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 conclusively that for a ganglionic 
nucleus 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 Centrurus has been studied by 
means of sections and maceration preparations. A horizontal section 
of an adult retina (Pl. 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 “ post-bacillar ”’; those in which the nucleus was in front of the bacil- 
lus, “ pre-bacillar.”’ 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. 
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