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 pi»obable. 



In the later stages (Figs. 11, 12, 21-24) it is not always easy t9 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 



