A plant stays partly embryonic its whole life; an animal never does
By the time an animal is born or hatches, it already has every body part it will ever have; from then on it simply grows and matures. A plant never finishes. Meristems at its tips keep producing new tissues and organs for life, so a living plant always carries embryonic tissue. Developmental biology studies how both strategies build bodies.
Animal embryos rely on three intertwined processes. Patterning turns a ball of look-alike cells into distinct regions, growth enlarges tissues at different rates, and morphogenesis folds and moves cell sheets into three-dimensional shapes. Patterning starts with molecules parked in one part of the fertilised egg. Cells that inherit them become signalling centres that release a factor which diffuses away and decays, forming a gradient; neighbouring cells read the local concentration and switch on different genes accordingly.
The early embryo splits rapidly without growing, so each daughter cell is half the size of its parent. Then gastrulation rearranges the cells into three sheets, the germ layers, and sets up the head-to-tail axis and structures such as the mammalian placenta. Finally cells differentiate into specialised types like neurons, muscle fibres and liver cells, each packed with a few characteristic proteins. Master switches drive the choice: NeuroD for neurons, myogenin for muscle, HNF4 for liver cells. How the whole sequence is timed is still unknown; it may follow a master clock or simply a chain of local events.
Some animals can rebuild lost parts. Hydra can regrow its entire body from a small fragment, and planarian worms usually regenerate both heads and tails, powered by stem cells that in planarians are at least partly pluripotent. Crickets regrow legs and salamanders regrow limbs, with each cell type replacing itself except connective tissues, which interconvert. Whether regeneration is an ancient general property or a trick evolved separately in each species remains debated, a question with real stakes for hopes of boosting it in humans.
Metamorphosis puts all these processes on display at once, as tadpoles lose their tails and become frogs, and fly larvae rebuild themselves from imaginal discs during the pupal stage. Plants manage shape differently because their cells mostly cannot move, so they sculpt form through uneven growth instead.
Source: Developmental biology
