ANIMAL DEVELOPMENT

the processes that lead eventually to the formation of a new animal starting from cells derived from one or more parent individuals. Development thus occurs following the process by which a new generation of organisms is produced by the parent generation. Additional reading General textbooks covering animal development include J. Brachet and H. Alexandre, Introduction to Molecular Embryology, 2nd totally rev. and enlarged ed. (1986); and Gerald M. Edelman, Topobiology: An Introduction to Molecular Embryology (1988). Hans Spemann, Embryonic Development and Induction (1938, reprinted 1988; originally published in German, 1936), is a classic exposition of the experimental method in embryology. Additional useful works are Robert Wall, This Side Up: Spatial Determination in the Early Development of Animals (1990); D.R. Johnson, The Genetics of the Skeleton: Animal Models of Skeletal Development (1986); John Phillip Trinkaus, Cells into Organs: The Forces That Shape the Embryo, 2nd ed. (1984); Elizabeth S. Watts (ed.), Nonhuman Primate Models for Human Growth and Development (1985), which compares both physical and behavioral growth among the primates, including humans; Matthew H. Kaufman, The Atlas of Mouse Development (1992); Claudio D. Stern and Phil W. Ingham (eds.), Gastrulation (1992); Brian K. Hall, The Neural Crest (1988); Brigid Hogan et al., Manipulating the Mouse Embryo: A Laboratory Manual, 2nd ed. (1994); and the work by Wilkins, cited above in the section on animal malformations. Organ formation Primary organ rudiments Immediately after gastrulationand sometimes even while gastrulation is underwaythe germinal layers begin subdividing into regions that will give rise to various parts of the body. Subdivision proceeds in stages: initially a mass of cells is set aside for an organ system (for the alimentary canal, for instance) and subsequently further subdivided into the rudiments of various parts of the organ system, such as the liver, stomach, and intestines. The initially formed larger units are referred to as primary organ rudiments; those they later give rise to, as secondary organ rudiments. Differentiation of the germinal layers The type of organ rudiment produced depends on the organization of the body in any particular group in the animal kingdom. In the vertebrates the earliest subdivision within a germinal layer is the segregation within the chordamesodermal mantle of the rudiment of the notochord from the rest of the mesoderm. During gastrulation the material of the notochord comes to lie middorsally in the roof of the archenteron. It separates by longitudinal crevices from the chordamesodermal mantle lying to the left and right. The material of the notochord then rounds off and becomes a rod-shaped strand of cells immediately under the dorsal ectoderm, stretching from the blastopore toward the anterior end of the embryo, to the midbrain level. In front of the tip of the notochord, there remains a thin sheet of prechordal mesoderm. The mesodermal layer adjoining the notochord becomes thickened and, by transverse crevices, subdivided into sections called somites. The somites, which later give rise to the segmented body muscles and the vertebral column, are the basis of the segmented organization typical of vertebrates (seen especially in the lower fishlike forms but also in the embryos of higher vertebrates). The lateral and ventral mesoderm, which remains unsegmented, is called the lateral plate. The somites remain connected to the lateral plate by stalks of somites that play a particular role in the development of the excretory (nephric) system in vertebrates; for this reason they are called nephrotomes. Rather early the mesodermal mantle splits into two layers, the outer parietal (somatic) layer and the inner visceral (splanchnic) layer, separated by a narrow cavity that will expand later to form the coelomic, or secondary, body cavity. The coelomic cavity extends initially through the nephrotomes into the somites; in the somites it is eventually obliterated. Endoderm completely surrounds the lumen of the archenteron (when present) and produces the cavity of the alimentary canal. If no archenteric cavity is formed during gastrulation, the cavity of the alimentary canal is formed by the separation of cells in the middle of the mass of endoderm (as in bony fishes) or by folding of the sheet of endoderm. The endodermal gut sooner or later acquires an extended anterior part called the foregut and a narrower and more elongated posterior part, the hindgut. Characteristic of chordates is the development of the nervous system from a part of ectoderm lying originally on the dorsal side of the embryo, above the notochord and the somites. This part of the ectodermal layer thickens and becomes the neural plate, whose edges rise as neural folds that converge toward the midline, fuse together, and form the neural tube. In vertebrates the neural tube lies immediately above the notochord and extends beyond its anterior tip. The neural tube is the rudiment of the brain and spinal cord; its lumen gives rise to the cavities, or ventricles, of the brain and to the central canal of the spinal cord. The remainder of the ectoderm closes over the neural tube and becomes, in the main, the covering layer (epithelium) of the animal's skin (epidermis). As the neural tube detaches itself from the overlying ectoderm, groups of cells pinch off and form the neural crest, which plays an important role in the development of, among other things, the segmental nerves of the brain and spinal cord. In developing the primary organ rudiments mentioned above, the embryo acquires a definite organization clearly recognizable as that of a chordate animal. Similar processes, which occur in the development of other animals, establish the basic organization of an annelid, a mollusk, or an arthropod.