180 Vertebrate Embryology 



wall of the optic cup becomes thinner, as the 

 inner wall increases in thickness, until it is re- 



Epen. 



f.b. 



FIG. 59. TRANSVERSE SECTION THROUGH THE ANTE- 

 RIOR REGION OF A CHICK EMBRYO WITH ABOUT TWENTY- 

 EIGHT SEGMENTS. (After Minot.) 



Ao.D., descending aorta. A0. 1 , first aortic arch. Ao. 2 . second 

 aortic arch, card., anterior cardinal vein. cl.I., first gill cleft. cl.II., 

 second gill cleft. EC., ectoderm. Ent., entoderm. Epen., roof of 

 hind-brain, f.b., fore-brain, h.b., hind-brain. L., invagination of 

 lens. Mdb., mandibular arch, mes., mesoderm. nch., notochord. 

 Op., optic vesicle. Ph., pharynx. Ret., retina. 



duced to a single layer of flattened cells which 

 soon become darkly pigmented, and form the 



