Final answer:
The substage of prophase in meiosis from which the egg is released monthly is called the primary oocyte.
Step-by-step explanation:
During meiosis, the substage of prophase from which the egg is released monthly is called the primary oocyte.
This stage occurs just prior to ovulation, when a surge of luteinizing hormone triggers the resumption of meiosis in the primary oocyte, leading to the formation of a secondary oocyte and a smaller cell called the first polar body. The secondary oocyte, which is much larger, eventually leaves the ovary during ovulation.