THE "organization GENTRE" 75 



about the development of insects yet, and it is rather 

 unUke the development of most other animals. The 

 egg is a banana-shaped object, containing a large 

 amount of yolk. The nucleus, after it is fertilized, 

 lies nearly in the middle. The ^'cleavages" consist of 

 divisions of the nucleus without any accompanying 

 divisions of the whole mass of yolk, so we find 

 gradually more and more nuclei scattered about 

 throughout the egg, till the whole yolk is full of them. 

 After a time the nuclei collect on the surface and 

 cell boundaries form round them, so that eventually 

 a stage is reached with the yolk nearly empty of 

 nuclei and covered with a cellular skin. This is called 

 the blastoderm stage and corresponds to the blastula, 

 only here the blastula cavity is filled up with yolk. 

 Then gastrulation takes place, starting with a sinking 

 in of the blastoderm-skin at one side of the embryo 

 (Fig. 20). 



Seidel tied thin hairs round the egg in the early 

 "cleavage" stages, and thus constricted the egg into 

 two parts. He then watched how each part developed 

 and in this way could discover the influences each 

 part had on the formation of the embryo. He found 

 that there is an essential region in the bottom end 

 of the egg. He called this an activation centre, and it 

 is not quite the same as an organization centre, 

 because it does not determine the way in which the 

 various parts of the egg will develop. What it does 

 do is to activate another centre which lies further 

 towards the front of the egg. This other centre, which 

 is called the differentiation centre, lies, in fact, just 



