Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers. Animals differ from both plants and fungi in their mode of nutrition. Animals cannot construct all of their own organic molecules and so, in most cases, they ingest them either by eating other living organisms or by eating nonliving organic material. Most animals do not feed by absorption; instead, animals ingest their food and then use enzymes to digest it within their bodies. Animals are eukaryotes, and like plants and most fungi, animals are multicellular. Animal cells are held together by structural proteins, the most abundant being collagen. Most animals reproduce sexually, and the diploid stage usually dominates the life cycle. In most species, a small, flagellated sperm fertilizes a larger, nonmotile egg, forming a diploid zygote.