INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 467 



action of the archenteron roof for a second time. It was hoped that by 

 prolonging the period of induction in this way some change would be 

 made in the regional character of the induced neural tissue, or even that 

 it would be converted into posterior mesoderm which, in normal develop- 

 ment, is the fate of that part of the neural plate which is longest underlain. 

 Little effect was, however, produced. Nevertheless it remains true that 

 during the induction of the neural plate we are dealing vdth a system 

 which involves a considerable amount of relative movement, and it 

 seems most probable that the time-relations both in the production of the 

 evocating substance and in the period of its action on the overlying ecto- 

 derm will eventually be found to play some part in the process. 



This summary of recent work on the induction of the neural plate, 

 incomplete though it is, has probably sufficed to show how complex a 

 problem is presented by even a comparatively simple instance of individua- 

 tion. The more one looks into the details of what actually occurs in neural 

 induction the more paradoxical the phenomena appear to be. For instance, 

 we have seen that the neural crest appears to represent a weak grade of 

 induction, yet the first visible sign of the formation of the neural plate 

 occurs not in the dorsal midline but at the margins from which the neural 

 crest will later develop; it is in this region that the cells first assume the 

 elongated columnar shape which will later be characteristic of the whole 

 neural epithelium (cf. Fig. 20.21, p. 451). Why should it be in a region of 

 apparently weak action that an effect is first produced? Again, is it not 

 paradoxical that it is at the anterior end of the neural plate, which is 

 underlain for the shortest time by the archenteron roof, that the neural 

 tube attains its greatest dimensions, while in more posterior regions it is 

 smaller in cross-sectional area, and the very posterior end of the plate, 

 which has been successively underlain by all the levels of the archenteron 

 from anterior to posterior, does not develop into neural tissue at all but 

 forms the mesoderm of the tail? We can certainly not pretend to have any 

 explanation of these facts as yet. 



In spite of all the work that has been done on the regional determination 

 of the neural plate, we still fmd ourselves forced to appeal to the myster- 

 ious process of 'self-individuation' to explain the appearance of pattern. 

 This is true both of the pattern in which the different inducing capacities 

 are arranged in the mesoderm sheet, and of the details of the form of the 

 neural organs such as the forebrain, midbrain, etc. By 'self-individuation 

 we in fact mean no more than that the pattern arises without any ascertain- 

 able antecedents. We have seen earlier that both Henke and Turing have 

 discussed ways in which such spontaneous appearance of pattern might be 

 supposed to occur; but it is unsatisfactory to have to rely on mechanisms 



