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, PI. Ill) ; 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 02<^folding 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 tliese scales 



