MUSEUM OF COMPARATIVE ZOOLOGY. . 213 



outward and the apex inward. The base of the cone is slightly con- 

 cave, corresponding to the convexity of the inner surface of the lens. 



The wall of the vesicle is very distinctly differentiated into lens (Ins.) 

 and retinal (rtn.) portions, the latter forming about two thirds of the 

 whole. The lens is slightly biconvex, the two convexities being very 

 nearly equal. The line of demarcation between the lens and the retina is 

 a sharp one, though the two portions are plainly continuous. The cells 

 composing the lens are large and distinct in outline, each one extending 

 entirely through its thickness (Plate II. Fig. 5, el. Ins.). Their nuclei 

 are large, easily stainable, and somewhat granular ; they are uniformly 

 situated near the internal ends of the cells. The lens is entirely with- 

 out pigment. 



Figure 5 represents a highly magnified portion of a longitudinal 

 vertical section of the vesicle taken from near the median plane. In 

 the retinal portion six regions or zones may be distinguished. Passing 

 from the external surface toward the cavity, we find (1) a basement 

 membrane {nib. ba. ex.). This is very thin, but uniform in thickness, 

 and is of a structureless nature. From many points on this membrane 

 fine processes radiate into the connective tissue enveloping the vesicle 

 (Plate I. Fig. 3, pre. r.). These processes do not appear to be of a mus- 

 cular nature, but rather the same in structure as the basement membrane 

 from which they arise. (2) A zone containing a few scattered nuclei 

 (nU), and fine-grained sparsely but evenly distributed pigment (pig.)* 

 No cell boundaries can be made out in this zone. The nuclei, few in 

 number, form a single layer, and are situated near the basement mem- 

 brane. They are very nearly round, exhibiting no tendency to elongate 

 in the radii of the vesicle. Areas in their centres, which are somewhat 

 more deeply stained than the rest of the nuclei, and which are probably 

 nucleoli, are to be seen. (3) A zone (z. n ) in which are distinguish- 

 able neither cells, nuclei, nor pigment ; only a uniform, fine-granular, 

 slightly stainable substance, of much the same nature, apparently, as 

 the cell substance in those regions of the retinal portion in which cell 

 boundaries can be distinguished. Whether or not this zone repre- 

 sents the centrally directed ends of a layer of cells, the nuclei of which 

 are the ones found in zone 2, I am unable to say, but it probably 

 does. (4, 5) The next two zones are distinguished from each other 

 only by the difference in the elements composing them, no distinguish- 

 able line of separation existing between the two. The most obvious 

 difference between the constituent elements of these two regions is in 

 the shape of the nuclei, those in zone 4 being approximately spherical 



