THE AMERICAN NATURALIST [Vol. XLII 



has found relatively few advocates and is regarded by 

 many as wholly untenable. 



The second general theory, as to the origin of the 

 vertebrate retina, is one which has much more in its favor 

 and which is presented in as complete a form by Balfour 

 (1881) as by any other. It has recently been independ- 

 ently advanced by Jelgersma (1906). According to this 

 view the vertebrate retina originated on the outer sur- 

 face of the ancestral vertebrate in much the way that the 

 eyes of many invertebrates have been produced. The 

 primitive retinas thus formed were implanted in that por- 

 tion of the surface of the animal from which the central 

 nervous system was destined to develop and when this 

 was infolded these retinas were carried in with it, and 

 came thus to be involved in the central organ. If the 



morphological position of a sensory cell such as may 

 have existed in the primitive external retina is supposed 

 to have been retained as this organ was carried from its 

 superficial location into the central nervous system and 

 out again almost to the external surface, the resulting 

 retina would be composed of inverted elements (Fig. 1). 

 Thus this theory at once offers an explanation for the two 

 most striking features of the vertebrate retina, namely, 

 its formation as an apparent outgrowth from the central 



