74 BULLETIN OF THE 



founded on the position of the bacilli, and those based on the presence 

 or absence of a tapetum, correspond,* one can hardly avoid the cotivic- 

 tion that these two features are in some way connected, and that the 

 dimorphism first pointed out by Grenacher is emphasized in other matters 

 than those to which his attention was directed. 



The origin of the tapetum and the exact method of its formation are 

 not yet sufi&ciently clear to me ; but I hope to be able before long to 

 acquire more information upon the obscure points. In connection with 

 the development of the eyes of the "post-nuclear" group, Locy ('86, 

 p. 89) has mentioned a structure which separates the two layers of the 

 retinal infolding, and he has described it as a " much-folded chitinous 

 layer, probably homologous with the cuticular covering of the body, with 

 which, in the earlier stages, it appears to be continuous." 



After renewed examinations of his preparations, and others of a simi- 

 lar nature from other spiders, I have arrived at the conclusion that this 

 layer is without any doubt the tapetum, and that there is no certainty of 

 its having been at first continuous with the external cuticula of the body. 

 As understood by Locy, it was a natural inference, with regard to its 

 formation, that it resulted, like the cuticula, from the secretive activity 

 of the ends of the cells composing one or both the layers of the retinal 

 infolding. This view seems at first to receive confirmation from the 

 early appearance of the tapetum, its apparent continuity (in many cases) 

 with the external cuticula, its greenish-yellow color, and the peculiar 

 shape of the separate elements which ultimately make up this layer. I 

 find also that in Theridium f it is composed of tolerably regular, elon- 

 gated, hexagonal plates (PL III, Fig. 17), neatly fitting edge to edge (as 

 though secreted by a pavement-epithelium) ; and in one instance I have 

 noticed distinct perpendicular markings in some of the scale-like plates 

 when seen edgewise. If the plates were really comparable with the 

 cuticula, these markings might be the equivalents of " pore canals." I 

 should add, however, that they were so strong as to suggest rather the 

 composition of the plates out of numerous perpendicular rods of uniform 

 size. 



But notwithstanding all this, the tapetum may be the result of a cell- 



tute the post-nuclear (Graber's pre-bacillar) group. If the relation suggested above 

 should be realized, " pre-nuclear " eyes might with equal propriety be designated as 

 non-tapetal, and "post-nuclear" as tapetal. 



* In Thomisus vulgaris, Hentz, I have not been able to find any evidence of the 

 existence of a tapetum either upon sagittal or transverse sections. However, the 

 only sections at my disposal are such as have not been depigmented. 



t Theridium tepidarioruin, C. Koch. 



