The word "胚胎" is formed by combining two characters, each constructed with a semantic-phonetic structure. "胚" uses the radical "月" (meaning flesh or body) for semantics and "丕" for phonetics, indicating an early developmental stage. "胎" similarly uses "月" for semantics and "台" for phonetics, referring to a fetus. Together, they create a compound word where both characters contribute related meanings to specify the concept of an embryo.