Sorensen, Study of Epiphysis and Roof of Dicncephalon. 23 



Miclucho-Maclay (58- jo) in his work on the selachian 

 brain overlooked the diencephalon and identified that organ with 

 the mesencephalon, thus introducing considerable confusion 

 into the nomenclature of the fish brain. Stieda, however, 

 showed that the position of the epiphysis is a fixed point serving 

 to identify the diencephalic roof. 1 



Leydig (49- '72) who was probably the first to find the or- 

 gan and give a detailed account of its structure, gives the his- 

 tology of an organ which lies above the thalamencephalon or 

 region of the third ventricle, but says that this organ is not the 

 embryonic pineal which arises later and is of a very different 

 structure. The specimens studied by Leydig were Lacerta 

 agilis, L. muralis, L. vivipara, and Anguis fragilis. In L. 

 agilis he found an organ which he called the parietal organ. 

 Writing of this section he says that it could easily be taken for 

 a sharply outlined part within the epidermis. The pineal gland 

 lies in a direct line with the parietal organ in a transverse sec- 

 tion. The relations in the parietal organ he gives in detail : 1. 

 horny layer of epidermis ; 2. mucus membrane with the organ 

 imbedded within it ; 3. the strongly pigmented portion of the 

 cutis vera; 4. the bone which opens toward the surface. Be- 

 neath the bone lie the strongly pigmented meninges and under 

 this the pineal, which is joined to the roof by means of two ner- 

 vous peduncles. In Anguis fragilis he describes the organ as a 

 dark spot, and the pineal gland as a dark streak. Of the epiphysis 

 he says that its stalk consists of two peduncles, and lies below 

 the spot and streak, and differs from both. Its surface has a 

 folded appearance. The pineal is entirely unpigmented. As 

 to the significance of the organ, Leydig thought it was related 

 to the parietal gland of batrachians. Leydig 2 (4.9-87) thinks 

 that the parietal organ belongs to the system of sense organs of 

 the lateral line, and finds a parallel in the luminous organs of 

 certain fishes described by him. 



'Ueber die Deutung einzelner Theile des Fischgehirnes. Zeitsch. f. wis. 

 Zoologie, XXIII. 



2 Fide Beard. 



