418 Avery E. Lambert. 



a condition which he first obser\'ed in Acilius, and fonnd afterward 

 in a more marked degree in scorpions and spiders. He was led 

 to homologize the thickenings on the inner margins of the pre-oral 

 segments, the cerebral ganglia, with the thickenings which occnpy 

 a similar position on the inner margins of the segments of the lateral 

 bands, and which form the neuromeres which later develop into 

 the nerve cord. 



According to Patten, the vesicles formed by the anterior and lat- 

 eral invaginations do not furnish the principal mass of the brain, 

 but form important lobes instead which become associated with the 

 optic tracts. 



In his discussion of the development of the brain of scorpions, 

 this author pointed out the fact that the entire area of the fore-brain 

 is covered by a fold which progressively extends inward from the 

 l^eriphery of the lobes. This fold, he found, bears a most important 

 relation to the formation of the median eyes, and also to what he 

 calls the "vesicle" of the fore-brain. 



The origin and development of the eyes have formed one of the 

 most important subjects of investigation in Arachnid embryology, 

 and furnish a problem which is closely associated with the study of 

 the development of the brain. 



The first contribution of importance to our knowledge of the 

 subject was made by Grenacher, whose paper was published in 1879. 

 Graber (13) immediately followed with an account of the morphology 

 of the eyes of Arachnids, in which he failed to accept Grenadier's 

 results. 



Patten (32), in 1886, called attention to the fact that the median 

 eyes of spiders consist of three layers, an outer, or corneal, a middle 

 or retinal, and an inner or post-retinal, layer. He also pointed out 

 that, in consequence of its manner of formation, the retinal layer 

 became inverted so that its elements presented the same relation to 

 the direction of the rays of light that we find in the vertebrate eye, 

 a fact which was afterward established by Locy and Schimkewitsch, 

 who made a careful study of their mode of development. 



These observations were confirmed by Mark (25) in 1887, who 

 discussed at length the morphological derivation and relations of the 

 eyes of Arthropods. 



