746 PROFESSOR W. C. M‘SINTOSH AND MR E. E. PRINCE ON 
sheath proper (cs, Pl. XI. figs. 14, 15) being very thin, and the mesoblastic perichordal 
sheath (pes) but little increased in thickness. This latter sheath, of protovertebral 
origin, is equivalent to the skeletogenous layer of Plagiostomes, though in them it is 
greatly thickened. In this perichordal sheath an outer lamina can be made out, especially 
when the rudiments of the vertebral bodies and arches are developed, as it forms their 
outer investment. Below the sheath and its elastica externa, a layer of cells in the sharks 
intrudes, coming, as BaLrour thought, from the outside, and forming the cartilaginous 
tube around the chordal sheath. From this intruding layer the future vertebre are 
formed, and it may be termed the inner skeletogenous layer: it is the inner half of the 
skeletogenous tube. Outside the membrana elastica externa, however, another meso- 
blastic layer is formed, viz., the outer half or outer skeletogenous layer from which the 
neural and hemal arches are developed. In the less primitive sharks, such as Mustelus, 
the Rays, and others, the inner skeletogenous layer is much reduced, and the elastica 
externa is considerably nearer the chorda than in Cestracion and Notidanus. If we 
consider the process of reduction to have affected the external portion until no outer half 
exists, we can then look upon the perichordal sheath in Teleosteans as the inner half of 
the skeletogenous layer, reduced, but still bounded by its outer limiting layer, viz., the 
membrana elastica externa (mel, Pl. XI. figs. 14,15). There is, of course, difficulty 
in separating the parts of a sheath so thin as that surrounding the notochord in 
Teleosteans, but in a few forms, e.g., Cyclopterus, in which cartilage develops somewhat 
precociously in the vertebral column, large chondral cells appear in this external layer, 
which passes upward, and over the spinal cord as a membrana reuniens superior. The 
cells likewise ascend up each side of the cord, forming the rami of the neural arch. 
Similarly the ventral arch is developed. In many forms, however, the arches and outer 
osseous laminze of the vertebral bodies are not preceded by preformed cartilage. In such 
cases (e.9., Gastrosteus) the osseous matter is clear, homogeneous, and brittle (PoucHET’s 
“ spicular substance,” KOLLIKER’s “osteoid matter”), and exactly resembles in its chitinous 
appearance the clavicular portion of the pectoral girdle, and the maxillary elements of the 
upper jaw. The presence or absence of this spicular substance seemed to KOLLIKER of 
diagnostic value for classificatory purposes, but as PoucuEr points out (No. 119, p. 274), 
both spicular substance and osteoplastic tissue may occur in the same form. PovucHET 
states, and seems to be the first to do so, that in some cases osteoid processes, and in 
other cases cartilage, with osteoplasts, form the superior and inferior vertebral arches. 
But whether arising as bars of regularly disposed chondroplasts, or as homogeneous 
spicular deposits, the vertebral bodies, and their projecting dorsal and ventral rami, are the 
products of the perichordal sheath, and arise within its definite limiting layer. The view 
that the main part of the sheath in Teleosteans is a thickened membrana elastica interna, 
and derived from the cells of the chorda itself, is not supported by sections, inasmuch as 
the hypoblastic notochordal sheath always remains extremely thin, and even when 
well developed, as in the Salmonoids, is still merely a single stratum of flattened cells. 
In Elasmobranchs W. MU uer recognised an elastica interna closely investing the 
