THE "organization centre" 77 



ferentiation centre starts a wave of contraction pass- 

 ing through the yolky core of the egg, and as the 

 yolk contracts away from the outer skin of the egg, 

 a hollow is left in which the blastoderm cells rapidly 

 collect and begin to develop into the embryo. If part 

 of the egg is tied off with a hair, this hinders the 

 progress of the wave of contraction, which adjusts 

 itself to the amount of space which is available to it. 



Seidel also found out something more about the 

 activation centre. He showed that it could only be 

 formed if the early cleavage nuclei are allowed to 

 wander down to the bottom end of the egg, where 

 they react in some way with the cytoplasm there 

 and set free a substance which diffuses forward again 

 through the yolk to the differentiation centre. 

 Neither the nuclei alone, nor the particular region of 

 cytoplasm, can act effectively. 



There is one very important respect in which all 

 the organization centres we have described are alike, 

 except the Activation Centre, which is rather in a 

 class apart. Not only do they all become active when 

 gastrulation starts, but they are all located at the 

 centre of the gastrulation movements, that is to say, 

 at the region of the blastopore. This is true both 

 when it is endoderm which is being formed, as in 

 the sea-urchin or the first chick organizer, and when 

 the important process is the organization of the meso- 

 derm, as in the newt or the second chick organizer. 

 The blastopore is not only the centre of the move- 

 ments which are going on but also of the formative 

 influences which are at work. 



