The progressive division of cells during embryonic growth is called cleavage, it is not accompanied by any increase in the embryo's overall size.The first cleavage occurs in human zygote about 30 hours after fertilization. It is holoblastic (complete) and gives rise to the blastomeres-one of which is slightly larger than the other.

Unequal holoblastic because yolk sac is present in mammalian egg in small amount and unequal.

Cleavage in Mammalian Egg

Holoblastic Cleavage

- Holoblastic cleavage results in the complete division of the fertilized egg into smaller cells, resulting in the formation of a compact ball of cells known as the morula.
- In mammals, holoblastic cleavage is unequal, meaning that the cells divide at different rates and produce cells of different sizes.

Unequal Holoblastic Cleavage

- In mammals, the first cell division is symmetrical, resulting in two equally sized cells.
- However, subsequent cell divisions are asymmetrical, resulting in daughter cells of different sizes.
- This is due to the presence of yolk in the mammalian egg, which is unevenly distributed.
- The smaller cells resulting from the earlier, faster cell divisions are known as blastomeres, while the larger, slower-dividing cells are known as macromeres.

Mammalian Embryonic Development

- After the morula stage, the mammalian embryo undergoes further development, including the formation of the blastocyst, which contains an inner cell mass that ultimately gives rise to the embryo proper.
- The blastocyst also contains a layer of cells known as the trophectoderm, which forms the outer cell layer of the placenta.

Conclusion

- In summary, cleavage in mammalian eggs is unequal holoblastic cleavage, resulting in daughter cells of different sizes due to the uneven distribution of yolk.
- This process ultimately gives rise to the blastocyst, which is a key developmental stage in mammalian embryonic development.