organisers: inducers of differentiation 171 



only just tight enough to prevent their passage), there is no de- 

 velopment. After the nuclei have reached this region, a substance 

 appears to be given off from the activating region, and to diffuse 

 through the egg in an anterior direction. As time goes on, increas- 

 ingly large portions of the hinder part of the egg can be destroyed 

 without interfering with development, which shows that as soon 

 as a region has received the diffused substance, it is no longer de- 

 pendent on the activating region^ (fig. 84). 



This activating region differs, however, from an organiser in that 

 it is not concerned with the differentiation of this or that structure 

 in any particular position : it is merely a starter or activator, con- 

 ferring on the remaining regions of the Ggg the power to undergo 

 development. 



Farther forward in the insect egg, in the region which will 

 normally give rise to the thorax, the existence of a differentiating 

 centre has been established. For the differentiation of the regions 

 anterior to this, it is necessary not only that the activating substance 

 from the activating centre should have reached the differentiating 

 centre, but there must be cellular continuity between the differ- 

 entiating centre and the regions of the blastoderm anterior to it. 

 The activating substance, on the other hand, diffuses freely through 

 the egg, whether the cells are in continuity or not. It follows, either 

 that the differentiating centre absorbs this substance in its cells and 

 distributes it from cell to cell, or that it initiates a new chain of 

 reactions. At all events, the differentiating centre is responsible for 

 the localisation and determination of the various regions of the 

 embryo, and its presence is necessary if a properly and harmoniously 

 proportioned embryo is to result from an egg in which an anterior 

 portion has been isolated by constriction, or by a discontinuity 

 between the cells of the blastoderm.^ 



Further results must be awaited before the question can be 

 answered as to whether the mode of action of the Insect differ- 

 entiating centre is comparable with that of the organiser in other 

 groups. 



It has been mentioned (Chap, v, p. 113) that in Chaetopterus and 

 in Tubifex, when the polar lobe or pole-plasms are equally dis- 

 tributed to the first two blastomeres, instead of being restricted to 

 ^ Seidel, 1929, 193 1. ^ Seidel, 193 1. 



