258 SCOTT. ' [Vol. I. 



cephalon consists entirely of cells (Fig. ii, PI. IX). In large 

 larvae, however, the optic thalami consist almost entirely of 

 fibres, though some layers of cells surround the ventricle, and 

 scattered cells may be seen among the fibres. These fibres 

 first appear as two bundles, one on each side, about midway 

 in the height of the thalami, but interrupted by the cells both 

 above and below (Fig. 8, PI. VIII). The fibres gradually en- 

 croach upon the cells until the latter are reduced to very small 

 proportions. 



The infundibulum appears very early (Fig. 2, PI. VIII) as a 

 diverticulum of the floor of the thalamencephalon, imme- 

 diately behind the optic chiasma. It is of large size from the 

 first, especially in the antero-posterior direction, but rapidly 

 increases in size. As Ahlborn (i) has remarked, it remains 

 single in larvae of 22 mm. in length, but in Ammoccetes of 26 

 to 30 mm. the division into lobus and saccus is to be found. 

 The saccus infundibiili has very thin walls, and its cavity is 

 much expanded below and somewhat contracted above, so that 

 in transverse section the saccus is seen to project much beyond 

 the sides of the thalamencephalon. This condition is reached 

 in larvae of 26 mm., and perhaps earlier. At a similar stage 

 the lobjis hifiiJidibuli extends backwards until it comes to un- 

 derlie the mid-brain, and in some transverse sections appears to 

 form a part of that region. Immediately in front of the infun- 

 dibulum there appears very early a constriction in the floor of 

 the brain (Fig. 4, PI. VIII) which encloses a small cavity, the 

 recesstts chiasniaticiis, formed by the union of the hollow bases 

 of the optic tracts. In the adult the optic tracts and the re- 

 cessus project very prominently from the floor of the brain (see 

 Ahlborn (i). Figs. 33-35, PI. XV), but during larval life all the 

 structures connected with the eye remain very small, and the 

 recessus retains essentially the sam.e characters as those shown 

 in Fig. 4, except that transverse fibres begin to appear in its 

 walls, and thus form the optic chiasma. _ 



The discoveries of Spencer (39) have latterly drawn great 

 attention to XhQ pineal gland, which Rabl-Riickhard, from a study 

 of the Teleosts (28), and Ahlborn (3), from his observations on 

 the lamprey, had already conjectured to be a rudimentary eye. 

 Its history in Petromyzon is of great interest. It first arises, as 

 was shown in my former paper, in embryos of about the seven- 



