42 Journal of Comparative Neurology. 



which thins out anteriorly. Next to this are repeated thicken- 

 ings of the roof of the diencephalon which contain the supra- 

 commissure. The pineal arises where these two sections meet. 

 It consists of a corkscrew-like stalk which winds forward ob- 

 liquely ; its posterior part is hollow, while its anterior is solid. 

 At the end, which is bent around horizontally, there is a pineal 

 vesicle, a little glandular sack which is occasionally filled with 

 sand. A communication of the cavity of the pineal stalk with 

 the third ventricle is not shown, but such is possible. In front 

 of the supra-commissure the roof of the diencephalon is ele- 

 vated and forms by its walls a support for the pineal, hence this 

 portion was called " Zirbelpolster, " by Edinger. Then the 

 roof bends down and forms the velum, a principal fold directed 

 inwards, which produces other accessory folds. Here the epi- 

 thelium arises again dorsally, among numerous plications and 

 forms a primitive vascular plexus, for which I have suggested 

 the term conarium ; its anterior wall decreases anteriorly and 

 passes in two diverging forms into the median wall of the fore- 

 brain. 



To sum up Burckhardt gives the following relations : 



1. Commissura posterior. 



2. Schaltstiick (Pars intercalaris) 



j. , , ( a. Zirbelstiel. 



\ b. Zirbelblaschen. 



3. Commissura superior. 



4. Zirbelpolster. 



5. Velum. 



6. Adergeflechtknoten (Conarium) 



p. j a. Inferior et hemisphaerium sin. 



' ' \ b. Inferior et hemisphaerium dex. 



The last four are included in the tela choroidea superior. 

 ("Zirbel" of authors). 



Comparative relations. According to the investigations of 

 Ehlers, (18-78), Edinger, (16(1-92), Rex, (76a- 91), as well 

 as his own, Burckhardt found the following to be the structure 

 of the roof of the diencephalon in selachians : anteriorly the 

 mesencephalon descends obliquely and is limited by the post- 



