THE PINEAL BODY 89 



its base he was able to discern fibers which seemed to cross to 

 the opposite side. These nerve fibers extended backward from 

 the cells in the parapineal organ. Leydig was unable to identify 

 any structure which he considered a retina or a lens. The stalk 

 in the adult form becomes reduced to a mere strand containing 

 fibers which by many authors are considered to be nerve fibers. 

 The primitive lumen present in the stalk of the parapineal 

 organ very early disappears and the proximal portion rapidly 

 becomes inconspicuous and finally is lost by the marked develop- 

 ment of the commissura habenularis. 



The majority of investigators who have studied this part of 

 the brain in cyclostomes are in accord along several general 

 lines. They believe that the cells found in the parapineal 

 end-vesicle are ependymal cells, spindle or rod cells, and some 

 sensory cells. It is also their opinion that there are nerve fibers 

 connecting these cells situated among which are larger ganglionic 

 elements from which the fibers may take their origin. In a 

 general way the same constituents occur in the retina of the 

 parapineal organ as are present in the pineal organ. The main 

 differences between these two structures consist in the size and 

 disposition of their respective elements. In the adult the para- 

 pineal organ is situated upon the most anterior portion of the 

 membranous forebrain roof while directly in front of it is the 

 paraphysis and above it the pineal vesicle. Situated in this 

 position the two end-vesicles of the epiphyseal complex have 

 the appearance of a pair of eyes which are rudimentary and 

 which, in attempting to assume visual function, have morpho- 

 logically fallen short in the attainment of that object. It should 

 be noted that their position places them in the midsagittal 

 plane, one behind the other, and that according to the most 

 reliable evidence concerning cyclostomes available at the present 

 time, there is no definite tendency toward lateralization in one 

 or the other of these elements in the epiphyseal complex. The 

 two end-vesicles, practically in contact with each other, occupy 

 a deep fossa formed by a depression on the inner surface of the 

 skull. This fossa is especially well marked in adults and more 

 particularly in Petromyzon marinus. 



