Modular organisms are organisms in which the zygote develops into a discrete unit which then produces more units like itself, rather than developing into a complete (or "unitary") organism. Modular organisms (e.g. plants, fungi, sponges, etc.) usually have a branching structure and an overall shape that is highly variable and determined mainly by environmental influences. Mutations of cells along branches may lead to heterogeneity between different parts of the organism.