Sorensen, Study of Epiphysis and Roof of Diencephalon 155 



fibres pass toward the roof of the mesencephal. From the de- 

 generate condition it would be impossible to determine whether 

 these are nervous or connective tissue elements. In the Amia 

 as in other ganoids, the epiphysis lies to the left of the meson, 

 owing to the greater development of the right habena. The 

 stalk can be traced through a tortuous path to its union with 

 the brain. In the lamprey, the connection of the stalk with the 

 brain is very clear. The pigment of the organ is white by re- 

 flected light, agreeing with Beard's description (^-'87). The 

 white pigment extends from the floor of the epiphysis into the 

 hollow stalk for some little distance, then the stalk becomes in- 

 conspicous and passes between the two habenae to its union with 

 the brain at the left of the middle, but on a morphological 

 meson. The paraphysis in Diemyctylus forms a very irregular 

 sac lying in the midst of the supra-plexus. The opening is 

 upon the meson between the blood vessels supplying the dia- 

 and auli-plexuses. This opening is not into either diaccele or 

 aula but marks the boundary between the two. In Amia the 

 dorsal sac is reflected from the supra-commissure with a diverti- 

 culum extending caudad over the supra-commissure and hab- 

 enae. The stalk of the epiphysis arising caudad of the supra- 

 commissure is thus brought into contact with the dorsal sac and 

 continues cephalad upon it or partly enclosed in it and from 

 this arrangement the term second vesicle of the Epiphysis has 

 been applied (Zirbel-polster of German writers). 



W. A. Locy {51-9Z) has studied the derivation of the 

 pineal and gives the following summary : 



1. There are preserved on the cephalic plate of Elasmo- 

 branchs (Squalus acanthias) at least two pairs of accessory optic 

 vesicles. 



2. These taken with the primary optic vesicles give to the 

 embryo three pairs of rudimentary eyes. 



3. The anterior pair develop into the lateral eyes and the 

 first accessory pair form the walls of the thalamencephalon and 

 give rise to the principal outgrowths from it. 



4. The epiphysis is therefore double in origin forming 

 from a united pair of accessory optic vesicles. 



