104 



TEXT-BOOK OF EMBRYOLOGY. 



disk, as in Reptiles and Birds, is confined to one pole of the egg. Thus in these 

 forms the embryonic body develops on the surface of the large yolk mass. As 

 the embryo develops the germ layers simply grow around the yolk and suspend 

 it from the ventral side of the embryo. At the same time a constriction appears 

 between the embryo and the yolk mass, thus forming the yolk stalk. In this 

 case the yolk is surrounded from within outward, by entoderm, visceral and 



Fig. 97. — Diagrams representing stages in the development of the foetal membranes 

 in the chick. Herlwig. 

 a, Transverse section; b, c, d, longitudinal sections; yolk represented by vertical lines, al., Allantois; 

 am., amnion; am. c, amniotic cavity; cce., ccelom; dh., vitelline area between two dotted lines 

 which represent the edge of the mesoderm (at s.t.) and entoderm (at z. g.); dg., yolk stalk; 

 ds., yolk sac; d. umb., dermal umbilicus; ect., ectoderm; ent., entoderm; ex. b. c, extraem- 

 bryonic body cavity; gh., area vasculosa; h.am.f., head amniotic fold; m., mouth; p.mes., 

 parietal mesoderm; s.t., sinus terminalis; set., serosa (chorion); t.am.f., tail amniotic fold; 

 umb., umbilicus; v mes., visceral mesoderm; z. g., dotted line represents edge of entoderm. 



parietal mesoderm, and ectoderm (Fig. 98) . The yolk furnishes nutriment for 

 the embryo. This is conveyed to the tissues by means of blood vessels. 

 Branches of the vitelline artery ramify in the wall of the yolk sac (in the meso- 

 dermal tissue) ; the branches converge to form the vitelline veins which carry the 

 blood back to the embryo. 



In the chick, while the amnion is forming, the inner germ layer gradually 

 extends farther and farther around the yolk (Fig. 97, a, b, c and d). At the 



