746 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



sheath proper (cs, PI. XL 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 Balfour thought, from the outside, and forming the cartilaginous 

 tube around the chordal sheath. From this intruding layer the future vertebrae 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 haemal arches are developed. In the less primitive sharks, such as Mustelus, 

 the Kays, 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 [met, PI. XL 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 laminae of the vertebral bodies are not preceded by preformed cartilage. In such 

 cases (e.g., 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 jDurposes, but as Pouchet points out (No. 119, p. 274), 

 both spicular substance and osteoplastic tissue may occur in the same form. Pouchet 

 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. Muller recognised an elastica interna closely investing the 



