STUDIES IN THE RETINA. 65 



membrane between this lajer and the internal limiting mem- 

 brane where the dark brown of the pigmented matter is very 

 dense. Individual streams can hardly be followed. 



In these eyes^ then, again, and in spite of the quantity of 

 the pigment absorbed, we find the same difficulty as we found 

 in the cod (of. fig. 21, i.r.) in establishing direct radial paths 

 for the escape of the absorbed matter through the cytoplasmic 

 reticulum ; that such paths do occur and may be very highly 

 specialised we know from the (presumably old) human retina 

 shown in fig. 32, a. These, seen alone, certainly appear as 

 if they were sustentacular. 



Other perfect radial tracks seem to occur normally in the 

 Amphibia, for as soon as the eye begins to function, that 

 is in quite young tadpoles, there arise distinct, smooth, 

 nearly straight radial fibres through the inner reticular 

 layer, and these become so tough in preservation that 

 they can be isolated intact if a section is teased up or crushed 

 on a slide. ^ Further, in tangential sections they often appear 

 running through the inner reticular layer within a clear 

 passage. It is possible that this clear passage may be delu- 

 sive, and due to the fact that the adjacent parts of the 

 reticulum are drained by them of any matter which would 

 render their delicate cytoplasmic membranes or threads 

 visible. Compai-e with these apparently clear courses of the 

 " Miiller's fibres " through the inner reticular layer, fig. 29, b, 

 where " Miiller's fibres " of a rabbit are shown cut trans- 

 versely, and the tangential threads or membranes of the 

 inner reticular layer thickened with matter are seen to be 

 running into them. 



But the important contrast comes later. The establish- 

 ment of direct radial streams through the inner reticular 

 layers in the young tadpole is quite natural, for we remember 

 that in other eyes the nearest approach we found to a straight 

 course was in young retinas (see fig. 24, h) when the inner 

 reticular layer is a close " Punktsubstanz." But whereas the 



> Many of them with nuclei of the middle layer attached to I hem (see 

 fig. 25, c). 



VOL. 46, PART 1. — NEW SERIES. E 



