MSH of pars intermedia of middle pituitary is responsible for

Options

A. darkening of skin in lower vertabrates
B. light colouration of skin in lower vertebrates
C. Both a and b
D. darkening of skin in human beings

Explanation

### Step-by-Step Solution:

1. **Identify the Hormone and Its Source**:
- The question refers to MSH, which stands for Melanocyte-Stimulating Hormone. This hormone is produced in the pars intermedia of the middle pituitary gland.

2. **Understand the Function of MSH**:
- MSH primarily acts on melanocytes, which are cells responsible for the production of melanin, the pigment that gives color to the skin, hair, and eyes.

3. **Determine the Role of MSH in Pigmentation**:
- The main function of MSH is to regulate skin pigmentation. It stimulates melanocytes to produce more melanin, leading to darker skin coloration.

4. **Consider the Context of Lower Vertebrates**:
- In lower vertebrates (like fishes and amphibians), MSH is crucial for adapting skin color to their environment, allowing them to camouflage and protect themselves from predators.

5. **Evaluate the Options Given in the Question**:
- The options provided include:
1. Darkening of the skin in lower vertebrates.
2. Light coloration of skin in lower vertebrates.
3. Both A and B.
4. Darkening of the skin in human beings.
- Based on the function of MSH, the correct answer is that it is responsible for the darkening of the skin in lower vertebrates.

6. **Conclude the Answer**:
- Therefore, the correct option is option number 1: "Darkening of the skin in lower vertebrates."