HIND-BRAIN 



109 



The upper part of the hind-brain, so far as its ventral area is concerned, 

 undergoes changes essentially similar to those described for the lower part. The 

 mass of transverse fibres forming the pons is very late in appearing. 



FIG. 148. MODEL OF THE BRAIN OF A HUMAN EMBRYO OF 6'9 MM. (FOUBTH WEEK) 



LATERAL ASPECT. (His.) 



Cerebellum. We have already seen that over the rhombic brain the epen- 

 dymal roof-plate is much thinned and expanded. It is reduced to a simple 



islhmu* 



FIG. 149. SAME MODEL AS THAT SHOWN IN FIG. 148; MESIAL ASPECT. 



The mouth of the hemisphere vesicle, the future foramen of Monro, is bounded below by the corpus 

 itriatum and behind by a fold separating the pallium from the lateral wall of the diencephalon 

 which becomes the thalamus. The niche above the optic recess where the corpus striatum is 

 continuous with the thalamus is the stalk of the primitive hemisphere-vesicle. 



epithelial layer, except along the line of attachment to the rhombic lip, where it 

 retains some nervous tissue and forms the ligula. Opposite the pontine flexure it 

 lornis a fold which projects into the cavity of the vesicle, and becomes much 



