Observations upon the Development of the Teleostean Brain. 486 



outline at this stage. It is furrowed by the wall of the third ventricle 

 for the posterior half of its length. No structural change is obser- 

 vable in its roof (figs. 9 and 10 pa.), which resembles that of Anar- 

 rhichas (cf. 6, p. 910, pi. XXIV, fig. 5 fbr.). 



The anterior commissure (figs. 9 and 10 c. a.) is situated further 

 forward and is broader than in the earlier stage. The third ventricle 

 (v. 3.) passes for a considerable distance in front of the commissure 

 referred to, and the cerebrum appears in transverse section as a vesi- 

 cular mass with dorso-lateral patches of white matter : from the latter 

 the olfactory nerves pass forwards and outwards to the nasal sacs, 

 now removed to some little distance from the cerebrum, and disposed 

 partly in front of it. 



In section the tips of the optic thalami appear as before, above 

 the hinder ends of the cerebral lobes. The fibrous bridge over the 

 third ventricle is now more fully marked, as is its posterior extremity, 

 the posterior commissure (fig. 8 b c.p.). The tips of the tectum lobi 

 optici (figs. 8 and 9 1. 1. o.) are seen above the foregoing , and the 

 optic thalami (t. o.) are rather broader than formerly. 



The tori semicirculares (fig. 8 t s.) appear as well-marked eminences 

 on either side of the fissure of the mid-brain, posterior to the opening 

 of the third ventricle. The optic lobes are seen in a more advanced 

 condition. The lateral optic ventricles (fig. 8b v.o.) extend outwards 

 and downwards, in posterior sections (fig. 8) almost severing the tectum 

 {t. I. 0.) from the cerebral mass. 



A few cells appear along the dorso-lateral white matter of the 

 tectum and a deeply staining tract of cells (fig. 8 and 10 t. c.) connects 

 the thick lateral elements of the tecta, arising from the dorsal surface 

 near their inner margins (see fig. 8). 



A noticeable feature at this stage is the great increase in the 

 size of the mid-brain. The sides of the valvula (fig. 8 f.g.) again 

 appear as ridges, external to the tori (t. s.), which unite further back. 

 The posterior ends of the lateral optic ventricles are broader, and 

 extend more inwards than in the earlier stage. 



The thick cerebellar fold (fig. 10 ch.) very soon, by the reduction 

 of its central region, takes on the appearance of bilaterally symmetrical 

 elements united by a fine band of cells. It passes (figs. 9 and 10) 

 into the thin roof of the fourth ventricle {t.v.4.). 



The dividing commissure is not as well-marked as in earlier 

 stages. The infundibulum has undergone a certain amount of vertical 

 flattening, the lateral expansions representing the future lobi inferiores. 



