186 BULLETIN OF THE 



when, as sometimes occurs, one finds a place where the niesodermie 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. II.) and 

 Fig. 11 (PI. III.) are undoubtedly mesodermic, and represent a thin 

 tissue on the outside of the sclera.* Those in his diagramatic figure 

 (PI. 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 imclei 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 retiiia than the lentigen (PI. II. 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 meaniiifj 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. 



