Final answer:
Primary urine is formed in Bowman's capsule by ultrafiltration.
Step-by-step explanation:
The primary urine is formed in Bowman's capsule by ultrafiltration. Bowman's capsule, which surrounds the glomerulus, filters blood based on particle size to produce a filtrate that lacks cells and large proteins. This filtrate, known as primary urine, is then modified through secretion and reabsorption as it moves through the nephron tubules.