Primordia

Primordium (plural primordia) is a structure or region of embryonic tissue that is incompletely differentiated at early development but will form an organ, tissue, or body part. The term derives from Latin primordium, meaning "first beginning" or "first formed."

In plants, primordia appear at the shoot apical meristem. Leaf primordia are small bulges that will become leaves as the stem elongates; floral primordia arise in the meristem and develop into flowers and their organs. The arrangement of primordia on the shoot apex influences phyllotaxis and organ patterning.

In animals, primordia (often called anlagen) are rudimentary structures that will differentiate into limbs, organs, or neural tissue; examples include limb primordia and neural primordia in early embryogenesis.

In fungi and other organisms, primordia can refer to early aggregations of cells that develop into fruiting bodies or other mature structures.

Significance: the concept provides a framework for describing developmental timing and tissue specification. It is used in comparative embryology, botany, and mycology to map how early cellular organization translates into mature anatomy.

Notes: the plural primordia is standard in scientific usage; primordium is the singular form, and primordiums is less common.