LATERAL EYES OF VERTEBRATES. 151 



In Vespa, after the ganglionic infolding has closed, the lateral eye placodes 

 are themselves deeply infolded and partly covered by thin membranous folds, 

 but the latter soon disappear and take no part in the formation of the eye. ' 



In insects, Crustacea, andLimulus, the eye proper, or ommataeum, including the 

 cyrstalline cone cells and retinulse, is formed from the single layer of columnar, 

 ectodermic cells that constitutes the lateral eye placode. The infolding described 

 by Reichenbach and others, in the crayfish, as forming the deeper layers of the 

 eye, is merely the infolding that produces the lateral eye ganglion. 



II. Lateral Eyes of Vertebrates. 



In my first contribution to the origin of vertebrates, 1889, I pointed out the 

 remarkable resemblance between the early position of the eye placodes in verte- 

 brates and arthropods, and the similar way in which the neural crests enclose the 

 forebrain vesicle. 



Many contributions have been made since that time, especially in regard 

 to the vertebrates, that confirm my observations and my interpretations of them, 

 yet no one appears to have clearly understood the facts or their significance. 

 It is hoped that a fuller description, with numerous additional figures, will make 

 these important data intelligible and convincing. 



While the lateral eyes of arthropods are never caught in the infoldings of 

 the embryonic forebrain, as the larval ocelli are, they lie very close to the edge 

 of such folds, so that any marked deepening or extension of them, brought about 

 by the increasing size and precocity of the brain and its gangha, would be likely 

 to include the lateral eye placodes in the infolding, and thus transfer them to the 

 inner walls of the brain chamber. In this new position, they would be subject 

 to entirely new conditions, and they would doubtless quickly undergo important 

 structural changes. 



The structure and development of the vertebrate eye indicate that, in some 

 of the intermediate forms between vertebrates and arthropods, these events have 

 actually taken place. 



Location. — It has long been known that the lateral eye placodes of verte- 

 brates are visible at a very early stage on the outer margin of the open medullary 

 plate, (selachians, amphibia, birds). (Figs. 34 and 35.) 



As the neural crests advance toward the median line, the placodes are trans- 

 ferred to the inner limb of the fold, and finally come to lie in the walls of the 

 brain chamber, in precisely the same manner that the parietal eye placodes reach 

 a similar position. 



Origin of the Choroid Fissure and the Blind Spot.— After they are thus 

 enclosed in the brain walls, they assume the shape and position which is so 

 characteristic of vertebrates, and which becomes so significant when compared 

 with the same features in the lateral eyes of arthropods. 



1 See the peculiar, hood-like fold over the lateral eyes of Apus and other phyllopods. 



