STUDIES IN THE RETfjS'A. 337 



Further, any healthy retina of any age, if properly fixed, will, 

 as already described, show the filaments running down from 

 the nuclei on to the rods coated or beaded with staining 

 raatter. So that we are justified in concluding that large 

 quantities of chromatin are not only used up in the develop- 

 ment of the rods in the first place, but continue to be used up 

 in the life activities of the rods.^ The immediate source of 

 all this chromatin is the rod-nuclei. 



Turning, then, to these rod-nuclei, we find that in spite of 

 this immense and continuous drain — immense because, apart 

 from the supply needed for its functional activity throughout 

 life, each rod, in the course of its production, must require 

 more than a single nucleus could contain, — in spite of this 

 drain, these very rod-nuclei remain especially rich in refrac- 

 tive staining matter. This richness in chromatin of the rod- 

 nuclei is one of the first facts that strikes a diligent student of 

 the retina, no matter what eye is studied ; although it is seldom 

 so marked as shown in fig. 25 above quoted, it is always striking. 

 In the other retinal nuclei the chromatin is fairly evenly dis- 

 tributed over the nuclear reticulum, with usually one, perhaps 

 two, conspicuous masses (nucleoli);^ but iu the rod-nuclei it 

 is invariably collected into large, sometimes immense clumps. 

 These coarse clumps of chromatin are specially striking in 

 those animals in which the rod-nuclei are long and spiudle- 

 shaped. Figs. 20 h, from the retina of a cat six weeks old, 

 and 20 c, from that of a very old cat, show the kind of banded 

 appearance which the nuclei acquire owing to the arrange- 

 ment of these chromatin clumps in single rows within nuclei 

 of that shape. 



In the rabbit there are frequenlly two clumps with a 

 single clear band. This appearance has frequently been 

 noticed by earlier observers, and thought to have some 

 peculiar physiological significance. It is, however, the 



* See Gustav !Munn, 'Journal Auat. Pliys.,' xxix (1S95), summary. 



- Tlie most conspicuous nucleohis is seen iu those nuclei in which tlie 

 chromatin is all balled together in the centre, liere regarded as a phase (see 

 p. 347). 



VOL. 47, PART 3. NEW SEIMKS. Y 



