The purpose of this book is twofold: it is meant to serve both as a practical manual for the study of animal development and as a general introduction to the subject. Central to our en deavour is the belief that developmental biology is best taught and learnt at the laboratory bench, with specimens which are either alive and can be seen to develop or with fresh material derived directly from the egg (as in birds) or mother (as in mammals). Once the dynamic nature of development is appreci ated and the overall structure of the developing organism discerned the more conventional study of sections and whole mounts is more likely to become a delight rather than a diffi cult, and often meaningless, chore. We have laid considerable stress on the early development of animal embryos and the ways in which they can be obtained from a relatively few, but reliable, sources. In addition, emphasis has been placed on fairly simple experiments which make use of the embryos and larvae chosen for the purpose of illustrating develop ment. Embryology ceased to be a descriptive science at the beginning of this century and any practical course, at what ever level, should attempt to reflect this change. It is true that the analysis of development, particularly the genesis of chor date structure, owed much to the invention of the microtome."

A bioaerosol is a colloidal suspension of liquid droplets or solid particles in air whose components contain or have attached to them one or more microorganisms. Bioaerosols are an exciting and vital object of study because the attached microbes play a critical role in human, animal and environmental health. In an era of genetically engineered microorganisms and the application of biopesticides, bioaerosols are increasingly an environmental problem, both indoors and outdoors, and can affect entire ecosystems. Atmospheric Microbial Aerosols examines naturally occuring bioaerosols, as well as bioaerosols generated by human activity. Included in this volume is a complete array of topics concerned with outdoor microbial bioaerosols ranging from the physical and chemical to the meteorological and microbial. It will be of great interest as a starting point for researchers interested in outdoor microbial bioaerosols as well as for those interested in atmospheric dispersion models, new equipment, and government regulations.

Antarctic Ecosystems comprises 55 papers presented at the Fifth Symposium on Antarctic Biology held under the auspices of the Scientific Committee on Antarctic Research (SCAR) in Hobart, Australia, 29 August - 3 September, 1988. Both short- and long-term changes in ecosystems and community structures caused by natural and human factors were discussed to help understand the ecological processes taking place in a changing environment. The variability of ecological factors must be known for the development of realistic monitoring strategies and sound conservation practices.

Yom 17. bis 21. September 1962 fand in der Vogelwarte Garmiseh- Partenkirehen ein Symposium fiber die Fragen der Orientierung der Tiere statt. Die Anregung zu diesem Treffen ging von Professor HENRI PIERON, Paris, aus. Die Analyse der Orientierung der Tiere im Raum und in der Zeit beginnt mit der - keineswegs aueh nur annahernd vollstandige- phanomenologisehen Bestandsaufnahme. Allein diese Bestandsaufnahme staBt auf erhebliehe Sehwierigkeiten, die nur mit kritisehen experimen- tellen und einwandfreien statistisehen Methoden fiberwunden werden konnen. Ein weiterer Sehritt ist die Bestimmung der sinnesphysio- logisehen Leistungen, die an den Orientierungsvorgangen beteiligt sind. Die Kenntnis der Phanomene und der reeeptorischen V oraussetzungen der Orientierung stellen aber erst den Anfang dar. In erster Linie sind es zentralnervose Vorgange, die eine Orientierung ermogliehen. Hier spielt die zentrale Verreehnung der sinnesphysiologisehen Daten, hier spielen Regelvorgange und es spielen - bisher kaum bekannt, noeh weniger analysiert - autonome zentralnervose Prozesse eine ent- seheidende Rolle. Daher sind Sinnesphysiologie, Verhaltensforsehung, Neurophysiologie, Kybernetik, Biophysik und mathematisehe Statistik in gleieher Weise an der Erforsehung der Orientierungsleistungen be- teiligt. Diese Arbeitsriehtungen zusammenzubringen und Anregungen auszutausehen, war eine der Hauptaufgaben des Symposiums. Die Phanomene und Probleme der Orientierung in der Zeit wurden ausgeklammert. Sie sind Gegenstand eines Cold Spring Harbor-Sym- posiums im Jahre 1960 gewesen. Ihre Behandlung hatte den Rahmen gesprengt und die fUr eine fruehtbare Diskussion notwendigerweise kleine Teilnehmerzahl allzu stark vermehrt.

Cholesterol is essential for normal cellular function. It is found in all cells of the body and is a major constituent of cell membranes. In addition, certain specialized tissues use cholesterol as the basic substrate for the synthesis of more complicated sterols such as bile acids and the steroid hormones. Like most lipids cholesterol has a very low solubility in water. Therefore, in order for these molecules to be transported be tween different organs of the body a series of lipid carriers known as lipoproteins has evolved. Over the last several years epidemiological evidence has accumulated suggest ing a strong relationship between the risk of atherosclerosis and high plasma concen trations of one of these lipoproteins, low density lipoprotein (LDL) 1). Thus, in popula tions with high plasma concentrations of LDL-cholesterol a correspondingly high incidence of vascular disease is seen. Since it has been demonstrated that reducing the steady-state plasma LDL-cholesterol concentration produces a proportional decrease in the risk of developing atherosclerosis 2), a major effort is now underway to develop safe and effective lipid lowering agents. In the past, these efforts have been hampered by a lack of knowledge concerning the physiological regulation of the plasma LDL-cholesterol concentration. However, recently there have been several major advances in these areas and it is now possible to precisely determine those factors that have the greatest effect on either increasing or decreasing the plasma LDL-cholesterol concentration."

There has been a tremendous increase in interest in the neuropathogenicity of viruses during the past decade as we have come to recognize that the human immunodeficiency virus, which causes the acquired immunodeficiency syndrome (AIDS), can infect glial cells and cause neurological disease. Yet this increase has not been limited to AIDS but has extended to viruses that infect either or both the central and peripheral nervous systems. The changes examined here include both neurological and psychological diseases or syndromes. Moreover, the chapters in this volume review the interaction of the host immune system with the viruses examined and how such interactions may increase or decrease the neuropatho genicity of the viruses. Questions regarding viral neuropathogenesis include: (I) What is the mode of transmission of virus to the nervous system? (2) What types of cells are infected, and do they contain receptors for the virus? (3) What is the extent of damage that results from viral infection? (4) What are the immunologic mecha nisms by which damage is mediated or limited? Many of these questions remain unanswered, but this volume delves into efforts to provide some answers.

Desert invertebrates live in an environment where resources alternate unpredictably between brief periods of plenty and prolonged scarcity. This book describes the adaptive strategies of desert invertebrates in acquiring energy and sustaining life with such fluctuations. Some cooperate in foraging; others compete for resources. Some are nomadic and migrate to more favorable sites as conditions change. Others conserve energy by going into a deep dormancy until better conditions return. Still others store food during plentiful periods so as to retreat underground during less favorable times. The adaptive modes of economizing on scarce energy resources are diverse and lead to an appreciation of the intricate interactions of animals living close to their environmental limits.

Ls book is an account of what physical chemistry h . . to say about the structural, electrical and transport properties of biological membranes and their simplest model-the lipid bilayer. The accent throughout is on basic ideas. In contrast to the essentially descriptive ap proach characteristic of texts on membrane biochemistry, our underlying themes are the role of force and entropy in maintaining membrane organization, in determining the electric fields and ionic environment of membranes, and in regulating the passage of molecules and ions across membranes. Although experimental findings will always be the touch stone against which theory will be tried, no attempt is made to present an exhaustive survey of experimental data. On the other hand, there is discussion of the nature and limitations of the results obtainable by the major laboratory techniques. The treatment is at the level of an advanced undergraduate course or an introductory survey suitable for post graduate students carrying out research in biochemistry, biophysics, or physiology. The mathematical demands on the reader are trivial. The few forbidding equations appearing in Chapter 7 are soon whittled away to simple practical expressions. Although the current-voltage characteristics of nerves are traditionally the province of biophysics rather than physical chemistry, certain aspects relevant to the electrical activity of nerves are nevertheless included in this text, namely, mem brane and diffusion potentials and conductivity fluctuations. Where rival theories exist, conflicting convictions have been presented, but not necessarily accorded equal approbation. The author has a viewpoint."

While diseases of free-ranging and captive mammalian and avian wildlife species have received considerable interest in the past 25 years, those of amphibians and reptiles (collectively, the herptiles) generally have been assigned lesser importance. The literature concern ing disease in herptiles is widely scattered, consisting chiefly of case reports and prevalence surveys, and with heavy emphasis on captive reptiles. The dynamics of the host-agent-environment relationship have been studied for only a few diseases. This diverse data base is primarily a function of the paucity of investigators whose chief interest is in diseases of herptiles. This first edition represents an effort to bring together some of the diffuse knowledge on infectious and non-infectious diseases of free-ranging and captive herp tiles. Issue may be taken with the choice of topics; how ever, predominant diseases, as well as some diseases of lesser prominence, are presented. The editors were forced to accept certain omissions, particularly with amphibian diseases, simply for lack of contributors. The resulting text, however, we hope will be of value to veterinarians, herpetolo, gists, wildlife disease investigators, wildlife managers, zoo curators, and university students. G.L. Hoff F.L. Frye E.R. Jacobson v CONTENTS Mycobacteriosis 1 D.G. Brownstein Pasteurella in Reptiles 25 K.P. Snipes Pseudomonas 37 E.R. Jacobson Areomonas 49 E.B. Shotts, Jr."

acids. The achievements of molecular biology testify to the success of material science in a realm which, until recently, appeared totally enig matic and mysterious. Further scientific developments should bring to mankind vast developments both in theoretical knowledge and in practical applications, namely, in agriculture, medicine, and technology. The purpose of this book is to explain molecular biophysics to all who might wish to learn about it, to biologists, to physicists, to chemists. This book contains descriptive sections, as well as sections devoted to rigorous mathematical treatment of a number of problems, some of which have been studied by the author and his collaborators. These sections may be omitted during a first reading. Each chapter has a selected bibliography. This book is far from an exhaustive treatise on molecular biophysics. It deals principally with questions related to the structures and functions of proteins and nucleic acids. M. V. Vol'kenshtein Leningrad, September, 1964 CONTENTS Chapter 1 Physics and Biology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Physics and Life. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Molecular Physics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Molecular Biophysics ................................... 9 Thermodynamics and Biology. . . . . . . . . . . . . . . . . . . . . 12 . . . . . . . Information Theory. . . . . . . . . . . . . . . . . . . . . . . . . . . 19 . . . . . . . . . . Chapter 2 Cells, Viruses, and Heredity. . . . . . . . . . . . . . . . . . . . . . . . 27 . . . . . . . . The Living Cell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 . . . . . . . . . . . Cell Division. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 . . . . . . . . . . . . Viruses and Bacteriophages ........... . . . . . . . . . . . . . . . 44 . . . . Basic Laws of Genetics . . . . . . . . . . . . . . . . . . . . . . . . . 50 . . . . . . . . . Mutations and Mutability ........, ................. " . . . . 60 Genetics of Bacteria and Phages " . . . . . . . . . . . . . . . . . . 66 . . . . . . Chapter 3 Biological Molecules. . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 . . . . . . . . . . Amino Acids and Proteins . . . . . . . . . . . . . . . . . . . . . . . 79 . . . . . . . . Asymmetry of Biological Molecules ....................... 87 Primary Structure of Proteins ............................ 94 Nucleic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 . . . . . . . . . . . . Some Biochemical Processes in the Cell. . . . . . . . . . . . . . . . 109 . . . . Chapter 4 Physics of Macromolecules. . . . . . . . . . . . . . . . . . . . . . . . 123 . . . . . . . . ."

The use of biotechnical processes in control of environmental pollution and in haz ardous waste treatment is viewed as an advantageous alternative or adduct to phys ical chemical treatment technologies. Yet, the development and implementation of both conventional and advanced biotechnologies in predictable and efficacious field applications suffer from numerous technical, regulatory, and societal uncertainties. With the application of modern molecular biology and genetic engineering, there is clear potential for biotechnical developments that will lead to breakthroughs in controlled and optimized hazardous waste treatment for in situ and unit process use. There is, however, great concern that the development of these technologies may be needlessly hindered in their applications and that the fundamental research base may not be able to sustain continued technology development. Some of these issues have been discussed in a fragmented fashion within the research and development community. A basic research agenda has been established to promote a sustainable cross-disciplinary technology base. This agenda includes developing new and improved strains for biodegradation, improving bioanalytical methods to measure strain and biodegradation performance, and providing an in tegrated environmental and reactor systems analysis approach for process control and optimization."

When phagocytes are exposed to a number of different stimuli, they undergo dra matic changes in the way they process oxygen. Oxygen uptake increases markedly, frequently more than 50-fold; the phagocytes begin to produce large quantities of superoxide and hydrogen peroxide; and they immediately begin to metabolize large amounts of glucose by way of the hexose monophosphate shunt. This series of changes has become known as the respiratory burst. It was first believed that the major function of this respiratory burst was to generate powerful antibacterial agents by the partial reduction of oxygen. It is becoming apparent that the respiratory burst has much wider application, and its physiological function in many different biolog ical areas is clear. In this volume, we have attempted to bring together the work of experts who have published extensively on the involvement of the respiratory burst in different physiological functions. In the first three chapters, Dr. Borregaard and Dr. Berton and co-workers and Dr. Roos and co-workers bring together what is known about the respiratory burst. They present up-to-date versions of the biochemical and metabolic activities associ ated with the burst. In Chapter 4, Dr. Styrt and Dr. Klempner discuss the respiratory burst as it affects cellular ion homeostasis. Dr. Cohen and Dr. Britigan (Chapter 5) present some interesting data on the competition between the respiratory burst and bacteria for oxygen. Dr. Dobrina and Dr."

Yury Verlinskyand Anver Kuliev Reproductive Genetics Institute, Illinois Masonic Medical Center, 836 W. Wellington chicago, IL 60657 Although introduction of a first trimester prenatal diagnosis by chorionic viIIus sampling (CVS) has considerably improved the possibility for prevention of genetic diseases, it requires a selective abortion in case of an affected fetus. Following the direction of an earlier prenatal diagnosis and to avoid the need for abortion, preimplantation genetic diagnosis has been initiated based on polar body removal and pre-embryo biopsy. The First International symposium on Preimplantation Genetics, Chicago, September 17-19, 1990, was organized to explore these important developments, to review the state of knowledge in the field, and to address existing problems to be solved for developing and improving current approaches for preimplantation diagnosis of genetic disorders. A growing interest in the subject was obvious from the wide attendance of the meeting: over 250 scientists from 19 countries participated. This was the first attempt to put together the advances in different areas of basic and applied research relevant to Preimplantation Genetic Diagnosis, with the multidisciplinary scientific program including the sessions on embryology, micromanipulation and biopsy, genetic analysis of gametes and pre-embryos, IVF, gene expression and gene therapy, and ethical and legal issues. The deliberations of the Symposium presented in the above mentioned sessions, which comprise the contents of correspond ing sections of the Proceedings, open a newarea in medical research based on the interaction of IVF and New Genetics."

3 There are other trends in the attempt to meld evolution and development which immodesty permits me to add. I have been concerned over the years with the selection forces which might have produced larger organisms and therefore a development of increasing complexity. This is nowhere more evident than in the multiple evolutionary origins of multicellularity and all the variety of developmental mechanics that have gone with it. (I discuss this and related themes in various places, but see especially The Evolution of Development (1958; (1)) and Size and Cycle (1965; (2)). To some degree these examples of the bringing together of evolu tion and development are exceptions. By far the most important historical trends in this century have been the success of in dividual disciplines. Embryology had its great flowering be ginning in the last century, coming to a climax with the work of Spemann on induction in the first two decades of this cen tury. Genetics has had an extraordinary continuing series of revolutions beginning with the rediscovery of Mendel and pro gressing through Morgan to the flash flood of molecular genetics and the structure of DNA. This flood was a flash only in the sense that it rushed upon us with amazing speed; its effects in the form of important and exciting work produced has not subsided and we are still in a peak period of molecular genetics."

Although sponges are one of the major components of littoral ecosystems, their systematics obviously lags behind that of the majority of other groups of marine invertebrates. Also the evolutionary trends in these most primitive of the Metazoans are poorly understood. The confused state of their taxonomy and the difficulties with their identification prevent their use in ecological studies. The emphasis of interest presently seems to have shifted towards chemistry, but many of the hundreds of interesting chemicals recently described in sponges pro bably have been found in mistakenly or imprecisely identified animals. This situ ation will become worse in the next few years, when a number of experts will retire and will not be replaced by new ones, owing to the difficulties that the science of systematics is facing in most countries - as exemplified during this meeting by an informal discussion on the endangered future of European Museums. Under these circumstances, it is the responsability of present sponge taxono mists to leave a less confused state and tools such as simple fauna or computerized data bases to generations following. These tools do not exist even in one of the best studied areas, the North-Eastern Atlantic (including the Mediterranean Sea).

Birth defects have assumed an importance even greater now than in the past because infant mortality rates attributed to congenital anomalies have declined far less than those for other causes of death, such as infectious and nutritional diseases. As many as 50 % of all pregnancies terminate as miscarriages, and in the majority of cases this is the result of faulty intrauterine development. Major congenital malformations are present in at least 2 % of all liveborn infants, and 22 % of all stillbirths and infant deaths are associated with severe congenital anomalies. Not surprisingly, there has been a great proliferation of research into the problems of developmental abnormalities over the past few decades. This series, Advances in the Study of Birth Defects, was conceived in order to provide a comprehensive focal source of up-to-date information for physi cians concerned with the health of the unborn child and for research workers in the fields of fetal medicine and birth defects. The first four volumes featured recent experimental work on selected areas of high priority and intensive investigation, including mechanisms of teratogenesis, teratological evaluation, molecular and cellular aspects of abnormal development, and neural and behavioural teratology. It seems logical and timely that the clinical aspects should now be presented. Accordingly, leading experts were invited to review a broad range of common problems from the standpoint of embryology, aetiology, clinical manifestations, diagnosis and management. This volume deals with genetic disorders and prenatal diagnosis."

49 about six months ... to well over a year. If there is only one part of the year that is favourable, such as spring and early summer in the temperate climates, then each species must make an evolutionary choice, so to speak, as to which parts of the reproductive cycle - conception, gestation, lactation or weanin- must be protected and which can come in less favourable times of the year. The rhesus and langur monkeys of northern India give birth during the time of year when temperatures are hottest and wells and tanks are often dry. However, gestation and the later months oflactation come during the monsoon season when food and water are abundant. In contrast the east African baboons give birth at the beginning of the small rains, and gestation and the late part of lactation occur during the six months dry season. Whether any pattern of relationship will be found to hold true for other species of primates is still not clear. It may be that a wide variety of patterns have evolved depending on the lengths of gestation and lactation and the particular ecological complex in which each species or even subspecies lives' (pages 503, 504).

The book provides a comprehensive account of ticks and tick-borne diseases occurring in tropical and subtropical areas. It begins with a complete up-to-date overview of the systematics of the Ixodida (Ixodidae, Argasidae and Nutalliellidae) and is followed by a review of the problem of ticks and tick-borne diseases of domestic animals world wide. This leads on to multi-disciplinary approaches to planning tick and tick-borne disease control and to contributions on calculating the economic impact of a tick species such as Amblyomma americanum on beef production systems. Heartwater fever (cowdriosis) and dermatophilosis are endemic in Africa and pose a threat to the North American mainland. The epidemiology of these two diseases is discussed in detail as is the role of frozen vaccines to control bovine babesiosis and anaplasmosis. The book also includes chapters on tick transmitted zoonoses such as Lyme borreliosis, tick typhus and ehrlichiosis. It concludes with a review of the acaricidal treatment of tick infestation.

William Trager has been an avid student of parasites for over 50 years at the Rockefeller University. Around the turn of this century, parasitology enjoyed a certain vogue, inspired by colonial responsibilities of the technically ad vanced countries, and by the exciting etiological and therapeutic discoveries of Ross, Manson, Ehrlich, and others. For some decades, the Western hemi sphere's interest in animal parasites has been eclipsed by concern for bacteria and viruses as agents of transmissible disease. Only very recently, initiatives like the Tropical Disease Research programs of WHO-World Bank-UNDP, and the Great Neglected Disease networks of the Rockefeller and MacArthur Foundations have begun to compensate for the neglect of these problems by United States federal health research agencies. Throughout that period, how ever, the Rockefeller Institute (later University) has given high priority to the challenges of parasitism, corresponding during a formidable period with Dr. Trager's own career. The present work then, is a distillation of the insight collected by our principal doyen of parasite biology, informed but by no means confined to his own research. It is addressed to the reader of broad biological interest and training, not to the specialist. The disarmingly unpretentious style makes the work readily accessible to college undergraduates or even to gifted high school students; but do not be deceived thereby, as it has an enormous range of factual information and theoretical insight, familiar to few, but potentially important to most biologists."

For many, the terms aging, maturation and senescence are synonymous and used interchangeably, but they should not be. Whereas senescence represents an endogenously controlled degenerative programme leading to plant or organ death, genetiC aging encompasses a wide array of passive degenerative genetiC processes driven primarily by exogenous factors (Leopold, 1975). Aging is therefore considered a consequence of genetiC lesions that accumulate over time, but by themselves do not necessarily cause death. These lesions are probably made more severe by the increase in size and complexity in trees and their attendant physiology. Thus while the withering of flower petals following pollination can be considered senescence, the loss of viability of stored seeds more clearly represents aging (Norden, 1988). The very recent book "Senescence and Aging in Plants" does not discuss trees, the most dominant group of plants on the earth. Yet both angiospermic and gymnospermic trees also undergo the above phenomena but less is known about them. Do woody plants senesce or do they just age? What is phase change? Is this synonymous with maturation? While it is now becoming recognized that there is no programmed senescence in trees, senescence of their parts, even in gymnosperms (e. g. , needles of temperate conifers las t an average of 3. 5 years), is common; but aging is a readily acknowledged phenomenon. In theory, at least, in the absence of any programmed senescence trees should -live forever, but in practice they do not.

This volume contains the papers presented at the symposion on Central Neurone Environment and the Control Systems of Breathing and Circulation held at Bochum, October 5-7,1981 in honour of Prof. Dr. Dr. h.c. Hans H. Loeschcke, who retired in March 1981. His discovery of ventral medullary substrates forming an essential drive for the ventilatory as well as for circulatory control systems, and the elaboration of this concept during the last 25 years have profoundly influenced the concepts in this field. In an age of proceeding specializa tion on partial mechanisms, his approach has always emphasized the integrative aspects of the control systems, especially the role of the respiratory control system for the ionic homeostasis of the brain and the close interrelation between neuronal respiratory and cardiovascular control. This general intention implies the combination of physico chemical and neurophysiological approaches as well as the study of peripheral function of respiration and circulation, topics which are usually handled separately according to the different methods."

Since the last symposium on "Neuronal Mechanisms of Hearing" held in Prague in 1980 and published in the volume of the same name (J. Syka and L. Aitkin, Eds., Plenum Press, 1981), remarkable progress has been achieved in the understanding of the auditory system. A variety of new ideas and new methods have emerged. This progress can be easily documented by comparing the volume based on the 1980 Symposium with the program for the 1987 Symposium. For example, there were 45 contributions to auditory physiology in each symposium but there were 27 contributions focusing on anatomy in 1987 as compared to 7 in 1980, and perhaps most telling, there were 12 contributions to the neurochemistry of the system in 1987 while there were only 3 in 1980. In terms of percentages of contributions, neuroanatomy rose from 13% to 32% and neurochemistry (or chemical anatomy) rose from 5% in 1980 to 14% in 1987. These increases in the numbers and proportions of anatomical and neurochemical contributions undoubtedly reflects the increasing availabil ity and rising expertise in the new neuroanatomica1 and biochemical techniques most notably, tract-tracing by exploitation of axonal transport or by intracellular micro-injection methods, and neurotransmitter identifi cation by use of immunocytochemistry or receptor-binding techniques. New ideas have emerged on the function of cochlear hair cells particularly in connection with olivococh1ear bundle stimulation and supported by findings of contractile proteins in outer hair cells."

It has been known for a long time that the majority of plant viruses contain RNA and in the past decade and a half it has been realized that many have genomes consisting of three molecules of single-stranded RNA with positive polarity. Among these are viruses belonging to four groups recognized by the International Committee for Virus Taxonomy: the Bromovirus and Cucumovirus groups whose genomes are encapsi dated in small icosahedral particles or the Ilarvirus and alfalfa mosaic virus groups with spheroidal or bacilliform particles. In addition to their tripartite genomes, these viruses share a number of other properties and it has been proposed that they should perhaps be grouped in a single virus family for which the name Tricornaviridae has been suggested, the tri indicating the tripartite nature of the genome, the co emphasizing the cooperation of the three genome parts required to initiate infection, and the rna indicating that the genome is composed of RNA. Viruses of this "family" are less uniform in their biological proper ties. A number of them are widespread, causing very destructive plant diseases. Viruses such as those of cucumber mosaic and alfalfa mosaic have very extensive host ranges and are responsible for serious crop losses in many parts of the world. Others such as prunus necrotic ringspot or prune dwarf viruses are more restricted in their host ranges but never theless infect important perennial hosts such as stone fruits and reduce productivity considerably."