80 BULLETIN OF THE 
a sharp bend in the middle (Fig. 24), and the inclination of the sides to 
each other may also vary several degrees. The tapetum does not carpet 
the whole fundus of the eye, being, even in its broadest part, much nar- 
rower than the latter (Figs. 13, 14, Pl. III) ; but it appears to be as exten- 
sive as the layer of bacilli developed in front of it. Corresponding in 
position to the keel of the canoe, is a narrow interruption, or fissure, which 
extends through the whole length of this layer. It is sometimes slightly 
curved, S-shaped, and its edges are not always clear cut. It is probable 
that the appearance which Leydig described as ‘‘a band of black pigment 
traversing the middle of the tapetum” was due to the presence of a 
similar fissure. In some instances the broad outer end of the tapetum 
appears to abut directly upon the inner surface of the external cuticula ; 
but even in such cases I have not found in its vicinity any modifications 
of the cuticula, neither an infolding, nor any marked interference with 
its regular course. In no case have I been able to trace a direct con- 
tinuity of cuticular and tapetal substances. Often the tapetum cannot 
be followed up to the external cuticula; but where the conditions of 
the sections were favorable for its study, I have never failed to find that 
the narrow, deep end of the tapetum reaches to, and is apparently con- 
tinuous with, the internal cuticula, or basement-membrane. This con- 
dition seems to afford confirmation of the opinion that the tapetum 
results from an owtfolding of cells which previously occupied a position 
at the bottom of an early hypodermal infolding, involving the “ retinal ” 
and ‘‘ post-retinal” tracts. For if the tapetal cells originally grew into 
the cavity of the hypodermal pocket from its deepest end, they would 
naturally retain a direct connection with that portion of the basement- 
membrane where they were at first situated. The region of this ingrowth 
into the cavity of the original pocket may have extended along the two 
margins of the pocket for a greater or less distance, and the interrup- 
tion in the tapetum (“fissure”) may possibly have resulted from the 
failure of these two regions of ingrowth to meet along the axis of the 
original pocket. The absence of a direct connection with the external 
cuticula is in itself a strong argument against considering the tapetum 
homologous with that layer; this is further strengthened by a consid- 
eration of the chemical differences between the two, referred to by 
Grenacher. 
The tapetum in Agelena consists of small, thin, slightly curved, scale- 
like, iridescent structures which are superposed and closely packed. 
The whole layer has a considerable thickness, and when viewed in lon- 
gitudinal section, a peculiar wavy, fibrous appearance. If these scales 
