476 



nerve fibres, we now see an irregular clumping together of the 

 fibres and even partial disruption of these. Into this degen- 

 erating mass now extend newly formed nerve fibres from the 

 recently developed nerve cells. In the larva eight hours after 

 defaecation these give off short processes, which soon become 

 longer and needle-shaped (fig. 228). These then extend into 

 the degenerating nerve strands as fibres of extraordinary fine- 

 ness, and as the old larval fibres degenerate the newly devel- 

 oped processes from the imaginal nerve cells replace them. 

 These events usually run so closely together that it is not 

 possible to observe either in progress; it is only when for 

 some reason there is a delay in the formation of new nerve 

 fibres, as often happens in the brain, that a marked globular 

 degeneration of the nerve strands can be detected. 



In the peripheral nerves, however, the process is much 

 more marked, "and" very fine instances of tissue disintegration 

 y^ in thevabsence of phagocytes can be observed. In the defae- 

 cating larva, as the splanchnopleural covering of the nerve 

 fibres is degenerating and is in process of rapid regeneration 

 (so that no discontinuity exists between the sheaths of the 

 peripheral nerves of the larva and imago), a total degenera- 

 tion of the contents of these nerve sheaths takes place. The 

 constituent nerve fibres disintegrate, and the products of 

 disintegration unite to form several large oval globules (fig. 

 223), which, perhaps as a result of the pressure of the sheath, 

 are forced along the nerve, and breaking out, evidently 

 through some point of weakness, float about as small rounded 

 globules in the blood stream. Here they may be in part 

 absorbed by phagocytes and in part simply dissolve in the 

 blood. Towards the end of the larval period the imaginal 

 nerve fibres, which have been growing down and replacing the 

 old larval nerve strands in the ventral nerve cord, enter the 

 emptying renovated sheaths of the old larval nerves. No more 

 profound tissue metamorphosis than this could be imagined, 

 and yet, as far as the gross anatomical changes are con- 

 cerned, no marked change occurs. It is probably these large 

 globules — degeneration products of the larval nerve fibres — 

 that Weismann observed. 



Meanwhile the neuroglia network within the nervous 

 system has been undergoing similar changes. This is especi- 

 ally clearly visible in the larva eight hours after defaecation 

 in the regions between adjacent ganglia. The larval neuroglia 

 cells are observed here forming a loose network of fibres 

 (fig. 229). Some of the cells are clearly in a senescent con- 

 dition, presenting large nucleoli; some are growing very pale, 

 evidently losing their cytoplasm, while others about them 

 are growing at their expense. 



