THE INSECTS 127 



the 128 which are formed in the first seven divisions — is adequate to 

 activate the centre. 



The chemical w^hich diffuses forwards from the formation centre does 

 not act directly on each separate part of the egg. It sets going the differen- 

 tiation centre, which is not only morphologically distinguishable, as the 

 site of the first steps in the development of the germ-band, but is also the 

 second centre of physiological activity. Once it is activated by the sub- 

 stance from the formation centre, it becomes a focus around wliich the 

 embryo is organised. The region over which its activity extends can also 

 be broken by constricting the egg with a hair loop, but in this case the 

 continuity is much more easily disrupted, and even a fairly loose con- 

 striction is enough to inhibit the passage of the differentiation centre's 

 influence. Thus if a loose loop is tied round the egg posterior to the 

 differentiation centre, between it and the formation centre, an embryo 

 develops only in the anterior region; if the loop lies anterior to the centre, 

 the embryo lies wholly posterior to the loop, while if the constriction is 

 located actually within the differentiation centre, twin embryos will form, 

 one in each part (Fig. 8.5). 



Figure 8.5 



The action of the Differentiation Centre in Platycnemis. If a loose constric- 

 tion is made in front of the Centre (a) the embryo forms wholly posterior 

 to it. If the constriction is behind the Centre, the embryo lies wholly in front 

 of it (c). If the constriction is at the actual site of the Centre, this may be 

 divided and embryo formation take place both in front and behind {b). 

 (After Seidel 1938.) 



The ease with which the influence of the differentiation centre is inter- 

 rupted shows that it is not transmitted by a diffusing chemical. Seidel 

 came to the conclusion, in fact, that the activity of the centre is funda- 

 mentally mechanical, and consists in a contraction of the yolk, which 

 leaves a space into which the cells of the blastoderm migrate, thus forming 

 the thickened region which becomes the germ-band. This contraction 



