In a process called cavitation, the trophoblast cells secrete fluid into the morula to create a blastocoel, the fluid-filled cavity. At this stage there is no cavity within the morula the embryo is still a ball of dividing cells. The external, surrounding cells develop into the trophoblast cells. The inner cell mass will go on to become the actual embryo. 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 morula is a solid ball of cells that has a small group of internal cells surrounded by a larger group of external cells. In mammals Īfter 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. In the Xenopus a very small cavity has been described in the two-cell stage of development. It is the first cell cavity formed as the embryo enlarges, and is the essential precursor for the differentiated gastrula. It develops following cleavage of the zygote after fertilization. At this stage in mammals the blastula develops into the blastocyst containing an inner cell mass, and outer trophectoderm. The blastocoel ( / ˈ b l æ s t ə ˌ s iː l/), 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.
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