Fix the mistake: Animal Development
Cleavage and Blastula Formation: The Beginning of Development
A fertilized egg undergoes repeated cell divisions (cleavage) and becomes a hollow sphere called a blastula. Cleavage has characteristics different from ordinary somatic cell division.
Basic Knowledge
Cleavage: Cell division of a fertilized egg. It is ordinary mitosis, but because it is not accompanied by cell growth (the G1 and G2 phases are short), the cells become smaller with each division.
The pattern of cleavage is determined by the distribution of yolk (nutrients):
・Holoblastic cleavage: Little yolk distributed uniformly → sea urchins, mammals, and others.
・Unequal cleavage: Yolk is concentrated at the vegetal pole → frogs (amphibians). The blastomeres on the animal-pole side are small, while those on the vegetal-pole side are large.
・Meroblastic cleavage: There is a very large amount of yolk, so only the blastodisc on the animal-pole side divides → fish and birds (yolk-dependent type).
Stages of development:
① Morula: The stage at which many small blastomeres gather in the shape of a mulberry.
② Blastula (blastula): A hollow sphere. The internal space is called the blastocoel (blastocoel).
③ Gastrula: The stage at which part of the blastula invaginates and an archenteron forms inside.
Comparison of sea urchins and frogs:
Sea urchin = isolecithal egg and equal cleavage; frog = telolecithal egg and unequal cleavage (large blastomeres at the vegetal pole).
Answer: (1) A frog's egg is telolecithal, with yolk concentrated at the vegetal pole. Because cell division is slow on the yolk-rich vegetal-pole side and the blastomeres become large, unequal cleavage occurs. (2) In a frog blastula, the blastomeres on the yolk-poor animal-pole side are small, and the blastocoel is displaced toward the animal-pole side. A space is more likely to form on the animal-hemisphere side than in the space surrounded by the large blastomeres on the vegetal-pole side.
- Cleavage = somatic cell division but without cell growth → blastomeres become smaller
- Equal cleavage = little, uniformly distributed yolk (sea urchins and mammals)
- Unequal cleavage = abundant, unevenly distributed yolk (frogs) → large blastomeres at the vegetal pole
- Meroblastic cleavage = extremely abundant yolk (birds and fish)
- Blastula = hollow sphere with a blastocoel
- Frog blastocoel = displaced toward the animal pole
Practice
- State how cleavage differs from ordinary somatic cell division.
- Explain why bird eggs undergo meroblastic cleavage.
- Explain what the blastocoel is.
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