STUDIES IN THE EETINA. 29 



most layers. But as soon as and ^Yhe^ever vesicles are pro- 

 duced and rods begin to be developed out of tlieuij a 

 process which always takes place first in the centre of the 

 retina and spreads outward from the centre, no divisions 

 normally take place. In order to ascertain this point, I 

 have examined the retinas of tadpoles (toads and frogs) 

 killed at almost all hours of day and night. ^ Nuclear 

 divisions were very numerous in tlie tadpoles killed in the 

 night, and sometimes in those killed in the daytime. A 

 study of them makes it quite safe to affirm that nuclear 

 division is normally confined to the edges of the retina, 

 that is, to those parts where there are no traces, or 

 only the faintest traces, of vesicular protrusions, although 

 one may just occasionally be seen dividing a short way within 

 the zone where the vesicular protrusions are beginning." This 

 result is obtained from so many amphibian retinas that I 

 have no hesitation in affirming that after the rotls have begun 

 to develop, nuclear divisions are never found normally in the 

 layer of rod nuclei. This is, of course, what we should have 

 theoretically expected, that cells specialised for some active 

 function are incapable of mitotic division. 



We are, then, debarred from finding the source of the nuclei 

 for new rods in the nuclear layer itself. Hence the new nuclei 

 required must come into the outer nuclear layer from without, 

 i.e. from the middle nuclear layer, by migration through 

 the outer reticular layer ; and these migrating nuclei, whether 

 the retina grows mainly at the edges or not, must be many 

 thousands, considering the great numbers of " cones " found 

 in the central regions in all stages of its growth. 



This, then, brings us face to face with the question. Whence 

 does the middle nuclear layer obtain the large supply 

 necessary to furnish the outer nuclear layer with so many ? 

 No one will suggest that the supply could be kept up from 

 the "ganglionic cell" layer, which in the central regions is 



' Viz. at almost every hour of the niglit, from 4 p.m. to G a.m. 

 ^ They are occasionally found ill young fish retinas, even witliin t.lie already 

 functioning central region! 



