The developmental stage of an embryo in which a single layer of cells is surrounded by a space filled with fluid is called the blastula stage. During this stage, the embryo undergoes a process called blastulation, where the zygote undergoes several rounds of cell division, resulting in the formation of a hollow, fluid-filled structure called a blastocyst. The blastocyst consists of an inner cell mass (ICM), which will eventually develop into the embryo, and an outer layer of cells known as the trophoblast, which plays a crucial role in implantation and formation of the placenta. The blastula stage is an essential step in early embryonic development, preceding the formation of germ layers and organogenesis.
