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



There is thus much less complexity in the stages just described than shown by Balfour 

 and Parker in the notochord of the Ganoids.* 



Skull. — Much has been done in regard to the development of the cranium of 

 Teleosteans, and the comparatively recent memoir of Mr PARKERt specially treats of the 

 skull of the salmon, so that it will be necessary to give only a brief account in these pages. 



On escaping from the ovum in January, the cranium of the wolf-fish is in a very 

 rudimentary condition. The greater part of the vault is covered by a thin layer — the 

 external cellular integument with a membranous layer beneath, the former showing a 

 considerable thickening above the prominent ocular region similar to the tissue — con- 

 sisting of pulpy columnar cells — in Gadoid larvae -^ inch long. In vertical section 

 the first skeletal elements in this region are the anterior ends of the trabecules (PI. 

 XXIII. fig. 1, and PI. XXIV. figs. 5 and 6, tr), which seem to have united in front, as in 

 the salmon of the first day, and form a kind of ridge, with a superior convexity. They 

 extend downward and inward, leaving a space behind the pituitary body, and appear 

 to merge in the parachordals which lie on each side of the notochord. The notochord at 

 its commencement abuts, in fact, on the cartilaginous plates just mentioned. The inner 

 ends slightly curve upward, and do not appear to touch. The parachordals in the 

 post-larval Pleuronectid, fa of an inch long, furnish a great contrast to this condition, for 

 their inner edges have coalesced, and form a dense plate of cartilage — into a cylindrical 

 cavity in which the anterior end of the notochord passes. From this dense central plate 

 thus pierced by the oral end of the chorda, two thin plates pass and unite with the 

 otocystic cartilage on each side. The basilar plate now forms a complete floor in the 

 posterior cranial region. Even more marked is the united condition of the parachordals 

 in the young Clupeoid, f inch long, at the point where the notochord passes into the 

 cranium — the coalesced part having, in transverse section, the form of a massive oblong 

 element penetrated centrally by the notochord. Posteriorly, as the diameter of the 

 notochord increases, the cartilaginous investing mass diminishes, until between the ears 

 it is represented merely by four angular nodules of cartilage, two at the upper and 

 two at the under side of the chorda. In the post-larval goby, -fa inch long, the 

 parachordals unite, but they form a comparatively thin, flattened, basilar plate, into which 

 the chorda passes, and their lateral extensions unite with the otocysts and continue 

 upwards over the hind brain — the posterior part of the chondro-cranium being, in fact, 

 now wholly cartilaginous, and this complete tube of thin cartilage continues into the 

 occipital region, and encloses the medulla oblongata. The small Gadoids, ^ inch long, 

 show a similar condition of the posterior floor of the skull, the thin cartilaginous 

 parachordals uniting with the floor of the otocyst on each side ; but the roof is still 

 membranous above the fourth ventricle. There is a remarkable development of black 

 pigment in the lining membrane of the otocystic cartilages — the corpuscles being situated 



* The development of the vetebral elements of Teleosteans at St Andrews has been undertaken by Prof. D. J. 

 Cunningham, of Dublin. 



t Phil. Trans., vol. clxiii., 1873, pp. 112-145, plates i.-v. 



