organisers: inducers of differentiation 163 



It seems that the endoderm can determine the polarity of the 

 embryo by determining the locahsation and polarity of the primi- 

 tive streak itself, in the blastoderm overlying it. We are here con- 

 fronted with a phenomenon which seems to be nothing less than 

 the determination of the organiser itself. The primitive streak is 

 dependent in some way ultimately on the endoderm, and it would 

 seem that we have to look for the morphogenetic expression of this 

 determination in certain streaming movements which take place in 

 the blastoderm. The direction of these movements is backwards 

 along the periphery on each side, and forwards along the central 

 line, immediately along which line the primitive streak is formed.^ 

 In some as yet undetermined way, the endoderm seems to control 

 these movements. 



If this should turn out to be correct, we have here an example 

 of the effects of "dynamic determination" referred to on p. 154. 

 From the theoretical point of view, the interest and importance of 

 these facts lies in the question whether dynamic determination can 

 be regarded as the causal antecedent of '* material" (chemical and 

 histological) determination. The answer to this question appears 

 to differ in different groups of vertebrates. In the amphibian 

 embryo the early stages are characterised by well-marked move- 

 ments (dynamic effects) of tissues ; and attempts made to test the 

 power of chemo-differentiation of other tissues which have been 

 prevented from undergoing such movements ^ have yielded results 

 which can only be regarded as negative (see Chap, vii, p. 250). For 

 the moment, therefore, the general significance of dynamic deter- 

 mination in birds must remain an open question. 



With regard to the physico-chemical nature of the action of the 

 organiser in the bird, it is interesting to note that it retains its 

 organising capacity although coagulated as a result of having been 

 dipped in a thin glass tube into boiling water for 30 seconds.^ 



From all these results, it is abundantly clear that the dorsal lip 

 of the amphibian blastopore, and its homologue the avian primi- 

 tive streak, possess the function of an organiser, and it is probable 

 that these structures will be found to have similar properties in 

 other groups of Vertebrates. 



^ R.Wetzel, 1925, 1929; Graper, 1929. 



^ Goerttler, 1927 ; Holtfreter, 1933 a. ^ Waddington, 1933 a. 



