THE DEVELOPMENT OF THE NERVOUS SYSTEM. 493 



by the condition of the peripheral nerves. All the imaginal 

 discs possess nervous pedicles which, during the life of the 

 larva, must be regarded as without functional activity as 

 nerves it is from these that the peripheral nervous system of 

 the imago is undoubtedly developed, a conclusion already 

 advocated by Van Rees [147]. With these entirety rudimentary 

 nerve-cords, functionally active nerve-fibres, which supply the 

 muscles and integuments of the larva, are intimately con- 

 nected ; and these, like the structures they supply, undergo 

 complete histolysis during pupation. In this there is a com- 

 plete harmony with the view I advocate, that the functionally- 

 active elements of the larval nerve-centres are intermixed with 

 embryonic elements in the nerve-centres, and that the former 

 only undergo histolytic changes. It is true that Van Rees 

 believed that the great thoracic muscles of the imago are 

 developed directly from certain larval muscles in the second 

 and third thoracic segments, and he held that the second and 

 third thoracic muscular nerves are derived directly from active 

 larval nerves ; but, as already stated, I cannot agree with this 

 exceptional origin of the parts in question. Otherwise, my 

 conclusions are in complete consonance with those of Van 

 Rees as to the manner in which the peripheral nerves of the 

 imago are developed. 



The Post-oral Nervous Chain. In the resting stage of the larva, 

 and in the pronymph stage, the ventral cone of the neuroblast 

 becomes distinctly segmented, and the infra-oesophageal ganglia 

 are closely related with the thoraco-abdominal centres. It is 

 only during the evolution of the head that the separation of the 

 cephalic and thoracic centres takes place, by the development 

 of the cephalo-thoracic nerve cord, and this consists at first 

 entirely of fusiform and elongated cells. Thus in the early 

 pupa state the hemispheres are separated from the infra- 

 ossophageal centres by the elongated crura, just as they are in 

 the less specialised Insecta. The permanent ganglia of the 

 imago are developed from groups of embryonic cells, totally 

 distinct from the stellate and branching cells which form the 

 active elements of the nervous system of the larva. In this 



