52 



VERTEBRATE PHOTORECEPTORS 



safe in drawing at least a close parallel if not a definite deriva- 

 tive relation between cilia and the primitive outer limbs of 

 the visual elements." 



A similar view apparently is held by Duke-Elder. With 

 respect to this matter, Arey (1928) says, ''An interpretation 



W^J W W 



Fig. 34. Stages in the development of the human retinal cones. (Redrawn 

 from Ida Mann, 1928, Cambridge University Press.) A, cone cells from fundus 

 of a 48 mm. embryo; B, cone cells from an 80 mm. embryo; C and D, further 

 stages of differentiation; E, cones from the retina of an 8 month old foetus. 



