The end of the beginning for pluripotent stem cells.

Pluripotent stem cells can be expanded seemingly indefinitely in culture, maintain a normal karyotype and have the potential to generate any cell type in the body. As such they represent an incredible resource for the repair of diseased or damaged tissues in our bodies. These cells also promise to open a new window into the embryonic development of our species.     

Epicycles,eccentrics, and ellipses: The predictive capabilities of Copernican planetary models

A theoretical analysis of the potential accuracy of early modern planetary models employing compound circles suggests that fairly simple extensions of those models can be sufficiently accurate to meet the demands of Tycho Brahe's observations in both ecliptic longitude and latitude. Some of these extensions, such as the substitution of the true sun for the mean sun, had already been taken by Kepler before he abandoned circular models. Other extensions, involving one or two extra epicycles, were well within the mathematical capabilities of sixteenth-century and seventeenth-century astronomers. Hence neither the failure of astronomers before Kepler to correct errors in planetary positions nor Kepler's decision to abandon circular models was a consequence of inherent limitations in those models.