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 4 inch long, in which a cartilaginous cup 
is already fairly formed beneath the olfactory organ; but in the goby, about 4 inch 
long ($4 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 Plewronectes 
flesus, 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 (Pl. XXIV. fig. 5) that, if it is no 
better developed in the Ganoids, WinpER 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 Pl. 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 (Pl. 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 
ereat size of the optic ventricles, and on the floor, in its progress to the median fold, are 
the tori semicirculares (Pl. 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 
