#embryonic development

• Agenetic — Relating to agenesis; characterized by the congenital absence of an organ, tissue, or body part, caused by the failure of its development during embryonic life. In a clinical sense, it describes an organism, anatomical segment, or process affected by this severe structural development anomaly.
• Agenesis — The congenital absence of an organ, tissue, or body part, caused by the failure of its development during embryonic life. Unlike aplasia, where the embryonic bud exists but fails to fully mature, in this severe structural anomaly the initial blueprint of the organ is completely missing from the genetic profile.
• Agonadism — A highly rare congenital medical condition characterized by the complete structural and functional absence of gonads (testes or ovaries), caused by an anomaly in early embryonic development; it results in a lack of sex hormones and requires life-long hormone replacement therapy.
• Amniogenesis — The biological process of formation, differentiation, and structural development of the amnion and the amniotic cavity during the early stages of embryonic growth. This phenomenon is essential for ensuring the protective fluid environment required for the subsequent growth of the fetus in amniote vertebrates.
• Anamniote — A lower vertebrate animal (such as fish or amphibians) whose embryo develops directly in the aquatic environment and lacks complex protective embryonic membranes, specifically the amnion and allantois. As an adjective, it describes a species, embryonic stage, or anatomical structure specific to these non-amniotic vertebrate organisms, which entirely depend on the presence of water for reproduction and egg survival.
• Aplacental — Lacking a placenta; a mammal or organism that does not develop a placenta during the embryo's gestation period, with reproduction occurring via eggs or through fetal development inside an external marsupial pouch (e.g., monotremes and marsupials).
• Arthromere — An anatomical segment or primitive development zone within an embryonic limb, directly involved in the formation of future joints and adjacent skeletal structures. In evolutionary biology and morphology, the term describes a repetitive structural unit or metamere of the limbs, acting as a genetic organizing center that coordinates the proper alignment of bones and synovial capsules during organogenesis.