Cuap. II. HISTOLOGY. 609 
the claw (fig. 20, a). They are sharply polygonal, and contain minute scattered 
granules, a single, large, round mesoblast, and a minute entoblast (fig. 20d). In 
the last phase, just before the Turtle is hatched, the cells of the epidermis of 
the shell are (Pl. 20, fig. 18, on the right) sharply polygonal; they have very 
thick walls, and appear to be perfectly homogeneous. Underneath these are 
very large, hyaline, excessively thin-walled, polygonal cells (fig. 18, on the left). 
Each cell contains a central heap of very dark granules, which extend in a 
scattered manner to the walls of the cell. 
The Eye. Soon after the eye has begun to develop, (Pl. 12, fig. 6; p. 550,) 
the cells of the retina (Pl. 21, fig. 28, a) are columnar, narrow, elongated, and 
arranged with their longer axes trending perpendicularly to the parietes of the 
cavity which they surround. Lach cell occupies the whole thickness of the 
retinal layer. At a much later period, when the shield has become quite con- 
spicuous, (Pl. 14, fig. 1,) and the toes begin to protrude from the paddle-shaped 
feet, the cells (Pl. 21, fig. 30) of the crystalline lens are excessively hyaline, 
and have homogeneous contents without a trace of a mesoblast. They are 
arranged in lines, which run from the anterior to the posterior side of the lens, 
and converge around a centre (a) where the cells are smallest, so that a fibre 
of the lens may be said to be an exceedingly long, fusiform body, composed of 
cells arranged in a single line. A short time before the Turtle is hatched, (PI. 
15, fig. 1, 2, 3,) the membrana pupillaris is composed of very large, polygonal 
cells (Pl. 21, fig. 29, a). The crystalline lens (fig. 32) is composed of long, 
ribbon-shaped, flat fibres, (fig. 52b, 32c,) containing scattered, faint granules. Each 
fibre runs from the anterior to the posterior side of the lens, those at the centre 
in a straight line, and those exterior to them more and more in a curve, accord- 
ing as they are nearer the surface. Every one is composed of excessively trans- 
parent, thick-walled, quadrilateral cells, (fig. 32a,) to display which requires 
the agency of water, since in a fresh state they cannot possibly be detected, 
except that portion of the wall which forms the edge of the fibre (fig. 32b, 
32c). A few exceedingly transparent, large, globular bodies are scattered among 
the fibres; but even these must be brought out by the agency of water. By the 
continued action of water, the cells swell up enormously (fig. 31, 32d). This 
can only be accounted for by the thickness of the walls of the cells, in a nat- 
ural state, which allows such great extension without tearing. At the time the 
Turtle is hatched, the retina is nearly as complicated as in the adult. A section 
of the thickness of the retina was made at a point halfway between the crys- 
talline lens and the back of the eye. 
Immediately behind the hyaloid membrane, (Pl. 22, fig. 8, /,) the surface of the 
retina is covered by a thin layer (fig. 8, 7) of excessively elongated fibres, which 
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