Step 1: After fertilization, the zygote undergoes cleavage to form a solid ball of cells called the morula.
Step 2: This morula transforms into a hollow ball of cells known as the blastula (or blastocyst in humans).
Step 3: The central, fluid-filled internal cavity that forms within the blastula is the blastocoel. It serves to separate the inner cell mass from the outer trophoblast in mammals.