THE PIGMENT-CELLS OF THE PINEAL BODY 6l 



dermal chromatophores are present in the connective tissue of the derma, 

 and the areolar subcutaneous connective tissue. These are capable of 

 amoeboid movement and are believed to serve as carriers of pigment which 

 is formed elsewhere, e.g. in the melanoblasts. The pigment granules in 

 these cells are larger and more irregular than in the basal epidermal cells 

 and melanoblasts. Moreover, their distribution in the cell-body is not 

 so even as in the epidermal melanoblasts. Pigment cells containing 

 melanin which has actually been formed in the cell-body are, however, 

 said by Maximow to occur, although rarely, in the derma, and have been 

 named by him " dermal melanoblasts." 



Like the pigment of the skin, the pigment of the eye may be considered 

 under two headings, viz. (i) epithelial pigment and (2) the pigment 

 present in connective tissue or mesenchyme cells. Thus both in inverte- 

 brates and vertebrates the pigment is chiefly found in the sensory receptive 

 cells or in specialized epithelial cells of the retina, which either surround 

 individual receptive sensory cells, as in the ommatidia of a composite 

 faceted eye, or form a layer such as the external layer of hexagonal pigment 

 cells in the retina of vertebrates. The connective tissue type of pigmented 

 cell is found either in the form of " intrusive " mesenchyme cells among 

 the sensory cells of the retina, as in the central eyes of the king crab 

 {Limulus polyphemus), or in a definite layer in one of the tunics of the 

 eyeball, e.g. the lamina chorio-capillaris, in which the spaces between the 

 vessels contain a large number of branched cells of the chromatophore 

 variety, or again in the anterior mesodermal layer of the iris. One of the 

 principal functions of this pigment appears to be to screen off or to absorb 

 superfluous rays of light. In the case of the iris in the lateral eyes of 

 vertebrates, the pigment contained in the epithelial cells of the pars 

 iridica retina; and the mesodermal anterior layer serves by means of the 

 sphincter and dilator pupillge muscles to regulate the amount of light 

 entering the eye through the pupil. The iris thus serves as a movable 

 curtain which not only prevents oblique or circumferential rays entering 

 the eye but controls the number of rays which fall upon the central part 

 of the lens and more sensitive portions of the retina. In those animals in 

 which the eye is not provided with a movable iris, pigment is nevertheless 

 deposited in a circular zone round a transparent area of the skin or cuticle, 

 which area serves as a cornea (Fig. 107, Chap. 12, p. 149). The clear 

 area surrounded by a rim of pigment will allow the passage of a beam of 

 light to any particular part of the retina, from approximately one direction 

 only, and an indistinct image will be formed, the direction of which is 

 well indicated, but there is often no means of focusing this image or 

 regulating the amount of light entering the eye : such eyes are represented 

 by the eye of Nautilus (Fig. 112, Chap. 12, p. 152), or the eyes round the 



