Answer :
Final answer:
The yolk sac plays a crucial role in providing nutritional support and enabling blood circulation in the early embryo. By three months, a fetus comprises numerous specialized cell types, and about 50% of fertilized eggs survive to birth. By birth, the infant is well-developed, capable of hearing but with limited vision.
Explanation:
The Role of the Yolk Sac
The yolk sac serves a vital purpose during early embryonic development. It is formed by cells of the hypoblast and is primarily responsible for providing early nutrition to the embryo through absorbed nutrients from the trophoblast. Additionally, during the third week of development, it facilitates primitive blood circulation, functioning as a site for nutrient absorption and blood cell generation. As the placenta matures around week 4, the yolk sac's function diminishes, and it becomes mainly a source of blood cells and germ cells.
Developmental Stages and Functions
By three months into the pregnancy, the fetus has formed hundreds of cell types, including ectoderm, mesoderm, and endoderm cells, each having specific roles in organogenesis and system development. Regarding survival rates, it is estimated that approximately 50% of fertilized eggs will survive to birth. Unique characteristics such as fingerprints begin to develop around week 16 of gestation. After birth, many physiological changes occur as the newborn adapts to the external environment.
Sensory Capabilities of the Fetus
While in the uterus, a fetus can hear sounds, including external noises and the mother's heartbeat, which may help it recognize familiar sounds postnatally. However, visibility is limited; the developing baby cannot see clearly but can perceive light and dark depending on gestational development.
Characteristics at Birth
At birth, a typical baby measures about 20 inches (50.8 cm) long and weighs around 7.5 pounds (3.4 kg), with a body composed of trillions of cells, ready for interaction with the outside world. Major organs are developed, though still maturing, and initial adaptations are crucial to postnatal survival.
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