LATERAL EYES OF VERTEBRATES. 153 



The Retinal Cell Pattern. In Limulus, and no doubt similar conditions 

 prevail in trilobites and merostomes, the lateral eyes consist of numerous chitenous 

 lenses, under each of which is an "ommatidium," consisting of a circle of fifteen 

 or twenty rod bearing cells, surrounding a central one that appears to be more 

 highly specialized and to have a richer nerve supply than the others. 



The ommatidia are separated from one another by circles of unspecialized 

 columnar epithelial cells. The crystalline cone cells and the corneagen cells of 

 other arthropods are absent. 



When such a simple kind of facetted eye was enclosed in the brain walls 

 of vertebrates, not only was the primitive shape of the whole eye retained, but 

 the characteristic pattern in the arrangement of the two different kinds of cells 

 was also retained. That is, the circles of rod-bearing cells surrounding a single 

 central one, is probably represented in vertebrates by the circles of rod cells sur- 

 rounding a cone cell. (Fig. 106.) 



The histological changes involved in this transformation are comparatively 

 small, the most important one being a transfer of the retinular rods from a lateral 

 to a terminal position. Such changes as this frequently occur in the arthropods. 

 Compare, for example, the striking differences in the structure of the retinal cells 

 in the parietal eye of Apus, Branchipus, Buthus, Galeodes, and Limulus. 



The Retinal Ganglion. In Limulus, there is a loose layer of ganglion cells 

 lying just beneath the inner surface of the lateral eye; and a similar one is present 

 in the eyes of many other arthropods, e.g., retinal ganglion of insects and Crustacea. 

 When the lateral eye of vertebrates was involved in the palial fold, this layer went 

 with it, forming the nerve cells that lie outside the stratum of rod and cone cells. 

 (Figs. 107 and 108, r.g.} 



The Lens. A striking feature of the lateral eye is the development of a lens 

 vesicle from the surface ectoderm and its union with a retinal placode which grows 

 out from the brain walls to meet it. 



The origin of the image forming organ at a remote time and place from that 

 of the sensory receptive surface, has led many writers to the conclusion that they 

 represent two originally different classes of organs, secondarily united into one. 

 Thus the lens vesicle has been interpreted as a specialized gill pocket, or as a 

 segmental sense organ serially homologous with those of the hindbrain region. 

 These views are untenable because they are not called for by the facts as we now 

 understand them. In Limulus and scorpion, we have shown that the cuticular 

 lens and the lentiginous ectoderm of the parietal eye are formed wherever the 

 vesicle reaches the surface ectoderm, no matter how remote that point may be 

 from the original position of the retinal placode. (Figs. 101 and 102.) It is 

 clear enough, in these cases, that the lens cups are an original part of the eye, and 

 cannot be thought of as existing apart from it. Precisely the same condition, it 

 seems to me, prevails in the lateral eye of vertebrates. We would therefore 



