Growth and Development

Development is an ordered sequence of irreversible steps, each step setting up the conditions for the next, and the sequence runs gamete formation, fertilisation, cleavage, gastrulation, organogenesis, growth. Fertilisation fuses the gametes to a single zygote; cleavage divides that zygote into blastomeres without the embryo growing; gastrulation lays down the germ layers; organogenesis builds the organs from those layers as the cells interact and differentiate; growth then enlarges the organs until the adult body form is attained. The last stage is the one students skip on paper, because a question on "growth" reads as plant material, when in this sequence it is the enlargement that closes organogenesis.

Regeneration is the separate process of rebuilding a part that has been lost or damaged, and the range across animals is the point of it. Sponges replace almost any lost piece, though the process is slow and may take months or years. A lobster grows a new pincer claw, a starfish regrows arms, an earthworm regrows a lost head, and salamander limbs are regenerated throughout life, more rapidly when the animal is young and small. The lizard is the sharpest case: it discards its tail, and the tail regrows from features the tail itself carries. Healing a fracture and repairing a skin wound are the same process at small scale. In plants regeneration is the basis of propagation, because a piece of stem, a leaf, or even a single tissue cell can develop into a full plant. That last claim is totipotency, and when the papers ask why plants are totipotent they key both halves of the answer: each cell holds the full genetic potential of the organism, and a single cell could become a complete plant.