THE PINEAL BODY 83 



The retina of the pineal organ in cyclostomes shows its most 

 marked development in embryonic and larval stages. Beard 17 

 in 1887 found in Ammocoetes rod cells, and Owsiannikow 295 in 

 1888 showed in Petromyzon fluviatilis that there were five dis- 

 tinct layers of cells and fibers in the retina. The first of these 

 layers consisted of nerve fibers; the second, of large nerve cells; 

 the third was fibrous; the fourth consisted of small cells inter- 

 spersed among the large rod-shaped cells, and the fifth was an 

 ependymal layer. Gaskell 145 in 1890 was able to find rod cells 

 only in the retina of Ammocoates, and he was of the opinion 

 that the so-called pineal eye in this form was a compound struc- 

 ture in which the light-receiving bodies were formations com- 

 parable to the rhabdites of the Arthropod eye. Studnicka 

 ('93) 384 recognized four layers of cells and fibers in the retina of 

 cyclostomes. The first of these was a layer of nerve fibers, the 

 second were basal cells, the third small cells, and the fourth, 

 large cylindrical cells. Leydig 239 in 1896 found two types of 

 cells, an inner cylindrical and an outer layer of round cells. 

 Retzius, 331B however, in 1895, could find no evidence of the sensory 

 organ in the so-called pineal eye of cyclostomes and he did not 

 consider it to be an eye. Mayer 264 in 1897 found ganglionic 

 cells in the retina, and Studnicka 388 in 1899 found still more 

 evidence of the retinal nature of the ventral wall of the end- 

 vesicle. 



The pellucida becomes best developed in Petromyzon marinus, 

 for the dorsal wall of the pineal organ appears in the more or less 

 constant form of a plane or convexed lens, the flattened surface 

 of which is ectally directed. In Petromyzon planeri and fluvia- 

 tilis, the pellucida is extremely irregular in its thickness as well 

 as in its form. It must not, therefore, be maintained that even 

 in those forms where the pellucida has a lenticular shape and 

 arrangement that it is actually a lens structure. One feature 

 about it, however, suggests that it is an organ designed for the 

 transmission of light rays, namely, its almost complete lack of 

 pigment except perhaps at the peripheral edges where it passes 

 over into the ventral wall or so-called retina of the pineal eye. 

 This lack of pigment led Carriere 57 in 1890 to call the dorsal wall 



