In a foetus, the insulin secretion begins by

Insulin is a peptide hormone, produced by beta cells of the pancreas, and is central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, skeletal muscles, and fat tissue to absorb glucose from the blood. In the liver and skeletal muscles, glucose is stored as glycogen, and in fat cells (adipocytes) it is stored as triglycerides. In mammals, insulin is synthesized in the pancreas within the b-cells of the islets of Langerhans. One million to three million islets of Langerhans (pancreatic islets) form the endocrine pa of the pancreas, which is primarily an exocrine gland. The endocrine poion accounts for only 2% of the total mass of the pancreas. Within the islets of Langerhans, beta cells constitute 65-80% of all the cellsRef: Ganong&;s review of medical physiology; 24th edition; page no-432