The fluid-filled sphere that floats freely in the uterus for a day or two and then implants itself into the wall of the uterus is called the

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The description fits that of a blastocyst, which is a fluid-filled sphere that forms during early embryonic development and implants into the uterus wall after a day or two of floating freely. The other options, such as embryo and fetus, refer to later stages of development. Chorion, on the other hand, is a membrane that surrounds the developing embryo and helps form the placenta.

The Fluid-Filled Sphere That Floats Freely in the Uterus for a Day