MUSEUM OF COMPARATIVE ZOOLOGY. 79 
corresponds to the superficial ends of the component hypodermal cells, 
and the bacilli accordingly occupy the ends of the cells which were © 
originally directed towards the light. 
Upon either supposition there is a difficulty in instituting a comparison 
with the eyes of the “pre-nuclear” group. Upon the first assumption, 
while the bacilli would occupy the originally deep ends of the cells, as in 
the other type, the retinal layer as a whole would have been only par- 
tially and temporarily inverted, — not permanently, as in that type, — and 
therefore a strict homology could not be claimed. But upon the second 
assumption, while the infolding would result in an inversion of the 
retinal layer, as in the simpler type, the bacilli would occupy the 
originally superficial ends of the cells, and this would also present a 
serious obstacle to a satisfactory comparison. 
I have not, perhaps, a sufficient number of successive stages to place 
the matter beyond question, but believe that the evidence from the ma- 
terial which I have, and also certain theoretical considerations, point 
towards the second assumption — that the retinal layer «is inverted —as 
the more probable. 
In the later stages (Figs. 11, 12, 21-24) it is not always easy to dis- 
tinguish at once between the nuclei of the first and second layers ; but 
careful attention to the shape and inclination of the nuclei, as well as to 
the intensity of their staining, allows one to determine fairly well the 
extent, if not the exact boundaries, of each layer. In Figure 22, for 
example, the nuclei of the “lentigen” were excessively flattened and 
apparently degenerating ; those of the retinal layer were much paler, less 
broken, and less granular. 
The origin of the third tract (tap.) is involved in the question just 
considered ; but whatever this origin, — whether it arise by delamination, 
or by an outfolding which affects only its own cells, or whether it result 
from an outfolding one wall of which is the retinal layer, — the ultimate 
condition of this tract can scarcely be called in question ; it produces the 
tapetum. Its nuclei (compare also Figs. 18-22) often undergo a remark- 
able elongation, and conform in shape to the curved direction of the layer. 
In all the eyes of the “ post-nuclear” group in Agelena the tapetum 
has the form of a short canoe, the cavity of which is directed towards 
the retina. Its greatest length corresponds with the direction of the ecto- 
dermic infolding. The end corresponding with the bottom of the pocket 
of involution is narrower than the opposite end, and does not approach so 
near to the surface of the head as the latter. The variations in the curva- 
ture from end to end are often considerable, amounting in some cases to 
