STUDIES IN THE RKTINA.. 59 



but tlie ordinary nuclei of tlie middle layer, and are used up 

 like the rest. The appearances which have led to the sup- 

 position that they are nuclei of fixed morphological strands 

 are due to the fact that single nuclei are not infrequently 

 involved in these streams of matter, and, indeed, may at 

 times apparently enter into some intimate physiological 

 association with them. They may often be seen drawn out, 

 and even at times robbed of their chromatic substance (see 

 fig. 25, c). That they are not the nuclei of preformed sus- 

 tentacular fibres follows from the fact that a comparative 

 study shows that no such preformed structures exist, and that 

 the so-called "Miiller's fibres" are mere expressions of 

 functional activity, and great numbers, even when best 

 developed, have no such involved nuclei (6g. 32, a). 



Coming to the inner reticular layer, this also, like the 

 middle nuclear layer, undergoes changes with age (cf. i.r., figs. 

 20, a and h, and 24, a and h). In very young eyes the reti- 

 culum is close, and forms what is called the " Punktsubstanz." 

 As soon as the eye begins to function, before which time 

 there are no "M filler's fibres," streams of refractive 

 matter begin to pass through it as very thin radial threads. 

 Under a high power these are seen to be a fine zigzag; they 

 are clearly not independent strands, but some staining matter 

 running along the threads of the inner reticular layer. Fur- 

 ther, they may branch or end suddenly in thin, taugentially 

 arranged layers, from which new radial strands arise to run 

 further in. Again, it is evident that these thin radial strands, 

 which every one would at once call the " Muller's fibres," are 

 not fixed structures, from the fact that in the retina of an 

 older animal of the same kind (cf. figs. 20, a and b, 24, a and 

 h) ^ they may have disappeared altogether, and instead there 

 occur thicker streams finding their way in much coarser zig- 

 zags (fig. 21) along the strandsand between the much more open 



' The specimens of llie vivi|)arous bleiiny were fixed in Bles' fluid 

 in the St. Andrews Marine Laboratory, and kindly given nic by Mr. Wallace, 

 who had prepared them for his own work. The trout were specially (ixed for 

 these researches by Dr. Kyle, aUo of St. Andrews. 



