MUSEUM OF COMPARATIVE ZOOLOGY. 83 



tion of bacilli, — the intervals between the markings corresponding very- 

 well with the intervals shown when the plane of sectioning is nearly 

 parallel with the direction of the fissure (compare Fig. 19), — but I am 

 uncertain whether it is to be concluded from this that the bacilli have 

 the shape of broad plates, or whether these plate-like structures are really 

 composed of row& of rods, which the method of preparation and mount- 

 ing (Canada balsam) has made incapable of optical resolution. There 

 is a suggestive resemblance between these plate-like markings and the 

 sinuous figure formed by the peculiar arrangement of the bacilli in the 

 posterior eye of Lycosa as given by Grenacher (78, Taf. Ill, Fig. 24) ; 

 but I was not able to satisfy myself that these plates presented the 

 folded-back-and-forth arrangement shown in Grenacher's figure. From 

 what is known of the form of the bacilli in other simple eyes, it seems 

 most reasonable to suppose, however, that the plates are composed of 

 rows of bacilli. 



The second section (Fig. 14) shows the remaining portion of the tape- 

 tum, belonging principally to the anterior end of that structure ; if there 

 were portions of the bacilli present upon this section, they were too faint 

 to be discerned. 



Finally, the third (deepest) section (Fig. 15) passes entirely below the 

 tapetum, cutting through the post-retinal layer. 



The presence or absence of a pre-retinal membrane in the eyes of the 

 present type is of some interest, and yet it may not be of radical impor- 

 tance. Whether the change in the relative positions of the retinal and 

 pre-retinal tracts during development is due to a true folding, or to a 

 slipping of one layer over the other, may depend simply upon how faith- 

 fully the original method of transposition (folding) is adhered to. With 

 the gradual substitution of a slipping for a folding, the opportunity 

 for the formation of a pre-retinal membrane may have gradually disap- 

 peared ; nevertheless, I am of opinion that evidence of such a membrane 

 will usually be found during some stage in the formation of the ocellus. 



In some spiders (Tegenaria, Theridium) the development of the re- 

 tinal infolding and the secretion of the lens are accompanied by a grad- 

 ual displacement of the deep ends of the " lentigenous " cells towards 

 the margin of the eye, so that in the adult the pre-retinal membrane is 

 almost in contact with the posteric)r surface of the lens, especially near 

 the margin opposite that towards which the nuclei of the "lentigen" are 

 displaced. This of course increases the difficulty of discerning the 

 membrane. 



