272 THE DEVELOPMENT OF THE CHICK 
ness appears. The transition from the thinner lenticular to the 
thicker retinal zone soon becomes rather sudden in the region of 
the future ora serrata. About the eighth or ninth day a further 
differentiation arises within the lenticular zone, marking off the 
regions of the iris and ciliary processes (Fig. 159). The region 
eo Mes RT 
157 158 
Fig. 157. — Section through the eye of a chick embryo at the 
beginning of the fourth day of incubation. (After Froriep.) 
ch. Fis. 1., Lip of the choroid fissure. Di., Lateral wall of the 
diencephalon. 1’, 1”, Distal and proximal walls of the lens. — st., 
Optic stalk. 
Fic. 158. — Section of the distal portion of the eye of a chick, 
second half of the fifth day of incubation. (After Froriep.) 
ce. ep. int., Internal epithelium of the cornea. Corn. pr., Cornea 
propria. Ect., Eetoderm. ep., Epidermis ir., Iris. mes., Meso- 
derm. p., Pigment layer of the optic cup. r., Retinal layer of 
the optic cup. 
of the iris is a narrow zone bounding the pupil in which the two 
layers of the optie cup become blended so that pigment from 
the outer layer invades the inner layer; the epithelia are decidedly 
