INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 44I 



rolls up into the neural groove the strength of the outer concave surface 

 becomes markedly greater than that of the inner convex surface. Such 

 an increase in the strength of the concave surface would be produced if 

 the coat there w^as undergoing contraction and thickening, and it seems 

 rather probable that this is occurring and is one of the major factors in 

 causing the change of the neural plate into the neural groove (Fig. 20.11). 

 The whole process of gastrulation cannot, however, be attributed to 

 changes in the coat. Amphibian gastrulation involves two main types of 

 movement for which an explanation has to be found. If one compares 

 the shape of the presumptive areas in the late blastula with the configura- 

 tions which they wiU have assumed at the end of gastrulation, the major 

 changes would be accounted for if we suppose that an area lying in the 



FiGXJRE 20.11 



The ukimate strength of various tissues in amphibian embryos. The figure 

 for the blastocoel roof of the young gastrula (indicated by a cross) was found 

 by pulling a small steel ball through the tissue with a magnet. The other 

 figures were derived from observations of the rate at which the cell surfaces 

 were disrupted when placed in the air-liquid interface of the three saponin 

 solutions whose surface tensions are given at A, B and C. The measurements 

 are approximate, but indicate the changes which occur, and the relative 

 strengths of the surfaces. (From data of Waddington 1939ft, i942f.) 



