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 convic- 
_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 sufficiently 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 ¢apetuwm, 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 + 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. 
+ Theridium tepidariorum, C. Koch. 
