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



lobes, as before stated, laterally overlap the basal region of the mid-brain, at an early 

 stage ; but this superposition is now considerably increased — indeed, it reaches fully to 

 the middle lateral line behind the eyes (PL XXIV. fig. 3). The fore-brain still forms a 

 narrow, laterally compressed mass projecting anteriorly to form the round snout or face 

 of the embryo. Its bulk is less than half that of the mid-brain, and it encloses a small 

 dorsal chamber (PL XXIV. fig. 5). The prosencephala floor is very dense, the side 

 walls less so, while the roof thins out greatly. A small fissure continued from the 

 chamber above passes into and partially divides the thick floor of the fore-brain; but 

 sections of this region show a condition much in contrast with the capacious hollow 

 vesicle of the Elasmobranch fore-brain. Several folds appear on the superficial aspect of 

 the fore-brain (PL VIII. fig. 6) about the time of hatching or even before: but not until 

 the second or third day after emerging does the deep fold appear which divides the fore- 

 brain into two parts, the cerebral or anterior fore-brain and the thalamencephalic or 

 posterior fore-brain (PL VI. fig. 7). A longitudinal fold passes over the dorsal surface of 

 the fore-brain (fb) before it is markedly separated into front and hind prosencephalic 

 regions, and it thus becomes longitudinally bifid at an early stage (PL VI. fig. 6). 

 No olfactory lobes proper exist as yet; indeed, as Marshall found in Salmonoid larvae, 

 these structures must be comparatively late in appearing. 



The changes which ensue when the primitive brain of three vesicles is finally divided 

 into a series of five, are very complex and difficult to follow ; but the main features may 

 be indicated. The five parts of the brain distinguished are as follows : — 



{(l) Anterior fore-brain or cerebral hemispheres, &c. (" Vorder- 

 hirn," Gegenbaur and Baer). 

 (2) Posterior fore-brain or thalami optici (" Zwischenhirn," Baer). 



n^r ii J (3) Mid-brain, or optic lobes, &c. (Mittelhirn, Baer, Zwischenhirn, 



I Gegenbaur). 



™ , 11 ( (4) Hind-brain or cerebellum (Hinterhirn, Baer, Mittelhirn, 



( Gegenbaur). 



Myelencephalon, (5) Medulla oblongata (Nachhirn, Gegenbaur). 



From the fourth ventricle a narrow fissure, the aqueductus Sylvii, leads into the third. 

 The base of the latter partly overlies, and partly abuts against, the mass now separated, 

 as already indicated, from the cerebral hemispheres or anterior fore-brain by a transverse 

 superficial cleft (PL XIII. fig. 1). This posterior portion of the fore-brain is the 

 thalamencephalon, and it is the part of the brain which, for the most part, overlies the 

 roof of the mouth. 



Very early a portion of the hind part of the thalamencephalic floor is directed back- 

 ward as a prolongation beneath the elevated medulla oblongata, and during its course its 

 direction is slightly downward. The cells composing this encephalic diverticulum have a 

 somewhat columnar arrangement, and surround a cavity continuous above with the third 



