XII.] THE FOKE-BKAIN. 371 



first visible on the appearance of an oblique transverse 

 furrow, by which the whole mid-brain is divided into an 

 anterior and posterior portion. The anterior portion is 

 further divided by a longitudinal furrow into the two 

 anterior tubercles (nates) ; but it is not until later on 

 that the posterior portion is similarly divided longitu- 

 dinally into the two posterior tubercles (testes). 



The floor of the mid -brain, bounded posteriorly by 

 the pons Varolii, becomes developed and thickened into 

 the crura cerebri. The corpora geniculata interna also 

 belong to this division of the brain. 



Fore-brain. The early development of the fore- 

 brain in Mammals is the same as in the chick. It forms 

 at first a single vesicle without a trace of separate 

 divisions, but very early buds off the optic vesicles, 

 whose history is described with that of the eye. The 

 anterior part becomes prolonged and at the same time 

 somewhat dilated. At first there is no sharp boundary 

 between the primitive fore-brain and its anterior 

 prolongation, but there shortly appears a constriction 

 which passes from above obliquely forwards and down- 

 wards. 



Of these two divisions the posterior becomes the 

 thalamencephalon, while the anterior and larger division 

 forms the rudiment of the cerebral hemispheres (Fig. 

 39 cer) and olfactory lobes. For a considerable period 

 this rudiment remains perfectly simple, and exhibits no 

 signs, either externally or internally, of a longitudinal 

 constriction dividing it into two lobes. 



The thalamencephalon forms at first a simple 

 vesicle, the walls of which are of a nearly uniform thick- 

 ness and formed of the usual spindle-shaped cells. 



242 



