Final answer:
The blastocyst stage of early embryonic development is characterized by a blastocoele, which is a fluid-filled cavity within the blastocyst, and it forms a few days after fertilization.
Step-by-step explanation:
The early embryonic stage characterized by a fluid-filled cavity with an inner cell mass surrounded by a layer of cells is known as the blastocyst stage. This occurs after the blastulation process, where a solid ball of cells, called the morula, changes into a fluid-filled ball of cells known as the blastocyst. The blastocyst contains a feature called the blastocoele, which is the fluid-filled cavity within it. This stage typically develops a few days after fertilization and is a key part of the germinal stage of embryo development.