17G THE FORE-BRAIN. 



which have been represented. It is at its maximum in stage Q. 

 This short preliminary sketch of the development of the brain 

 as a whole will serve as an introduction to the history of the 

 individual divisions of the brain. 



Fore-hrain. In its earliest condition the fore-brain forms 

 a single vesicle without a trace of separate divisions, but buds 

 off very early the optic vesicles, whose history is described with 

 that of the eye (PL xiv. fig. 3, op. v). Between stages I and K 

 the posterior part of the fore-brain sends outwards a papiiliform 

 process towards the exterior, which forms the rudiment of the 

 pineal gland (PL XIV. fig. 1, _2^?i). Immediately in front of the 

 rudiment a constriction appears, causing a division of the fore- 

 brain into a large anterior and a small posterior portion. This 

 constriction is shallow at first, but towards the close of stage K 

 becomes much deeper (PL xiv. fig. 2 and fig. 16a), leaving 

 however the two cavities of the two divisions of the fore-brain 

 united ventrally by a somewhat wide canal. 



The posterior of the two divisions of the fore-brain forms 

 the thalamencephalon. Its anterior wall adjoining the cerebral 

 rudiment becomes excessively thin (PI. xiv. fig. 11); and its base 

 till the close of stage K is in close contact with the mouth 

 involution, and presents but a very inconspicuous prominence 

 which marks the eventual position of the infundibulum (PL XIV. 

 fig. 9a, 12, 16, in). The anterior and larger division of the 

 fore-brain forms the rudiment of the cerebral hemispheres and 

 olfactory lobes. Up to stage K this rudiment remains per- 

 fectly simple, and exhibits no signs, either externally or 

 internally, of a longitudinal constriction into two lobes. From 

 the canal unitino' the two divisions of the fore-brain (which 

 eventually forms part of the thalamencephalon) there spring 

 the hollow optic nerves. A slight ventral constriction separating 

 the cerebral rudiment from that part of the brain where these 

 are attached appears even before the cJose of stage K (PL xiv. 

 fig. 11, op.n). 



During stage L the infundibulum becomes much produced, 

 and forms a wide sack in contact with the pituitary body, and 

 its cavity communicates with that of the third ventricle by an 

 elongated slit-like aperture. This may be seen by comparing 

 PL XV. fig. la and Ic. In fig. Ic taken along the middle line, 



