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 

 layer 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 



^Pft" Sph. 



B. 



Fig. 160. — Two sections of the pupillary margin of the eye of a chick of 13 



days' incubation. A., X 260. B., 130. (After Lewis.) 



c. P., CiHary process. E. B., Epithelial bud. P., Margin of pupil, p. 1., 

 Pigment layer of Iris. r. 1., Retinal layer of iris. 8ph., Bud for the forma- 

 tion of the sphincter 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- 



