Hatching embryo Day - Here is an embryo squeezing out of its shell. Due to the thickness of this particular embryo's shell, it was most likely hatched artificially in the laboratory - this is called Assisted Hatching!
Day 5 Blastocyst - Now the embryonic cavity is full of fluid. The placental (trophoblast) cells are the ring of cells around the outside. The large ball in the center is the baby part (inner cell mass). The pressure from the fluid causes the embryo to expand like a water balloon. This causes the shell to thin. Eventually, the shell is thin enough for the embryo to "hatch" out.