Final answer:
The chemical messenger responsible for stimulating the production of hydrochloric acid in the stomach is gastrin, which is secreted during the gastric phase when food reaches the stomach.
Step-by-step explanation:
The chemical messenger that stimulates the production of hydrochloric acid is gastrin. Gastrin is a hormone that is secreted by G cells in the stomach's gastric glands in response to certain stimuli, such as the presence of proteins in food entering the stomach. This hormone plays a crucial role in the digestive process as it signals the parietal cells to secrete hydrochloric acid, which aids in the digestion of proteins. The gastric phase, which involves the actions of gastrin, begins when food reaches the stomach, as this is when the protein-induced secretion of gastrin is initiated.