MUSEUM OF COMPAKATIVE ZOOLOGY. 81 



were the product of a cuticular secretion on the part of the cells of the 

 tapetal layer, one would rightly expect the nuclei of the cells to retain 

 some constant relation to the scales. They should all be located on one 

 or both surfaces of the scale-like layer, or they should all lie in the 

 middle between two sheets of such structures. But I have been unable 

 to find any such constancy of relation, the few nuclei being distributed 

 through the layer apparently without any regard to their distance from 

 either surface of the tapetum (Figs. 15, 18-22, tap.). For these reasons 

 I believe it must be admitted that the tapetal scales are formed by a 

 metamorphosis of the cell-substance of the cells forming what I have 

 called the third, or tapetal tract, and not by a process of cuticular secre- 

 tion. I have not traced the development of the separate scales within 

 the body of a cell, but from the small number of nuclei present it is 

 evident that each cell must give rise to a large number of the scale-like 

 elements. 



The fourth or deepest layer apparently corresponds with the deepest or 

 third layer in the eyes which present the simpler structure, — the pre- 

 nuclear ocelli. There is no doubt that it owes its formation here, as well 

 as there, to a process of hypodermal infolding (Figs. 2, 5-9, 16, 18, 19), 

 and it retains, even after the formation of the tapetum, an evident con- 

 tinuity with the indifferent hypodermis immediately in front of it. Like 

 the deep layer in the eyes of the " pre-nuclear " group, it also becomes 

 the seat of an early and intense pigmentation. That it subserves the 

 ordinary functions of a pigment-layer to the retina can scarcely be 

 doubted ; but instead of progressively diminishing in thickness and indi- 

 viduality, as in the pre-nuclear eyes, it here seems to increase in thick- 

 ness, and may perhaps fulfil important functional relations not shared 

 by the corresponding layer in the simpler ocelli. In the more advanced 

 stages (Figs. 20-24) this layer is considerably augmented in bulk as com- 

 pared with earlier stages and in comparison with the mass of nuclear 

 material. Its anterior border overlaps the anterior margin of the other 

 layers (Fig. 22), much as the superficial layer {pr r.) at an earlier stage 

 (Fig. 12) envelops the posterior margin of the layers underlying it. 

 From its connection with the optic nerve it has acquired a somewhat 

 conical shape (Figs. 23, 24). A portion of the nuclei still forms a more 

 or less continuous layer near the surface (Fig. 20) ; others (Fig. 22) lie 

 near its axis. Throughout the whole of its substance very fine striations 

 are now distinguishable. The direction of the striations makes it evi- 

 dent that they are due to the radiation of the fibres of the optic nerve, 

 towards which they all tend. But I am not yet entirely certain about 



VOL. XTII. — NO. 3. 6 



