THE INSECTS 



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actions between the various germ layers, and tlius suggest similarities 

 between insect development and the phenomena which we shall find 

 in the development of the vertebrates. According to the available 

 information, there are certain differences in the epigenetic reactions in 

 the various forms, but it seems not improbable that these may tend to 

 disappear as the territory opened up by the recent pioneering work 

 becomes more fully explored. 



In all three forms, Chrysopa, Leptinotarsa and Tachycines, die differen- 

 tiation centre begins to be active in the anterior part of the germ-band, 

 which will later develop into the prothoracic region. Its presence is 

 demonstrated by the fact that parts of the embryo removed from contact 

 with the centre do not continue differentiating. Haget shows that in 

 Leptinotarsa, the centre is at first localised in a small region, and gradually 

 spreads in all directions (Fig. 8.6). In Tachycines, Krause finds that the first 



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Figure 8.6 



The increase in size of the 'Differentiation Centre* in Leptinotarsa, from 18 

 to 27 hours of age. If the embryo is cut in half through the black region, both 

 parts develop partial embryos ; if the cut is in the dotted region, the larger 

 part forms a partial embryo and the remainder a germ-band which fails to 

 develop further; while if the cut is in the white areas, only the larger part 

 shows any signs of development. (From Haget 1953.) 



result of the action of the centre is a tendency for the mesoderm to be 

 invaginated. If, following injury, the mesoderm does not form, the ecto- 

 derm fails to develop ; and Krause concludes that it is the mesoderm which 

 endows the ectoderm with the capacity to differentiate. He points out, 

 however, that the manner in which the ectoderm develops (i.e. the organs 

 it forms) is not dependent on the nature of the mesoderm underlying it. 

 In fact, once the ectoderm has been set going, the main inductive influence 

 goes the other way, the character of the ectoderm determining what type 

 of mesoderm shall be produced. Bock and Haget quite independently also 

 obtained clear evidence for this induction of mesodermal tissues by the 



