CHAPTER 8 



Growth 



The phenomena of growth and development in insects are, in broad 

 outline, like those of higher animals. At the posterior pole of the egg 

 there may an 'activation centre' where, it is suggested, some active 

 substance is liberated which sets in motion the subsequent processes 

 of development. In all insect eggs there can be recognized in addition 

 a 'differentiation centre', lying in the anterior third of the germ- 

 band region, which acts as the leading point in all the succeeding 

 stages of embryonic development. These 'centres', the nature of 

 which is quite unknown, are often active in the cortical plasma of the 

 egg before its division into cells, and they cause this to become 

 transformed into an invisible 'mosaic' of regions 'determined' or 

 committed to form a given part of the embryo. The cleavage nuclei 

 which migrate through the yolk and settle down at the surface to 

 form the cellular blastoderm are thus induced to contribute to the 

 formation of different organs according to the region in the plasma 

 where they chance to come to rest. Those which arrive outside the 

 germ-band zone form only the extra-embryonic blastoderm. Once 

 differentiation has begun, development, co-ordinated by some un- 

 known mechanism, proceeds until the insect hatches. Then follows 

 a period of growth ending in maturity and reproduction. 



Growth ratios 



As in other animals, there is often a change in the proportions of the 

 body as it increases in size; and when this is the case, the dispropor- 

 tion generally follows the allometric law, that is, the logarithm of the 

 dimension of the part is proportional to the logarithm of the dimen- 

 sion of the whole : y = Kx a , where x is the dimension of the whole, v 

 the dimension of the part, a the 'growth coefficient' and K another 

 constant - each part increasing in size geometrically at a rate peculiar 



