STUDIES IN THE RETINA. 339 



nuclei. They are also not the clumps of pigmentary matter 

 which sometimes terminate the rod-fibres, as shown in Part V, 

 fig. 21^ but they are the equivalents of the stellate bodies 

 seen in chambers in the rod-fibres shown in fig*. 20 h on the 

 same plate in Part V. After a long and careful comparative 

 study of these bodies, I feel fairly safe in affirming that they 

 are large extra-nuclear masses of chromatin, attached as they 

 should be by threads to the surrounding nuclei. The one 

 figured on the right in fig. 18 a has been selected because it 

 is the very largest I had ever seen in any retina. But 

 almost any good section of a frog's retina will show a score 

 or two of them all along the outer reticular layer. Owing to 

 the thinness of the layer of rod-nuclei in the frog there are 

 seldom any conspicuous rod-fibres, which are only really pro- 

 nounced in animals in which the rod-nuclei are many layers 

 deep. In these cases the rod-fibres, on reaching the outer 

 reticular layer, end either in a knob or in a chamber-like 

 expansion of the fibre. The knob is, I believe, as already 

 stated, a mass of pigmentary matter arrested against the 

 outer reticular layer, while the chamber is an expansion of the 

 fibre, in some way due to the presence of one of these masses of 

 extra-nuclear chromatin which takes up its position within it. 



At the present moment these masses of extra-nuclear 

 chromatin are interesting as evidence for the passage of this 

 matter outwards. It looks as if, just as the pigmented 

 matter, passing inward from the rod, is arrested on reaching 

 the outer reticular layer, the chromatin also, passing outwards, 

 may be arrested temporarily by the same layer. I have, for 

 instance, frequently seen, in the human retina, a little plate 

 of staining matter like that shown in the diagram fig. 22, 

 where the fibre of the rod which is marked with an asterisk 

 passes through the outer reticular layer; this plate is resolv- 

 able into minute granules. 



Passing on, we have to ask — whence do the nuclei of the 

 middle layer obtain their supply ? Capillaries are only 

 known to penetrate to this layer in the higher vertebrates. 

 Hence we are driven to conclude that it must come through 



