Proceplialic Lobes of Epeira Cincrca. 449 



become secondarily connected with the optic ganglia, this connection 

 taking place after the eyes are fully formed. 



His observations on the manner on which the accessory eyes are 

 formed appear to be confirmed by my own observations of the way in 

 which they arise in Epeira, and probably hold true for all spiders. 

 He finds that these eyes do not arise by a process of infolding, but 

 as simple depressions of the ectoderm, which is thickened in the optic 

 area to form the retina. The walls of this depression grow inward 

 so that they finally cover the retinal portion of the eye. 



Patten was the first to point out that the eyes of Arachnids consist 

 of three layers, an outer, or corneal layer, a middle, or retinal layer, 

 and an inner, or post-retinal layer. He also called attention to the 

 fact that, not only in the manner in which light traverses the retinal 

 elements, but in their mode of origin as well, the anterior median 

 eyes of Arachnids bear a close resemblance to the pineal eyes of 

 vertebrates, with which they are compared (35). 



One of the most important contributions which this author has 

 made to our knowledge of the morphology of the Arachnid eye, is his 

 discovery that the optic elements do not arise originally in the median 

 position, but first appear on the margins of the cephalic plate. Their 

 ultimate median position is brought about by the reflexing of the 

 margins of the cephalic plate over the optic ganglia and the cerebral 

 lobes. 



Mark's paper (25) is an important contribution to the discussion 

 of the way in which the mode of the formation of the Arachnid eye 

 bears on the question of the morphology of the eyes of Arthropods 

 in general, following for his account of the Arachnid eye the work 

 of Locy on A galena. 



The Anterior Median Eyes of Epeira. — The anterior median eyes 

 originate as sensory thickenings of the antero-lateral margins of the 

 cephalic plate. (Figs. 19-29; o. r//.). As the development of the 

 cephalic lobes progresses, that portion of the margin upon which 

 the optic plate is located is tnrned upward and backward in such a 

 manner that a fold is formed which covers the anterior portions of the 

 brain (Figs. 27-31; o. /".). The edge of tliis fold advances over the 

 cephalic plate in the medio-posterior direction, the two halves of the 

 fold meeting, as they progress downward, in the median line. 



