The blastocoel (), also spelled blastocoele and blastocele, and also called cleavage cavity, or segmentation cavity is a fluid-filled or yolk-filled cavity that forms in the blastula during very early embryonic development. At this stage in mammals the blastula develops into the blastocyst containing an inner cell mass, and outer trophectoderm. It develops following cleavage of the zygote after fertilization. It is the first cell cavity formed as the embryo enlarges, and is the essential precursor for the differentiated gastrula. In the Xenopus a very small cavity has been described in the two-cell stage of development. In mammals After fertilization, the zygote undergoes mitotic divisions during rotational cleavage resulting in daughter cells known as blastomeres until they are at the 16-cell stage called the morula. The morula is a solid ball of cells that has a small group of internal cells surrounded by a larger group of external cells. Following further cleavages and compaction the blastomeres undergo cellular differentiation that gives rise to an internal group of cells called the inner cell mass, (also embryoblast) and an external layer of surrounding cells known as trophoblasts. The inner cell mass will go on to become the actual embryo. The external, surrounding cells develop into the trophoblast cells. At this stage there is no cavity within the morula; the embryo is still a ball of dividing cells. In a process called cavitation, the trophoblast cells secrete fluid into the morula to create a blastocoel, the fluid-filled cavity. The membranes of the trophoblast cells contain sodium (Na+) pumps, Na+/K+- ATPase and Na+/H+ exchangers, that pump sodium into the centrally forming cavity. The accumulation of sodium pulls in water osmotically, creating and enlarging the blastocoel. The oviduct cells stimulate these trophoblast sodium pumps as the fertilized egg travels down the fallopian tube towards the uterus. As the embryo further divides, the blastocoel expands and the inner cell mass is positioned on one side of the trophoblast cells forming a mammalian blastula, called a blastocyst. In amphibians An amphibian embryo in the 128- cell stage is considered a blastula as the blastocoel in the embryo becomes apparent during this stage. The fluid-filled cavity forms in the animal hemisphere of the frog. However, the early formation of the blastocoel has been traced back to the very first cleavage furrow. It was demonstrated in the frog embryo that the first cleavage furrow widens in the animal hemisphere