<P> Loss of multicellularity occurred in some groups . Fungi are predominantly multicellular, though early diverging lineages are largely unicellular (e.g. Microsporidia) and there have been numerous reversions to unicellularity across fungi (e.g. Saccharomycotina, Cryptococcus, and other yeasts). It may also have occurred in some red algae (e.g. Porphyridium), but it is possible that they are primitively unicellular . Loss of multicellularity is also considered probable in some green algae (e.g. Chlorella vulgaris and some Ulvophyceae). In other groups, generally parasites, a reduction of multicellularity occurred, in number or types of cells (e.g. the myxozoans, multicellular organisms, earlier thought to be unicellular, are probably extremely reduced cnidarians). </P> <P> Multicellular organisms, especially long - living animals, face the challenge of cancer, which occurs when cells fail to regulate their growth within the normal program of development . Changes in tissue morphology can be observed during this process . Cancer in animals (metazoans) has often been described as a loss of multicellularity . There is a discussion about the possibility of existence of cancer in other multicellular organisms or even in protozoa . For example, plant galls have been characterized as tumors but some authors argue that plants do not develop cancer . </P> <P> In some multicellular groups, which are called Weismannists, a separation between a sterile somatic cell line and a germ cell line evolved . However, Weismannist development is relatively rare (e.g. vertebrates, arthropods, Volvox), as great part of species have the capacity for somatic embryogenesis (e.g. land plants, most algae, many invertebrates). </P> <P> One hypothesis for the origin of multicellularity is that a group of function - specific cells aggregated into a slug - like mass called a grex, which moved as a multicellular unit . This is essentially what slime molds do . Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a coenocyte . A membrane would then form around each nucleus (and the cellular space and organelles occupied in the space), thereby resulting in a group of connected cells in one organism (this mechanism is observable in Drosophila). A third hypothesis is that as a unicellular organism divided, the daughter cells failed to separate, resulting in a conglomeration of identical cells in one organism, which could later develop specialized tissues . This is what plant and animal embryos do as well as colonial choanoflagellates . </P>

Do all cells in a multicellular organism have the same function