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Why is cleavage in placental mammals unusual compared with cleavage in other animals?

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embryos of different animals — Animal development The embryos of many animals appear similar to one another in the earliest stages of development and progress into their specialized forms in later stages.

Cleavage in placental mammals is considered peculiar compared to other animal groups due to the unique structural and functional adaptations required for viviparous development.21

Unlike many other animals where cleavage patterns are heavily influenced by the distribution of yolk, placental mammals possess tiny, yolkless (oligolecithal) eggs. Because there is no yolk to hinder cytoplasmic movement or suppress division, cleavage is holoblastic (complete) and results in blastomeres of nearly equal size.213

The process is distinct in its progression and final organization:
* Formation of the Morula and Blastocyst: Cleavage results in a solid ball of cells called a morula, which subsequently develops into a hollow blastocyst.13
* Differentiation of Cell Fates: Early in development, the cells segregate into two distinct groups: the trophoblast, which forms the fetal portion of the placenta, and the inner cell mass, which gives rise to the embryo itself.13
* Implantation: Unlike many other species, the mammalian blastocyst is specifically adapted to attach to and often sink into the uterine wall, a process known as implantation.13