im 
586 EMBRYOLOGY OF THE TURTLE. Part III. 
that body, but its capsule also. The lens proper (Pl. 22, fig. 9, ¢) is formed 
by a hollowing of the solid mass which fills the depression in the retina, so as 
to leave a thin layer or outer wall attached to, and on a level with, the 
surface of the head, and a deeper layer or wall, from which a thick swelling 
(fig. 9, ¢) protrudes into the cavity (c') between these two walls. The outer 
wall (fig. 9, ¢) contains the elements of the conjunctiva and of the cornea, but 
we have not ascertained what becomes of the inner wall; it may turn out to 
be the iris. : 
The two walls of the retina (PI. 22, fig. 9, d, d') have separated from each other; 
the outer one (d') remains closely pressed against its orbit, but the imner one 
(d) forms a partition behind the crystalline lens, (c’,) and separates it from the 
cavity of the retina, which contains the vitreous humor. The fold of the retina, 
at the lower side of the eye, continues to grow narrower, but elongates as the 
eye increases in size, thus forming a narrow, white streak, (Pl. 24, fig. 11,) from the 
crystalline lens to the point of attachment of the optic nerve. The crystalline 
lens becomes a perfectly independent body, and develops its cells m a shape and 
with an arrangement peculiar to itself (Pl. 21, fig. 30, a, 6). As yet there are no 
eyelids, (Pl. 15, fig. 13,) and the eye is unprotected, as in Fishes. In a little 
later phase, (Pl 18a, fig. 3,) the skin adjacent to the eyes begins to encroach 
upon their anterior surface, in the form of a narrow rim. At this time, too, the 
white streak on the lower side of the eye has disappeared; at least, it cannot 
be seen externally, since several new coatings have~been developed over it. The 
narrow rim around the eyes grows broader, (Pl. 18a, fig. 1,) and assumes more 
distinctly the appearance of an eyelid, and the eyes become less prominent. Soon 
the rim ceases to broaden at two opposite sides of the circle which it forms, but 
continues to increase in the intermediate space, (PI. 25, fig. 10,) so as to pro- 
duce a broad, oval opening between the approaching opposite edges, the upper 
half of the rim forming the superior eyelid, and the lower half the inferior eye- 
lid. The eyelids continue broadening until they touch each other, and may be 
opened or shut at the pleasure of the embryo (Pl. 15, fig. 3). They never 
become agglutinated to each other, as happens among Birds and Mammals. The 
state of. the membranes of the eye at this stage has already been sufficiently 
described (p. 570, Pl. 21, fig. 29, 31, 32-32d). Beyond this we have not had 
time to trace the development of the eye, and must leave the subject for future 
investigation. 
At the time the Turtle was hatched, a very full examination of the structure of 
the eye was made, the results of which we will now proceed to give. On 
account of the softness of the different parts of the eye at that age, it was 
thought advisable to begin with some preliminary studies upon specimens hardened 









