ORIGIN AND STRUCTURE OF THE HEAD 



33 



g. opt. 



"I can only suggest that the development of a primary 

 optic vesicle, and its conversion into an optic cup, is due 

 to the retinal part of the eye having been involved in the 

 infolding which gave rise to the canal of the central nervous 

 system. The position of the rods and cones on the posterior 

 side of the retina is satisfactorily explained by this hypothesis, 

 because, as may be 



easily seen from retina 



fig. 285, the poste- 

 rior face of the retina 

 is the original exter- 

 nal surface of the 

 epidermis, which is 

 infolded in the for- 

 mation of the brain, 

 so that the rods and 

 cones are, as might 

 be anticipated, si- 

 tuated on what is 

 morphologically the 

 external surface of 

 the epiblast of the 

 retina." (cf. fig. 10). 



Carriere (1885, 

 p. 89) joins BAL- 

 FOUR in the assump- 

 tion that in Verte- 

 brates "der Theil 

 des Ektoderms, aus 

 welchem sich die 

 Augenanlage bildet, 

 in den Bereich der Fig- 

 Gehirneinstiilpung 

 gezogen wurde," 

 and in 1888 BEARD 

 (1888, p. 68) declares: 



ret. 



g.opt. 



retina 



10. Diagrams of groove-eyes, eye- 

 vesicles and the development of 

 inverted Craniate eyes. g. opt. optic 

 ganglion. 



Most of US now accept the view 

 of BALFOUR, Carriere and others, that the eyes were once 

 structures opening dorsally on the surface of the unclosed 

 neural plate." V. KENNEL (1891) made the further step to 

 derive the primary eyecups from the vesicular eyes of preda- 

 tory Annelids, though his attempt cannot be called very 

 successful. He imagines the cerebral ganglia to have receded 

 along the circumoesophageal commissures and to have 



3 



