Final answer:
During the embryonic stage, major body organs form, including the central nervous system, heart, and initial parts of the digestive and respiratory systems.
Step-by-step explanation:
During the embryonic stage, the major body organs are formed, which is a process called organogenesis. This developmental phase includes the formation of the central nervous system through neurulation, the rudimentary heart and the beginnings of the circulatory system, and the initial development of the pulmonary system. Following gastrulation and embryonic folding, the embryo establishes the three primary germ layers: endoderm, mesoderm, and ectoderm, all of which contribute to the formation of various organs and tissues. For instance, the endoderm gives rise to the lining of the digestive system and respiratory tract, while the mesoderm will form structures including the circulatory system.
The embryonic liver starts producing red blood cells temporarily before this function is taken over by the bone marrow during fetal development. Meanwhile, the early cardiovascular system starts operating when the heart begins beating around the fourth week, although full blood pumping commences later. By the end of the embryonic stage, essential organ systems are outlined, preparing the embryo for the subsequent fetal development phase where these systems will mature and become functional.