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 rows 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. III, 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 toa 
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 posterior 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. 
