The Questions of Developmental Biology

networks of each organ arise within the organ itself, and are not extensions from larger vessels
(Auerbach et al. 1985; Pardanaud et al. 1989).
The aggregation of hemangioblasts in extraembryonic regions is a critical step in amniote
development, for the blood islands that line the yolk sac produce the vitelline
(omphalomesenteric) veins that bring nutrients to the embryo and transport gases to and from
the sites of respiratory exchange (Figure 15.16). These cells are first seen in the area opaca of
chick embryogenesis, when the primitive streak is at its fullest extent (Pardanaud et al.
1987).These cords of hemangioblasts soon become hollow. The outer cells become the flat
endothelial cell lining of the vessel.