PIGMENT-CELLS. 



133 



Peculiarities of Form of Pigment- cells. 



Generally, pigment-cells present no peculiarities of form; but 

 those (the pigmentum nigrum) of the choroid membrane of the eye 

 are of a hexagonal form, resembling pavement epithelium. (Fig. 

 69.) Those between the choroid and the sclerotica are somewhat 



Fig. 39. Fig. 70. 



Pigmentum nigrum of adult human subject. A. Cells 

 forming epithelium of the choroid. B. Irregular cells 

 from substance of choroid. Nuclei visible at a; pig- 

 ment granules, &. 



Cells between the choroid and sclerotic 

 of the sheep. (Qutcltett.) 



Fig. 71. 



fusiform, and sometimes have bifid extremities. Their appearance, 

 5 found in the sheep, is shown by Fig. 70. In both these figures 

 the nuclei are seen to be white. In 

 albinoes there are no pigment-granules 

 in the cells on the choroid, and hence 

 they are not pigment-coils. Fig. 71 

 shows the appearance of the cells of 

 the pigmentum nigrum of a black rab- 

 bit at A, and of the white (albino) rab- 

 bit at B. In the human foetus, also, 

 the granules are less numerous than in 

 the adult. The pigment-cells of the 

 human skin also often present the 

 hexagonal form; they being here, in 

 fact, epithelial (epidermic) cells. 



A. Cells on choroid of black rabbit. 

 B. Cells on choroid of white rabbit, des- 

 titute of pigment granules. (Queckett.) 



Distribution of Pigment-cells in the Lower Animals. 



In the lower animals, pigment-cells present a variety of forms. 

 In the skin of the lamprey they resemble the lacunae and pores of 

 bone. (Fig. 72.) In the skin of the frog, and some other reptiles, 

 they are of a more stellate form. (Fig. 73.) In the iris of the tiger 



