646 C. JUDSON HERRICK AND JEANNETTE B. OBENCHAIN 



icus and above the sulcus limitans is a tegmental swelling which 

 is directly continued forward into the lobus medius thalami. 



Figure 11 illustrates the appearance of this region in cross sec- 

 tion at the le\^el of the rostral end of the posterior commissure and 

 the caudal end of the mammillary recess. The sulcus limitans 

 is clearly marked both by a ventricular groove and by a thick- 

 ening of the ependyma. The cell plate which forms the ventri- 

 cular grey is more irregularly arranged below the sulcus limitans 

 and its neurones are larger and more angular. Above this sul- 

 cus is the tegmental swelling referred to, which appears to be 

 formed by a thickening of the cell plate. Still farther dorsally 

 the section passes through the caudal end of the metathalamic 

 recess, which is bounded by a still thicker cell plate. In fact, this 

 recess seems to have been formed by a slight total fold of the 

 brain wall occasioned by rapid growth of the dorsal part of the 

 cell plate. Above the recess is the postcommissural eminence 

 and laterally of it the caudal end of the lateral geniculate body 

 which contains the optic tract laterally and internally a dense 

 neuropil containing numerous neurones which have apparently 

 been derived by migration from the underlying thickened cell 

 plate in the floor of the metathalamic recess. 



The recessus pinealis is a narrow lateral pit in front of the pos- 

 terior commissure whose roof and side walls are membranous. 



The recessus rnetathalamicus is a wide saucer-shaped depres- 

 sion whose deepest part is immediately below the pineal recess. 

 It is more extensive on the left side than on the right. Atten- 

 tion has already been called to the fact that this recess, like the 

 optocoele, is apparently caused by a total outward fold of the 

 brain wall, this part of the wall being roughly comparable with 

 the metathalamus of higher brains. The grey matter in the 

 wall of the anterior part of this recess, together with a part of 

 the lobus subhabenularis adjacent, constitues the nucleus se- 

 cundus thalami of Schilling ('07, p. 433). 



The designation of this region as metathalamic recess is based 

 chiefly on its topographic relations. It is crossed laterally by 

 the optic tracts, among which are a few scattered neurones, 

 which probaly represent a primordium of the corpus geniculatum 



