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 ^ 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 PI. 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 (PI. 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 (PI. XXIV. fig. 4, i) 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 ^ inch in length, the optic 

 nerves are solid ; in the gurnard, ^ 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, T 7 F 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 

 in certain stages. Thus, after they have separated from thef ore-brain in the manner 

 described in Anarrhichas, they may pass downward to penetrate the lateral wings of 



