Multiple choice In the course of development, the blastocoele becomes __________. gestational sac intraembryonic coelom septum primum primary yolk sac chorionic cavity Reveal answer Fill a bubble to check yourself D Correct answer Explanation Around the 9th day, cells from the hypoblast spread around the blastocoele, forming the exocoelomic or Heuser's membrane. When the blastocoele is surrounded by this membrane, it is referred to as the primary yolk sac or exocoelomic cavity.