1010 REPOET OF COMMISSIONER OF FISH AND FISHERIES. [30] 



Amiurus, is continuous with the mesoblast of the skeletogenous invest- 

 ment of the notoehord, by way of a thick band of mesoblast, which 

 thus fills up a wide space between the lateral muscular laminse of the 

 opposite sides of the body. So far as it is possible to decide from very 

 thin sections (j-^oo ii^ch thick), no part of the epiblast takes a share 

 in the formation of the primitive blastema, from which the proximal 

 axial elements of the fin are derived, the corium, Malpighian, and epi- 

 thelial layer not being involved in the formation of the basal actino- 

 phoral elements ; the basal cartilages of the iiroximal ends of the rays, 

 as well as the rudiments of the basilar interneural and interhaemal 

 pieces of Cope, or median actinophores, being evidently laid down from 

 mesoblastic rudiments.* 



This evolution of cartilage actually occurs before the development of 

 the ribs in cartilage and contemporaneously with the formation of a 

 cartilaginous basal plate in the pectoral fold, upon which the divided 

 osseous sheaths of the rays are afterwards superimposed. This basal 

 or basipterygial plate bp, Fig. 5, PI. X, becomes segmented distally into 

 at least the basal "cartilaginous nodular portions of the rays, there 

 being no long basipterygial bar formed in tlie Teleosts, as in Elasmo- 

 branchs, but the proximal portion of this plate is thickened in some 

 Teleosts, and this thickening doubtless repr/Bsents the basipterygial bar 

 of cartilaginous fishes. 



The continuity of the tract from which the axial portion of tbe verti- 

 cal fins are derived, with the skeletogenous investment of the notoehord, 

 shows that the mesoblast from whence the fin-rays and interspinous 

 pieces are derived has a common origin with the former in the embryo. 



The vertical epiblastic folds from which the unpaired fins are derived, 

 are at first, throughout the whole fin-bearing series of the Vertehrata, 

 as well as the larvae of Amphibians, almost devoid of mesoblast: that 

 layer beijig insinuated into the folds secondarily, either to furnish the 

 material for the limiting walls of vessels or else to supply the material of 

 which the basilar and all of the medullary substance included by the 

 rays is formed. . How much of the rays themselves are formed from with- 

 out or by the epidermis, it is difficult to say, but the deposition of new 

 material, we would, on account of the relations of the vessels, expect 

 to occur from within or on the internal faces of the opposite halves of 

 the rays. So that after all it is not so clearly demonstrated that the 

 rays of osseous fishes are wholly of dermal origin, though their outer 



*It may also be here stated that the medullary substance of the barbels of embryd 

 Catfislies consists of mesoblast, and that shortly after the barbels have appeared, or 

 about the sixth day of developmeut, they contain a central cartilaginous rod, which 

 appears contemporaneously with the cartilaginous elements of the larval cranium. 

 The presence of cartilaginous internal supports in the barbels of larval Catfishes, aN 

 well as their extremely eaily appearance, as compared in these respects with th( 

 chin barbel of the Codfish and the labial barbels of the young Carp would indicate 

 ihat these barbels of the Nematognathi appeared very remotely in the past in the 

 ancestral form whence the modern forms are derived. 



