MALFORMATION

in biology, irregular or abnormal structural development. Malformations occur in both plants and animals and have a number of causes. The processes of development are regulated in such a way that few malformed organisms are found. Those that do appear may, when properly studied, shed light on normal development. The science of teratologya branch of morphology or embryologyis concerned with the study of these structural deviations from the normal, whether in animals or plants. In general, abnormalities can be traced to deviations from the normal course of development, often in very early embryonic stages. Such deviations may be caused by abnormal (mutant) genes, by environmental conditions, by infection, by drugs, and, perhaps most frequently, by interactions between these sets of causes. A general interpretation has been that one factor in many cases is reduction of the rate of development, the kind and degree of deformity depending upon the stage at which the retardation occurs. This interpretation is supported by the results of descriptive studies of anomalies, and especially by evidence from experimental teratology. L.C. Dunn Additional reading Malformations in the plant world are the topic of George N. Agrios, Plant Pathology, 3rd ed. (1988), a college-level text that presents the effect of pathogens on host growth and functioning in chapters 35; R. Heitefuss and P.H. Williams (eds.), Physiological Plant Pathology (1976); P.G. Ayres (ed.), Effects of Disease on the Physiology of the Growing Plant (1981), a compilation of seminar papers; William F. Bennett (ed.), Nutrient Deficiencies & Toxicities in Crop Plants (1993); R.D. Durbin (ed.), Toxins in Plant Disease (1981), the role of microbial toxins on plant growth; John A. Bailey and B.J. Deverall (eds.), The Dynamics of Host Defense (1983), which includes an account of the responses of plants to disease-causing organisms, pp. 3368; and James G. Horsfall and Ellis B. Cowling (eds.), Plant Disease, vol. 3, How Plants Suffer from Disease (1978), with a thorough discussion of the alteration of plant growth as a result of disease, pp. 183200. Animal malformations including human malformations are discussed in Birth Defects Original Article Series (monthly), authoritative articles on congenital malformations and abnormalities; Mary Louise Buyse (ed.), Birth Defects Encyclopedia: The Comprehensive, Systematic, Illustrated Reference Source for the Diagnosis, Delineation, Etiology, Biodynamics, Occurrence, Prevention, and Treatment of Human Anomalies of Clinical Relevance (1990); Congenital Malformations Worldwide: A Report from the International Clearinghouse for Birth Defects Monitoring Systems (1991); Adam S. Wilkins, Genetic Analysis of Animal Development, 2nd ed. (1993); Karl Theiler, The House Mouse: Atlas of Embryonic Development (1972, reissued with changes, 1989); and Uta B. Schambra, Jean M. Lauder, and Jerry Silver, Atlas of the Prenatal Mouse Brain (1992).