has developmental contribution from
endoderm
yolk sac
http://upload.wikimedia.org/wikipedia/commons/1/17/Gray22.png
In therians, a structure homologous to the yolk sac is is present, but contains no yolk platelets. Instead it is filled with fluid [ISBN:0073040584 (Vertebrates, Kardong)]
consider splitting class
splanchnic layer of lateral plate mesoderm
extraembryonic tissue
extraembryonic membrane
epithelial sac