i6o organisers: inducers of differentiation 



lacks the primitive streak which the upper layer possesses. The 

 lower layer is therefore in the same case as a ventral half of an 

 embryo of an amphibian. The organising action of the primitive 

 streak on the lower layer is shown by the fact that the upper layer is 

 capable of inducing the lower layer to give rise to the fore-gut in 

 the correct position with regard to the notochord, from tissue 

 which would normally not have given rise to fore-gut at all. This 

 is shown by experiments in which an upper and a lower layer are 

 cultivated together in such a way that the primitive streak overlies 

 a region of the lower layer other than that which represents the 

 presumptive fore-gut. 



i.n.s- ^^ u.n.g. 



Fig. 77 

 Induction by organiser in birds. Two blastoderms of the chick grafted together. 

 u.n.g. normal neural plate of upper blastoderm ; i.n.g. secondary induced neural 

 plate in upper blastoderm, formed in relation to Iji.g. normal neural plate in 

 lower blastoderm. (From Waddington, Phil. Trans. Roy. Soc. B, ccxxi, 1932.) 



It is clear, therefore, that the primitive streak is an organiser. It 

 has further been found that it possesses regional differences of 

 potency, both as regards self-differentiating capacities and in- 

 ductive power. Anterior pieces of the primitive streak differentiate 

 into neural tube, notochord, and mesodermal somites; middle 

 pieces produce mesoderm with or without neural tube; posterior 

 pieces never produce neural tube. In other words, there is a 

 gradient in developmental potencies along the primitive streak.^ 



It should, however, be noted that when portions of primitive 



streak are cultivated in isolation, they give rise to considerably 



more than their presumptive fate'^ (fig. 78). 



* See also Hunt, 1932. 



- Waddington and Schmidt, 1933. 



