STUDIES IN THE RETINA. 27 



that the migration of the nuclei from the middle nuclear 

 layer into the outer, which I described for the Amphibia, occurs 

 also in the human retina ; and indeed, I mny add, it occurs in 

 all the vertebrate eyes I have yet examined. 



Borysiekiewitz's own theor}^ of the essential structure of 

 the retina, in the light of which this migration finds no ex- 

 planation, is very different from mine. According to him, 

 these nuclei are inside the " Miiller's fibres," in the more fluid 

 axial portions of which they can move. The outermost ends 

 of these radial fibres are, according to his view, the rods and 

 cones. So that the migration of the nuclei beyond the outer 

 reticular layer is a kindred phenomenon with their movement 

 beyond the membrana Umitans externa into the basal limbs 

 of the '^ cones," both being mere shiftings outward along the 

 axes of the " Miiller's fibres." The comparative study of the 

 " Miiller's fibres," which will be found in Part V, makes the 

 acceptance of this description impossible, I may add that 

 the only difference which Borysiekiewitz can see between the 

 rods and cones of the human retina, is that the latter are 

 those tips of the Miiller's fibres into which nuclei have 

 migrated beyond the membrana limitans externa. There is 

 no observable difference in the lengths of their outer limbs. 



Confining ourselves to the migration of the nuclei, we may 

 review the position of the argument as far as it was advanced 

 in Parts I and II. In the Amphibia migration from the middle 

 to the outer nuclear layer can not only be seen — (1) in the 

 actual passage of nuclei through the outer reticular layer 

 (Part I, PI. 8, fig. 5, e,f, and Part II, PL 31, figs. 23, 24, 25), 

 and (2) in the exact similarity of those in the outer nuclear 

 layer which are not yet rod nuclei, but still close up against 

 the outer reticular layer with certain nuclei in the outermost 

 edge of the middle nuclear layer (see Part II, PI. 30, 

 fig. 16, 6), but is a necessary assumption in order to account 

 for the number of new rods required by the growing retina. 

 A short migration within the outer nuclear layer can be 

 seen in the fact that the " cone " nuclei, as the cones assume 

 the definitive rod-form, move outwards from near the outer 



