Final answer:
The period from beginning day 15 to approximately the eighth week after conception is known as the embryonic period. This stage is crucial for the development of the embryo, where major organs and body structures begin to form.
Step-by-step explanation:
The period from beginning day 15 through approximately the eighth week after conception is called the embryonic period. During this time, a developing human is referred to as an embryo and experiences significant development. This encompasses cell division, migration, and differentiation. By the end of the embryonic period, all organ systems are laid out in a rudimentary form. Although the organs are not fully functional yet, structures such as the heart begin to beat, and basic body parts start developing from the primary areas that will form the head, chest, and abdomen.
During the pre-implantation embryonic development, which is part of the germinal stage before the embryonic period, the zygote undergoes several mitotic cell divisions without increasing the total volume of the conceptus. These divisions continue until the structure known as a blastocyst implants into the uterine wall. The gestational time frame for the embryonic period is a crucial part of human development, as it sets the stage for the functions and health of the organism.