FORMATION OF THE OPTIC CUP 



107 



sure of the developing lens; nor is the surface ectoderm passively sucked 

 inward, to form the lens vesicle, by the cupping of the optic vesicle. 



The conversion of the optic vesicle into the optic cup is more than 

 a simple indentation or invagination (Fig. 38). At first, the dilated 

 vesicle lies largely above the level of the optic stalk, but after the com- 

 pletion of the optic cup the stalk is found to be attached to the center 

 of its back. Figure 38b shows what really happens — a growth of the two 

 sides of the base of the vesicle laterally and downward, closing in below 

 the attachment of the stalk. The closure is not at first complete, so that 

 a slit, the 'embryonic fissure of the optic cup' is left in the ventral 



Fig. 39 — Cell-lineage 

 of the retina. Modified 

 from Fiirst. At the extreme 

 left is the initial coluninar-epithel- 

 ioid condition of the inner layer of 

 the optic cup. Germinative cells, occupy- 

 ing a position comparable to that of the 

 ependymal cells of the brain wall, proliferate a 

 pseudo-stratified tissue, some of whose elements 

 eventually retract one or both of the processes con- 

 necting their cell-bodies with the limiting surfaces. The ••*" -^lJ^::^^ /7 

 oldest, most vitread of the elements (bottom-most in the drawings) are about the first to 

 differentiate, and maturation proceeds outward toward the germinative cells, which at last 

 become the rods and cones. 



r- portion of rod; c- cone; h- horizontal cell; m- Miiller fiber; b, b- bipolar neurons; 



a- amacrine cell; g- body of ganglion cell; n- nerve fiber (axon of ganglion cell). 



meridian of the cup, running from its rim to the cup end of the optic 

 stalk. Along the under side of the stalk, nearly all the way to the brain 

 wall, there is now a deep groove which has invaginated during the form- 

 ation of the optic cup. This groove opens into the cavity of the optic 

 cup and here forms the apex of the embryonic fissure. The old cavity 

 of the optic vesicle has been nearly obliterated by the indentation of the 

 vesicle. It, through its continuation in the optic stalk, still opens into 

 the forebrain cavity but of course has no communication with the new 

 cavity of the optic cup or with that cavity's continuation, the ventral 

 groove of the optic stalk. 



