Final answer:
Major organogenesis occurs during the embryonic stage, specifically from the 2nd to the 10th week of pregnancy, with the heart being one of the first functioning organs to develop.
Step-by-step explanation:
The period of major organogenesis, which is when the internal organs and body structures begin to develop, occurs during the embryonic stage of prenatal development. This stage is specifically from the 2nd to the 10th week of pregnancy. By the end of this period, all of the organ systems are structured in rudimentary form, even if the organs are not yet fully functional.
Organogenesis begins during the third week after fertilization and continues through the end of the embryonic stage. The heart is one of the first functional organs to develop and starts beating by the end of the third week. However, organ development together with cell division, migration, and differentiation characterizes the entire phase of embryonic development.