56 



VERTEBRATE PHOTORECEPTORS 



The first stage in the development of the amphibian 

 visual cell is the production of a protoplasmic bud and a 

 clear achromatic globule from the cells of the external 

 nuclear layer. The globule, which becomes the paraboloid 



B 



Fig. 37. Later stages in the development and differentiation of the visual 

 cells in Amblystoma punctatum. A, visual cells from the retina of a larva 

 21 days after tail-bud stage; B, 26 days after tail-bud stage; C, 31 days after 

 tail-bud stage; D, 40 days after tail-bud stage. X 920. (From Detwiler and 

 Laurens, 1921, J. Comp. Neur., v. 33.) 



of the inner segment, appears to be of cytoplasmic origin 

 and is not extruded from the nucleus as asserted by Cameron 

 (op. cit.). Distal to this globule there appears a mass of 

 deeply staining granules in iron-hematoxylin preparations 

 (Figure 36, A). These granules contribute mainly to the 

 formation of the ellipsoid of the inner segment and to the 

 granular material of the outer segment (Figure 36, C, D). 



