Abstract: The neural crest is a group of cells that has a transient existence in the developing vertebrate embryo. Arising at the edges of the neural plate during nervous a formation, these cells initiate life within the central nervous system but soon leave to invade the far reaches of the body (Fig. 7.1). They undergo an epithelial to mesenchymal transition, emigrate from the neuroepithelium, and become a highly migratory cell type that follows distinct and characteristic pathways to numerous sites. They populate most of the peripheral nervous system, parts of the adrenal gland, and much of the facial region. Upon reaching their final destinations, these cells differentiate into a myriad of cell types ranging from pigment cells to sensory and autonomic neurons to cartilage and bone of the face.

Abstract: RNA tumor viruses are enveloped animal viruses whose virions contain an RNA genome and a DNA polymerase. RNA tumor viruses have been isolated from reptiles, birds, and many orders of mammals, including primates. However, none has been isolated from man. In susceptible animals, RNA tumor viruses cause leukemias, sarcomas, carcinomas, rapid lethal infections with anemia and necrosis, slow infections of the nervous system and lung, and, most frequently, no disease. (The nonpathogenic RNA tumor viruses are assayed by the formation of virion components.) The name RNA tumor viruses describes the oncogenicity of some of the members of the virus group. It does not mean that all of the viruses that belong to this group cause tumors. RNA tumor viruses are also called leukoviruses, rousviruses, oncor­ naviruses, retraviruses, retroviruses, ambiviruses, rnadnaviruses, oncoviruses, and ribodeoxyviruses. I prefer the name ribodeoxyviruses. RNA tumor viruses are very popular organisms for study for several reasons: I. all RNA tumor viruses replicate using RNA to DNA to RNA information transfer, a previously unknown mode of information transfer discovered in this group of viruses; 2. some RNA tumor viruses are the apparent cause of much "natural" cancer in chickens and in mice; and 3. some RNA tumor viruses, for example the strongly transforming Rous sarcoma virus, are the most efficient carcinogenic agents known in animals or in cell culture. (Transformation is used in animal cell culture