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