PARIETAL EYE AND ADJACENT ORGANS IN fcJPHENODON. 123 



With the foregoiug description of a series uf transverse 

 sections should be compared the sagittal section represented 

 in fig. 15^ especially with regard to the position of the 

 commissures. This figure shows very clearly the strong 

 upward arching of the thin roof of the third ventricle, which 

 is doubtless due to the compression of the thalamencephalon in 

 the straightening out of the cerebral flexure. The thin-walled 

 upward extension of the third ventricle thus caused is bounded 

 anteriorly by the commissura fornicis {Com. F.), and pos- 

 teriorly by the superior commissure (Com. Sup.). 



The Parietal Stalk. — The parietal stalk (figs. 14, 15, 16, 

 27, Pa. S.) has now become considerably elongated. It is 

 attached to the roof of the brain, immediately behind the 

 superior commissure, and in front of the posterior commissure 

 (fig. 15). From its point of attachment it first runs upwards and 

 slightly backwards ; it then curves forwards over the bay formed 

 by the roof of the third ventricle, to which its anterior wall is 

 closely applied. Thus, in a transverse section, such as is 

 represented in fig. 27, the proximal part of the stalk is cut 

 longitudinally and tangentially below the thin-walled upper 

 division of the third ventricle, while the distal portion is cut 

 transversely above it. 



At the comraencemeut of Stage R the blind distal extremity 

 of the parietal stalk ends in a not very strongly marked and 

 somewhat flattened expansion close beneath the parietal eye 

 (fig. 14). In somewhat older embryos of the same stage it 

 ends in a glove-finger-shaped extremity separated by a con- 

 siderable interval from the parietal eye (figs. 15, 16), this 

 separation being due apparently to a forward shifting of the 

 parietal eye itself. 



At its proximal end the parietal stalk becomes greatly 

 narrowed, as if by compression from the strong development 

 of the superior commissure in front of it; and its lumen, else- 

 where wide, is here completely obliterated, though its walls 

 can still be traced into direct continuity with the epithelial 

 lining of the brain. Except at its proximal extremity the wall 

 of the parietal stalk is thick and densely packed with large 



