A fluid-filled sac- or sausagelike, extraembryonic membrane lying between the outer chorion and the inner amnion and yolk sac of the embryos of reptiles, birds, and mammals. The allantois eventually fills up the space of the extraembryonic coelom in most of these animals. It is composed of an inner layer of endoderm cells, continuous with the endoderm of the embryonic gut, or digestive tract, and an outer layer of mesoderm, continuous with the splanchnic mesoderm of the embryo. It arises as an outpouching of the ventral floor of the hindgut and dilates like a filling balloon into a large allantoic sac which spreads throughout the extraembryonic coelom. The allantois remains connected to the hindgut by a narrower allantoic stalk which runs through the umbilical cord. See also: Amnion; Germ layers

A thin, cellular, extraembryonic membrane forming a closed sac which surrounds the embryo in all reptiles, birds, and mammals and is present only in these forms; hence the collective term amniotes is applied to these animals. The amnion contains a serous fluid in which the embryo is immersed. See also: Amniota

A collective term, also referred to as Anamniota, for the vertebrate animals that lack an amnion in development, including the Agnatha, Chondrichthyes, Osteichthyes, and Amphibia. The amnion is a protective embryonic envelope that encloses the embryo and its surrounding liquid, the amniotic fluid, during fetal life. An amnion is present in mammals, birds, and reptiles, but is absent in fishes and amphibians. In early classifications, the vertebrates were commonly separated on this basis, and the expressions Amniota and Anamnia (Anamniota) are still useful in grouping higher and lower vertebrates. It should be recognized that these terms represent grades of development, however, and do not carry the connotation of established classificatory ranks. Anamnia, then, is a group name that includes the Holocene (Recent) members of the Agnatha (jawless fishes), Chondrichthyes, Osteichthyes, and Amphibia; by presumption, the class Placodermi, which is known only from fossils, should be included in the Anamnia as well. See also: Amnion; Amniota; Amphibia; Chondrichthyes; Jawless vertebrates; Osteichthyes; Placodermi; Vertebrata

The origin and development of blood vessels. Angiogenesis is the process through which new blood vessels are formed. Blood vessels (Fig. 1) are composed of two basic cell types: vascular endothelial cells and periendothelial cells (including vascular smooth muscle cells and elongated contractile cells called pericytes, both of which support the underlying endothelial cells). The inner epithelial lining of all blood vessels, adjacent to the lumen, is a single layer of endothelial cells. In larger blood vessels, such as arteries and veins, the inner endothelial lining, called the tunica intima (Fig. 2), is surrounded by a medial layer, the tunica media, composed of multiple layers of vascular smooth muscle cells embedded in an elastin-rich extracellular matrix. The tunica media layer is surrounded by an extracellular matrix–rich layer called the tunica adventitia (also referred to as the tunica externa) [Fig. 2]. In contrast, capillary walls consist of only a single layer of endothelial cells, sometimes surrounded by pericytes. See also: Blood; Blood vessel; Elastin; Vascular development

The increase in size or weight of an animal. Animal growth is an increase in an animal’s size or mass over its lifetime. Growth, which is often associated with progressive development, may extend throughout the animal's life or it may be limited to when that species is fully mature (see illustration). Various processes are involved in animal growth. These processes include hyperplasia, that is, increase in cell number due to cell proliferation (cell division or mitosis) and recruitment from stem cell populations; hypertrophy, thatis, increase in the size of cells; and differentiation (in which precursor cells achieve mature functioning; often, this process cannot be reversed). See also: Animal; Cell differentiation; Cell division; Cell senescence; Developmental biology; Embryonic differentiation; Mitosis; Morphogenesis; Stem cell

The formation of new animal individuals, which may occur by asexual or sexual means. All animals must reproduce asexually or sexually to create new individuals of their species. In asexual reproduction, a single parent can produce offspring that are genetically identical to itself. In contrast, sexual reproduction requires the participation of two individuals or parents (Fig. 1) that produce gametes: namely, sperm and eggs. Most animals use sexual reproduction as a means of maintaining the genetic diversity of their species. See also: Developmental biology; Genetics; Reproductive system; Speciation

The symmetrical disposition of organs and other constituent parts of the bodies of living animals with respect to imaginary axes. Animal symmetry relates the organization of parts in animal bodies to the geometrical design that each type suggests. The term asymmetrical applies to most sponges and some protozoa because the body lacks a definite form or geometry, and it cannot be subdivided into like portions by one or more planes. Spherical symmetry is exhibited by some protozoa, including the Heliozoia and Radiolaria. The body is spherical, with its parts concentrically around, or radiating from, a central point. Radial symmetry is exemplified by the echinoderms and most cnidarians. The body is structurally a cylinder, tall or short, having a central axis named the longitudinal, anteroposterior, or oral-aboral axis (see illustration). Any plane through this axis divides the animal into like halves. Often several planes, from the axis outward, can divide the body into a number of like portions, or antimeres, with five being most common in echinoderms. Ctenophores and many sea anemones and corals possess biradial symmetry, which is basically radial but with some parts arranged on one plane through the central axis. Most animals have bilateral, or two-sided, symmetry, in which a median or sagittal plane divides the body into equivalent right and left halves, with each being a mirror image of the other. See also: Animal; Bilateria

The formation of a segmentation cavity or blastocoele within a mass of cleaving blastomeres and rearrangement of blastomeres around this cavity in such a way as to form the type of definitive blastula characteristic of each species. The blastocoele originates as an intercellular space which sometimes arises as early as the four- or eight-cell stage. Thus blastulation is initiated during early cleavage stages, and formation of the definitive blastula is thought to terminate cleavage and to initiate gastrulation. Accordingly, cleavage and blastulation are simultaneous events which follow activation of the egg and precede the next major step in development, namely, gastrulation. Initially the diameter of the blastula is no greater than that of the activated egg; subsequently it increases. See also: Cleavage (developmental biology); Gastrulation

The process whereby cells interact and attach to other cells or to inanimate surfaces. Cell adhesion is mediated by cell surface proteins and associated macromolecules. Cell adhesion is important because it forms the physical basis of multicellularity, morphogenesis and embryonic development, tissue integrity, host–pathogen interactions (Fig. 1), immune system function, and the ecological integration of microbial communities. In addition to attaching cells to one another, cell adhesion systems act to coordinate a wide range of cellular behaviors. In animal cells, sites of adhesion serve to organize the system of intracellular protein filaments, known as the cytoskeleton, responsible for cell shape and movement. Adhesive interactions are critical for juxtacrine signaling, by which neighboring cells communicate. Juxtacrine signals regulate gene expression and cell behavior, and they determine whether a cell survives, divides, differentiates, or undergoes programmed cell death (apoptosis). See also: Apoptosis; Cell (biology); Cell biology; Cell differentiation; Cell division; Cytoskeleton; Embryogenesis; Embryonic induction; Gene; Immunology; Microbial ecology; Morphogenesis; Protein

The condition in which the entire body of an adult animal or plant consists of a fixed number of cells that is the same in all members of the species. This phenomenon is also called eutely. The largest group of animals exhibiting eutely are the nematode worms, one of the largest of all animal phyla, and of great medical and agricultural importance as parasites of plants, animals, and humans. A plant that exhibits eutely is usually called a coenobium. Many species of semimicroscopic aquatic green algae exist as coenobia, such as the common Volvox and Pandorina.