Final answer:
At about 8 weeks of gestation, when neural cell differentiation begins, a developing human organism is termed an embryo until the end of the 8th week. From the 9th week until birth, it is called a fetus, and this period involves significant developments in organs and bodily functions that prepare the fetus for survival outside the womb.
Step-by-step explanation:
The term given to a developing human organism at about 8 weeks gestation, when the first neural cells start differentiating, is an embryo. This term is used up until the end of the 8th week of gestation. From the ninth week of gestation until birth, the organism is then referred to as a fetus. During this fetal period, significant development occurs, including cell growth and differentiation, which develop the structures and functions of immature organ systems formed earlier.
Neurulation, the formation of the central nervous system, begins in the third week post-fertilization and continues over approximately two weeks. Once an embryo transitions into the fetal stage, sexual differentiation starts, wherein male and female gonads differentiate, and other processes such as facial feature development and organ system maturation take place.
By the completion of the fetus' growth and development, which includes the functioning of most organs and sensory systems, the organism is prepared to survive outside the womb, resulting in a newborn.