466 



PRINCIPLES OF EMBRYOLOGY 



by the late gastrula stage it would presumably have attained a higher con- 

 centration in the more anterior mesoderm, which has been invaginated 

 for a longer time, than in the more posterior regions. Similarly one might 

 expect it to spread out laterally from the dorsal midline. Processes of this 

 kind will give rise to a graded field of evocator concentration within the 

 mesoderm (Waddington 1940^, Waddington and Yao 1950) (Fig. 20.28). 



lit 1 1 1 > 



I V ^ /III 



/^ 



/ I 





> 1 1 

 f > ' I 



V 



V 



V 



^^^_- 



Figure 20.28 



To illustrate the time relations of organiser action. The drawing on the left 

 represents the archenteron roof in a late gastrula, the black circle being the 

 blastopore. If we suppose that the evocator is liberated in the median strip 

 (close dots), and diffuses laterally, it will have spread into a field represented 

 by the dashed contours. The drawing on the right represents the pre- 

 sumptive neural plate, which has been moving down towards the blasto- 

 pore, passing over the already invaginated mesoderm. The concentration of 

 evocator which has diffused into it from below will be approximately as 

 indicated by the contours. (From Waddington and Yao 1950.) 



As these authors have also pointed out, the movement of invagination 

 will have another consequence in that at any given time the posterior end 

 of the neural plate will have been underlain by mesoderm for a longer 

 period than the anterior end. It may be surmised that the period of time 

 for which the inducing mesoderm acts may have an influence on the 

 regional character of the neural tissue which is produced. Waddington 

 and Deuchar (1952) attempted to demonstrate this. From a late gastrula 

 stage, pieces of presumptive neural plate were removed and transplanted 

 into young gastrula hosts in such a way that they were exposed to the 



