274 



HISTOLOGY 



when magnified five hundred diameters or more, is easily resolved 

 as a group of oval to rounded pigment granules (Fig. 237, pg.). 



The cells covering 

 the surface of the man- 

 tle of a mollusk, like 

 Mytillus, are colum- 

 nar, with oval nuclei 

 near their base. The 

 distal ends are naked, 

 but their cytoplasmic 

 structure is hidden by 

 numerous pigment 

 granules. These gran- 

 ules become less nu- 

 merous near the nuclei 

 and are wanting at the 

 bases of the cells (Fig. 

 238). 



Connective-tissue 

 cells are the most com- 

 mon elaborators of pig- 

 ment. In many cases 

 they become specially 

 modified as pigment- 

 bearing cells. Such 

 cells are found in the 

 skin and peritoneum of 

 the fish Ammodytes. 

 These pigment cells 

 are stellate with 



cow. v 



FIG. 23 7.-Individual of the protozoon Metopus. p g ., segre- 



gated pigment granules ; MM, nuclei; con. v., con tractile vacuole. shaped rays. The One 



or two conspicuous 



nuclei lie within the central cell mass. It is within this centrally 

 disposed cytoplasm that the pigment granules are most numerous 

 (Fig. 239). This is an example of what is known as a pigment cell. 



In the last three examples the pigment was mostly confined to a par- 

 ticular region of the cell. In certain hepatic cells this is not the case. 

 The hepatic cells of Cryptobranchus are polygonal with a well-rounded 

 nucleus inclosed in a cytoplasmic layer. The cytoplasm, when not 

 highly pigmented, contains many vacuoles. Pigment granules are elabo- 

 rated more or less uniformly throughout the cytoplasm (Fig. 240, B}. 

 This pigmentation may be so great as to conceal all cytoplasmic struc- 

 ture (Fig. 240, C). 



