Development of the Third Day 177 



While the cells of the inner wall are elongating 

 until they form what might almost be called 



Ao.D 



Fig. 58. — Transverse section through the ante- 

 rior REGION OF A CHICK EMBRYO WITH ABOUT TWENTY- 

 EIGHT SEGMENTS. (After Minot.) 



j4(?., trunk of the aorta. ^(7.Z?., descending aorta. ..4^.*, second 

 aortic arch, card.^ anterior cardinal vein. cl.II.^ second entodermal 



gUl-pouch. Ec.y ectoderm. /,d„ fore-brain. h.S.^ hind-brain. Ht.^ 

 eart. w^J., mesoderm. ./I/y., muscle plate. wcA., notochord. Op.^ 

 optic vesicle. /%., pharynx. 



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

 periphery of the lens, where they become con- 



