Final answer:
The embryonic development period spans from implantation until the end of the eighth week after fertilization, during which the organism is known as an embryo and significant developmental changes occur.
Step-by-step explanation:
The embryonic development period lasts from the time of implantation until the end of the eighth week after fertilization. During this period, the developing organism is known as an embryo. Significant growth and changes occur during the embryonic stage, including the formation of three germ layers, development of the nervous system, and the initial formation of organs.
The embryonic stage is a critical phase of development where all of the organ systems begin to take rudimentary form, setting the groundwork for all subsequent stages of life. After the embryonic stage, the developing organism is referred to as a fetus, starting from the ninth week of gestation until birth.