158 VERTEBRATE PHOTORECEPTORS 



rod outer segments may regenerate following the replacement 

 on a recovery diet with cod-liver oil supplementation, the 

 evidence was not conclusive, for there was no way of knowing 

 what the condition of the retina was before the animal was 

 placed on the recovery diet. In order to obtain critical data 

 it became necessary to enucleate one eye when the animal 

 was in an extreme state of vitamin A-deficiency, and the 

 other eye after a certain length of time on a recovery diet. 

 Such experiments have been carried out recently by Miss 

 Johnson (1943), who showed that retinas with complete 

 degeneration of the rod outer segments, partial degeneration 

 of the inner segments and some damage of the visual cell 

 nuclei, are capable of a remarkable degree of recovery, but 

 only after ten to eighteen weeks of adequate feeding. Some 

 of the rods in these cases (those whose nuclei have degener- 

 ated) apparently are lost. Although these 'repaired' rods 

 were found to be thinner and less heavily stained than normal 

 ones at the time of examination (after ten to eighteen weeks 

 of adequate diet) , it is entirely possible that they might have 

 become completely normal after a longer period of recovery. 

 Miss Johnson is of the opinion, from her findings, that in 

 certain cases the rods have regenerated completely new 

 outer segments, and perhaps, in some cases, new inner seg- 

 ments as well. No regeneration of visual cell nuclei was 

 found in any of her cases. It is apparent, therefore, from 

 her findings that the outer and inner segments of the rat 

 visual rods are capable of regeneration following the pro- 

 longed administration of an adequate diet. 



Hart and Guilbert (1937) have reported cases of permanent 

 night blindness in cattle and sheep resulting from severe 

 A-deficiency, even though ample amounts of vitamin A 

 were administered for months with the view of effecting a 

 cure. No histological examination of the eyes was made, 

 and the permanence of the night blindness was attributed 

 to optic nerve degeneration which was followed by stenosis 

 of the optic canal. Guilbert, Howell, and Hart (1940), on 

 the other hand, have been able to cure night blindness in 



