Final answer:
The embryonic stage with a fluid-filled cavity and surrounding cell layers is called the blastocyst stage. Its inner cell mass contains totipotent cells capable of differentiating into any cell type in the body.
Step-by-step explanation:
Embryonic Cell Stages and Totipotency
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 stage happens approximately 3 days after fertilization when the conceptus, called a morula, reaches the uterus and starts to secrete fluid, forming the blastocoel.
The inner cell mass of this conceptus is composed of totipotent cells during the early stages of development. These totipotent cells have the remarkable ability to differentiate into any cell type necessary for developing the organism. This stage leads onto subsequent stages of development where cells start to specialize and form the endoderm, mesoderm, and ectoderm in a process called gastrulation, eventually developing into the various tissues and organs.