MUSEUM OF COMPARATIVE ZOOLOGY. 187 



over, that one would expect to find mesodermic elements if any were 

 present, and naturally enough it is in this region that the exceptional 

 nuclei previously referred to occur (PI. II. fig. 8). 



The fact that mesodermic tissue is incorporated in the preretinal mem- 

 brane makes it highly probable that the mesodermic cells noticed on the 

 sclera really contribute to that layer, and that the sclera is in part meso- 

 dermic and in part ectodermic. To summarize, then, the preretinal 

 membrane, like all parts of the basement membrane, appears first as an 

 ectodermic cuticula. Mesodermic elements may be included between its 

 two layers, but this is the exception. The most of the membrane is in 

 any event cuticula, of which the greater part is produced by the lentigen, 

 the lesser by the retina. 



The retinal and post-retinal layers. — The intimate connection into 

 which these two layers enter in forming the retina is a sufficient reason 

 for considering them together. Grenacher's ('79, pp. 39-57) researches 

 on the eyes of arachnids led him to believe that the retina consisted of 

 a number of similar elements, each of which contained a rod-like body, or 

 bacillus, and a nucleus. Each element was, therefore, to be considered a 

 single cell. He also discovered that there were two different types in 

 the disposition of the nucleus and bacillus. Either, as in the anterior 

 median eyes of Epeira ('79, pp. 43-45, fig. 18 A), the bacilli were in 

 front of the nuclei, or, as in the posterior median eye (fig. 18 B), they 

 were behind the nuclei. In the structure of the eye this interesting 

 dimorphism, as Grenacher has termed it, has proved to be a feature of 

 common, if not universal, occurrence with spiders. 



Graber, who based his conclusions on the study of the retina in scor- 

 pions as well as spiders, opposed Grenacher's views, and claimed to have 

 found in the median eye of Buthus ('79, pp. 71, 72) at least two nuclei 

 to each element, — a large, basal, ganglionic nucleus, and, near the outer 

 end of the element, a smaller apical one. The equivalent of Grenacher's 

 bacillus lay between these. In the case of the lateral eyes * of Buthus 

 (PI. V. fig. 5), as well as in the anterior and posterior median eyes of 

 Epeira (PI. VII. figs. 25, 26), Graber figured a third small nucleus 

 directly behind the bacillus. 



This discovery, if corroborated, would invalidate Grenacher's view of 

 dimorphism in the eyes of spiders, and one would be forced to admit 

 that the retinal elements are multinuclear, and therefore not single cells. 

 Grenacher's ('80, pp. 415-430) reply to Graber, at least so far as the 



* When the discussion of the lateral eyes is reached, the subject of their nuclei 

 will be considered more at lengtli. 



