MUSEUM OF COMPAIIATIVE ZOOLOGY. 57 



must have been exceptionally frail, since separations of this kind are 

 rare in adjacent regions. 



The thin layer of tissue next to the lens — in fact closely applied 

 to it in many cases (Figs. 5 and 17, cm.) — is part of a layer that en- 

 velops the entire eye, in many places lying close upon the pigmented 

 layer of the retina. The portion in the region of the retina is inidoubt- 

 edly the sclera, and will hereafter be designated as such. The portion 

 in the region of the lens may be regarded as representing all there is of 

 a cornea excepting its epithelial layer ; but on this subject I shall speak 

 further presently. The strands of connective tissue that have been 

 spoken of as intervening between the epidermis and the eye in the 

 smaller specimens are distinctly fibrous, and contain numerous small, 

 much flattened connective-tissue nuclei. These strands are directly 

 continuous with the sub-epidermal connective tissue of the surrounding 

 regions, and are not largely continuous with the sclera, though in part 

 they are (Fig. 17a). 



In addition to the small flattened connective-tissue elements in these 

 bands of connective tissue, a few much larger cells are found {cl. conH.). 

 They have distinct round nuclei, and each nucleus has a nuclear mem- 

 brane and a nucleolus. The membraneless cell body is drawn out into 

 one or more processes, usually two or three, which become lost among and 

 are apparently continuous with the fibres of the connective-tissue strands 

 in which the cells are situated. They are probably embryonal connect- 

 ive-tissue cells concerned in the production of the thick layer of this 

 tissue that intervenes between the eye and the epidermis in older speci- 

 mens (Plate II. Fig. 6). In this older specimen (Fig. 6) a large num- 

 ber of nuclei are seen, in part immediately over the eye, and consequently 

 in the same position as the cells regarded as embryonal connective-tissue 

 cells in the young specimens ; but they are mostly at one side of the 

 eye (Fig. 6, cont. (is.), and although some of them are undoubtedly 

 cells of connective-tissue character, at the same time many of them are 

 certainly not of this nature, but are probably leucocytes. As shown in 

 Figure 6, there is over the eye in the large specimens a well defined 

 dermal layer, drm., which usually remains adherent to the epidermis 

 when the latter is removed. This layer is nearly structureless, though 

 fine fibres are not uncommon in it. In the specimen shown in Figure 6, 

 the entire thickness of the tissues over the eye is about 392 fi, of which 

 103 fi is epidermis and 289 n sub-epidermis. About midway between 

 the epidermis and the eye there is a thin stratum of formed connective 

 tissue (st. coiit.), much denser than the surrounding tissue ; and imme- 



