484 -p. W. GAMBLE AND J. H. ASHWORTH. 



and tlie effect is limited to tlie sheath, while the contents 

 remain translucent and colourless. In sections of the nerve- 

 cord taken from pieces fixed with vom Rath's (osmic-acetic- 

 platinic chloride) mixture and subsequently treated with 

 pyrogallol or crude pyroligneous acid, the sheath is stained 

 brown. The nature of this sheath is demonstrated by sections 

 prepared by the various methods described on p. 468. There 

 is first, the neurilemma sheath which envelops the entire 

 nerve-cord j but within this, directly enclosing the giant-fibre 

 and sending tag-like processes into its contents, is another 

 sheath, which is slightly laminated or fibrillated, and composed 

 of nucleated cells of what we consider neuroglial nature 

 (PI. 27, fig. 66). This is the true sheath of the giant-fibre, 

 and from its reactions, evidently contains a certain amount 

 of myelin,^ though not so much as in the earthworm or in some 

 members of the family Maldanidse. 



The contents of the fibi'es appear in transverse section to 

 be extremely finely granular, staining faintly with iron hsema- 

 toxylin. In longitudinal sections, and also at points where 

 the lateral fibres anastomose with each other, or with the 

 central one, they present a distinctly fibrillar structure, but 

 so fine are the fibrillte that they are irrecognisable when cut 

 transversely. The fibrill^e are embedded in an almost homo- 

 geneous protoplasm, which in many places shrinks away some- 

 what from the neuroglial sheath. This is due to the myelin 

 sheath being dissolved by most of the reagents employed with 

 the exception of osmic acid. In the fresh condition and in 

 many series, however, the contents of the giant-fibre almost 

 fill the sheath. 



The giant-fibres arise from giant-cells of the cord. Many 

 of our preparations give direct evidence of this mode of origin 

 (PI. 28, figs. 70 — 73), and although on account of the sinuous 



' The fullest proof t.liat myelin does occur in the sheath of Chsetopods is 

 given by Friedlander (1894). The presence of this substance is a very impor- 

 tant fact, for it furnishes an exact parallel to the medullary sheath of Verte- 

 brates. Not only so, it will now be possible to trace the course of the giant, 

 ftbres by the degeneration method. 



