INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 459 



Even at the neural plate stage, the regional specificity is not completely 

 fixed, either in the neural tissue or in the mesoderm underlying it. This 

 was clearly shown in extensive experiments by ter Horst (1948). She 

 separated the neural plate firom the archenteron roof, cut each into five 

 transverse strips, and cultured these after wrapping them in pieces of 

 young gastrula ectoderm, and observed both the differentiation of the 

 isolate and the character of the induction it produced. The neural plate 

 fifths on the whole developed into their presumptive fate, but showed a 

 tendency to produce the next most anterior and most posterior regions 

 as well — thus they still have a capacity to regulate towards the formation of 

 a more complete neural system. The differentiation of the mesoderm 

 showed evidence of another kind for a lack of full determination — the 

 anterior parts of it sometimes developed some neural cells. In their 

 induction effects, the two tissues gave even more evidence of flexibility, 

 each fifth inducing a certain region in greatest frequency, but also calling 

 forth neighbouring regions in considerable numbers (Fig. 20.24). A re- 

 markable fact is that a given fifth of the archenteron roof tends to induce 



Figure 20.24 



The open neural plate of the newt is divided into five equal zones, from 

 anterior (1/5) to posterior (5/5); the neural plate is dissected free from the 

 archenteron roof; and neural plate and mesoderm are then separately com- 

 bined with flaps of young gastrula ectoderm. The thick lines show the 

 frequency of inductions of fore-, mid-, and hind-brains by the neural plate, 

 and the thin lines that by the mesoderm. Note that the neural plate has a 

 more 'anterior' performance than the corresponding mesoderm. (After ter 



Horst 1948.) 



