3. Which kind of cleavage is shown in mammals?
   (1) Holoblastic rotational
   (2) Meroblastic rotational
   (3) Holobastic radial
   (4) Meroblastic radial

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Introduction

Embryonic cleavage is the initial phase of rapid cell division in the fertilized egg, leading to the formation of a multicellular embryo. The cleavage pattern in mammals is distinct from many other animals due to the small amount of yolk and the complex developmental requirements of mammals. Mammalian embryos undergo holoblastic rotational cleavage, a specialized type of cleavage that plays a key role in early embryogenesis.

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Features of Holoblastic Rotational Cleavage in Mammals

• Holoblastic cleavage means the entire egg is divided during cleavage. Unlike eggs with large yolk where cleavage is partial, mammals have small, evenly distributed yolk allowing complete division.

• The rotational cleavage pattern is characterized by a unique orientation of cleavage planes during the second division:

  • The first cleavage is meridional, splitting the zygote into two blastomeres along the polar axis.

  • During the second cleavage, one blastomere divides meridionally, while the other divides equatorially (rotated 90 degrees relative to the first).

• Subsequent divisions are asynchronous, often resulting in embryos with odd numbers of cells before the blastocyst stage.

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Biological Importance

• This cleavage pattern facilitates the formation of the inner cell mass and trophoblast, crucial for implantation and placenta formation.

• Rotational cleavage supports the early segregation of embryonic and extraembryonic lineages.

• The asynchronous cleavage tempo allows for proper gene activation and protein synthesis necessary for mammalian development.

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Comparison with Other Cleavage Types

• Meroblastic cleavage (options 2 and 4) occurs in species with large yolk (e.g., birds), where only part of the egg divides.

• Holoblastic radial cleavage (option 3) is typical in amphibians and echinoderms, featuring cleavage planes at right angles but lacking the rotational aspect seen in mammals.

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Conclusion

Mammalian embryonic cleavage is distinguished by its holoblastic and rotational nature, ensuring complete division of the egg and a unique second cleavage orientation. This specialized cleavage pattern underpins mammals’ complex developmental trajectory from zygote to blastocyst.

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This article provides a detailed understanding of mammalian cleavage, an essential foundation for studying reproductive and developmental biology.

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Final Answer:
(1) Holoblastic rotational