PARIETAL SENSE-ORGANS OP GEOTRFA. 7 



subsidiary atrial cavities lying behind the principal one, and 

 donbtless representing a further remnant of the original 

 lumen of the pineal outgrowth ; another series of sections 

 shows that this appearance may be due to ciii'vature of 

 the atrium, whereby its lumen may be seen twice in the same 

 section. 



The proper cavity of the optic vesicle is well developed 

 and usually of the shape shown in longitudinal section in 

 fig. 7, with a funnel-shaped depression in the middle of the 

 lower surface, probably indicating the original connection 

 with the atrium. The funnel-shaped depression may, how- 

 ever, be almost, if not quite, unrecognisable (fig. 2). The 

 peculiar network of protoplasmic strands which occupies the 

 cavity of the optic vesicle will be described later on. 



The whole organ is enclosed externally in a thin and ill- 

 defined layer of fibrous connective tissue, which may be 

 regarded, for the most part at any rate, as an extension of the 

 p i a mater. 



Histology of the pellucida (fig. 7, Fell.). —The 

 outer surface of the pellucida is smooth and even, but its 

 inner surface is produced into large, irregular villi or pro- 

 cesses which project into the cavity of the optic vesicle and 

 are connected by thin strands of tissue with the retina 

 (fig. 7, P.St.). The pellucida is composed, for the most part 

 at any rate, of a single layer of columnar cells, which are 

 enormously elongated to form the projecting villi. These 

 cells contain conspicuous oval nuclei (fig. 7, N.G.C.) situate 

 near their inner ends. Between the villi the inner surface of 

 the pellucida, though uneven, is smooth, but at the iuner ends 

 of the villi the cells appear drawn out into threads, which go 

 to form the strands of tissue connecting the pellucida with the 

 retina. The columnar cells themselves appear to contain but 

 little cytoplasm, which is only slightly granular and stains 

 lightly with Ehrlich's haematoxylin and with acid fuchsin. 

 Their outlines are well defined, but with a characteristic wavy 

 appearance, which I attribute to shrinkage. Amongst these 

 columnar cells, in the outer part of the pellucida, one finds a 



