122 ARTHUR DENDY. 



thalamencephalon one of the cerebral hemispheres (C. H.), 

 with its large lateral ventricle {L. V.), is cut through. 



Fig. 26 represents a section through the hinder part of the 

 third ventricle, the upper and lower divisions of which are seen 

 to be completely separated from one another in this region by 

 the development of a strong band of commissural fibres {Com. 

 Sup.) which runs across from side to side, between and beneath 

 the hinder ends of the two ganglia habenulse. This commissure 

 I identify with the " superior commissure " described by 

 Burckhardt (9) in Lacerta, and by Hill (17) in various bony 

 fishes. Hill further identifies it with the superior commissure 

 found by Osborn^ in Amphibia, and with a commissure found 

 by Balfour^ in Elasmobranchs. Its position corresponds very 

 closely with what Beraneck (5) calls the " parietal centre/' and 

 one can hardly help identifying it with this structure as 

 represented in his fig. 5. 



Owing to its very strong development iu Sphenodon, I at 

 first mistook the superior commissure for the posterior com- 

 missure, which at this stage is very feebly developed.^ 



The upper division of the third veutiicle is bounded beneath 

 by the ganglia habeuulse, aud arched over by the here much 

 folded choroid plexus [Ch. P.). The lower division is bounded 

 as before by the two optic thalami, and beneath it lies the 

 infundibulum {Inf.). 



Fig. 27 represents a transverse section through the entrance 

 to the iter, and shows the very feebly developed posterior 

 commissure {Com. Post.) running across just above the latter. 

 The extension of the upper division of the third ventricle 

 {V. 3), which runs back above the superior commissure, is cut 

 through between the anterior extremities of the two optic 

 lobes (0. L.). 



* 'Journal of Morphology,' vol. ii, pp. 51 — 96 (quoted by Hill). 



^ Vide Balfour's 'Comparative Embryology,' vol. ii, p. 356, fig. 254. 



' This mistake occurs in my summary of results (10), iu which the inferior 

 commissure is referred to as the posterior. I shall show later on, however, 

 that there is some reason for believing that the inferior commissure may unite 

 with the posterior commissure in the adult. 



