754 ‘ MR FRANK J. COLE 
C. Tue Cranium anpD NEuRAL TUBE. 
The neural tube is a double-walled structure. It consists of an inner cylinder often 
selectively staining red with methyl-blue-eosin, and the base of which rests on the 
elastica externa of the somewhat flattened roof of the chorda, and an external layer, 
present only at its sides and roof, formed by the skeletogenous layer of the chorda. 
The former is comprised of fine transverse fibres very closely packed together, and, in 
fact, bears some resemblance to an elastic membrane. The cavity of the neural tube is 
very much larger than the spinal cord which it contains. There is sometimes seen 
wedged in between the fibres of the inner cylinder, in the mid-dorsal region, a wide 
conspicuous mass of longitudinal fibres. The neural tube is perforated laterally below 
at intervals by the roots of the spinal nerves. 
The cranium is entirely membranous, and J. MULLER is of course quite wrong in 
describing an infiltrmg cartilaginous substance in the cranium of Bdellostoma. It is 
perforated in front by the olfactory nerves, and laterally below by the cranial nerves. 
It is simply an expansion of the neural tube, and consists of the same two layers. 
Between the cranium and the olfactory capsule the skeletogenous layer is very 
extensive, as it also is ventrally and laterally at the termination of the chorda in the 
parachordals. The brain almost fills the cavity of the cranium, and in this respect may 
be contrasted with the spinal cord. The skeletogenous layer of the cranium is 
continued into the median dorsal longitudinal septum between the myotomes of the 
head, as in the spinal region. Anteriorly, the floor and roof of the cranium are joined 
up by a median vertical septum consisting of a double sheet of the inner wall of the 
cranium with the skeletogenous layer between, and which divides the anterior extremity 
of the cranial cavity into two chambers, each containing an olfactory lobe. 
D. THe HisroLogy oF THE SKELETON. (Figs. 3 and 4.) 
I do not propose to consider in any detail the finer structure of the myxinoid 
skeleton, which is a somewhat complex and contentious subject, but simply to discuss 
such facts as bear directly on the morphology of the skeleton. Consequently I leave 
over for the present my observations on the histogenesis of the cartilage. 
The first observer to work at the histology of the myxinoid skeleton was J. MULLER, 
who distinguished two kinds of cartilage,* which he refers to as “ yellowish” or “ brown” 
(hard) and “grey” (soft) cartilage, and which he describes as “cellular cartilage.” 
Its peculiar structure, which he roughly worked out, differing apparently from any 
other variety of cartilage known, “greatly surprised” him. G. Rerzrus in 1881, m 
his work on the auditory organ, figures and briefly describes the cartilage of the 
auditory capsule of Myxine; and he states that it consists of a substance containing 
closely opposed oval or rounded cell cavities, and a weak intercellular substance which 
* 7, in Bdellostoma. In his concluding remark (p. 340) he wrongly excludes Myzxvne. 
