Sorensen, Study of Epiphysis and Roof of DiencepJialon. 5 5 



is impossible to distinguish it from the black pigment. It is 

 distinctly recognized, however, by turning the mirror and observ- 

 ing with the illumination from above. Then it is distinguished 

 from that in the envelopes by its snowy whiteness. The pigment 

 was found both in the cells and also intercellularly in the retinal 

 layer, although this is a rare case. The pigment appears very late 

 in the parietal organ. It was found only in larvae 50 mm. long 

 and in Petromyzon. Pigment was found in larvae 6 mm. long but 

 this had nothing to do with that found later filling the organ. This 

 pigment reappears often in the tissue of the nerve centres of em- 

 bryos. The youngest larvae in which the white pigment is 

 developed are 60 mm. long. Only little pigment is found there 

 and this only with a high power. It is only in the highest devel- 

 oped Ammocoe'tes more than a decimeter long that a completely 

 pigmented retina was found and this only in two out of five 

 specimens examined. The specimens of Petromyzon exam- 

 ined had each its pigmented organ except one preserved in 

 nitric acid. In Petromyzon it is more developed and thicker 

 than in Ammoccetes, sometimes penetrating the entire thickness 

 of the retina. The pigment is ordinarily diffused in the entire 

 extent of the retina. Passing laterally it sometimes becomes 

 rarer and disappears. From the retina it descends into the 

 atrium and fills most of the posterior edge in entering it. It 

 often passes into the nerve and fills it throughout its entire dia- 

 meter, or it may only enter it scattered along the nervous fila- 

 ments. In Ammoccetes the pigment was generally found in the 

 cells, more rarely intercellularly in all the layers of the retina. 

 The principal base of the pigment is composed of long cylin- 

 drical nutritive cells bordering the superior surface of the retina. 

 In other cells there is less pigment. The sensitive cells (rods) 

 in Ammoccetes sometimes do not contain pigment and are easily 

 recognized in stained preparations, among the other pigmented 

 cells. In Petromyzon all the cells contain the pigment which 

 descends by fine nervous prolongations and which in conse- 

 quence make the retina appear transversely striated. In the 

 layers situated above the cylindrical cells the pigment is at times 

 very diffuse. The cells thus filled by pigment have the appear- 



