96 PEIICIVAL HAILEV 



111 the wall of the third ventricle, the habeiiula is well marked 

 (fig. 20, n.h.) and from its posterior end a ridge extends downward 

 to the tegmentum (fig. 20, f.r.). Just below the habenula, the 

 wall of the diencephalon is much thicker than in the 19 mm. 

 embryo, and extends anterior to the velum transversum to form 

 the posterior wall of the foramen interventriculare. 



3. The 32 mm. embryo (H. 4I) 



Just in front of the velum transversum, as in the 28 mm. 

 embryo, the roof plate forms a low arch (figs. 22 and 25, p.a.) 

 to the sides of which are attached the lateral choroid plexuses. 

 Immediateh^ anterior to this arch the roof plate becomes very 

 thin (fig. 25, tx.t.m.). Anterior to this thin lamina, the roof 

 plate is thickened again in the manner noted m the case of the 

 28 mm. embryo (see fig. 6, t.r-p.). At the anterior end of this 

 region (fig. 25, l.sJ), there is an indication of the median notch 

 on the ependymal surface (fig. 8, t.r-p). It is, however, not 

 nearly so well marked as the similar notch in the 28 mm. embryo 

 (fig. 7, t.r-p.) At the extreme anterior end of this region is 

 a structure not found in any of the other embryos. A deep 

 narrow pit extends into the roof plate from the pial surface, 

 which causes the roof plate to project into the ventricle (figs. 13, 

 14, and 25, d.p). This pit is found only in one section, with 

 indications of it in the two adjacent sections. Immediately 

 anterior to the pit, the roof plate thickens markedly and extends 

 as a thickened lamina (fig. 22, l.t.) to the recessus preopticus. 

 There is, however, a short distance anterior to the pit a shallow 

 notch on the ventricular surface of the lamina terminalis (fig. 

 25, r.n.f). 



The tela chorioidea diencephali shows only slight indications 

 of longitudinal folding. The outwardly curved and very promi- 

 nent thalamic lip (fig. 12, t.l.) is not in contact with the lateral 

 wall of the thalamus. At the anterior end, the tela chorioidea 

 diencephali is very broad, and a pouch (fig. 25, a.p.) arises which 

 extends forward over the velum transversum. The whole tela 

 resembles very closely the same structure in the 28 mm. embryo. 



