypothesis fits in well with the ideas developed by Toivonen 

 from his studies on the inducing powers of adult organs (p. 216). However, 

 one cannot help feeling that the results described by Nieuwkoop scarcely 

 prove his hypothesis. The phenomena in the ectoderm flaps, which take 

 on the regional character of the point at which they are attached and 

 exhibit the structures which would normally he anterior to this, rei^^ 

 one very much of the appearances in a limb regenerate, the base of which 

 develops the regionahty of the cut face on which it forms and which 

 contains all the structures lying distal to this. In the case of the limb 

 regenerate, it scarcely seems plausible to suggest that the first phase is the 

 determination of the distal tip of the limb, and the second phase a trans- 

 formation into more posterior regions; and such a hypothesis, though 



