640 C. JUDSON HERRICK AND JEANNETTE B. OBENCHAIN 



dorsal portion of the stria medullaris complex which passes 

 through the primordium hippocampi (tractus cortico-habenularis 

 mediaHs, etc., see beyond). This is the structure which is marked 

 em.th. in figure 9 of Johnston's paper ('12, p. 381), and which is 

 not the same thing as the structure similarly marked in some of 

 his other figures (p. 660). 



Immediately ventrally of the subhabenular sulcus is another 

 eminence of the lateral surface, the lobus subhabenularis thalami, 

 which is also visible on the ventricular surface (figs. 1, 2, 3, 

 l.sh.). This eminence is separated by a very shallow external 

 groove from a similar elevation farther forward, the primordium 

 hippocampi, most of which is concealed in the stem-hemisphere 

 fissure (fig. 2). All these features related to the habenula are 

 much more prominent on the right than on the left. 



The dorsal sac is much larger and more widely dilated in this 

 specimen than in Johnston's model of the 120 mm. specimen. 

 Its highest point is at the junction of the pineal stalk and vesicle 

 above the anterior end of the right habenula. 



The pineal vesicle is large and approximately circular in out- 

 line as seen from the dorsal surface (fig. 1, ep.). A small para- 

 pineal organ is apparently concealed beneath the pineal vesicle; 

 but, as the tissues of this region were poorly preserved, no attempt 

 was made to model it. A slender solid pineal stalk extends 

 backward from the pineal vesicle to a point above the pineal 

 recess. The size of this stalk is somewhat exaggerated in the 

 model. 



The wall of the dorsal sac becomes continuous forward with 

 the lamina supraneuroporica within which is the dorsal commis- 

 sure (fig. 4). Above this commissure is a transverse fold or 

 wrinkle of the membranous roof (figs. 2, 3, v.tr.) which probably 

 represents the velum transversum, forming the boundary be- 

 tween the dorsal sac and the lamina supraneuroporica. This 

 fold extends only a short distance lateralward and then fades 

 out in the wall of the dorsal sac. Its ventral (rostral) limb lat- 

 erally joins the massive primordium hippocampi along the taenia 

 fornicis (fig. 5). 



