404 THE INTERMEDIATE PRIMATES 



shaped process which contains embedded within it the nuclei of the fourth 

 and third cranial nerves. As the diencephalon is approached the ventral 

 extension of the central gray matter rapidly increases until the third ven- 

 tricle appears where the central gray matter becomes continuous with the 

 diencephalic subependymal gray matter and the mesial mass of the thalamic 

 nuclei. 



The floor of the fourth ventricle shows practically no modeling in its 

 lower half. Above the level of the lateral recess, however, there appear a 

 relatively well-marked eminences on either side of the midline, separated 

 from each other by a rather deep sulcus, representing the underlying con- 

 densation of the gray matter to form the nucleus of the sixth cranial nerve. 



