156 VERTEBRATE PHOTORECEPTORS 



Still further centrally, in some cases, the outer molecular 

 layer became entirely lost, and the inner nuclear layer was 

 reduced in width, showing definite degenerative changes. 

 In some regions only 2 or 3 rows of bipolar cells remained. 

 These had lost their chromatin and exhibited a pale pink 

 stain with erythrosin (Figure 105). 



It is of interest and possibly significant to point out that 

 the epithelial pigment layer is always present and apparently 

 normal except where the visual cells and the outer nuclear 

 layer are absent. The loss of these two layers is always 

 accompanied by a loss of the pigment epithelial layer (Figure 

 105). When this layer is not present, what remains of the 

 retina is attached firmly to the choriocapillaris by a layer 

 of connective tissue, which in many places is as thick as the 

 sclera (Figure 105). 



In all of the above cases, the blood vessels throughout 

 the retina and choroid were engorged with blood, and the 

 connective tissue of the choriocapillaris was increased in 

 amount. 



Five retinas in this group (2) showed the presence of ro- 

 settes similar to those produced experimentally by Tansley 

 (19336) and by Glucksmann and Tansley (1936). These reti- 

 nal rosettes are produced by infoldings of the outer nuclear 

 layers to form irregular-shaped pockets, which are lined with 

 visual cells (Figure 110). The condition is frequently ac- 

 companied by a corresponding disarrangement of the re- 

 maining layers of the retina. The rods lining the rosettes 

 were found to have undergone degeneration, so that the 

 lumen became more or less filled with debris (Figure 110). 



In group 3 the animals were fed on an adequate diet with 

 supplements of cod-liver oil for 3 to 10 days after showing 

 signs of vitamin A-deficiency. The retinae of rats killed 

 3 days after replacement on an adequate diet showed 

 degenerative changes typical of severe vitamin A-deficiency, 

 although there was less edema and less engorgement of blood 

 vessels than in those killed in the acute stage of deficiency. 



Retinae of animals killed 5 days after being on a recovery 



