Animal Development and Growth

A fertilized egg looks uniform but isn't. The mother deposits mRNA and proteins unevenly in the egg cytoplasm before fertilization even happens. In frogs, a maternal mRNA sits concentrated in the vegetal half of the egg, and sperm entry triggers the cortex, the outer layer of cytoplasm, to rotate roughly 30 degrees. That rotation shifts the determinants further, creating a gradient that switches on different genes in different regions of the embryo. Every cell has the same genome, but cells in different positions read different instructions, and that is what sets the head-to-tail and back-to-belly axes before a single cell division driven by the zygote's own genes has occurred.

Once the embryo has divided into a hollow ball of cells, the blastula, it has to become something with a gut and distinct tissue types. Gastrulation does this: one region of the blastula buckles inward, a process called invagination, pulling cells into the interior and forming a tube called the archenteron. That inward fold becomes endoderm, which gives rise to the gut lining and the glands that bud off it, such as the liver and pancreas in vertebrates. Cells left on the outside become ectoderm, the future skin and nervous system, and a third layer, mesoderm, forms between them and builds muscle, bone, and blood.

Growth after gastrulation is not always a smooth scale-up. Many animals pass through a larval stage built for a different job than the adult: a caterpillar eats, a butterfly disperses and reproduces. Larvae grow in increments, often multiplying mass at each molt rather than adding fixed amounts. Metamorphosis then rebuilds the body suddenly, so a plotted growth curve shows a sharp step or even a brief dip at pupation, not a smooth line.