In developmental biology, an embryo refers to an organism in its earliest, highly formative stages of development following fertilization. This period encompasses a series of critical events, beginning with cleavage, which involves rapid cell division, and advancing through gastrulation, where three distinct germ layers—ectoderm, mesoderm, and endoderm—are established. The formation and patterning of these germ layers are responsible for the subsequent morphogenesis and differentiation that lead to the establishment of the basic body plan. These early developmental processes are fundamentally driven by intricate biochemical signaling cascades and gene expression patterns, culminating in the initial formation of all major organ systems and tissues.