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 



ep ^es p r 



157 1.38 



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', \", Distal and proximal walls of the lens, st., 

 Optic stalk. 



Fig. 158. — Section of the distal portion of the eye of a chick, 

 second half of the fifth day of incubation. (After Froriep.) 



c. ep. int., Internal epithelium of the cornea. Corn, pr., Cornea 

 propria. Ect., Ectoderm, 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 optic cup become blended so that pigment from 

 the outer layer invades the inner layer; the epithelia are decidedly 



