Spreading of presumptive ectoderm on underlying mass of cells is

Options

A. Epiboly
B. Invagination
C. Ingression
D. Delamination.

Answer

### Step-by-Step Solution:

1. **Understanding Cell Movement in Gastrulation**: Gastrulation is a crucial phase in embryonic development that involves various types of cell movements. There are five main types of cell movements during this process: involution, invagination, ingression, delamination, and epiboly.

2. **Identifying the Correct Movement**: The question specifically asks about the spreading of presumptive ectoderm on an underlying mass of cells. To answer this, we need to analyze the definitions of the five types of cell movements.

3. **Defining Epiboly**: Epiboly is a type of cell movement that involves the spreading of epithelial layers of cells. This is the movement that allows the presumptive ectoderm to spread over the underlying mass of cells.

4. **Examining Other Movements**:
- **Invagination**: This movement involves the folding of an area of the exterior sheet towards the inside of the blastula, which does not pertain to the spreading of ectoderm.
- **Ingression**: This refers to the formation of mesenchymal cells, which is also not related to the spreading of ectoderm.
- **Delamination**: This involves the splitting or migration of one sheet of cells into two, which again does not describe the spreading of ectoderm.

5. **Conclusion**: Since epiboly is the only movement that describes the spreading of the presumptive ectoderm on an underlying mass of cells, it is the correct answer.

### Final Answer:
The spreading of presumptive ectoderm on an underlying mass of cells is called **Epiboly**.

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