DIFFERENTIATION OF RETINAL ELEMENTS 53 



of the rods and cones as robust cilia, homologous with those 

 of the central canal of the embryonic neural tube, likewise 

 does not appeal." 



According to Mann the visual cones develop as shown in 

 Figure 34. A in this figure shows the condition at the pos- 

 terior pole in a 48 mm. embryo. Each cell has a very slight 

 protoplasmic extension which indicates the origin of the 

 inner segment. Each contains a very fine filament attached 

 to the blunt apex of the cell and which lies within the cavity 

 of the primary vesicle. This filament, according to her, repre- 

 sents the earliest recognizable outer limb of the cone, and 

 shows the same relation and structure as did the cilium of the 

 basement membrane in the earlier stages. B shows the condi- 

 tions in an 80 mm. foetus. 



The nuclei have become f J / - , ■ , / 



spherical, the protoplasmic 1 1 / i \ ) | / / / 



protrusions are longer and / * 

 each contains a pair of darkly 

 staining particles, the dip- 

 losome. The outer segment, 

 which is more marked, ap- 

 pears to be attached in the 

 region of the diplosome. 

 According to Leboucq (1909), 



Magitot (1910), and Seefelder ^ig. 35. Section showing rods and 



/irv-irix 1 J.1 1 1 cones at the end of the seventh 



(1910), both rods and cones ^^onth. (Redrawn and modified from 



in the human retina develop Mann, 1928, Cambridge University 



simultaneously, and are dis- ^^ 



tinguishable mainly by the fact that in the cone the axis 

 of the diplosome (centrosomes) is perpendicular to the long 

 axis of the cell, whereas in the rod it is parallel to the long 

 axis of the cell. This is not entirely borne out in the illustra- 

 tion which is reproduced in Figure 35. C and D (Figure 34) 

 show further development of the cones and E shows the 

 condition of young cones during the eighth month. At this 

 time the outer processes of the Miillerian fibers have reached 

 the basement membrane and have united with one another 



