MUSEUM OF COMPARATIVE ZOOLOGY. 65 
a continuous cell-layer in front of and concentric with the retina. The 
critical region — where the pigmented hypodermis passes into the layers 7 
behind the lens — is not satisfactorily portrayed in the figure. On one 
side (the right) the hypodermis seems to be directly continuous with the 
retinal layer ; upon the other side it is continuous with the layer form- 
ing the wtreous body, the retina being on this side more detached from 
it. Not finding nuclei in the vitreous layer, Grenacher admits that they 
may have entirely disappeared ; but he is more inclined to the opinion 
that they are grouped with nuclei of the ring-shaped pigmented zone at 
the anterior border of the retina, — where the nuclei are too numerous 
to be supposed to belong exclusively to the pigment zone, —- and that 
the finely attenuated posterior ends of the cells, bent outward towards 
the nuclei, escaped direct observation. 
If the nuclei of the ‘‘ vitreous” have completely disappeared, it is diffi- 
cult to see how this could be regarded as a monostichous eye. There is 
nothing, it is true, in the second assumption which precludes the idea 
that the ommateum consists of a single layer of cells; but it is equally 
clear that it does not preclude the possibility that the nuclei of the 
“vitreous ” have been displaced towards the margin of the lens ; and this 
would be compatible with a true involution of the retinal cells. I think 
that such a displacement of the nuclei from the central portion of the 
“vitreous? — in a manner analogous to that which Grenacher believes to 
have taken place with the retinal nuclei in the case of Salticus (Grenacher, 
79, Fig. 25 K)—1is more probable than either their total disappearance 
or their having primitively held a marginal position. 
In all these cases there is the opposing argument that no third layer of 
cells was discovered. 
The ocelli of the emagines also seem from previous descriptions to be 
destitute of a third layer, — at least no one, so far as I am aware, has 
claimed it. From one of Grenacher’s figures (that of Crabro, /. ¢., Fig. 34) 
I infer that a third layer may nevertheless exist as a thin sheet of cells, 
forming, as in spiders, the matrix of the so-called sclera.* 
The only observations on the development of the simple ocelli of the 
imago are those of Carriere already alluded to. They are too incomplete 
to serve as a safe guide. I am, moreover, persuaded, from the examina- 
* Grenacher (79, p. 60) speaks of the nuclei as belonging to this fine cuticula, and 
in the copy of his paper which I have, the (blue) nuclei lie on the inner side of the 
cuticula. Since the ‘‘ registering” appears to be very accurate for the ‘‘ vitreous” 
cells, I have no doubt that the nuclei of the sclera are printed as Grenacher intended, 
although no mention of their position in relation to the cuticular membrane 
(“‘sclera’’) occurs in the text. 
VOL. XIII. — NO. 3. 5 
