What stage of development refers to the early phase of any organism?

The term "embryo" refers to the early stage of development for many organisms, including humans. During this critical phase, the fertilized egg begins to divide and develop into a complex structure that will eventually form the various systems and organs of the organism. This stage is crucial in the overall developmental process, as it lays the foundation for growth and differentiation into specialized tissues.

In contrast, emphysema is a chronic lung disease that predominantly affects adults and is not related to early development. Epiphysis pertains to the end part of a long bone, specifically in adults after growth has ceased, and is not connected to the initial developmental stages. Empyema describes a collection of pus in a body cavity, often due to infection, and does not pertain to an organism's developmental phase either. Thus, "embryo" is the only term that accurately describes the early phase of development across various species.