 The spaces around the embryo change dramatically over the course of development. These spaces are quite hard to visualize with only 2D images. So in this video we will look at the amniotic cavity and the yolk sac in 3 dimensions, specifically how they both change shape over time. We will start at day 14 post conception. The embryo at this stage consists mainly of a bi-laminar disc, two layers known as epiblast and hypoblast. The epiblast develops an internal cavity which is the amniotic cavity. To avoid over complicating things the yolk sac is a space that sits beneath the hypoblast. The parts of the embryo in between will be represented by this simple blue object here. As time goes on these cavities expand. Each element grows slightly longer in the longitudinal direction than in the transverse. This is obvious because we are all longer than we are wide, at least at birth. Around the middle of the third week of development the amniotic sac begins to fold downward like so. It sort of looks like a mouth enveloping these parts of the embryo and some of the yolk sac. It then starts to tighten around the neck of the yolk sac constricting it like so. This part is now known as the yolk stalk. This part below is now the yolk sac. And this, this is now the gut, what will become the gastrointestinal tract. We divide it at this point into hind gut, mid gut and fore gut. This must be the head part of the embryo and this the tail. The amniotic cavity continues to expand around the embryo until it is finally burst at the breaking of the waters usually just prior to delivery. The yolk sac typically degenerates around the seventh week of development. And the yolk stalk is to become part of the connection between embryo and mother, the umbilical cord. Click the link below to have a look around this model yourself. Hit the like button if you found this video useful. Thanks for watching and we will see you next time.