EYE OF NOTORYOTES TYPHLOPS. 555 



is only one layer of ordinary columnar cells wliicli are cub 

 obliquely, so that there appear to be two or three layers of 

 nuclei; whereas there is but a single layer of cells. 



It is interesting to compare the state of development of this 

 gland mass, conjunctival sac, and ducts in other forms. Thus 

 we find in Typhlomolge [2, p. 52] , whose eye that of 

 Notoryctes resembles in many points of structure, that there 

 is no sign of a conjunctival sac, nor is there any glandular 

 structure connected with the eye. In Ehineura likewise [4, 

 p. 536] there is no conjunctival sac, but Harder's gland is 

 well developed ; its secretion, on the other hand, is poured 

 directly into the tear-duct, and so into the nasal cavity. In 

 Siphonops [6, p. 101 — 105] we have a very well-developed 

 gland, and also a conjunctival sac, into which, in one case, 

 Dr. Kohl found the lachrymal glands emptying, and from 

 which the naso-lachrymal duct passed off through the sur- 

 rounding tissues and nasal wall, to open into the nasal cavity 

 far forwards, as in Notoryctes. More generally, however, in 

 Siphonops, the gland tubes open directly into the naso- 

 lachrymal duct, while in Typhlops [6, p. 119 — 121] they open 

 into a duct which leads into the back of the mouth cavity. 



The relations of the glands, which are apparently true 

 Harderian and lachrymal glands are similar in Typhlops to 

 those in Notoryctes, as also the presence of blind ducts 

 ending in the subcutaneous tissues. 



In Scalops, Talpa, and Typhlotriton we find instructive 

 stages in the closing up of the conjunctival sac, intermediate 

 between that of Notoryctes and the normal eye. In both 

 Scalops [8, pi. xviii, figs. 4, 7, 8 ; and pi. xix, fig. 16] 

 and Talpa [6^, Taf. i, figs. 3, 4, 5] the eyelids have closed 

 over to such an extent that the only connection between this 

 pre-corneal space and the surface of the body is by an open 

 though small canal, which is of very little if any use for the 

 passage of light rays. 



In Typhlotriton [3, p. 41, figs. 1 and 1 ft], the lids are 

 merely overlapping slightly, a shallow groove indicating the 

 position of the eye cleft. The comparatively small amount 



