186 BULLETIN OF THE 
when, as sometimes occurs, one finds a place where the mesodermie ele- 
ment with its nucleus has been loosened from the sclera, and both nucleus 
and sclera remain uninjured. 
The nuclei which Schimkewitsch has drawn in Fig. 4 (PI. IL.) and 
Fig. 11 (Pl. III.) are undoubtedly mesodermic, and represent a thin 
tissue on the outside of the sclera.* Those in his diagramatic figure 
(Pl. III. fig. 4), if they are, as Schimkewitsch says, identical with those 
of the other two figures, are mesodermic nuclei drawn on the wrong side 
of the sclera ; if, on the other hand, their position is correct, they are not 
the same nuclei as those in Figs. 4 and 11, but, as Mark (87, p. 70) 
maintains, the nuclei of the post-retinal layer. 
From what has been said it will be inferred that in the adult neither 
the sclera nor preretinal membrane contains nuclei. It is conceivable 
that in some cases mesodermic nuclei might be surrounded in either of 
these structures. Such, of course, would be exceptional. Of the twenty ~ 
preretinal membranes studied in section only one has shown nuclei ; but, 
strange as it may seem, half t of this one contained no less than fourteen. 
They were uniformly distributed, and always elongated parallel to the 
striations of the membrane. In position they were appreciably nearer 
the retina than the lentigen (PI. Il. fig. 8, nl. ms d.). This instance 
shows that the preretinal membrane may at least have a central layer of 
mesodermic tissue, although the greater part of it is ectodermic cuticula. 
The great thickness of the preretinal membrane in scorpions has al- 
ready been noticed by Graber (’79, p. 67). In Centrurus, as in others, it 
presents a fibrous laminate appearance, and in specimens treated with 
potassic hydrate it is slightly swollen and vacuolated. 
At the edge of the preretinal membrane, where its two constituents 
separate, the one which passes around the retina is much thinner than 
the one which continues under the hypodermis. This is an indication 
of the relative amount of substance contributed respectively by the retina 
and the lentigen in the formation of the membrane. The line which 
would separate the lentigenous from the retinal part must be drawn 
somewhat nearer the retina than the lentigen. It is on this line, more- 
* This limitation of the meaning of the word “sclera” seems desirable in view 
of the possibility that a mesodermic covering may be altogether wanting, but it is 
not intended as a criticism of Schimkewitsch’s statement that the sclera contains 
nuclei; for the “sclera” as previously understood may evidently be in part meso- 
dermic, and therefore cellular, as Schimkewitsch has claimed. 
+ The remaining half of this membrane was mounted on a second slide, and 
treated by a method which did not make its nuclei distinguishable. 
