412 
My own observations by no means agree with those of SARGENT. 
I found early in 1909, that in Gasterosteus aculeatus the filum ter- 
minale, reduced practically to an epithelial tube (Pl. I, Fig. 2 f.£.) 
devoid of surrounding nervous matter, projects freely from the neural 
canal of tbe vertebral column for an appreciable distance between the 
fin-rays of the caudal fin (Pl. I, Fig. 1). It is here enclosed in a 
slight connective tissue sheath which represents, presumably, the con- 
fluent meninges and, except for this covering, it is protected only by 
the skin. 
Subsequently, somewhat similar conditions were found to obtain 
in the terminal sinus of other teleosts, and also of various cyclostomes, 
elasmobranchs and larval amphibians. In all these cases REISSNER’s 
fibre can be followed through the terminal foramen and into the sinus 
terminalis, in which, in some instances, it expands into a conical ter- 
minal plug (Pl. I, Fig. 2 ¢p.) and becomes inserted into that part 
of the wall of the sinus terminalis which is constituted by the menin- 
geal sheath. 
III. The supposod Function of REISSNER’s Fibre. 
The early theories of the function of REISSNER’s fibre were all 
based upon a study of preserved material only and the works of 
SrupniéKA (1899) and of Sargent (1900) while going far towards 
establishing firmly the fact of its preformed character, had done 
nothing to help us to an understanding of its function, which, indeed, 
could only be certainly ascertained by experimental work. 
In his second contribution to this subject, however, SARGENT 
(1901) put forward his “optic reflex” theory and, in support of this 
view gave an account of certain observations made by him upon the 
behaviour of a number of elasmobranchs in which he had intentionally 
broken the fibre. He stated (1904) that, upon recovering from the 
shock of the operation, the subjects of the experiments apparently 
swam about normally but would collide with bodies interposed suddenly 
in their paths, and he remarks that their efforts to avoid such ob- 
stacles were made too late to prevent a collision. This reaction he 
attributed to a “slowness of response to optical stimuli” and he drew 
from his observations the inference that ReissNeR’s fibre actually forms 
a short circuit for optic reflexes. 
Such an explanation does not appear to account for other obser- 
vations of SARGENT’s; thus not only did he find that his specimens ran 
into obstacles unexpectedly encountered but, also, and apparently as 
frequently, into stationary objects such as the walls of the aquarium; 
