VEETEBEATES FEOM A OEUSTAOEAN-LIKE ANCESTOR. 423 



osmic, and had then been mounted after passing through the 

 series of alcohols in the ordinary way, were full of pigment, 

 as in fig. 20, PL XXVII ; while all those which contained no 

 pigment had been killed in Perenyi^s fluid or in picric acid, 

 and then stained with various staining reagents. Clearly the 

 pigment had been removed during the preparation of the speci- 

 mens in these latter cases ; and doubtless one agent in its 

 removal is the nitric acid, which is so important a constituent 

 of the Perenyi's fluid. I have dissected out the eye and placed 

 it in Perenyi's fluid, and watched how the white pigment was 

 dissolved away. Ahlborn says that these white granules are 

 composed of calcium phosphate, and are the same as the brain- 

 sand found in the pineal body of man and the higher Verte- 

 brates. Ahlborn also lays stress on the absolute untraus- 

 parency of these particles. 



Clearly, then, all the eyes are pigmented, and it is equally 

 clear that there is no black pigment ; it is always white. Beard's 

 observations of the presence of black pigment are due to a 

 want of care on his part ; what he figures as black is in 

 reality white, as he will see at once if he will look at his 

 specimens with reflected light instead of transmitted light. 

 The white shining particles of pigment are so opaque that they 

 appear, when the section is viewed by transmitted light, quite 

 black, as in figs. 20a — d. If now the sub-stage be darkened, 

 and a ray of light from a condenser be thrown on the section 

 so as to view it as an opaque object, the only part of the 

 section which is visible are these pigment particles, and they 

 shine and glitter as white particles on the uniform blackness 

 of the rest of the section. Very curious is it to follow, in 

 this way, the changes of the distribution of the pigment in a 

 series of sections through the eye, and to see how, as we pass 

 from section to section, the compact group of brilliant white 

 crystalline-looking particles flashes suddenly into view from 

 out of the uniform blackness. 



We see, then, that the pigment which Beard considered to be 

 black is really white, and that this white pigment is always pre- 

 sent, but is easily dissolved out by reagents such as nitric acid. 



