Development of the Third Day 17? 



While the cells of the inner wall are elongating 

 until they form what might almost be called 



Ao.D. 



EC. 



mes. 



f.b. 



FIG. 58. TRANSVERSE SECTION THROUGH THE ANTE- 

 RIOR REGION OF A CHICK EMBRYO WITH ABOUT TWENTY- 

 EIGHT SEGMENTS. (After Minot.) 



Ao., trunk of the aorta. A o.D., descending aorta. Ao.*, second 

 aortic arch, card., anterior cardinal vein. cl.IL, second entodermal 

 gill-pouch, EC., ectoderm, f.b., fore-brain, h.b., hind-brain. Ht., 

 heart, mes., mesoderm. My., muscle plate, nek., notochord. Op., 

 optic vesicle. Ph., pharynx. 



fibres, the cells of the outer wall, except at the 

 periphery of the lens, where they become con- 



