EVOLUTION OF EYE FROM BRAIN 119 



SO (whereas in mammals they later separate, owing to the eye's growing 

 faster than the lens, so that the suspensory ligament is thereby put in a 

 state of tension, forcing the lens to become flatter during its growth). 

 In the snakes, the course of development of many parts has been pro- 

 foundly modified, as is explained in detail in section D of Chapter 16. 



(B) Evolutionary 



In its simplest terms as seen in the lamprey, the vertebrate eye has 

 only a very few essential living parts: retina, uvea, fibrous tunic, and 

 lens. The problem of the origin of the eye is merely the problem of the 

 status of each of these parts previous to their present association. Yet 

 though when thus stated the matter appears simple, it has baffled a great 

 many astute morphologists. The great German anatomist Froriep once 

 likened the 'sudden' appearance of the vertebrate eye in evolution to the 

 birth of Athena, full-grown and fully-armed, from the brow of Zeus. 



The Eye a 'Part of the Brain' — From the embryology of the eye it 

 appears that there could have been no complex retina until the chordates 

 had evolved an internal, tubular brain. The foveolae opticae have been 

 interpreted as an ancestral stage in which the eyes were essentially a pair 

 of photosensory epithelial pits in the skin, analogous to those of a 

 modem Nautilus. Another possibility is that the foveolae are develop- 

 mental precocities without phylogenetic meaning. Before we can decide 

 how to interpret them, we shall have to try to determine how far back 

 the rods and cones may have been photosensory. 



If the retina is thought of as a photosensory portion of the brain wall, 

 outpocketed to keep it near the skin in an ancestor whose body was 

 becoming larger and more opaque as evolution proceeded, then the scle- 

 rotic and uveal coats are easily disposed of by homologizing them with 

 the meningeal envelopes of the central nervous system, the dura mater 

 and the pia-arachnoid. The sclera is actually continuous with the dura 

 via the sheath of the optic nerve. The latter also possesses a continuation 

 of the pia-arachnoid, though this ends outside the eyeball and does not 

 merge with the chorioid even in the embryo. The vascularity and pig- 

 mentation of the chorioid are however strongly pia-like characteristics, 

 and in lampreys there are even striking histological similarities between 

 chorioid and pia. 



