PARIETAL EYE AND ADJACENT ORGANS IN SPHEN(1D0N. 121 



mantle walls of the cerebral hemispheres. I identify this with 

 the mantle commissure or corpus callosum mentioned by 

 Hoffmann/ and I think a comparison of his fig. 4<, pi. 

 clxiii, representing a corresponding section of an embryo 

 of Lacerta, leaves no doubt as to the correctness of this 

 identification. Beneath the third ventricle is seen the optic 

 chiasma [Op. Ch.). 



The section represented in fig. 23 passes through the 

 foramina of Monro, and is drawn to show especially the 

 choroid plexus of the lateral ventricle on either side springing 

 from a common origin between the inner mantle walls of the 

 cerebral hemispheres. 



Fig. 24 represents a similar section immediately behind the 

 foramina of Monro, and the only feature about it which calls 

 for special comment in this place is the presence of a con- 

 spicuous transverse commissure [Com. F.) running across 

 above the third ventricle from the inner mantle wall of one 

 cerebral hemisphere to that of the other. This commissure, 

 here very strongly developed, appears to be identical with the 

 rudiment of the fornix discovered in 1881 by Rabl-Riickhard 

 (27) in Psammosaurus.^ I shall refer to it in future as the 

 Commissura fornicis. 



Fig. 25 represents a transverse section across the third 

 ventricle in the plane of the parietal eye,^ The ventricle 

 itself (fig. 3) is partially divided into two parts, upper and 

 lower. The lower is a deep, slit-like space, bounded on either 

 side by the thick optic thalamus. The upper is a much wider 

 cavity, having its roof formed by the thin choroid plexus, and 

 its floor by the two ganglia habenulse [Gang. Hah, R. and L.), 

 which lie upon the right and left optic thalami respectively, 

 on either side of the slit-like opening into the lower division of 

 the ventricle. There is no noticeable difference as regards 

 size between the two ganglia habenulse. On either side of the 



1 (18), pp. 1977, 1979, et seq. 

 ' Vide also HofFmaun (18), pp. 1975 et seq. 



' The slightly asymmetrical position of the eye itself is evidently due to an 

 artificial crease in the integument. 



