Which event occurs around day 15 after fertilization?

• A. Fertilization
• B. Implantation
• C. Limb buds form
• D. Heartbeat begins

Answer: (D)

The timing of early developmental events is laid out in a precise sequence as the embryo moves from fertilization to organ formation. Implantation occurs about a week after fertilization as the blastocyst adheres to the uterine lining. By around day 15, the embryo is in the phase of gastrulation, where the three germ layers are forming and the foundations for organ systems are being established. The heart, as one of the first organ systems to function, starts as a simple tube that forms a little after this early tissue organization and begins to beat a bit later in the first month of development, typically around the end of the third week (roughly day 22–23 after fertilization). Limb buds form much later, in the fourth week to early fifth week. Fertilization itself happens at day 0. So among the options, the onset of heartbeat reflects the start of functional organ activity within the early organogenesis window, even though the exact day is a bit later than day 15.