definitiveという言葉は発生学においては、特別な意味合いで使われます。ChatGPTに説明してもらいましょう：
In embryology, the term “definitive” is used to describe structures or stages that are mature, final, or fully developed. This contrasts with earlier, transient, or intermediate forms that occur during development. Here are some examples of how “definitive” is used in this context:
- Definitive Yolk Sac: The final form of the yolk sac, as opposed to the primitive yolk sac that appears earlier in development.
- Definitive Blood Circulation: The mature circulatory system that is established after the temporary, embryonic circulatory systems have been replaced.
- Definitive Structures: Refers to the adult forms of tissues, organs, or body parts that have completed their development.
In summary, “definitive” in embryology signifies the end stage of a developmental process, when the structure has reached its final, functional form.