i8o Vertebrate Embryology 



wall of the optic cup becomes thinner, as the 

 inner wall increases in thickness, until it is re- 



Epen. 



Fig. 59. — Transverse section through the ante- 

 rior REGION OF A CHICK EMBRYO WITH ABOUT TWENTY- 

 EIGHT SEGMENTS. (After Minot.) 



Ao.D.^ descending aorta. Ao.^, first aortic arch. Ao."^. second 

 aortic arch, card.^ anterior cardinal vein, cl /., first gill cleft. cl.II.y 

 second ^ill cleft. Ec, ectoderm. Ent.^ entoderm. E^en.^Tooi of 

 hind-brain, y.d.y fore-brain, h.b,, hind-brain. /-., invagination of 

 lens. Mdb.^ mandibular arch. 7nes.^ mesoderm, nck.^ notochord. 

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



duced to a single layer of flattened cells which 

 soon become darkly pigmented, and form the 



