76 HOW ANIMALS DEVELOP 



where the embryo begins to be formed by the thick- 

 ening and sinking-in of the blastoderm. One might 

 almost say that it lies at the blastopore, like the other 

 organization centres, but the gastrulation of the 

 insects is so a-typical that such a comparison is 

 doubtful. In any case, the differentiation centre is 



DIFFERENTIATION 

 CENTRE 



ACTIVATION 

 CENTRE 



b c 



Fig. 20. — Diagram of the development of the dragon-fly. {a) Shows 

 the egg- and sperm-nuclei just about to unite, (b) Is an early cleavage 

 stage with the activation centre, (c) Is the "blastula" stage, with many 

 nuclei lying on the surface, when the differentiation centre is active 



much more like an organization centre in its activity 

 than is the activation centre. As soon as it has been 

 activated it starts to form a complete embryo round 

 itself. If part of the tgg is tied off with a hair, so 

 that only a small region remains connected with the 

 differentiation centre, nevertheless a complete em- 

 bryo may be developed in the part. This means that 

 the centre alters the fate of the tissues around it and 

 determines how they shall develop so as to give a 

 complete embryo. Seidel discovered how at least 

 the first step of this determination is done. The dif- 



