by Müller) and instantly realized the importance of connecting the two phenomena. The resemblance was confirmed without delay by both observers. In further experiments, Schwann examined notochordal tissue and cartilage from toad larvae, as well as tissues from pig embryos, establishing that animal tissues are composed of cells, each of which has a nucleus. Schwann published his observations in 1838 in the Neue notisen geb. nat.-heilk. This was followed in 1839 by the publication of his book Mikroskopische Untersuchungen über die Uebereinstimmung in der Struktur und dem Wachsthum der Thiere und Pflanzen (Microscopic investigations on the similarity of structure and growth of animals and plants). It is considered a landmark work, foundational to modern biology. In it Schwann declared that "All living things are composed of cells and cell products". He drew three further conclusions about cells, which formed his cell theory or cell doctrine. The first two were correct: The cell is the unit of structure, physiology, and organization in living things. The cell retains a dual existence as a distinct entity and a building block in the construction of organisms. By the 1860s, these tenets were the accepted basis of cell theory, used to describe the elementary anatomical composition of plants and animals. Schwann's theory and observations created a foundation for modern histology. Schwann claimed that "there is one universal principle of development for the elementary parts of organisms, however different, and this principle is the formation of cells." Schwann supported this claim by examining adult animal tissues and showing that all tissues could be classified in terms of five types of highly differentiated cellular tissues. cells that are independent and separate, e.g. blood cells cells that are independent but compacted together in layers, e.g. skin, fingernails, feathers cells whose connecting walls have coalesced, e.g. cartilage, bones, and tooth enamel elongated cells forming fibers, e.g. tendons and ligaments cells formed by the fusion of walls and cavities, e.g. muscles, tendons and nerves His observation that the single-celled ovum eventually becomes a complete organism, established one of the basic principles of embryology. Schwann's third tenet, speculating on the formation of cells, was later disproven. Schwann hypothesized that living cells formed in ways similar to the formation of crystals. Biologists would eventually accept the view of pathologist Rudolf Virchow, who popularized the maxim Omnis cellula e cellula—that every cell arises from another cell—in 1857. The epigram was originally put forth