abnormal visceral yolk sac morphology small visceral yolk sac MPheno.ontology small secondary yolk sac reduced size of the extraembryonic tissue membrane, formed from the visceral endoderm and the extraembryonic mesoderm, which is located ventral to the embryonic disc and is connected to the presumptive midgut of the embryo MP:0012131 2013-10-08T15:46:33Z small umbilical vesicle csmith small vitelline sac small yolk sac