64 THE PINEAL ORGAN 



clearly seen in preparations from which the " white pigment " had been 

 removed. 



Concerning the nature of the white pigment seen in the " pineal eye " 

 of cyclostomes, there is a considerable difference of opinion with regard 

 to its true nature. Thus, according to Studnicka, Mayer in 1864 stated 

 that the epiphysis of Petromyzon contained many concretions of lime. 

 Ahlborn considered the small white corpuscles to be a special white 

 substance, which he described as white pigment somewhat similar to 

 the " brain sand " of higher vertebrates, and thought that the substance 

 with which they were dealing might be calcium phosphate. 



These small corpuscles compactly fill the retinal cells, and they give, 

 with reflected light, a snow-white appearance to the organ. When it is 

 deposited in large masses, so as not to be transparent, it appears on 

 examination with the microscope by transmitted light to be a deep black, 

 which circumstance has, according to Studnicka, given rise to many 

 erroneous statements, certain authors having described the retina as being 

 filled with black pigment. The substance, however, was observed by 

 Gaskell and also Studnicka to be completely removed from specimens 

 fixed in Perenyis fluid, which contains nitric acid, and also from specimens 

 fixed with picric acid. Under high magnification, Studnicka observed the 

 material in the form of separate pigment granules. These varied in size, 

 were round or oval in form, with well-defined, sharp contours. In his 

 opinion they are present in the protoplasm of the retinal cells and not on 

 their surface, as maintained by some authors. These granules are present 

 also in the ganglion cells, which, when they are completely filled, have the 

 appearance of irregular clumps of pigment. Finally, in the deepest part 

 of the retina, many of the pigment corpuscles are found in the intercellular 

 tissue, in the wall of the atrium, and prolonged into the pineal nerve. 

 The pigment is absent in the larval form, or ammocoetes, when smaller 

 than 50 mm. in length. In older ammocoetes it is generally present, and 

 it is always present in the adult animal. The absence of the pigment in 

 the young ammocoetes and its absence in specimens prepared with acid 

 fixatives have given rise to the view expressed by certain authors that the 

 pigment is present in some species of Petromyzon, but is absent in others. 

 Gaskell maintained that the " brain sand " so commonly present in the 

 human pineal organ was not only an indication of its vestigial nature, 

 but that it also pointed to the mammalian " epiphysis " being derived from 

 the " pineal eye." 



It will be of interest to mention here that white pigment has also been 

 described in the eyes of invertebrates. Thus G. H. Parker figures in 

 Gammarus " accessory " retinular cells, which lie outside the pigment 

 cells that surround the clear axial rod or rhabdome. These cells are said 



