No. 501] THE ORIGIN OF VERTEBRATE EYES 603 



from the outer ectoderm and, secondly, these elements 

 are inverted in their relation to the stimulus. 



As a consequence of these peculiarities of the verte- 

 brate retina numerous theories have been proposed to 

 account for its present condition. It must be quite clear 

 from what has already been stated, that the chances of 

 a simple course of development for the vertebrate retina 

 are not great, and the first theory to be considered em- 

 phasizes this feature. This theory was first proposed 

 by Sharp (1885) ; subsequently it was independently ad- 

 vanced by Beraneck (1890) whose views were supported 

 by v. KuprTer ( 1894 ) and especially by Burckhardt ( 1901 ) . 

 According to this theory the primitive vertebrate retina 

 is what we now recognize as the lens. As is well known, 

 this organ develops as a pocket of ectoderm in the region 

 superficial to the forming eyeball. In its early stages, 

 as v. Kupffer pointed out, it has a striking resemblance 

 to the beginnings of the nose and the ear and its first 

 steps of development recall those of the eyes of many 

 invertebrates. This primitive retina is supposed by the 

 advocates of this theory to be metamorphosed gradually 

 into a dioptric organ and the dee]>er ganglionic parts are 

 believed to assume receptive functions and thus establish 

 a new retina from tissue that is an outgrowth from the 

 brain. By this process of replacement the primitive 

 retina is converted into a lens and a secondary retina 

 established. It will be observed that while this theory 

 offers an explanation for the lens and for the origin of 

 the retina from a part of the central nervous system, it 

 does not take into account that very striking character- 

 istic of the vertebrate retina, its inversion. Not only is 

 this theory thus defective, but any candid reviewer must 

 agree, I believe, with Keibel that the resemblance that the 

 vertebrate lens has to a sense organ is only of the most 

 superficial kind and that when one attempts to picture the 

 steps whereby a retina is to be converted into a lens and 

 a ganglionic mass into a retina, the difficulties come to be 

 almost insurmountable. For these reasons this theory 



