184 BULLETIN OF THE 



Graber ('79, p. 59) raised the question whether the lens consists of 

 only the normal cuticular layers thickened, or contains additional layers. 

 This is a question upon which evidence is not easily obtainable, for 

 it deals with layers which in the lens naay be of considerable thickness 

 and yet remain so thin as to be almost imperceptible in the indifferent 

 cuticula. The condition of the lens in young individuals offers some 

 evidence. About the time a young scorpion leaves the mother's back, the 

 indifferent cuticula appears to consist conclusively of what in later stages 

 corresponds to the external hyaline layer. Careful search has failed to 

 show any subjacent cuticula, and yet in the region of the lens a very per- 

 ceptible layer of stainable cuticula is visible. This seems to indicate 

 that the lentigen has the power of producing cuticula independently of 

 that produced by the indifferent hypoderrais. Admitting this, it seems 

 probable that of the many lamellae in the lens some may be peculiar to 

 the lens itself and unrepresented in the adjacent cuticula. 



The separation of the retina and lentigen, as discovered by Grenacher, 

 was further emphasized by Graber's discovery ('79, pp. 64-67) of a 

 limiting membrane (" praeretinale Zwischenlamella ") between them. 

 This preretinal membrane, as Graber showed, is continuous with the 

 " sclera " and the basement membrane of the hypodermis. The explana- 

 tion of these structures which is offered by the formation of the eye from 

 a hypoderraal sac, has already been discussed by Mark ('87, p. 71). He 

 has claimed that the sclera is the basement membrane of the post-retinal 

 layer, and that the preretinal membrane is the fused basement membranes 

 of the lentigen and retina. The explanation given in the case of the 

 infolded eyes of spiders applies equally well to the median eyes in 

 scorpions. 



As to the nature of the basement membrane, especially in the region of 

 the sclera and preretinal membrane, two opposing theories have been 

 advanced. Graber ('79, pp. 63, 64) maintains that the basement mem- 

 brane including the sclera is cuticular, not cellular, and as its matrix he 

 claims cells whose nuclei were found both by Grenacher ('79, p. 60, fig. 

 34) and himself ('79, p. 64, fig. 18). For the preretinal membrane 

 Graber ('79, pp. 64, 65) also claims a cuticular nature and states that it 

 contains no nuclei. Lankester and Bourne ('83, p. 189) describe the 

 omraateal capsule or sclera as laminate and devoid of nuclei. Mark ('87, 

 p. 71) believes that the basement membrane with its modifications is a 

 cuticula derived from the basal ends of the hypodermal cells. 



Opposed to these views Schirakewitsch ('84, pp. 8, 9, 11, 12) main- 

 tains that the basement membrane and its modifications are connective 



