Yolk sac

From Freepedia

The yolk sac is the first element seen in the gestational sac during pregnancy, usually at 5 weeks gestation. It is a critical landmark, identifying a true gestation sac. It is quite echogenic (light) to ultrasound, and reliably seen early. In the mouse, the yolk sac is the first site of blood formation, generating primitive macrophages and erythrocytes.