3o 4 SCIENCE PROGRESS 



are constructed upon the so-called " invertebrate type," with the 

 optic nerve spreading out over the back of the retina, while 

 the latter are constructed upon the " vertebrate type," in which 

 the optic nerve pierces the retina and spreads out over its inner 

 face. If we study the development of these organs, however, 

 we shall find that there is really no very fundamental difference 

 between them. Both originate as hollow outgrowths of the fore- 

 brain ; in both the actual sense-cells or "rods" are formed from the 

 innermost layer of cells, next to the original cavity of the optic 

 vesicle (in other words, they are modified ependymal cells), 

 and in both the original optic stalk is replaced by nerve-fibres 

 constituting the optic nerve. The apparent difference in the 

 relation which the nerve bears to the retina is due simply to the 

 invagination of the wall in the ordinary paired eye to form an 

 optic cup, whereby the original cavity of the optic vesicle is 

 obliterated, and a space which is really altogether outside the 

 optic vesicle becomes enclosed as the posterior chamber of the 

 eye. Thus it comes about that the portion of the original optic 

 vesicle which becomes the " pellucida " in the pineal eye of the 

 lamprey and the " lens " in that of the lizards and the tuatara, 

 becomes invaginated and forms the retina in the ordinary paired 

 eyes, while the part which forms the " retina " in the " pineal 

 eyes " of both types forms merely the pigment epithelium at the 

 back of the retina in the ordinary paired eye. The development 

 of the paired eyes teaches us that some, at least, of the fibres 

 of the optic nerve grow out from the front face of the retina 

 {i.e. the morphological outer surface of the original optic vesicle) 

 and over the lip of the optic cup at the choroid fissure, and 

 thence to the brain along the optic stalk. It is thus evident 

 that the primary relations of the optic nerve and the retina are 

 precisely the same as in the " pineal eye." It is hardly neces- 

 sary to point out that the lens of the paired eyes is an altogether 

 new development not represented at all in the pineal eye. 



There is, therefore, nothing in the structure or development 

 of the "pineal" and "paired" eyes respectively to prevent us 

 from regarding them, with Locy, as being serially homologous 

 with one another. 



In reptiles other than Rhynchocephala and Lacertilia, as 

 already stated, the pineal organs no longer show any trace of 

 eye-like structure. In view of the apparently close relation- 

 ship of the Ophidia to the Lacertilia, we might have expected to 



