USA Biology Olympiad (USABO) Practice Exam

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What type of cleavage do protostomes exhibit during early development?

  1. Radial and indeterminate

  2. Spiral and indeterminate

  3. Spiral and determinate

  4. Radial and determinate

The correct answer is: Spiral and determinate

Protostomes exhibit spiral and determinate cleavage during early development. In spiral cleavage, the cells divide at an angle to one another, resulting in a spiral arrangement of cells. This type of cleavage is characteristic of protostomes, which are often classified as having a more complex developmental program compared to other groups. Additionally, determinate cleavage means that the fate of each cell is fixed early in development. This contrasts with indeterminate cleavage, where the fate of the cells can change based on the developmental context, allowing for the potential for identical twins in some organisms. In protostomes, the developmental pathway of each cell is predetermined, which contributes to the specialized functions of cells that will form various tissues and organs. This distinction is crucial for understanding the overall development process in protostomes and how their early embryonic development influences later stages of formation and development.

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