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



the trabecule, as in the post-larval gurnard, ^ inch in length, in which species the 

 fronto-nasal cartilage is very dense and massive. Through this cartilage the nerve 

 goes by a distinct canal — a similar canal, it may be noted, passing along a parallel 

 course slightly external to the nerve, and giving transit to an artery. In the cod the 

 nerves pass along the floor of the cranium, and, without piercing the anterior trabecular 

 outgrowth, reach the olfactory pits. The nasal pits show considerable variation in their 

 rate of development, e.g., in a Clupeoid barely \ inch long, in which a cartilaginous cup 

 is already fairly formed beneath the olfactory organ; but in the goby, about \ inch 

 long (^ inch), the walls of the chamber, though very thick and composed of elongated 

 radially-arranged sensory cells, have merely a very thin outer membranous support similar 

 to the delicate layer lining the pit. In the outer portion of the wall of the organ large 

 loosely connected cells appear, forming a distinct prominence, probably indicating the 

 transverse bridge, the later development of which in Anarrhichas is described on p. 918. 

 The cartilaginous optic cup is well developed in the early post-larval stage of Pleuronectes 

 fiesus, that is when the young fish measures ^ inch in length. 



Behind the optic commissure in the wolf-fish a strong band of fibres passes from side 

 to side along the ventral edge of the brain (PL XXIV. fig. 6, fa), forming a broad bridge 

 of communication. It disappears about the commencement of the succeeding region. 

 The roof of the anterior cerebrum is composed at first of a layer of nerve-cells, which 

 becomes thinner as the chamber above the inferior pale median streak becomes larger. 

 So thin is it in a line with the anterior commissure (PI. XXIV. fig. 5) that, if it is no 

 better developed in the Ganoids, Wilder would very readily suppose it to be absent. 

 There can be no doubt, however, of its presence in the Teleostean embryo, for the spindle- 

 shaped cells can be followed upward to each edge, as a diminishing column that runs into 

 the thin layer of more flattened cells forming the roof. It is certainly remarkably thin in 

 some parts. It rapidly increases in thickness as the thalamencephalon is approached (PI. 

 XXIV. fig. 6, and also in PI. XXIII. figs. 3 and 3a). As in the Ganoids, a transverse 

 commissure appears in front of the pineal gland on the roof of the vesicle formed by the 

 anterior portion of the thalamencephalon. Posterior to the gland are other bands of 

 fibres (PI. XXIII. fig. 3) crossing over the arch beneath the optic ventricles, and above 

 the continuation of the third ventricle. Behind the foregoing is the large posterior 

 commissure, which in front commences over the infundibular region, and it increases in 

 size when traced backward. 



The optic lobes posteriorly form a conspicuous vesicular region on each side, from the 

 great size of the optic ventricles, and on the floor, in its progress to the median fold, are 

 the tori semicirculares (PI. XXIV. fig. 3), fusiform thickenings very characteristic of the 

 region over the commencement of the notochord. The arrangement of these folds in 

 Anarrhichas is noteworthy, and differs from the same folds in the salmon whether one, 

 thirteen, or forty-five days old, but the nature of the preparations may account partly for 

 the divergence. The latter species (salmon) shows a nearly straight section of the floor of 

 each optic lobe posteriorly, and the fusiform enlargements and distinctness of the median 



