Introduction:
MSH, or melanocyte-stimulating hormone, is a hormone that plays a role in regulating pigmentation in the skin. It is secreted by the pituitary gland, which is a small gland located at the base of the brain. The pituitary gland is divided into two main parts: the anterior pituitary and the posterior pituitary. In order to determine which part of the pituitary gland secretes MSH, we need to examine the functions and hormones produced by each part.

Anterior Pituitary:
The anterior pituitary, also known as the adenohypophysis, is responsible for producing and releasing several hormones that regulate various bodily functions. Some of the hormones secreted by the anterior pituitary include growth hormone, prolactin, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, and luteinizing hormone. MSH, however, is not secreted by the anterior pituitary.

Posterior Pituitary:
The posterior pituitary, also known as the neurohypophysis, does not actually produce any hormones itself. Instead, it stores and releases two hormones that are produced by the hypothalamus: oxytocin and vasopressin (also known as antidiuretic hormone). MSH is not secreted by the posterior pituitary either.

Middle Lobe of Pituitary:
The middle lobe of the pituitary gland, also known as the intermediate lobe, is present in some animals but is not well-developed in humans. It does not play a significant role in hormone secretion in humans, and therefore does not secrete MSH.

Conclusion:
Based on the information provided, MSH is not secreted by any part of the pituitary gland. Instead, it is produced and released by another structure in the brain called the hypothalamus, specifically by the pars intermedia region of the hypothalamus. The hypothalamus produces a precursor molecule called pro-opiomelanocortin (POMC), which is then cleaved to produce MSH among other hormones. MSH then acts on melanocytes in the skin to regulate pigmentation.

B. Middle lobe of pituitary