Sorensen, Study of Epiphysis and Roof of Dienceplialon. 43 



commissure ; then there is a segment of the roof in front of 

 which the pineal arises in form of a funnel, which, with its knob- 

 like end enters the skull roof. In front of the pineal is a trans- 

 verse thickening traversed by the supra-commissure. In front 

 of the supra-commissure the roof of the brain thins out into a 

 superior tela choroidea, in which we can distinguish ; (1) the 

 "Zirbelpolster," (2) the Velum, directed into the ventricle as a 

 fold of membrane, (3) an anterior segment from which (4) the 

 plexus hemisphaerium descends. In front of this we find the 

 fore-brain. If we now compare these relations with the forego- 

 ing relations in the roof of the diencephalon of Protopterus, it 

 appears that the segment between the commissures is the same 

 as in Scyllium. A complication, however, is encountered in 

 the tela choroidea in that it is raised and forms a vascular plexus 

 which has the appearance of manifold plications ; likewise the 

 lateral 'plexus inferiores appear in selachians as a segment of 

 the tela. We have already seen that the velum in Protopterus 

 has folds. The fore-brain which is two-lobed in Protopterus, 

 has undergone in its median opening a strong constriction. Its 

 median lamella is folded vertically to the tela and not horizon- 

 tally. The elementary plications in the Protopterus brain are 

 increased in amphibians. In the roof of the diencephalon of 

 Ichthyopsis the segment between the post-commissure and the 

 pineal has become somewhat extended ; The pineal has aban- 

 doned its relation to the skull roof and forms only a small pear- 

 shaped vesicle; the "Zirbelpolster" is elongated ventrally, 

 and the velum has been transformed into an intricately ramified 

 plexus choroideus, (ventriculi III), which reaches to the aquae- 

 ductus Sylvii. The vascular plexus (conarium) has become a 

 complicated tubular system, whose anterior extremity lies be- 

 tween the hemispheres, while the posterior limit covers the en- 

 tire roof of the diencephalon and envelops the pineal and "Zir- 

 belpolster." The inferior plexus has also become ramified and 

 enters the third ventricle, in which it lies close to the plexus infer- 

 ior of the other side. The plexus hemisphaerium begins to de- 

 scend into the first ventricle in front of it and not more medially, 

 as in Protopterus. The ontogenetic development of the roof 



