480 ERNEST W. L. HOLT, 



in a position exactly similar to that occupied by the structure, alluded 

 to by Orr (Notes on the development of some Amphibians, Q. J. Mic. 

 Sei., vol. XXIX, 1889, p. 310) in the embryo of AmUy stoma as the 

 optic groove ; the latter, however, appears to be comparatively shallow. 

 In the Herring the walls of this fold or recess consist of vesicular 

 matter, and its lumen undergoes rapid transverse extension; further 

 back, bands of fibres pass down from the optic thalami to the lateral 

 walls, reducing the width ot its lumen, and restricting the vesicular 

 area to the central region. The white matter disappears, and the recess 

 (fig. 4 r.v. 3.) passes back beneath the point of decussation of the optic 

 nerves (IT, fig. 4), and ends blindly as an ill-defined band of deeply 

 staining cells. Rabl-Rückhard (7, pp. 125, 126) describes a similar 

 structure in Salmo, in which form, however, the recess does not 

 appear at any time to extend as far back as in the Herring. He con- 

 cludes: „Die dünne ihn abschliessende Lamelle kann man nur der 

 Lamina terminalis homologisiren". Comparison with the brain of the 

 larval and adult amphibian (Frog and Toad) certainly supports the 

 view that the thin cellular floor of the recess is identical with the 

 postero-ventral wall of the bilobed eminence which forms the lamina 

 terminalis in the adult frog; but Rabl-Rückhard's theory, if I have 

 not misinterpreted him, that it represents the whole lamina, appears 

 somewhat improbable. The homologue of the thickened anterior portion 

 of that structure must, I think, be looked for, in Teleosteans, in that 

 part of the cerebrum (fig. 3 ce.) which lies between the recess (r. v. 3.) 

 and the anterior commissure (c. a.). 



The thin-walled part of the recess (fig. 4 r. v. 3) seems to have 

 its origin as a part of the cerebral floor which (like the pallium) fails 

 to become converted into true brain tissue, whilst the growth of the 

 adjacent parts, viz. the anterior commissure in front and the optic 

 thalami and optic commissure behind, tends to thrust it out in the 

 form of a fold. The cranial flexure is probably also to some extent 

 instrumental in bringing about this condition, as we shall see later 

 that with the diminution of the flexure, the fold becomes less strongly 

 marked. 



The cerebrum (figs. 2 and 5 ce.) consists for the most part of 

 vesicular matter. Ventro-laterally it bears patches of white matter, 

 divided anteriorly into dorsal and ventral limbs, by the intervention 

 of irregular strands of cells. 



The nasal sacs (fig. 2 and 5 ol) are closely apposed to this region 

 of the cerebrum, and from the ventral patch of white matter the short 



