STUDIES IN THE RETINA. 31 



the day, they might take phice during the night, when 

 the eye is at rest. This, however, as above stated, proves 

 not to be the case ; no divisions occur in the middle nuclear 

 layer, except near the edges of the retina, where it is not 

 possible to speak of the middle layer because the two reticular 

 layers which separate the retinal nuclei into zones only begin 

 where the rods and cones are themselves commencing to 

 form. 



We have, then, no other source, except this undifferentiated 

 rim, for the enormous number of nuclei required by the middle 

 nuclear layer in order to keep up the supply of rod nuclei 

 required by the growing retina. A few, one here and there, 

 as we have seen, may be obtained from the innermost, 

 or the ''ganglionic cell" layer, but none from divisious of 

 those already present. The real supply must come, as stated, 

 from the rim of the retina ; and however startling the idea may 

 at first appear, we have to assume a stream of nuclei from 

 the undifferentiated edges of the retina towards 

 the base of the cup. Further, as long as growth lasts, 

 this streaming must be considerable, for in addition to the 

 supply of nuclei for the formation of new rods, the thickness 

 of the middle layer is kept up,^ even though the layer itself 

 has to expamd greatly as the eye grows larger. Indeed, it 

 has not only to extend as the eye grows, but, as compared 

 with the bulk of the layer in very young eyes, it may also 

 greatly thicken. Sections of small retinas (of tadpoles) about 

 0'5 mm. in diameter may show the middle layer in the central 

 region only three nuclei deep, while eyes over 1 mm. iu diameter 

 may show it six nuclei deep ; in the adult frog a layer four 

 deep is very common. But I do not think that much import- 

 ance can be laid upon these variations in thickness, as they 

 are probably accidents of nourishment and growth. It is 

 quite possible that at times the supply of fresh nuclei may be 

 greater than the immediate demand, in which case the layer 

 would temporarily thicken; or in times of bad nourishment 



' Apparently in all eyes, except in the " fovea centralis " of human and 

 ape reliuat). 



