The development of a fully functional animal body begins with the transformation of the single-celled zygote into a mul-ticellular embryo. Part of the definition of an animal (i.e., a metazoan) is that it possesses true multicellularity, which it acquires during the process of embryogenesis. True multi-cellularity is defined not only as the possession of multiple cells in the body, but also a specific division of labor among those cells. In particular, the body of an animal must have somatic (body) cells separated from the reproductive (germ) cells. But, beyond that the somatic cells must be differentiated into different functional groups. During the process of embryogenesis, the new individual first develops numerous cells, then differentiates them according to space, structure, and function. Among the lower metazoans, we find a range of structural complexity, from simple double-layered groups of cells in the phylum Placozoa to complex organ systems in such phyla as the Platyhelminthes, Nemertea, and Nemata. Many other lower metazoan phyla fall at various places along this spectrum.

The first stage of embryogenesis is cleavage, which creates the simplest form of multicellularity. Cleavage is simply a series of mitotic cell divisions that take the new individual from a unicellular zygote to a multicellular mass of cells generally known as a morula. Depending on the phylum and species, this cell mass may consist of a few dozen to hundreds of cells. The actual pattern of cleavage varies in terms of the cells' spatial relationships to one another, the plane of the cell axis at which cleavage occurs, and the degree to which the cytoplasm is divided. The subject of cleavage is complex, but the type of cleavage is important in defining a group's evolutionary relationships to other groups. There is also great variation in the degree of cellular differentiation at this stage. In most lower metazoans, however, the cells do differentiate during cleavage into large macromeres and small micromeres; some groups also have intermediate-sized mesomeres. In most cases, these classes of cells are destined to become specific layers within the later embryo. In turn, each layer will give rise to certain tissues of the adult body. Prior to forming layers, most embryos go through a stage of minor reorganization known as blastulation, which provides the spatial framework in which the actual layering takes place.

The vast majority of animals develop either two or three layers of cells, known as germ layers. These germ layers develop out of the dramatic reorganization of the cells that were formed during cleavage and slightly reorganized during blas-tulation. The stage of radical reorganization is called gastru-lation. Possession of two or fewer germ layers is found only in certain lower metazoan phyla. A very few simple phyla (e.g., the Placozoa, Monoblastozoa, Orthonectida, and Rhombo-zoa) do not have specific germ layers. Sponges (phylum Porifera) are often interpreted as having no germ layers, but some biologists regard them as having two. Members of the phyla Cnidaria (jellyfishes, corals, etc.) and Ctenophora posses two well-developed germ layers, and thus are described as diploblastic. All other lower metazoans, and indeed all other animal phyla, possess three well-developed germ layers, and are correspondingly described as triploblastic. The actual mechanisms of layering vary widely among different phyla; they range from the migration of cells into the interior of the cell mass to the infolding of an entire hemisphere of the spherical blastula. In all cases, however, the embryo is left with an endoderm layer on the inside and an ectoderm layer on the outside. In triploblastic phyla, a layer of mesoderm forms between the other two. The endoderm typically develops into the digestive system of the adult animal, while the ectoderm develops into the epidermis and nervous system. The mesoderm, if one is present, gives rise to such structures as excretory systems and the tissues that line body cavities. Other organ systems develop from various layers, depending on the phylum, and most often involve contributions from two germ layers working cooperatively.

At the end of gastrulation the embryo is fully formed; embryogenesis is complete. The new organism now looks a bit like an animal with a skin and a gut; all the basic layers are present, ready to differentiate further into a more definitive animal having the recognizable characteristics of its taxo-nomic group.

Most higher animals, above the platyhelminthes, can be divided into two groups based primarily on embryonic features. These two major branches are known as the proto-stomes and the deuterostomes. Protostomes undergo determinate cleavage or mosaic development, in contrast to the indeterminate cleavage or regulative development of deuterostomes. The determinate cleavage of protostomes results from a plane of cell division, usually visible after the second division, that cuts diagonally across the original zygote axis, thus compartmentalizing different regulative and nutritive chemicals in each of the resulting cells. This is referred to as spiral cleavage, since the cells dividing diagonally appear under the microscope to spiral around the original axis. In contrast, the indeterminate cleavage of deuterostomes results from a planes of cell division that cut alternatively longitudinally along the zygote axis, then transversely across the axis, thus leaving each resulting tier of cells with similar regulative and nutritive chemicals. This is referred to as radial cleavage, since the cells dividing at alternating parallel and right angles to the original axis appear under the microscope to radiate in parallel planes from that axis. The most important thing is not whether the resulting cell masses appear to spiral or to radiate, but that only the spiraling cells of the protostomes show determination of specific germ layers as early as the first cell division, and almost universally by the third. Thus, at the very earliest stages of cleavages, specific cells of protostomes have already been determined to a fate of forming a specific one of the three germ layers. During gastrulation, the embryo is left with an opening to the outside called the blastopore, which will develop into an opening into the gut in the adult animal. The precise nature of the opening is the second major feature differentiating deuterostomes from proto-stomes. In protostomes, the blastopore becomes the adult mouth, whereas in deuterostomes, the blastopore becomes the anus. Shortly or immediately after gastrulation is complete, higher animals form their body cavity, the coelom. By definition, the true coelom is always a body cavity within meso-dermal tissue. The mechanism by which the coelom is formed is the third primary distinction between protostomes and deuterostomes. In most deuterostomes, the coelom forms by outpocketing from the original archenteron, a process known as enterocoely since the coelomic cavities are thus derived directly from embryonic enteric cavities. In protostomes, the coelom forms from a split in the previously solid mass of mesodermal cells, a process thus known as schizocoely. There are some exceptions to this rule, but it applies well to most.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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