On the 5th day some very significant changes take place inside the embryo. It begins to form a fluid-filled cavity in between the cells. In the microscope, you can now distinguish between two different kinds of cells.The cells located along the edge of the cavity, close to the zona pellucida, are the cells, which later will form the placenta. They are called trophoblast cells. The cells of the becoming fetus (arrow) consist of a small lump of cells located in the middle. The embryo is now called a blastocyst.The trophoblast cells continue to pump fluid into the cavity. The pressure inside the cavity increases, and therefore extends the blastocyst itself. It grows in size and the zona pellucida becomes thinner and thinner. The blastocyst is ready to be transferred back into the womb.