DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 909 
dorsally-rounded whit estratum above. The apex of the grey substance in this early 
post-larval stage quite separates the white matter into two masses, and this condition 
is still more marked in a later post-larval stage, the vesicular matter intruding to a 
larger extent in the goby $4 of an inch in length. 
The superior fold of the mid-brain (optic lobes) in Anarrhichas forms from side to 
side a semicircle, and is therefore larger than in the salmon. Moreover, the prolongations 
of this fold on each side towards the cerebellum are longer than in the salmon. The 
adjoining edges of the optic lobes (forming the sulcus longitudinalis superior) are some- 
what regularly and deeply crenated, as if indicating rudimentary convolutions. The 
posterior mesial fold (valvula) of the same region is likewise larger, and it gradually 
widens out posteriorly until it merges in the furrow of the medulla. 
Anteriorly the brain presents two somewhat short cerebral lobes, with a large median 
ventricle, which terminates a little behind the anterior border. In vertical transverse 
section they are seen in Pl, XXIV. fig. 5. 
The lower margin of the cavity of the fore-brain is closed by a thin cellular layer in 
the middle line in front of the optic commissure. A well-marked commissure passes 
between the two lobes a little above the inferior border posteriorly, and the fissure in 
transverse section is thus closed up inferiorly in the region (Pl. XXIV. fig. 5, ac). In the 
section, however, only a portion of the commissure is visible. This may be, as BaLFour 
and PARKER suggest in Lepidosteus, the homologue of the anterior commissure. The 
olfactory nerves (Pl. XXIV. fig. 4, 1) spring from the anterior end of the cerebral lobes, 
and their separation is well shown in the same figure. 
Very soon afterwards the optic fibres from the lower part of the brain cross (without 
decussation) in the middle line. 
At the end of March the optic nerves are hollow in some, a considerable chamber 
occurring in the centre from the choroidal fissure to the optic commissure. It must be 
stated, however, that in earlier stages this was not always visible, probably because the 
degree of development varied so much. 
While in many post-larval forms, e.g., the goby, when $4 inch in length, the optic 
nerves are solid ; in the gurnard, 53, inch in length, they exhibit a well-marked longi- 
tudinal fissure shortly before penetrating the sclerotic layer to enter the optic chamber. 
This chamber is even more distinct and capacious in Labrus, 3%; inch in length, and at the 
point where the nerves pass over each other the lumen is of an irregular form, as its 
walls are much wrinkled, the hollow nerve showing a series of folds, which disappear as 
it passes outward, the walls approaching so as to enclose merely a narrow median slit, and 
this can be traced only to the aperture through which the nerve enters the eye. In 
Callionymus, + inch in length, the optic nerves are surrounded by a layer of fibrous 
tissue having a slightly metallic lustre. 
Some further points of difference present themselves in regard to the olfactory nerves 
im certain stages, Thus, after they have separated from thefore-brain in the manner 
described in Anarrhichas, they may pass downward to penetrate the lateral wings of 
