JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The National Institute of Environmental Health Sciences (NIEHS) and Brogan & Partners are collaborating with JSTOR to digitize, preserve and extend access to Environmental Health Perspectives.The distinction between histiotrophic nutrition in which local macromolecules are chiefly responsible for the maintenance of the embryo > and hemotrophic nutrition Zwhich results from a transfer of material between the maternal and fetal circulationst is made. Placentation in a number of commonly used laboratory animals and in man is described, and it is shown that dependence upon histiotroph and hemotroph varies greatly, not only between species but also at different stages of gestation in a single species. These facts are likely to be reflected in considerable differences in response to certain teratogens; they must be carefully considered when experimental results are extrapolated between species. The significance to man of an agent which has been shown to be teratogenic in a single species of experimental animals should be evaluated in terms of possible differences in placental function between man and that species. This is par ticularly so if there is a suspicion that the potential teratogen may affect the fetal membranes. mass of yolk. Gradually the ectoderm, mesoderm, and underlying periblast extend from the blastoderm around the yolk mass and surround it in a vascularized trilaminar yolk sac. Embryonic nutrition is due to absorption of yolk by the periblast and transfer of nutrients to mesodermal blood vessels for transmission to the embryo. The requirements of terrestrial life lead to the development of further fetal membranes by reptiles (Fig. 2). The shell membranes are modified to prevent desiccation, but this and other factors inhibit gaseous interchange and exeretion. A diverticulum of the hind-gut-the allantois-is formed. Its cavity is lined by endoderm, and it is covered by vascularized mesoderm. This large sac receives and stores fetal urine and lies in the extraembryonic coelom. The latter develops as a result of a split in the extraembryonic mesoderm which separates an outer chorion (ectodermal trophoblast lined by avascular mesoderm) from the endodermal yolk sac covered by vascularized mesoderm. Allantoic mesoderm fuses with chorionic mesoderm to form the chorio-allantois, and by this means the allantoic circulation becomes greatly concerned with gaseous interchange. As will be seen later the chorio-allantois forms the December 1976 5
