46 HISTOLOGY. 



pigment cells. In man this type of cell is of limited occurrence, being 

 found chiefly around the eye (Fig. 48, A). The same sort of pigment 

 may be found in epithelial cells. Thus it appears in the epithelium of 

 that part of the conjunctiva which covers the bulb of the eye in the guinea 

 pig (Fig. 48, B), and as has recently been noted, it occurs there in all human 

 races but the European. The pigment of the skin in the negro races and 

 of the nipple in others is of this sort. It has been discussed whether such 

 pigment arose in epithelial cells or was transferred to them from underlying 

 connective tissue cells, or actually remained in such underlying cells 

 (Fig. 48, C). The retina affords positive evidence that pigment may 

 develop in epithelial cells, and it has even been said that some of these 

 become detached and send out branches. The term "pigment cell" as 

 ordinarily used refers to a branched cell of mesenchymal origin. Others 

 are said to " contain pigment granules," or to be " pigmented epithelial cells." 

 Finally, it should be added that the melanin series of pigments is one of 

 three which give color to the body. The others are the fat pigments, or 

 lipochromes, and the blood pigments, or haemoglobin derivatives. Cells 

 containing these other pigments are seldom called pigment cells. 



A B 



FIG. 48. 



A, Two pigment cells from the deep, peripheral part of the cornea of the rabbit. B, Pigmented epi- 

 thelium from the conjunctiva of the guinea pig. The pigment is chiefly in the basal layer. C, 

 Pigment cells sending processes between the epithelial cells of the skin of an embryo lizard, Lacerta. 

 (After Prenant.) 



Besides the pigment cells, fat cells, and fiber-producing cells (fibro- 

 blasts) several other forms occur in the meshes of connective tissue. These 

 are free from one another and are merely lodged in the connective tissue 

 meshes. Some of these cells emigrate from the blood vessels in adult life. 

 Others may be descendants of cells which emigrated from the vessels in 

 the young embryo, or else they may have arisen directly from mesenchyma 

 in the neighborhood of the vessels. A more definite statement concerning 

 them is not justified. The free cells in connective tissue have been recently 

 classed as lymphocytes, plasma cells, "resting wandering cells," mast cells, 

 and eosinophiles. All of these types except the resting wandering cells 

 are well known and generally recognized. 



