ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. coe 
The histology of the notochord of Myxinoids has been described by J. MUuzr, 
W. Muuurr,* G. Rerzius,t v. Epner,t and Ayers and Jackson. In transverse section, 
the chorda of Myxine is almost circular in the living condition, but is somewhat flattened 
dorsally in parts. . The dorsal involution, however, so common in preserved material, and 
described by J. MtLrEr, is an artifact produced by the fixing reagent. According to 
G. Rerztus, the chorda contains no chondrin, glutin, or mucin, but albumen is always 
present. When the chordal tissue or “jelly” is removed the stout sheath does not 
collapse, as mentioned by J. MULLER. 
Skeletogenous Layer (sk. 1.).—This layer consists of a few coarse fibres surrounding 
the elastica externa. As may be seen where the fibres are torn across (as shown in 
fig. 18), each fibre is formed of a large number of very fine fibrille. The fibres course 
almost straight round the chorda, and are usually closely packed together. External to 
these there may or may not be bundles of fibres coursing in various directions, and 
which often contain true elastic fibrils. With methyl-blue-eosin the skeletogenous 
layer stains a faint purplish blue. Arrived at the dorsal surface of the chorda, the 
skeletogenous layer forms the greater part of the pad of tissue filling in the angle 
formed by the meeting of the chorda and neural tube. At this place incipient patches 
of cartilage may be developed in bdellostoma, according to Ayers and Jackson. The 
layer is then continued over the neural tube as the external sheath of the latter. In 
_ Bdellostoma, according to AyErs and Jackson, the skeletogenous layer splits at the top 
of the chorda so as to roof over the chorda as well as the neural tube. I have seen only 
very slight indications of this in Myxine; and, assuming of course that the whole of 
the neural tube is not formed by this layer, it is practically not represented between 
the chorda and the neural tube, and it thus forms one tube, enclosing both the chorda 
and the spinal cord. J. MU ter’s account differs both from mine and Ayers and 
J ackson’s, and is clearly inaccurate. The dorsai vertical longitudinal septum separating 
the myotomes in the median plane is formed by the skeletogenous layer, and the latter is 
also continued into the septa intermuscularia and into the fascia superficialis interna 
(which latter is thus analogous to ribs), at these junctions the thickness of the layer 
being greatly increased. It thus also forms the connective tissue support of the 
myotomes. In the tail it further encloses the large blood-vessels, and is continued 
over the caudal cartilages. The skeletogenous layer is non-cellular (:.e. has no nuclei), 
but it is well infiltrated with blood-vessels. It is not, strictly speaking, part of the 
chorda, and internal to it no nerves or blood-vessels exist. © 
Elastica Eaxterna (el. ext.).—This is a relatively thin cuticular-looking membrane, 
staining intensely with eosin, and which closely invests the notochordal sheath as its 
outermost covering. According to G. Rerztus, it exhibits the chemical reactions of an 
elastic membrane. Its edges are doubtless by an optical effect sharply defined, and it 
may give off elastic fibrille into the skeletogenous layer and sparely into the external 
layer of the notochordal sheath (the latter according to v. Esner). There are no 
' * Jena, Zeits., vi., 1871. t Arch. Anat. Phys., Anat. Abt., 1881. t Z. f. w. Z., 62, 1896 (complete paper). 
