A bundle of 70-100, mostly undifferentiated human cells is known as a

• A. Zygote
• B. Blastocyst
• C. Morula
• D. Embryo

Answer: (B)

During early development, cells divide without growing, forming stages that move from a solid ball to a hollow structure. When you have about 70–100 cells, the structure is a blastocyst, which includes a fluid-filled cavity and begins to show differentiation into the inner cell mass (which will become the embryo) and the surrounding trophoblast (which helps form the placenta). This distinguishes it from the morula, a solid cluster of 16–32 cells with no cavity, and from the zygote, a single cell. The term embryo refers to the developing organism after implantation, later in development. So a bundle of roughly 70–100 mostly undifferentiated cells is best called a blastocyst.