THE FORE-BRAIN. 399 



flexure than any of the previously mentioned points, is the 

 appearance of a bend in the hind-brain which tends to correct 

 the original cranial flexure. The gradual growth of this fresh 

 flexure can be studied in the longitudinal sections 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-brain. 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 (PI. 15, fig. 3 op. v). Between stages I and K 

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

 process towards the exterior, which forms the rudiment of the 

 pineal gland (PI. 15, fig. !,/) 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 (PI. 15, fig. 2 and fig. i6#), 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. 15, 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 (PI. 15, 

 figs, ga, 12, 1 6, 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 perfectly 

 simple, and exhibits no signs, either externally or internally, of a 

 longitudinal constriction into two lobes. From the canal uniting 

 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 close of stage K (PI. 15, fig. 1 1, op. ). 



During stage L the infundibulum becomes much produced, 

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



