128 PRINCIPLES OF EMBRYOLOGY 



normally begins in the middle of the differentiation centre, and spreads 

 out in each direction from there; a comparatively slight mechanical abnor- 

 mality, such as is caused by a loose constriction, will either inhibit the 

 spread of the wave of contraction, or split into two. Thus the central yolk 

 mass of the egg, wliich might at first sight be taken to be a merely passive 

 supply of nutrients, is actually not so at all, but by its contraction plays an 

 essential role in the epigenetic system which gives rise to the embryo. 



After the action of the differentiation centre is completed, the various 

 regions of the embryo seem to be fairly definitely determined, and only 

 very slight regulation remains possible. 



In the period since Seidel carried out his pioneer work, a number of 

 other insect eggs have been found to follow a more or less 'indeterminate' 

 type of development; that is to say, they are capable of some degree of 

 regulation, and provide evidence of a sequence of epigenetic processes. 



In the Coleoptera (beetles) it was shown long ago that there are special 

 'pole cells' at the posterior end of the blastoderm, and that if these are 

 killed, no reproductive cells are produced by the embryo. In other 

 respects, however, some regulation is possible. In the mealworm Tenebrio 

 (Ewest 1937), there is a first stage wliich lasts from fertilisation till the 

 sixth cleavage (sixty-four nuclei). If, during this stage, a posterior region 

 (about 20 per cent of the total length) is removed or cauterised, the move- 

 ment of the nuclei into the cortex ceases and not even a beginning of 

 development occurs; but if a similar defect is made after the 64-nuclei 

 stage, blastoderm formation proceeds although no embryo will develop. 

 Thus this posterior region must exert some action in the very early cleav- 

 age stage, and it therefore seems to be similar to a formation centre, al- 

 though it does not seem to require activating by the presence of a nucleus, 

 as does that of Platycnemis. 



In another beetle, Leptinotarsa (the 'Colorado beetle'), Haget (1953), 

 in a very thorough and technically accomphshed study, could discover 

 no evidence for the existence of a formation centre active in the early 

 stages. He is inclined to doubt whether Seidel's evidence in Platycnemis 

 really proves the existence of such a centre, but in this he seems perhaps 

 unduly sceptical. 



Most recent workers have entirely confirmed the existence of a differ- 

 entiation centre, lying further anteriorly, and operating some time after 

 the beginning of development. Moreover our knowledge of the successive 

 steps in the action of this centre has been greatly extended recently, par- 

 ticularly by the work of Bock (1942) on the neuropteran Chrysopa, 

 Haget (1953) on Leptinotarsa, and Krause (1953) on the grasshopper 

 Tachycines. All these forms give evidence of a series of inductive inter- 



