MUSEUM OF COMPARATIVE ZOOLOGY. 61 
have hitherto received. He has especially defended the cuticular inter- 
pretation of the “sclera,” and in connection therewith has urged the ex-— 
istence of a cuticular matrix. The nuclei of this matrix he has very 
distinctly, and I am inclined to think very truthfully, figured (Fig. 18, 2) 
and described (pp. 64, 84) for Scolopendra. Even Grenacher (’80, p. 441) 
has granted a conditional assent to their presence, although maintaining 
that he did not feel entirely convinced.* 
To anticipate a conclusion, the grounds of which will be presented 
later, —in connection with a discussion of the nature of the pre-retinal 
membrane, —I may say here that the existence of a distinct cell-layer 
posterior to the retina, and inside the cuticular “sclera,” appears to me a 
strong argument in favor of the view that the retina in the Scolopendride 
has been formed by an involution with inversion. If Graber had realized 
the probable identity of these posterior cell-layers in Myriapods and 
scorpions, it is possible he might have been saved the expression of his 
sixth conclusion: ‘The ends of the retinal sacs [cells] appear to form, 
at least in part, the matrix of the sclera.” 
There is a very palpable difference between the figures of the “ vitre- 
ous” by Graber, and the figures and descriptions by Grenacher (’80, 
p. 484); nor is there any room to doubt that Grenacher’s work is, in 
most particulars, incomparably the more satisfactory and reliable. But 
Grenacher finds, if not a layer of uniformly fashioned cells, at least in 
some individuals of one species (Branchiostoma) a vitreous composed 
of an uninterrupted layer of cells, which differ from the vitreous cells 
of spiders, for example, only in the more central position of their nuclei, 
and the inclination of their axes towards (deep ends away from) the axis 
of the eye. This exceptional condition of the vitreous — found only in 
a few individuals — Grenacher brings into relation with the fact that the 
lens in these cases was only partially developed, and deduces the con- 
clusion that these animals had recently suffered a moulting, and that the 
increased thickness of the hypodermis and vitreous is simply evidence of 
increased functional activity. He recognizes the difficulty in the way of 
* According to Grenacher (’80, Fig. 8) the pigment-cells which invest the eye 
have the character of a continuous epithelium such as the posterior layer of the re- 
tinal infolding in spiders does at an early stage; but their relation to the thick strati- 
fied cuticula (viz. outside the latter) forbids a comparison. If Grenacher’s account is 
correct, the Myriapods stand quite alone in having such a continuous mesodermic 
investment of the eyes. 
Compare also Sograff (’80), Pl. III, fig. 17, where a nearly continuous layer of 
cells is represented outside the thick cuticula of the eye, but inside only isolated 
nuclei scattered among the nerve-fibres which occupy the space between the cuti- 
cula and the basal ends of the retinal cells. = 
