350 H. M. BERNARD. 



proximal nuclei might be expected to fill up fastest, and a 

 difference in this small matter, viz. in the speed with which the 

 nuclei again become vesicular, would be enough to account 

 for the apparent fixity of the zones. A larger number will 

 always be found in a condition to discharge in the inner than 

 in the outer zones, in which the nuclei remain longer in the 

 contracted condition. 



Putting the evidence together, I think that there can be little 

 doubt but that the outward stream of chromatin above 

 described is accompanied by a stream of fluid, and that the 

 active agents in causing the movement of the latter are the 

 nuclei that swell and become vesicular, and then contract, 

 either as wholes or by the chromatin network within them 

 contracting and expelling the accumulated fluid. The ex- 

 istence of some close association between the fluid and the 

 chromatin can be gathered from other facts as well, viz. from 

 the normal structure of the nucleus itself, which is a turgid 

 fluid vesicle and apparently a storehouse for chromatin, which 

 latter substance is thus bathed in fluid ; and also from the 

 fact that the refractive chromatin masses, the so-called 

 " centrosomes,^' are invariably accompanied by a shell of fluid. 

 On the other hand, however, it is to be noted that no fluid 

 seems to accompany the non-refractive chromatin which 

 streams along the filaments, and even accumulates in clumps, 

 e. g. in the outer reticular layer (see figs. 18 a and b). 



It is possible that the above is not quite the right inter- 

 pretation of the phenomena, but, speaking generally, I do not 

 think that there can be any doubt in the minds of those who 

 have followed the descriptions of rod-formation given in former 

 parts but that the rods are protruded by forcible discharges 

 of fluid, and all the microscopic appearances are in favour of 

 these discharges coming from the nuclei. 



Some Concluding Observations on the Nuclei. 



In Part 111 of these retinal studies, I described in detail a 

 migration of nuclei on a large scale through the retina. 



