EYE 



141 



all round, as they possibly did originally, have become crowded 

 together during the course of evolution into a single large bundle on 

 the ventral side of the cup. In accordance with the general principle 

 of economy of tissue this bundle of nerve-fibres has become sunk into 

 a deep notch in the wall of the cup — the choroid fissure — so that it 

 passes directly to the optic stalk. 



While this is probably a correct statement as regards phylogenetic 

 evolution, matters are somewhat simplified in the development of the 

 individual, inasmuch as the choroid fissure is brought about not by 

 the notching of the already formed cup rim but by the rim ceasing 

 to develop at the site of the choroid fissure while it grows actively 

 everywhere else. 



As regards the development of the actual nerve-fibres, all that 

 need be said is that they first make their appearance in the wall of 

 the optic stalk ventrally and that they increase rapidly in number, 

 passing between the 

 epithelial cells of the 

 stalk, loosening them 

 out, and causing them 

 in great part, if not 

 entirely, to degener- 

 ate. The individual 

 fibres certainly for 

 the most part become 

 differentiated in a 

 centripetal direction 

 i.e. from the retina to- 

 wards the brain, but- 

 whether this means that they are actually sprouting out from ganglion- 

 cells of the retina as is generally believed, or on the other hand that 

 their fibrils are simply becoming differentiated centripetally within 

 a continuous pre-existing protoplasmic connexion, remains to be 

 demonstrated. 



Embryology and the Evolutionary Origin of the Eye. — The 

 peculiar reversed position of the Vertebrate retina may perhaps be 

 taken as an indication that that organ had already come into exist- 

 ence, though no doubt in a very simple form, at a stage in Vertebrate 

 phylogeny when the central nervous system had not yet sunk down 

 below the surface. It is therefore of interest that in certain Verte- 

 brates the rudiment of the retina does actually become apparent 

 during embryonic development at a period when the medullary plate 

 is not yet closed in. Thus Eycleshymer (1895) has described in 

 Rana palustris and in Ambly stoma how a patch of pigment appears 

 for a time on the surface of the medullary plate (see Fig. 79) in the 

 position which will later on form the optic outgrowth. 



Although we are perhaps justified in believing that the eye of 

 existing Vertebrates was already present as a patch of epithelium 

 sensitive to light in the far back evolutionary period when the fore- 



Fig. 79. — Transverse section through the still open neural 

 plate of Rana palustris near its anterior end, showing the 

 position of the optic rudiments (E) already marked out by 

 the formation of pigment (after Eycleshymer, 1895). 



