STRUCTURE AND DEVEEOrjrENT OF REISSNER’s FIBRE. 25 
killed material it must possess a remarkable degree of elasticity. 
It would appear to exist in life under considerable tension, and, 
being subject to continually varying strain with every 
a,lteration of the position of the long axis of the body, it must 
be exceedingly liable to accidental bre.akage. In such an 
event, or in the case of artificial section in the fresh condition, 
the elasticity of the fibi'e brings about a sharp recoil of the 
broken ends to form large masses or snarls, as shown, for 
example, in fig. 19. Even when partially fixed by reagents, 
however, the resilience of the fibre is such that it will still 
contract if severed. Under these circumstances the retrac- 
tion appears to be a comparatively g-i-adual one, and the fibre 
will then be found twisted into a more or less regular spiral 
(fig. IG), which is often extraordinarily I'eminiscent of the 
retracted stalk of a Vorticella. Even where the recoil has 
been an abrupt one and a lai-ge knotted mass has formed, a 
careful examination practically always discovers the existence 
of such a spiral winding of the fibre (figs. 15, 19). These 
knots thus have the character of tangled heaps such as would 
result from the continued twisting in one direction of 
one end of a thin elastic thread of which the other end is held 
fast. In evei-y such case there is a marked decrease in length, 
and an accompanying and vei’y considerable increase in the 
diameter of the fibre. 
Seen in transverse section, Reissner’s fibre seems to show 
a very thin outer sheath investing an apparently homogeneous 
cetitral core which possesses a very high i-efractivity. I 
believe that this thin dark encircling rim is nothing but an 
optical eifect consequent upon the difference of refractivity 
of the fibre and that of the surrounding medium. That it is 
a medullary sheath of myelin, as Sai-gent declares (’04, p. 145), 
I can find no reason for believing. 
According to that author the central part of the fibre 
shows, in transvoivse section, a punctate appearance, which he 
interprets as the effect of the cut ends of the constituent axis 
cylinders (’04, p. 140). As stated above, I fiiid no trace of 
any such structui-e, and if the fibre is, as I believe, the result 
