Abstract
The equipment and techniques used in the study of adult microcirculation have been adapted to observations on mammalian fetuses. The initial report is concerned with the dynamics of flow in the changing vascular patterns of the posterior limb bud of the mouse fetus between 12 1/2 and 14 1/4 days of gestation. It is suggested that the temporary portal systems observed here may provide a mechanism for transfer of substances between interacting portions of the developing limb bud. It would appear that the method has potentialities for the study of many aspects of mammalian development.