32 



THE ANCESTRY OF VERTEBRATES 



Rana palustris and on Amblystoma. Here the open cerebral 

 plate shows two shallow depressions in which the epithelium- 

 cells produce in their distal 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 

 embryo of Rana palustris. 



au optic pits, en endoderm, ep superficial layer of 

 the ectoderm, med. cerebral plate, ms mesoderm 

 (after Eycleshymer, 1895). 



Encephalogenetic origin and inversion of 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 early 

 as 1881 BALFOUR in his Treatise (Vol. II, p. 419) 

 suggested the following explanation of the phenomena 

 mentionned above: 



