2/0 HISTOLOGY 



these cases the color does not appear until the blood is exposed to the 

 air or to the action of other external conditions. 



A more conspicuous form of pigment from an histological stand- 

 point is that due to the segregation of the pigment material in the tissues 

 or their products. In this condition the coloring matter is assembled 

 in little granules within the cell, usually in the cytoplasm which it some- 

 times fills completely, and at other times it gathers in certain localities 

 called the pigmented areas. These granules appear to play the same 

 rdle for the animal pigments that the chloroplasts play for the chlorophyll 

 in most plants. 



The so-called stigmata of certain Protozoa are rather conspicuous 

 bodies, which bear a bright red segregated pigment. In other Protozoa 

 there are to be found granules that bear a brown pigment that is quite like 

 that of the brown Algae, as, for example, Crysamceba; other granules 

 may be found that contain a dark to black pigment resembling greatly 

 the melanin of higher forms (Metopus}. In the invertebrates these 

 dark pigment granules are very common. It remains to be seen if this 

 resemblance to the melanin is superficial or a real one. Melanin is the 

 most common form of the segregated pigments found among the higher 

 vertebrates. It gives the characteristic color to the choroid of the eye 

 and to the skin in the darker members of the human race. Other cells 

 inside the body show it occasionally, as the nerve cells in age (see Fig. 

 163) and other cells in disease. In the nerve cells the granules may not 

 be a true melanin, but if it is, its presence could be accounted for by the 

 ectodermal origin of these cells in which the pigment was a vestigial 

 character. 



In the tissues of the lungs segregated pigments are found, which are 

 important retainers of oxygen. Segregated pigments are character- 

 istic of the tissues of the alimentary tract and of the liver. These are 

 quite active as retainers of digestion products. 



Diffused pigments, like urochrome, may be considered as excretions 

 that have been taken up from the blood along with the other constitu- 

 ents of the urine. Bilirubin, on the other hand, is to be taken as a 

 secretion of the liver cells. The pigment of the ink of the cephalopods 

 is also a secretion. In the secretion of this matter the entire secreting 

 cell is destroyed in the process. In certain Protozoa diffused pigments 

 arise as direct products of alimentation. In Vampyrella, for instance, 

 the red color of well-fed specimens is due to the color of the digested 

 food held within the protoplasm of the cell. M. von Linden indicates 

 that the red pigment of the intestine of the larva of Vanessa is the result 

 of a peptic digestion of the chlorophyll of the larva's food. In a series 

 of his experiments it is also suggested that the red pigment in the epi- 

 dermis has the same origin. 



