I50 THE LOWER PRIMATES 



between the mesial aspects of the inferior coiliculi. Continuing upward in 

 the mesencephalon, the central gray matter becomes more rounded about 

 the ac|ucduct of Sylvius, being surrounded Idv a dorsal prolongation of the 

 reticular formation which separates it from actual contact with the super- 

 imposed superior coHicuIi. In the middle portion of the mesencephalon the 

 central gray matter rapidly begins to extend ventrally forming a long slit-like 

 structure which is surrounded by thick walls of gray matter which then 

 become continuous with the interpeduncular gray matter. Contnniing 

 upward, the central gray matter of the mesencephalon becomes continuous 

 with the central gray matter of the diencephalon surrounding the slit-like 

 third ventricle. The interpeduncular gray matter presents a rather massive 

 development immediately above the pontile nuclei. This presents consider- 

 able proportions and contains a central ventricle. Continuing upward, with a 

 definite ventral evagination, the interpeduncular gray matter fuses with the 

 ventral prolongation t)f the central gray matter as the junction of the mesen- 

 cephalon with the diencephalon is approached. The interpeduncular gray 

 matter presents at a relatively low level the condensations representing the 

 mammillary bodies which appear at a level between the superior and inferior 

 coiliculi. 



The Geniculate Bodies 



These bodies since they form a definite part of the brain stem of tarsius, 

 are described in connection with the gray matter of the brain stem. 



The mesial geniculate body is the first one to appear and the first evi- 

 dences of its existence are found caudal to the termination of the inferior 

 colliculus. It appears as an oval mass of gray matter, situated lateral to the 

 substantia nigra and directed outward and backward. It increases in size, 

 shifts somewhat laterally and then dorsally and continues up as a rapidly 

 increasing mass of gray matter in contact laterally with the ventral aspect 



