274 THE DEVELOPMENT OF THE CHICK 
Nussbaum, Szily, and Lewis to arise from epithelial buds of the 
pupillary margin and the adjacent portion of the pigment layer 
of the iris. The marginal buds (Fig. 160) begin to form during 
the seventh day, the more peripheral ones somewhat later; the 
former are less numerous and larger than the latter. The 
observations are well supported, and appear to leave no doubt 
that the specificity of the ectoderm cells of the iris are not fixed. 
According to Lewis the wandering pigmented cells of the ante- 
rior portion, at least, of the choroid also arise from the pigment 
laver of the optic cup. 
The ciliary processes begin to form from the ciliary region 
of the lenticular zone on the eighth day (Fig. 159); the epithelium 
A 
Via. 160.—Two sections of the pupillary margin of the eye of a chick of 13 
days’ incubation. <A., X 260. B., 130. (After Lewis.) 
ce. P., Ciliary process. E. B., Epithelial bud. P., Margin of pupil. p. L., 
Pigment layer of Iris. r. 1., Retinal layer of iris. Sph., Bud for the forma- 
tion of the sphineter muscle of the iris, derived from the margin. Sph.’, 
Sph.”, Submarginal buds of the sphincter. 
becomes thrown into folds projecting towards the posterior cham- 
ber, the cavity of the folds being filled by the mesenchyme of the 
developing choroid coat. The muscles of the ciliary body de- 
velop from the mesenchyme of the processes, which acquire a 
connection with the lens through a special differentiation of the 
vitreous body, the zonula ciliaris (zonula Zinnii). 
In the retinal portion of the optic cup the inner layer forms 
the entire retina proper from the internal limiting membrane 
to the rods and cones inclusive. The outer layer forms the pig- 
