34 bulletin: museum of comparative zoology. 



• 



Figures 3 and 6 (Plate 1) and 44 (Plate 5) will make clear a condition 

 that is constantly met with. It will be seen that at the anterior margin 

 of the central cell there is plainly a separation of what appears (Plate 5, 

 Fig. 44) to be a single membrane at a little distance from the eye. 

 One of the two portions of it evidently is dorsal and one ventral as 

 regards the paired portion of the eye. It seems to me that no other 

 interpretation of this condition is possible than to regard these mem- 

 branes as belonging to the ectoderm, especially since in a number 

 of cases the membrane, which is to all appearances single, becomes 

 so closely applied to the ectoderm that the two are indistinguishable. 



The sheath of the optic nerve may be traced in sagittal sections 

 (Plate 5, Figs. 44, 48, vih. pin.) for some distance over the dorsal 

 surface of the eye, but on the ventral side of the nerve (in a sagittal 

 section) it seems to be interrupted at the posterior margin of the 

 central cell (Fig. 44). It appears to fuse with the basement mem- 

 brane of the h}^odermis at this point. I cannot say whether the 

 membrane of the optic nerve is reflected over the hypodermis in a 

 posterior direction or not, for it is impossible to distinguish the two.. 

 Likewise, I have been unable to find any evidence that the nerve- 

 sheath passes forward over the basal plate of the ventral eye with the 

 basement membrane of the hypodermis. In Figure 44 the membrane 

 of the optic nerve is shown passing over the dorsal surface of the 

 lateral eye for a short distance, but it cannot be followed over the 

 whole eye. It is possibly continuous with the membrane that passes 

 over the dorsal surface of the eye, and in front of the eye it seems to 

 unite with another membrane from the ventral side of the paired por- 

 tion of the eye (Plate 5, Fig. 44; Plate 1, Fig. 3). The resulting, appar- 

 ently single, membrane shortly becomes indistinguishable from the 

 hypodermal basement membrane; but if so it cannot be objectively 

 identified by any differential staining reaction. The membrane of 

 the optic nerve {mh. pi'n.) may also be seen in cross sections (Plate 5, 

 Figs. 45 and 47); in Figure 46 (Plate 5) it (mb. pi'n.) is shown envelop- 

 ing the lateral eye upon the sides, at least partly. 



My belief is that the membrane around the optic nerve does not 

 envelope the entire eye, as has been maintained by Grenadier ('79) 

 and Claus ('91). All the direct evidence that I can acquire goes to 

 show that it rests something like a cap over the posterior portion of the 

 eye, not passing over the dorsal surface of the ventral eye at all. How- 

 ever, the relation of this membrane to the eye is not so important in a 

 general consideration, as the relation that the parts of the eye bear to 

 the body ectoderm; and the further discussion of this subject may 

 therefore be deferred until later. 



