288 GENERAL ANATOMY OR HISTOLOGY OF BLOW-FLY. 



Many of the muscle fibres exhibit two or three concentric 

 layers of fibrilla;, especially at the ends of the fibres (PL XVII., 

 Fig. 6), where these are separated by tendinous tissue derived 

 from the hypodermis, into which they are inserted. The 

 larval muscles are also developed from rows of cells, and are 

 at first non-striated. I have not succeeded in seeing the 

 manner in which the fibre is subsequently converted into 

 bundles of sarcostyles. 



The wing muscles are developed from rows of muscle cells, 

 but these are at first imbedded in a mass of parablast 

 (PI. XVII., Fig 4, fl). During their development the nuclei 

 of the muscle fibre disappear ; but those of the parablast 

 cells remain between the fasciculi. The origin and develop- 

 ment of the wing muscles will be further considered in the 

 next chapter. 



b. The Nerves.* 



The Structure of the Peripheral Nerves. — My observations on 

 the structure of the larger nerve trunks, from the centres to 

 their primary divisions, agree with the description given by 

 Waldeyer. Each nerve trunk is surrounded by a nucleated 

 sheath (PI. XVII., Fig. lo) continuous with the capsule en- 

 closing the ganglion from which it arises. The sheath is sub- 

 divided by branching longitudinal septa. The spaces enclosed 

 by these septa contain fasciculi of exceedingly fine nerve fibrils, 

 which in transverse sections appear as dark specks surrounded 

 by a granular fiuid. The whole closely resembles a gray or 

 sympathetic nerve from a Vertebrate, consisting of several 

 fasciculi of fibrillae. It may be described as a number of fine 

 axis cylinders surrounded by plasma and enclosed in a connec- 

 tive-tissue sheath. 



Viallanes [27] , I believe, first noticed that the nerves of 

 Insects contain nuclei at intervals, chiefly at the angles of 



* For a general description of the nerve centres, see p. 66. A more 

 detailed account will be given in the chapters devoted to the nervous and 

 sensory organs. 



