OCR GCSE Biology Practice Exam

An embryo is classified as a fetus when significant developmental milestones are reached, particularly the formation of critical structures. The transition from embryo to fetus primarily occurs after the formation of physical and functional structures that distinguish the later stages of development. When muscle and skin cells begin to form, it signifies that the embryo has progressed into a more advanced stage of development, reflective of the development of organ systems and the general readiness for the subsequent growth phases associated with fetal development. The process of fetal development follows the embryonic stage, which encompasses the initial formations that occur after fertilization, including the development of the heart and early organ systems. While the heartbeat and certain cellular formations are crucial steps in overall development, it is primarily the formation of the muscle and skin cells that allows for the classification of the developing organism as a fetus. This change marks an important transition in the developmental timeline, highlighting the complexity and advancement of the organism.