ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES, 753 
towards the middle of the chorda, where they are largest. The cells are, roughly, 
modifications of a spherical type—that is, they present the same appearance in transverse 
and longitudinal sections. ach cell is, I believe, quite independent—z.ec. the fibrous- 
looking septa forming the network are always double, being formed by the walls of two 
adjacent cells, and there is no connection at the angles. This accords with the figures of 
G. Rerzius and Ayers and Jackson. Between the walls of adjacent cells there is a 
very narrow space, occupied probably by a cement substance, and each wall is marked 
by very fine closely applied striations (as shown in the figures and as first pointed out by 
G. Rerztvs), indicating that the wall is fibrillar. Lying on the wall of the cell is the 
flattened oval nucleus, which is surrounded by a delicate membrane, is very coarsely 
granular, and contains one or more nucleolar spots each surrounded by a clear area. 
In Bdellostoma, according to AyERs and Jackson, the nucleus may lie in the centre of 
the cell, when it is connected with the walls by protoplasmic strands. According to 
v. Epner and Ayers and Jackson, the walls of the cell bear a very thin, encrusting 
film of granular protoplasm. I find the nuclei in all the cells without exception, 
although they have so far only been recognised peripherally. The body of the cell 
is occupied by a clear, homogeneous fluid substance to such an extent that the chordal 
cells must be regarded as the most intensely vacuolated cells known, or possible. The 
fluid contains a granular substance in some forms, according tov. EpnER. Embedded in 
the centre of the chordal cells, or generally, perhaps, somewhat nearer the dorsal surface, 
is a condensed area known as the fibrous core (=the chordastrang of v. EBNER). Its 
extent varies very greatly in different individuals, and in some parts of the chorda 
may even be absent. In one specimen the fibrous core was cross-shaped in transverse 
section, but it is generally greatly flattened from above downwards and wide from 
side to side. As first suggested by KOuuIKEr, and ascertained by v. Esner, the fibrous 
core consists simply of chordal cells elongated im a longitudinal direction, and having 
relatively thick walls. That this explanation of the fibrous core is the correct one is 
obvious from an examination of thin longitudinal sections. Ayers and JacKSoNn state 
that in Bdellostoma it is entirely fibrous, but they were evidently unaware that it had 
previously been correctly described. | 
As the chorda enters the parachordals its sheath may gradually thin down (except 
at one place, ventrally, where for a time it is even thicker), until at the extreme tip it is 
covered only by the now irregular elastica externa, and even this is wanting for a short 
space ventrally. The tapering of the chorda and the condition of its sheath is evidently 
very little disturbed by the growth round it of the parachordals. In the tail, as the 
caudal cartilages surround the chorda the fibrous sheath gradually disappears, leaving the 
elastica externa; but even after the chorda is largely invaded by soft cartilage the 
elastica externa and a portion of the fibrous sheath remain. Finally, however, first the 
fibrous sheath and then the elastica externa vanish, and there is a fusion, though never 
quite complete ventrally, between the now largely cartilaginous chorda and the median 
ventral bar of the skeleton of the tail. 
