PARIETAL SKNSE-ORGANS OF GEOTRTA. 11 



polav character. The nuclei contains a few well-defined, 

 darkly-staining chromatin granules. In the thinner parts of 

 the retina (fig. 5) the ganglion cells are comparatively few in 

 number^ and occur chiefly towards the outside, just within 

 the connective-tissue capsule of the eye. In the neighbour- 

 hood of the atrium, however, they are accumulated in large 

 numbers, as already stated (fig. 7, G.C.). 



The nerve fibres are extremely delicate and form a netAvork 

 (together with connective-tissue fibres ?) in which the ganglion 

 cells are embedded (fig. 5, N.F.N.) . It is probable that there 

 is a special layer of nerve fibres between the ganglion cellls 

 and the connective-tissue sheath (fig. 7, C.T.S.), but I have 

 not found it possible to distinguish it clearly from the latter. 

 The connective-tissue cells of the retina are distinguished by 

 their elongated and very darkly-staining nuclei (fig. 5, C.T.N'), 

 resembling those found in the connective-tissue sheath ; they 

 seem to indicate the presence of connective -tissue fibres, 

 running more or less vertically through the retina. 



Irregular masses of pigment granules, similar to those 

 found in the pigment cells of the epithelial layer, occasionally 

 occur in the nervous layer, but these can hardly be regarded 

 as essential constituents of this layer. 



Histology of the wall of the atrium. — The atrium 

 is lined by a single layer of columnar ependymal cells, none 

 of which contain pigment, and I have not been able to demon- 

 strate the existence of sensory cells in this region. 



Contents of the optic vesicle. — Much discussion has 

 taken place as to the nature of the irregular network which 

 so constantly appears in the interior of the pineal eye (fig. 7, 

 P.St.). The researches of Studnicka leave no room for doubt 

 that it is a normal constituent of the organ and not merely an 

 artifact, although probably it undergoes much alteration during 

 the processes of hardening and in the preparation of sections. 

 It is probably partly due to coagulation of the albuminous[ 

 contents of the optic vesicle, but it is also undoubtedly in part 

 cellular in nature. As Studnicka has shown in Petromyzon, 

 the columnar cells of which the pellucida is composed are 



