914 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



The commissure on the roof of the lower division of the mid-brain (thalami) forms a 

 uniform deep band after this, and then it ceases, so as to open up the whole central region 

 into a single chamber (common ventricle). 



In longitudinal sections the roof of the tbalamencephalon shows a large fold of cells 

 (tela choroidea) in front of the pineal gland. The latter is cellular (on 1st May), and is 

 connected by strands of fibres at its base with the centre of the optic thalamus, the 

 direction of the fibres being generally downward and backward. 



At this stage the posterior region of the fore-brain is protected externally by a special 

 plate of cartilage, which stretches over and extends for some distance down each side, 

 passing over the anterior end of the optic lobes. The black pigment lies beneath it, while 

 externally is a hyaline ossific stratum. In transverse sections a deep dimple is usually 

 present over the pineal gland. The next feature of note is the formation of the optic 

 commissure. This is produced by a great band of fibres arising at the upper border of 

 the thalamus, and apparently continuous with the optic lobes above. This massive band 

 passes down on each side of the central region of the thalamus and the fibres simply 

 cross each other beneath. A streak in the centre of the band inferiorly is probably the 

 lumen of the nerve-trunk, and it can be traced to the choroidal fissure. The succeeding 

 sections show oblique bundles of longitudinal fibres on each side of the central region 

 inferiorly. These are isolated by the vertical bands, some of which form a commissure 

 beneath the central chamber of the thalamus. In the line of the great transverse com- 

 missure, just behind the pineal gland and at the upper part of the thalamus, a circular 

 region appears in the centre of the lateral mass of the organ superiorly, which increases 

 in size as it proceeds backward. Its outer (cellular) region stains deeply, and in the 

 preparation it has separated from the surrounding tissue. Vertical bands of fibres pass 

 across each region in front of the pituitary body. Shortly after the latter appears the)' 

 diminish in size, and, keeping at the upper part of the infundibulum, disappear before 

 the pituitary slices do. By the differentiation posteriorly of the upper region into 

 crura cerebri, these rounded bodies are placed in transverse section at the upper part of 

 the infundibulum. The lateral fissures from the infundibulum appear before the 

 former cellular differentiations cease, and they by and by attain a larger size than the 

 central one, apparently from constriction of the latter. This arrangement indicates 

 a possible homology with the trifid infundibulum of the Elasmobranchs, and the so- 

 called lobi inferiores are thus apparently closely connected with the infundibular 

 apparatus. The central fissure of the infundibulum has a very definite contour, and, 

 inferiorly, it leads to the pituitary body, above which a lozenge-shaped dilatation occurs, 

 the margin of the aperture being directly continuous with the hypophysis, the centre 

 of which is dimpled superiorly by the tip of the lozenge-shaped space. By and by 

 (proceeding backward) this central infundibular aperture becomes trifid by the protru- 

 sion of lateral diverticula, each forming a separate pouch after the connection with the 

 pituitary body is broken. These diverticula soon disappear and the central aperture 

 becomes very small, and at the same time its walls are folded and defined by cylindrical 



