152 VERTEBRATE PHOTORECEPTORS 



Johnson (1939) published a rather detailed histological 

 description of the retina of rats suffering from mild and 

 extreme conditions of vitamin A-deficiency. Her studies 

 were conducted upon rats grouped into three series as 

 follows: 1) animals on a vitamin A-free diet from the time 

 of weaning; 2) animals on vitamin A-deficient diet from 8 days 

 after birth (suckled by vitamin A-deficient mothers until 

 weaning and then continued on an A-deficient diet) ; 3) ani- 

 mals on a vitamin A-deficient diet during suckling, and 

 then weaned on an adequate diet. In all three series, control 

 litters fed on adequate diets were studied along with those 

 receiving deficient diets. These experiments were designed 

 to test the effects of A-deficient diets on the retina when the 

 deficiency is induced at a critical period of development 

 (Series 2 and 3), and upon the fully differentiated retina 

 (Series 1). 



Her results showed that the retinae of rats kept upon a 

 vitamin A-deficient diet during suckling are not markedly 

 different from those which were not placed on a deficient 

 diet until after weaning. In this respect her results are in 

 agreement with those of Tansley (op. cit.) in showing that 

 moderate vitamin A-deficiency during suckling retards 

 development, but produces no marked structural defects, 

 especially up to the time when the outer segments develop. 



Miss Johnson's observations were made primarily upon 

 the retinae of four groups as follows: 1) those exhibiting 

 moderate vitamin A-deficiency, 2) those suffering from severe 

 deficiency, 3) those fed adequately for 3 to 10 days following 

 symptoms of severe A-deficiency, and 4) those kept on a 

 vitamin A-deficient diet during suckling and then weaned 

 on an adequate diet. 



The retinae of all the animals with symptoms of mild 

 vitamin deficiency showed essentially the same condition. 

 In the peripheral portion, the rod outer segments stained 

 very lightly as compared with the normal. In the fundus, 

 there was definite evidence of degeneration changes. Here 

 the retina tended to detach more readily from the pigment 



