STUDIES IN THE RETINA. 33 



isolated from membrane to membrane. All that is i-e:illj 

 important at the present moment is to note that the nnclei in 

 the region of active division are attached to one or other of 

 the " limiting membranes " by definite strands which are only 

 found in this undifferentiated rim. 



Now these strands can often be seen showing the following 

 interesting arrangement : — On the axial side of this area of 

 nuclear division, and just where the differentiation of the 

 retina into zones is commencing, the nuclei, still for the most 

 part having retained their spindle shapes, are seen to be 

 arranged in slight curves (figs. 1 and 2) ; the two ends of the 

 curves are attached by these strands to the limiting mem- 

 branes, and their middle parts bulge outwards towards the axis 

 of the eye. This curving might easily be passed over, and 

 when seen it might be considered as a purely accidental phe- 

 nomenon. It is far more probable, however, that it is normal, 

 and due to the process we are discussing, viz. the tendency of 

 the nuclei to travel from the i-im towards the functional 

 axial region of the retina. It is clear that the curving could 

 be so explained. 



Again, comparisons of the different thicknesses of the middle 

 nuclear layer at different parts of the retina and at different 

 stages in its growth tell the same story of movement. We 

 always find that the layer is thickest near the rim where the 

 nuclei produced by division are crowding into it, and thinnest 

 in and near the centre where the nuclei are presumably in 

 most demand. Further, the variations in thickness of this 

 layer in the central region at different stages of growth 

 clearly show fluctuations in the numbers and changes in the 

 positions of its component nuclei. 



If the nuclei from the undifferentiated rim have, then, this 

 tendency to stream inwards towards the axis, it is clearly 

 those occupying the middle ranks in the retina which would 

 be the freest to move, and which therefore would travel 

 fastest. Those of the innermost ranks will be more firmly 

 attached to the internal limiting membrane, and may, 

 perhaps, be further entangled by the developing nerve-layer, 



VOL. 46, PAUT 1. — NEW SElilKS. C 



