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retinal cup, near its junction with the lens, and here accessory cavities 

 are not infrequently developed in the retina, each surrounded by radi- 

 ating streaks of pigment granules. The lens contains only occassionally 

 a very few pigment granules. The vitreous body also usually con- 

 tains very little if any pigment, but occasionally a good deal. 



At stage R, when the pigment first appears, it is found only in 

 very minute granules, chiefly, if not entirely, in the inner part 

 of the retina. In the adult much coarser granules appear, though 

 the small ones can still be recognised in the innermost part of the 

 retina. 



Perhaps the most novel results obtained are those which concern 

 the lens of the pineal eye, which is shown to be a glandular organ, 

 secreting part, at any rate, of the vitreous body. At a very early 

 stage in development we can recognise two zones in the lens, an outer 

 or marginal zone, in which the cells remain undifferentiated and con- 

 tinue to divide actively by mitosis; and a central portion in which 

 the cells become greatly elongated at right angles to the two surfaces 

 of the lens, which thereby becomes greatly thickened in the middle. 

 Growth of the lens is probably effected mainly by the marginal zone 

 of actively dividing cells, but it is not impossible that the cells may 

 continue to divide after elongation. The distinction between the central 

 and marginal zones of cells persists to a very late stage in develop- 

 ment, though possibly not in the adult. 



In the adult the arrangement of the elongated cells becomes far 

 less uniform, and they are irregularly curved so as to appear cut 

 through in various directions in vertical sections. They probably extend 

 right through from surface to surface of the lens, but their inner ends 

 are somewhat specially differentiated, and project as small rounded 

 protuberances into the cavity of the eye. The nuclei are situated at 

 various levels, and the cytoplasm of the inner portions of the cells is 

 very distinctly fibrillated in a longitudinal direction, while darkly 

 staining bodies resembling centrosomes can sometimes be seen close 

 to the inner extremities of the cells. 



In the adult lens, about the middle, one usually, if not always, 

 finds one or more irregular masses of a finely granular, deeply staining 

 substance. It was the observation of a large mass of this kind, with 

 a centrally-placed nucleus, which led to my description of a "central 

 cell" in the lens, and it was chiefly with a view to further investi- 

 gation of this remarkable structure that this research was undertaken. 

 I now find that such central central masses are very constant features 

 of the adult lens, and their true nature was indicated by the fortunate 



