key concept 43.2 Mitosis Divides Up the Early Embryo

β-Catenin plays a major role in the cell–cell signaling cascade that begins the process of cell determination and the formation of the embryo. But cell–cell signaling requires more than one cell. The single-celled zygote must become a multicellular embryo.

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  • The three main types of cleavage are complete, incomplete, and superficial.

  • Mammalian cleavage is unusual because of the requirements associated with the connection between the embryonic and maternal circulatory systems.

  • Determination can arise from informational molecules present in the egg or by information received by their environment and neighboring cells.

  • Twins can form in several ways.

  • Reproductive germ cells are determined very early in cleavage.