1 62 



prospective 

 fate 



actually 

 ' obtained 



reacts by forming a 

 head neural plate 

 I 



induced head 

 neural plate 



Induced head 



The influence of the host 

 overcomes that of the graft 



Fig. 78 



Diagrams illustrating some of the properties of the organising centre in birds. 

 A, The developmental potencies of a portion of the organiser region are greater than 

 its prospective fate. B, Analysis of the problem presented by the fact that when 

 a piece of the organiser region, the prospective fate of which is trunk mesoderm, 

 is grafted into the head region of another blastoderm, it itself gives rise to head 

 mesoderm, while at the same time inducing the formation of neural folds (B i, 

 B 3). The conversion of the graft into head mesoderm may be explained by 

 assuming either: B 2, that after the graft has induced the formation of a head 

 neural plate the latter in turn acts upon the graft and determines it to give rise to 

 head mesoderm; or B 2^, that the conversion of the graft into head mesoderm 

 is due to a process of interaction between the graft and the host's own organising 

 centre, to which latter the property must be ascribed of exerting an influence 

 over an area of given extent, termed an " individuation-field ", in which the whole 

 complex of tissues are controlled in such a way as to lead to the formation of a 

 complete individual. It is, further, an effect of the host's individuation-field that 

 the neural plate which trunk mesoderm induces out of the host- tissues in the head 

 region is head neural plate. That alternative B 2^ is the correct interpretation 

 follows from the cases, C i-C 3, in which the grafted trunk mesoderm in the 

 head region of the host becomes converted into head mesoderm without inducing 

 the formation of a neural plate at all: here, the graft can only be under the in- 

 fluence of the host organising centre. (From Waddington and Schmidt, Arch. 

 Entwmech. cxxviii, 1933.) 



