38 H. M. BERNARD. 



in the production of i*ods. This seems to be a possible 

 explanation of the difficulty. Some traces of an outer 

 reticular layer, however, there always are, and doubtless 

 here again the retinal cytoplasm forms its basis, and only its 

 basis. In a future paper I shall show that neither of the 

 reticular layers is a homogeneous structure; the outer layer, 

 indeed, presents several difficult problems. In fig. 3, o.r., we 

 already see signs of accumulations of deeply staining matter 

 along the line of the future outer reticular layer. These 

 accumulations, which we shall meet with again in Part V, 

 are apparently in some way due to the functioning of the 

 nuclei, for it is obvious they cannot, from their position, be 

 due to any merely mechanical streaming movements. 



In the very existence of these two reticular layers, as well 

 as in their stratified texture, in tlieir attachments round the rim 

 by threads to the membrana limitans interna, and in the shapes 

 they assume during early growth, we find strong evidence of 

 the migration of the nuclei, which is the subject we have 

 specially in hand.' We may sum up the arguments briefly: 

 (1) the nuclei of the adult rods protrude a little beyond the 

 membrana limitans externa ; (2) the nuclei of the cones, Avhicli 

 (in Amphibia) are early stages in the formation of new rods, 

 move gradually outwards from near the outer reticular layer 

 towards the ujembrana limitans externa as their rods de- 

 velop ; (3) no nuclear division takes place in this layer where 

 rods and cones are developing ; the nuclei for the further pro- 

 duction of rods come through the outer reticular layer from 

 the middle nuclear layer; (4) no nuclear division takes place 

 in this middle layer anywhere near the axial portion of the 

 retina, and the supply must be kept up by migration from 

 the sides. A very few may come through the inner reticular 

 layer from the layer of " ganglionic cells," but the bulk of 



> We sliall refer to some of tlie very discordant views which liave been put 

 out, as lo tlie origin and constitution of tlicse hiyers when we come to deal 

 wilh them in detail. In tlie meantime a useful summary may be found in the 

 Liieratur-Verzeiehniss to Borysiekiewilz's iirst paper, ' Untersuciiungen 

 iiljcr den feineren Bau der Netzhaut,' iS87, notes 19 — 27. 



