Observations upon the Development of the Teleostean Brain. 495 



siderably flattened, and pointed anteriorly, having thus a leaf -like 

 contour in vertical longitudinal section (fig. 10 pn.). As previously 

 noticed (p. 492) it receives a few fibres from the superior commissure 

 (fig. 11 CS.), and numerous fibres (fig. 10 fi.) pass from the optic 

 thalami to the hind wall of its stalk by way of the labium invaginatum, 

 (fibrous tract over the third ventricle). 



At the I inch stage, the pineal body (fig. 20 pn.) has grown 

 greatly and assumed a sac-like form by the development of a wide 

 lumen. The wall consists of two or three layers of small roundish 

 cells with large nuclei, and are much crenated. Preparations give the 

 appearance of fibres crossing the lumen, but this is probably due to 

 the presence of a coagulable fluid, as in the pineal eye of Fetromyzon 

 (cf. Beard, 4, p. 62, fig. 9). 



There is at this stage a fairly well marked constriction of the 

 lumen of the pineal body into proximal and distal vesicles (fig. 20 pn). 



At the lyV ioch stage the body is still larger. Very little coagu- 

 lable fluid is present. The walls are thicker, and consist of an outer 

 layer of small roundish cells, a middle fibrous, and an internal epi- 

 thelial layer of small cells, the outlines of which are difficult to deter- 

 mine. Externally the walls are comparatively smoother. Internally, the 

 epithelial layer is thrown into innumerable folds or crenations (cf. fig. 19) 

 by the varying thickness of the middle layer, which is thickest dor- 

 sally, giving the pineal wall in that region an internally convex appear- 

 ance in vertical longitudinal section. The cartilage of the tegmen 

 cranii {t. cr. of figs.) completely overhangs the pineal body at this 

 stage. The walls of the pineal stalk are wholly vesicular except for 

 a delicate bridge of longitudinally disposed fibres (fig. 19 /î.) passiog 

 up along the posterior wall from the fibrous tract (labium), and thus 

 from the optic thalami. 



The constriction of the pineal lumen, to which I have alluded 

 above, is still present, but is comparatively inconspicuous, owing to 

 the crenation of the lining membrane (cf. fig. 19). Beard, speaking 

 of Ammocoetes and Petromyzon, remarks (4, p. 58) that „very early 

 in development the pineal body is divided into two vesicles, a dorsal 

 one, the parietal eye, and a more ventrally situated one, which never 

 possesses black pigment, and never presents any resemblance to an 

 eye". The last part of Beard's remark applies well enough, in the 

 stages which 1 have studied, to the whole pineal body of the Herring. 

 There is never any approach to an eye-like structure, and I do not 

 think that the distal part of the body in the Herring has the same 



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