Developmental biology is the study of the processes by which organisms grow and develop. It expanded on classical embryology by using modern molecular techniques and genetic approaches. Some key areas of study include differentiation, morphogenesis, growth, reproduction, evolution, and environmental influences. Early debates centered around whether embryonic structures were preformed in the egg (preformationism) or arose de novo through epigenesis. Landmark studies in the 1800s by scientists like Pander, von Baer, and Rathke established germ layer formation and embryonic development principles. Lineage tracing techniques using dyes were later developed to track cell lineages in living embryos of organisms like tunicates, amphibians, zebrafish, and frogs.