abnormal visceral yolk sac morphology small visceral yolk sac 2013-10-08T15:46:33Z MP:0012131 MPheno.ontology csmith 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 small secondary yolk sac small umbilical vesicle small vitelline sac small yolk sac