organisers: inducers of differentiation i6i 



Anterior and middle pieces of primitive streak (corresponding 

 to the dorsal and lateral lips of the amphibian blastopore) grafted 

 beneath an upper layer induce the formation of neural folds from 

 host-tissue, but posterior pieces seem to be unable to do this. Thus, 

 in the primitive streak, there appears to be a graded distribution of 

 organising power. Since the induced neural tube is usually situated 

 immediately above the mesodermal tissue of the graft (corre- 

 sponding to the primitive gut-roof of the amphibian organiser), 

 the latter is probably responsible for the inductive effect. The 

 notochord in the bird is apparently unable to induce.^ 



When a graft which in the normal course of development would 

 have formed trunk mesoderm is implanted into the head region, it 

 produces only head mesoderm there, whether or not it succeeds in 

 inducing the formation of a secondary embryo. This shows that 

 there must be some influence of the host-tissues on the fate of the 

 grafted organiser.'^ 



The homoiogenetic power of the neural tube has been demon- 

 strated in birds, for a grafted portion of neural tube will induce the 

 formation of neural tube^ (fig. 77). 



The organising action of the avian primitive streak is not species 

 specific, for the primitive streak of the duck is functional when 

 grafted into the blastoderm of the chick, and vice versa} 



The orientation of the avian embryo is found to be dependent 

 on the polarities of both the primitive streak and of the lower layer. 

 The influences of the upper and of the lower layer are tested by 

 rotating the one relatively to the other through 90° or 180°, and 

 culturing them together. As in the comparable experiments in 

 Amphibia (p. 149), in which rotated or reversed organisers are 

 grafted, the results vary considerably in different individual cases. 

 In some, the orientation of the primitive streak, and therefore of 

 the upper layer, determines that of the embryo. In other cases, 

 however, the embryo is developed in relation to the polarity of the 

 lower layer or endoderm. The polarity of the upper layer is then 

 either deflected or obliterated. This is very remarkable, for, as 

 already stated, the lower layer lacks the primitive streak from which 

 all the axial structures of the embryo are formed.^ 



1 Waddington, 1933 b. See also Umanski 1932 a. '^ Waddington, i933 b. 

 - Waddington and Schmidt, 1933. ^ Waddington, 1933 C 



