THE PIGMENT-CELLS OF THE PINEAL BODY 59 



slime, secreted as droplets either in the substance of the cell-body or making 

 its way to the surface, was the first step towards lens-formation. Later 

 certain of the hypoderm-cells became elongated and their cytoplasm 

 becoming clear, they formed a cellular lens of the vitreous type (Fig. 43), 

 or the clear viscous secretion of the cells formed a non-cellular vitreous 

 lens of the type seen in certain molluscs : e.g. the limpet {Patella), in 

 which a simple optic pit is present the cells of which secrete a viscous 

 fluid into the hollow of the pit ; Trochus, in which the pit is converted into 

 an unclosed vesicle, completely filled with highly refractile secretion ; 

 and Murex (Venus 's comb), in which the secretion has become con- 

 solidated and shrunk away from the retina, thus forming a non-cellular 

 spherical lens ; and the optic vesicle has become closed (Figs. 33-35). 

 A " primary cornea " formed from the unpigmented distal segment of the 

 optic vesicle and a " secondary cornea " derived from the superficial 

 covering layer of hypoderm-cells and subjacent mesenchyme have also been 

 evolved (Fig. 36, p. 50, and Fig. 121, Chap. 12, p. 164). These eyes, more- 

 over show the differentiation of an outer clear refractile zone, a middle pig- 

 mented, and an inner nuclear-zone in the cells of the retina. In higher 

 types of eye a specialization of individual cells of the retina is observed 

 into refractile, pigment, and sensory cells (Figs. 11, 81, pp. 16, 119), and in 

 many of the arthropods and insects these cells are grouped together into 

 the separate units or ommatidia of a faceted eye (Fig. 37, p. 52). 



The Pigment-cells of the Pineal Body 



The occurrence of pigment in the pineal body is of interest not only 

 from the light which it throws on the nature of this organ, but is of practical 

 importance to the surgeon, as cases have occurred in which melanotic 

 sarcoma l and other pigmented tumours have arisen in or in association 

 with the pineal organ. The nature of the pigment contained in the human 

 and mammalian pineal organ and its exact situation have been studied in 

 detail by Quast - ; and pigmentation in the pineal organ of vertebrates 

 in general, but more particularly cyclostomes and reptiles, by Beard, 

 Dendy, Gaskell, and Studnicka. 



As is well known, the colour of the skin in man depends chiefly on 

 the presence in the skin of the pigment melanin. This is deposited as 

 fine granules which are found in the cells of the stratum germinativum, 

 more especially in its basal layer next the derma. As, in course of time, 

 these deeper cells move towards the surface to replace the more super- 



1 C. Ogle, " Sarcoma of the Pineal Body : Diffused Melanotic Sarcoma of 

 the Surface of the Cerebrum." Trans. Path. Soc, London, 50, 4-6, 1898. 



2 P. Quast, " Beitrage zur Histologic und Cytologie der normalen Zirbeldriise 

 des Menschen," Zeitschr.f. mikr. anat. Forsch., Bd. 23, s. 335, 193 1. 



