Final answer:
By week eight, the embryo measures about 20 mm (0.79 in.) in length and undergoes considerable development, such as limb and facial formation, brain structure development, and the onset of ossification. The head is nearly as large as the rest of the embryo's body, and by the end of this period, the embryo measures about 3 cm (1.2 in) from crown to rump and weighs approximately 8 g (0.25 oz).
Step-by-step explanation:
During the transition from embryo to fetus around 8 weeks, the embryo is quite developed in its size and complexity. By this time, the organism measures about 20 mm (0.79 in.) in length, having undergone significant development over the preceding weeks. For example, from week five to week eight, the embryo grows from a length of approximately 4 mm to 20 mm. During this period, the embryo begins to exhibit more human-like features, like forming arms and legs, developing facial structures such as nostrils and outer ears, and initiating the ossification process where bone begins to replace cartilage.
Significant developments also include the emergence of major brain structures, the disappearance of the embryonic 'tail,' and the beginnings of distinguishable external genitalia by the eighth week. The embryo's head grows to nearly match the size of the rest of its body and the gastrointestinal system sees rapid development, with intestines looping temporarily into the umbilical cord due to space constraints. By the conclusion of the eighth week, just before it is termed a fetus, the embryo measures approximately 3 cm (1.2 in) from crown to rump and weighs about 8 g (0.25 oz).