DIFFERENTIATION OF RETINAL ELEMENTS 59 



period they average about 3 ju in length at the fundus and 

 1.5 jLt at the region of the ora serrata. 



The visual cells develop very slowly up to ten or eleven 

 days after birth. At nine days, the internal and external 

 segments are discernible, and the entire rod has attained a 

 length of about 5-6 ju- From ten to sixteen days after birth. 



12 3 4 5 



DAYS AFTER BIRTH 



6 7 8 9 10 



12 13 H 15 16 17 18 



Fig. 



39. Growth curves of developing rat rods in micra. D, animals reared 

 in complete darkness; L, animals reared in daylight. 



the growth of the rods becomes greatly accelerated (Fig- 

 ure 39). After this period, growth is again slow. The 

 figure shows that from 16 to 18 days after birth the rods 

 measure from 23 to 25 m- It shows also that there is no dif- 

 ference in the growth rate of the rods whether the young are 

 reared from birth in light or in darkness. The rods of an 

 adult rat were found to measure approximately 36 \x. Just 

 as in the human eye, the rat eye differentiates its ganglion 

 cells and the bipolar conducting element's prior to the visual 

 cells. 



Although in the developing rat eyes, the presence of visual 

 purple could not be recognized in fresh, isolated retinae 

 younger than eleven to twelve days, rats eight days old 

 showed a definite avoiding reaction to a beam of heat-filtered 

 light. The same observation was made by Crozier and 

 Pincus (1927). 



