RECONSTRUCTION OF PAPIO CYNOCEPHALUS 347 



the midline of tlie mesencephalon. Laterally the central gray matter forms a 

 heavy structure which gradually becomes more circularly arranged about 

 the aqueduct of Sylvius. In the lower portion of the mesencephalon the 

 nucleus trochlearis appears as a specialization in the ventral prolongation 

 of gray matter which we have found passing forward from the ventral 

 aspect of the central gray matter. A similar arrangement is found also in 

 the upper part of the mesencephalon where the nucleus oculomotorius like- 

 wise develops in a ventral prolongation on either side oi the midline. As 

 the interbrain is approached the central gray matter elongates ventrally, 

 forming the long, thin sheet of gray matter surrounding the aqueduct 

 of Sylvius in its dorsal portion. This extension ventrally is produced by 

 its approach toward its conlluence with the caudal portion of the third 

 ventricle. Where the mesencephalon becomes continuous with the dien- 

 cephalon the gray matter surrounding the aqueduct of Sylvius becomes con- 

 tinuous with the subependymal gray matter lining the third ventricle and 

 the gray matter forming the mesial group of nuclei of the diencephalon. 

 The central gray matter forming the roof of the Sylvian aqueduct becomes 

 continuous with the structures in the roof of the diencephalon, while the 

 floor of the aqueduct and the undifferentiated medial interpeduncular gray 

 matter merges with the hypcncephalic region. 



