Which of the following statements regarding fertilization and implantation is incorrect ?

Options

A. Fertilization can occur only if sperm and ovum simultaneously reach the ampullary isthmic junction of the fallopian tube .
B. During fertilization , the sperm comes in contact with the corona radiata cells of the ovum first and start degrading it.
C. The division of the morula leads to the formation of blastocyst.
D. The embryo is formed by the differentiation of inner cell mass.

Answer

To determine which statement regarding fertilization and implantation is incorrect, we will analyze each option provided:

1. **Fertilization occurs only if sperm and ovum simultaneously reach the ampullary isthmic junction of the fallopian tube.**
- This statement is **correct**. The ampullary-isthmic junction is indeed the preferred site for fertilization, where the sperm and ovum meet.

2. **During fertilization, the sperm comes in contact with the corona radiata cells of the ovum first and starts degrading it.**
- This statement is **incorrect**. While the sperm does encounter the corona radiata, it does not start degrading it first. Instead, the sperm first penetrates the zona pellucida, which is the layer beneath the corona radiata. The degradation process primarily occurs at the zona pellucida, allowing the sperm to reach the ovum.

3. **The division of morula leads to the formation of blastocyst.**
- This statement is **correct**. The morula, which is a solid ball of cells formed after several divisions of the zygote, eventually develops into a blastocyst as it continues to divide and differentiate.

4. **The embryo is formed by the differentiation of the inner cell mass.**
- This statement is **correct**. The inner cell mass of the blastocyst is what eventually develops into the embryo, while the outer layer contributes to the placenta.

After analyzing all the statements, the incorrect statement is option number 2.

### Summary of the Solution:
- The incorrect statement regarding fertilization and implantation is:
**"During fertilization, the sperm comes in contact with the corona radiata cells of the ovum first and starts degrading it."**