This 50th volume continues the tradition of this series in addressing developmental mechanisms in a variety of experimental systems. The conceptual sequence of topics begins with totipotency by examining germ plasm, germ cell fates, and gonocytes, continues with sperm-mediated transgenic approaches and endomembrane dynamics within eggs, and then considers embryonic patterning in both plants and animals as well as pluripotency of hematopoietic stem cells. The chapter by Charles A. Ettensohn and Hyla C. Sweet reviews current knowledge of the molecular mechanisms of embryonic patterning in sea urchins and provides molecular mechanisms for animal–vegetal as well as oral–arboral axes. Alan J. Davidson and Leonard I. Zon examine the development biology of “Turning Mesoderm into Blood: The Formation of Hematopoietic Stem Cells during Embryogenesis.” The implications of developmental medicine are truly revolutionary.