423". 
VII. Observations upon the Microscopic Appearances in the 
Preserved Material, 
a) Normal Material. 
A general account has already been given of the relations of the 
fibre to the sinus terminalis, so that there will be need for me, here, 
to add only such further details as are essential to an understanding 
of the particular conditions which obtain in elasmobranchs. 
Fig. 4 (Pl. II) is a reproduction of an actual photomicrograph of 
a section through the end of the tail of a skate (Raja blanda), as 
seen under a comparatively low magnification. In this, the terminal 
filament is seen to be serially constricted in a manner which strongly 
recalls the marked neuromery to be observed in the early development 
of the central nervous system. This constriction is not easy to re- 
cognize in transverse sections and this may account for the fact that 
I have found in the literature no reference to its occurrence, but it 
is quite pronounced in every one of the seven series examined which 
were cut sagittally. At the very extremity of the tail the canalis cen- 
tralis of the filum terminale widens out ventrally into a relatively large, 
sub-spherical space, the sinus terminalis, which lies directly behind the 
end of the vertebral column (Pl. II, Fig. 5 s.¢). The columnar epen- 
dyma comes to an abrupt end ventrally, but dorsally it tapers away 
gradually so that in this animal the terminal neural foramen has a 
postero-ventral presentation. Behind this point the sinus terminalis 
is bounded only by the connective tissue of the meninges, into which 
the fibre of REISSNER is inserted.. 
In material which has been thoroughly hardened before the spinal 
cord is cut, REISSNER’s fibre in some cases passes back perfectly straight 
into the sinus terminalis and is there continued into the apex of a 
conical mass, the base of which is continuous with the hinder wall of 
the terminal sinus. For this mass, which almost always stains much 
less darkly than Reıssner’s fibre and has the appearance of being 
hollow, I have proposed the name ‘terminal plug’. Apically, it 
seems structureless, and this portion I interpret as the flaring trumpet- 
shaped end of Retssner’s fibre, of which the constituent fibrillae may 
be supposed to be separating centrifugally to become attached to the 
tissue of the hind wall of the terminal sinus. It is probable that 
such a mode of insertion would afford a very much stronger attachment 
than would be obtained by the insertion of the fibre as a whole at 
one circumscribed spot. The apical portion of this conical membrane 
is almost certainly very thin and frequently presents a rather crumpled 
