MUSEUM OF COMPARATIVE ZOOLOGY. 89 
the large elongated nuclei of the nerve-end cells and the flattened nuclei 
of the inner layer. In this stage (Pl. X. fig. 69) the essential features of 
the eye are established, and it is possible to affirm positively that the 
anterior median eyes in Agelena nevia belong to the type in which the 
nuclei of the retinal cells are post-bacillar. 
The three remaining pairs of eyes originate somewhat later, but in sub- 
stantially the same way as the pair just described ; a hypodermic thick- 
ening, a backward directed infolding which inverts the thickened region 
and carries in beneath it a thin layer of hypodermis, the closure of the 
orifice of involution, and the detachment of the involuted mass from the 
hypoderm. The lens is also produced from modified hypodermic cells 
resembling, though shorter than, those forming the lens of the median 
anterior pair. But the two layers of the infolded mass do not undergo 
the same changes as do the corresponding layers in the pair of eyes pre- 
viously described. In the first place, the two layers remain permanently 
(up to my latest stage, ten days after hatching) separated by the devel- 
opment of a (in hardened specimens) much folded chitinous layer, which 
is probably homologous with the cuticular covering of the body, with 
which in the earlier stages it appears to be continuous. Secondly, while 
the retina is developed as in the anterior eyes, from the cells of the 
inverted portion of the infolded region, the bacilli do not arise in the 
ends of the cells which adjoin the vitreous body, but at the opposite or 
posterior ends. They are, therefore, found in the immediate vicinity of 
the chitinous substance. The nuclei, in the latest stages examined, still 
continue to occupy the anterior portion of the layer. Whether they are 
ultimately displaced to the margin of the retina, I am not at present able 
to say. Clearly, however, the retina is developed out of the middle layer, 
as in the previous case, but the nuclei of the retinal cells are pre-bacillar 
in position. About the time of hatching nerve filaments grow out from 
the brain, and thus connect the cerebral ganglia with the retinal portion 
of the eye. 
3. The lungs arise as a pair of extensive invaginations at about the 
same time as the proctodeum. In sagittal sections of early stages the 
lungs appear as oblong plates of cells, the large oval nuclei of which are 
arranged in parallel rows (Pl. XI. fig. 73). The cells forming the ven- 
tral wall of the floor over the lung sacks, however, are several layers 
deep, and their nuclei are not arranged in parallel rows as the other 
nuclei are. The nuclei of the parallel rows undergo a change of form, 
becoming flattened on one side and very convex on the other. In each 
single row the convex faces look in the same direction, but the rows are 
