The fluid in the fertilized egg, the cytoplasm, is uniform and there is only a small amount of yolk: it is called an isolecithal egg. After fertilization, the vitelline membrane that surrounds the egg undergoes the cortical reaction. In this reaction, the membrane is pushed away from the egg and is now called the fertilization membrane. This keeps other sperm from being able to penetrate the egg.
The egg then divides into two cells, called blastomeres. This division is equal holoblastic: the egg divides into two halves by splitting through the center. The second division is at 90 degrees to the first, forming four blastomeres. The third division is horizontal, forming eight blastomeres.
Eight cells divide to form 16, 16 cells then divide to form 32 cells. At this stage, the cells are packed into a tight ball that resembles mulberries, which gives it the name morula.
To form the blastula, the cells of the morula get pushed out to the periphery, and a central cavity forms. This is the blastocoel. At this stage, the cells roughly resemble a donut, with the donut hole being the blastocoel.
Next, the blastula flattens at one end. These cells continue to push inward into the blastocoel, forming the gastrocoel, also called the archenteron. To visualize this, picture pushing your fist into a half-inflated balloon. The area that your fist takes up is the archenteron. The opening in the archenteron is called the blastopore, which eventually becomes the anus. This stage is called the gastrula. Throughout this process, there has been no growth phase of the dividing cells, so the gastrula is roughly the size of the original egg.
The anterior end of the archenteron pinches off and forms the body cavity. The mouth forms from a segment of the body wall and becomes continuous with the archenteron. Bands of cilia form. The embryo is now a free-swimming bipinnaria larva. The larva will attach to a rock or muddy bottom and undergo metamorphosis into a juvenile starfish.