Parietal and visceral yolk sac. (A) Necromys in early pregnancy (MAV/CEMAS 04). Masson's Trichrome. The parietal yolk sac (PYS) was one layered and associated with a well-developed Reichert's membrane (arrowhead). (B) Necromys in early pregnancy (MAV/CEMAS 05). HE. Adjacent to the labyrinth and its maternal blood channels (MBC), trophoblast giant cells (arrow) were present. (C) Cerradomys in mid gestation (MZUSP/APC 1177-2). HE. The visceral yolk sac (VYS) was villous and supplied by vitelline blood vessels (arrows). It was close to the placental disc and the very thin parietal yolk sac that covered it (arrowheads). (D) Cerradomys in mid gestation (MZUSP/APC 1177-2). Masson's Trichrome. Positively reacting endodermal cells that were partly binucleate.