219 
“1 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 
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). 
CARRIÈRE (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. 10. ge bee 
bat vesicles an e aeveropm 
ended inverted Craniate eyes. g. opt. optic 
gezogen wurde, ganglion 
and in 1888 BEARD 
(1888, p. 68) declares: “Most of us now accept the view 
of BALFOUR, CARRIÈRE and others, that the eyes were once 
structures opening dorsally on the surface of the unclosed 
neural plate” Vv. 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 
