218 
Rana palustris and on Amblystoma. Here the open cerebral 
plate shows two shallow depressions in which the epithelium- 
cells produce ín their dístal parts fine pigment granules 
in such a quantity that, according to the statement of the 
author, these optic areas are already recognizable on examina- 
tion of the complete egg, as two pigment-spots on the cerebral 
plate. In more advanced stages, when the cerebral plate 
closes, the pigment gradually disappears and at the same 
spots the optic vesicles now evaginate. 
Fig. 9. Transverse section through the cerebral plate of an 
em js. 
palustris 
au optic EEn en endoderm, ep superficial layer of 
the ectoderm, med. cerebral plate, ms mesoderm 
(after Rotsen 1895). 
Encephatogenetic origin and inversionof Craniate eyes. — The 
attention of investigators has been long drawn to the fact 
that the Vertebrate eye takes its origin from a place not 
directly exposed to the light which it will have to perceive. 
Thus it differs from all other sense-organs in that it is not 
derived from the surface of the body which is first impres- 
sed by all kinds of external stimuli, No less remarkable is 
the fact that the optic ganglion in Craniate eyes does not 
lie under the retina, as in Invertebrates, but on it, and that, 
as a consequence, the light rays must pass first through 
the ganglion to reach the retina in which the rods and 
cones are averted from the light. No wonder, then, 
that a phylogenetic significance has been attributed to 
the pigmented eye-pits on the cerebral plate. As wird 
as 1881 BALFOUR in his Treatise (Vol. Il, p. 419) 
suggested the following explanation of the phenomena 
mentionned above: 
