MUSEUM OF COMPAKATIVE 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 

 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 tlie other side it is continuous with the layer form- 

 ing the vitreous 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) — is 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 imagines 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, I. c, 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 whicli I have, the (bhie) 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 tlie nuclei of the sclera are printed as Grenacher intended, 

 although no mention of their position in relation to the c'uticular membrane 

 ("sclera") occurs in the text. 



VOL. XIII. — NO. 3. 6 



