THE DEVELOPMENT OF THE NEKVOUS 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 entirely 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—lIn the resting stage of the larva, 
and in the pronymph stage, the ventral cone of the neuroblast 
becomes distinctly segmented, and the infra-cesophageal 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- 
cesophageal 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 
