242 THE EYE IN EVOLUTION 



THE EMERGENCE OF THE VERTEBRATE EYE 



Since Wilhelm Miiller (1875) first put forward his view that the 

 pigment-spot in Amphioxus represented the forerunner of the vertebrate 

 eye, many hypotheses have been advanced to explain its sudden and 

 pecuhar appearance, but even today no theory can be said to be 

 completely convincing and each raises difficulties in interpretation. 

 These theories we shall now briefly discuss. 



Ray Lankester (1880-90) was among the first to appreciate the 

 importance of the cerebral origin of the vertebrate eye and reasoned 

 that, with the visual cells buried in the central nervous system, the 

 original pelagic pre -vertebrate must have been transparent, as indeed 

 are Ascidians and Lancelets, so that the light could traverse their 

 bodies. As the body became opaque the eye was then forced to travel 

 nearer and nearer to the surface until eventually it became separated 

 from it only by a layer of ectoderm which retained its primitive 

 transparency. In this view the light-sensitive cells originally associated 

 with the medullary tube migrated to the surface bringing with them 

 their associated pigment cells, and were multiplied and differentiated 

 to form the retina ; meantime, the surface epithelium in the correspond- 

 ing area remained transparent and ultimately became differentiated to 

 form the dioptric apparatus (cornea and lens). 



This view seemed a reasonable explanation of the phenomenon and 

 was crystallized by Balfour (1881) who pointed out that although the 

 retina appeared to derive from the brain it did not originate there but, 

 like the photoreceptors of Invertebrates, was really of integumentary 

 origin, appearing initially as patches of photosensory epithelium on the 

 area of the dorsal ectoderm which happened to become involuted with 

 the neural tube (Figs. 248 to 254). Such a theory accounted for the 

 inversion of the retina as well as its cerebral origin — a characteristic 

 unique among vertebrate sense organs. The concept that the vertebrate 

 eye ultimately derives from the skin was supported by a number of in- 

 vestigators,^ while Schimkewitsch (1921) carried the theory further by 

 suggesting that the lateral eyes were merely a pair of a series of homo- 

 logous pit-like sense organs, the more anterior of which Were photo- 

 sensory, a series in which were included other evaginations of the roof of 

 the diencephalon such as the pineal and parietal eyes. In these latter eyes 

 there is no secondary invagination so that a verted retina is formed ; and 

 Sleggs (1926) and Estable (1927) explained the secondary invagination 

 of the optic vesicles as a positive evolutionary step taken in order that 

 abundant nourishment might be available from the choroid to allow a 

 high degree of differentiation and activity in the sensory mechanism. ^ 



1 von Kennel (1881), Dohrn (1885), Keibel (1906), Froriep (1906), Lange (1908), 

 Franz (! '] •-), and others. 



