588 EMBRYOLOGY OF THE TURTLE. Part III. 
it is closely pressed upon it. That part of the ciliary processes (¢) which covers 
the posterior surface of the iris (e') is very probably identical with that which 
is said to “exist over the posterior surface of the iris and the projecting ends 
of the ciliary processes” in the human eye. Between the membrana_ iridis 
(fig. 8, e') and its posterior lining, (¢,) there is a layer of black pigment, which 
extends backward till it reaches the posterior border (e’) of the ciliary processes, 
where it becomes one with the pigment layer (/) which lines the choroidea 
proper (e). It is evident, that, whatever may be their connections in the higher 
animals, in the Turtle, even when full grown, the ciliary processes (see Pl. 22, 
o 
fiz. 7, &, e) are not an integral prolongation of the choroidea, but a separate 
layer, with a layer of pigment between the two membranes. The pigment layer 
(fig. 8, f) just mentioned as liming the choroidea, (e,) and its prolongation, the 
membrana iridis, (¢',) constituting the so-called pigmentum nigrum, forks at the pos- 
terior edge (¢*) of the ciliary processes, and sends off a layer along the inner 
surface of the latter (¢). The pigmentum nigrum is loosely coherent to the 
choroidea, yet presses very closely upon it. It disappears where the choroidea 
sheathes the optic nerve. 
In specimens preserved in alcohol, this layer, the pigmentum nigrum, has a 
leaden aspect with a peculiar lustre, and clings to the membrana Jacobi (g) in 
preference to the choroidea, (e,) when the former is lifted off from the latter. 
The papille of the outer surface of the membrana Jacobi, plunging into the 
pigment layer, adhere to it much more firmly than the smooth surface of the 
choroidea. Next within the pigmentum nigrum (f/f) is the retina, (yg, g’, h,) 
which, in a section of its thickness, appears transversely striated over nearly its 
whole extent,’ and seemingly divided into three layers. These apparent layers 
are the expressions of so many different strata of peculiar nervous cells, which 
will be described in detail hereafter; it may suffice here to say that they are 
not distinct layers. The outermost stratum (y) separates so easily from the inner 
one, that it has, until within a few years, been considered as a distinct membrane, 
under the name of “membrana Jacobi.” In the Turtle, this peculiar stratum pro- 
jects singly beyond the anterior edge (7') of the rest of the retina, and, pressing 
against that part of the pigmentum nigrum which covers the anterior extension 
of the ciliary processes, (¢,) extends halfway to the edge of the iris (e'). It 
is much thinner here than where it is connected with the imner strata of the 
retina; in the latter position it is quite thick, and uniform throughout (v). The 
exterior surface of the membrana Jacobi has a beautiful golden orange color, of 
? Sharpey and Quain’s Human Anatomy, Lon- * By mistake these striw of the retina were not 
don, 1848, vol. 2, p. 921. drawn perpendicular to its thickness, in front. 
