THE INSECTS 



131 



It is a most remarkable fact that in the insects it is the ectoderm which 

 takes the lead in the determination of development, while in vertebrates 

 this function belongs to the inner layers, endoderm and particularly 

 mesoderm. Whether this has any connection with the *upside-down* 

 morphology of the adult insect as compared with the vertebrate (ventral 

 nerve cord instead of dorsal) is an intriguing question. One might, per- 

 haps, think that it was more likely that conditions in the insects should be 

 similar to that in annelids; but again, in Tubifex as we have seen, the devel- 

 opment of the ectoderm is dependent on influences from the mesoderm 

 (p. 99). There seems to be no close parallel in other groups to the situation 

 found in the insects. 



Haget has also evidence of a later inductive process, by which the 

 mesoderm, after submitting to the influence of the ectoderm, itself 

 induces the differentiation of the gut endoderm. 



The Hymenoptera (Fig. 8.8) are a group which can be considered as 

 intermediate between the indeterminate and fully determinate types, 



Figure 8.8 

 a, b. Longitudinal sections through eggs of the ant Camponotus: a, shortly 

 after laying, with cortex more or less evenly spread over the whole surface 

 except for a sHght thickening at the posterior end; h, 6-12 hours later, show- 

 ing the differentiation of regions within the cortex. P is the collection of 

 granules which will pass into the pole-cells (future germ-cells) ; C.N., cleav- 

 age nuclei. (After Reith I93I-) 

 c, d, Longitudinal sections of eggs of the bee, Apis: c, just after laying; d, 

 stage with 512 nuclei. C, cortex, C.Z.; central zone; D.C., Differentiation 

 Centre; M.p., maturation plasma; S, sperm nucleus. (After Schnetter 



1934a.) 



