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Jan 6
Patterns of cleavage refer to the way in which the zygote (fertilized egg) undergoes cell division during early embryonic development. Cleavage patterns can vary among different organisms. In the case of frogs and chicks, the patterns of cleavage and blastulation are as follows:
1. Cleavage in Frogs:
- Frogs exhibit holoblastic cleavage, which means that the entire zygote undergoes cell division.
- The first cleavage occurs along the animal-vegetal axis, resulting in two equal-sized blastomeres.
- The second cleavage is also equal and perpendicular to the first, resulting in four blastomeres.
- Subsequent cleavages continue to divide the blastomeres, resulting in a solid ball of cells called a morula.
- The morula then undergoes blastulation, where a fluid-filled cavity called the blastocoel forms in the center, resulting in a hollow ball of cells called a blastula.
2. Cleavage in Chicks:
- Chicks exhibit meroblastic cleavage, which means that only a portion of the zygote undergoes cell division.
- The yolk present in the chick egg restricts the cleavage to a small disc of cells called the blastodisc, located on top of the yolk.
- The first cleavage occurs along the animal-vegetal axis, dividing the blastodisc into two unequal-sized blastomeres.
- The second cleavage is also unequal, resulting in four blastomeres.
- Subsequent cleavages continue to divide the blastomeres, but due to the presence of the yolk, the cells do not separate completely.
- Blastulation in chicks is different from frogs. Instead of forming a blastula, the blastodisc undergoes gastrulation directly, forming the three germ layers (ectoderm, mesoderm, and endoderm) and the primitive streak.
It is important to note that while the general patterns described above are common, there can be variations in cleavage and blastulation among different species of frogs and chicks.
1. Cleavage in Frogs:
- Frogs exhibit holoblastic cleavage, which means that the entire zygote undergoes cell division.
- The first cleavage occurs along the animal-vegetal axis, resulting in two equal-sized blastomeres.
- The second cleavage is also equal and perpendicular to the first, resulting in four blastomeres.
- Subsequent cleavages continue to divide the blastomeres, resulting in a solid ball of cells called a morula.
- The morula then undergoes blastulation, where a fluid-filled cavity called the blastocoel forms in the center, resulting in a hollow ball of cells called a blastula.
2. Cleavage in Chicks:
- Chicks exhibit meroblastic cleavage, which means that only a portion of the zygote undergoes cell division.
- The yolk present in the chick egg restricts the cleavage to a small disc of cells called the blastodisc, located on top of the yolk.
- The first cleavage occurs along the animal-vegetal axis, dividing the blastodisc into two unequal-sized blastomeres.
- The second cleavage is also unequal, resulting in four blastomeres.
- Subsequent cleavages continue to divide the blastomeres, but due to the presence of the yolk, the cells do not separate completely.
- Blastulation in chicks is different from frogs. Instead of forming a blastula, the blastodisc undergoes gastrulation directly, forming the three germ layers (ectoderm, mesoderm, and endoderm) and the primitive streak.
It is important to note that while the general patterns described above are common, there can be variations in cleavage and blastulation among different species of frogs and chicks.
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