Here's why:
* Cleavage: After fertilization, the zygote undergoes rapid cell divisions called cleavage. These divisions are characterized by increased cell number but no increase in overall size.
* Morula Formation: As cleavage continues, the resulting cells form a compact ball known as the morula. While the cells in the morula are still undifferentiated (they haven't specialized yet), they are already starting to establish positional relationships with each other.
* Blastocyst Formation: The morula later develops into a blastocyst, a hollow sphere with an inner cell mass (ICM) and an outer layer called the trophoblast. The ICM is the source of the embryo proper, while the trophoblast forms the placenta.
* Differentiation Begins: While the morula doesn't represent full differentiation, it's a crucial step in the process. The cells in the morula are already exhibiting spatial organization and setting the stage for future specialization.
Therefore, the morula is considered the first visible evidence of differentiation in an embryo, even though the cells are not yet fully specialized. This early stage of development lays the groundwork for the complex processes of cell differentiation that will continue throughout embryonic development.
