188 BULLETIN OF THE 



retina in scorpions is concerned, is based on a study of the median eyes 

 only. He ('80, pp. 422-425) shows conchisively that for a ganglionic 

 nucleuij Graber has described and figured a body which is not a nucleus. 

 Grenacher, after a careful search for Graber's middle and anterior nuclei, 

 positively denies their existence. This, as Grenacher says, leaves the 

 retinal elements in scorpions devoid of nuclei ; he then proceeds to show 

 that in the region of Graber's so-called ganglionic nucleus there exists a 

 true nucleus essentially unlike the latter. Therefore, according to Grena- 

 cher, the retinal elements in scorpions are to be placed in the category 

 to which the anterior median eyes of Epeira belong.* 



Lankester and Bourne ('83, p. 188) agree with Grenacher that each 

 retinal cell contains a single nucleus ; but they also maintain that 

 Graber's anterior and middle nuclei are to be found in the retina. 

 These nuclei, however, do not belong to the retinal elements proper, but 

 to small intrusive pigment cells. 



The composition of the adult retina in Centrums has been studied by 

 means of sections and maceration preparations. A horizontal section 

 of an adult retina (PI. I. fig. 1) presents a concavo-convex outline; a 

 portion of the convex face occupies the median plane of the body, and is 

 fused to the corresponding part of the opposite retina. The concave face 

 is limited by the preretinal membrane. The concave region is composed 

 of a series of deeply pigmented club-shaped masses, which taper off into 

 the lighter middle region. Behind the lighter area, which occupies fully 

 half of the thickness of the retina, many of the bands and lines of pig- 

 ment become thickened into irregular dark blotches, which make up a 

 poorly defined mottled area. This soon merges in the median plane into 

 the retina of the opposite side, and elsewhere into a densely pigmented 

 zone limited behind by the sclera. This pigmented zone can be traced 

 around the side of the retina till at the edge of the latter it becomes 

 confluent with the pigmented area first mentioned. 



After removing the pigment and staining the section in Grenacher's 

 alcoholic borax-carmine, the outermost pigmented region (PI. I. fig. 2) is 

 seen to consist of a very granular substance, in which cell-walls can be 

 traced from the preretinal membrane backward to the pointed, rod-like 

 structures, or rhabdomes. The latter cause the lightness of the large 



* Graber ('79, p. 69) designated those elements in which the nuclei were behind 

 the bacillus as " postbacillar " ; those in wiiicli tlie nucleus was in front of the bacil- 

 lus, "prebacillar." Mark ('87, p. 73) has proposed for these terms pre- and post- 

 nuclear, respectively. Since these present advantages over the older terms, they 

 will be adopted in the following pages. 



