78 THE PINEAL ORGAN 



pigment granules of a golden-brown colour first appear in the neuro- 



epithelium which forms the outer wall of the optic cup in human embryos 



at the 8-io-mm. stage of development. They are deposited at first in 



patches or small groups in the cytoplasm between the nucleus and the 



inner margin of the cell, namely the border of the cell next the cleft between 



the outer and the inner walls of the optic cup, which represents the 



original cavity of the primary optic vesicle. By increase in number of 



the granules they soon completely fill the inner part of the cell, whereas 



in the outer part of the cell the granules are fewer in number and with 



low-power magnification this part of the cell appears clear. A significant 



circumstance is that the deposit of pigment in the outer wall of the cup 



appears at the same time as the development outside it of the plexus 



chorio-capillaris. At first the plexus is in close relation with the neuro- 



epithelium ; later, however, at the 14-mm. stage of development, a 



continuous clear membrane, the membrane of Bruch, is formed between 



the epithelium and the choroid. It is probable that this membrane 



would prevent the passage of pigment granules from choroid to epithelium, 



but would act as a dialysing membrane with reference to the fluid 



constituents of the pigment, namely tyrosin, which has been shown to be 



converted into melanin by the action of the ferment tyrosinase. Little 



is known with regard to the formation of pigment granules, but it seems 



probable that these are developed in the cytoplasm of epithelial cells 



(Fig. 51), and grow to a definite size and shape ; and that the relation of 



the colouring matter to the granule is much the same as that of haemoglobin 



to a blood corpuscle — that is to say, under ordinary circumstances the 



colouring material is carried by the granule, but it may under certain 



conditions escape from the granule and become diffused in the cytoplasm 



or intercellular spaces. Returning to the further development of the 



pigment layer of the human retina : this continues and remains throughout 



life as a single layer of cells, which in the definitive form are seen to be 



hexagonal when viewed in tangential sections and of an oblong 



quadrangular shape in sections made vertical to the surface. The inner 



end is prolonged into delicate, tapering processes which surround the 



outer ends of the rods and cones. These are specially well seen in the 



retina of certain fishes and Amphibia, in which the pigment granules 



appear as minute oval plates arranged in series and in parallel rows, their 



colour when seen by transmitted light being a resinous-brown or amber 



tint. The two layers of the optic cup are each carried forward as a single 



layer of cells beyond the ora serrata, on to the inner or posterior surface 



of the ciliary region and iris, as the pars ciliaris retinae and pars iridica 



retinae. The outer or anterior layer of the pars ciliaris retinae only is 



pigmented, and this is the case in the early stages of development of the 



