Final answer:
The placenta serves as the biochemical barrier allowing substances to pass to the fetus and blocking harmful agents. It connects the mother to the fetus, transferring nutrients and oxygen, while filtering waste products. The answer is option A.
Step-by-step explanation:
In the context of the development of the conceptus, the placenta serves as a biochemical barrier, allowing dissolved substances to pass to the fetus but blocking some kinds of viruses and bacteria from passing into the fetal circulatory system. The placenta develops throughout the embryonic period and during the first several weeks of the fetal period, providing nutrition, excretion, respiration, and endocrine functions between the mother and the fetus.
The placenta forms a connection between a developing embryo (and later the fetus) and the mother, functioning as a conduit for the transfer of nutrients, oxygen, antibodies, hormones, and other necessary substances, while also filtering out waste products for excretion. It allows certain substances to move across by simple or facilitated diffusion, and some, like amino acids and iron, which the fetus has a high demand for, are moved by active transport.