Johnston, The Brain of Pctrotnyzon. 35 



Studnicka ('00) has described sense cells bearing rods or 

 cones in the epiphysis. These are probably present in Lam- 

 petra, as noted above. He describes the paraphysis as only a 

 pouch of the choroid plexus with a single layered wall through- 

 out and containing no sense cells. This description will not 

 apply in any particular to the Zirbelpolster of Lampetra, and I 

 know of no other structure in this region of which Studnicka 

 could be speaking. 



In addition to the structure of the epiphysis and paraphy- 

 sis, their position and the structure of the overlying tissues 

 should be taken into account in order to determine their func- 

 tion. The epiphysis is separated from the paraphysis by a thin 

 and imperfect membrane. The epiphysis is covered dorsally 

 by the skeletogenous layer of the cranium, the outer portion of 

 which is connected with the myocommata. This is a thick layer 

 of fibrous connective tissue, containing many flattened and stel- 

 late nuclei. The layer is thicker here than elsewhere and in 

 GoLGi sections there are seen many nerve fibers on their way 

 to the skin. Immediately outside of this layer is a layer of pig- 

 ment cells which form a dense and continuous pigment layer 

 everywhere except in the cornea of the eyes and here over the 

 epiphysis. Outside this is the thinner connective tissue layer 

 of the dermis, and outside that the epidermis which is thicker 

 and better supplied with free nerve endings here than elsewhere. 

 The epidermis contains many deeply stained goblet cells which 

 are lacking over the epiphysis. These facts show that provision 

 has been made here for the passage of light as in the case of 

 the lateral eyes, although the thickness of the layers offers 

 greater resistance than in the cornea. The form and structure 

 of the epiphysis make it an organ for focussing the light, al- 

 though not as efficient as the lens of the eye. Finally, the 

 nerve cells of the paraphysis are doubtless capable of receiving 

 stimuli from the light which easily passes through the thin dor- 

 sal wall and cavity filled with fluid. These light stimuli would 

 then be carried to the left ganglion habenulae. The fibers from 

 the paraphysis seem to be more than half as numerous as those 

 which end in the left ganglion from the tractus olfacto-haben- 



