The Amphibia has been one of the most important animal groups for the study of developmental biology, and a huge descriptive and experimental literature has accumulated over the years. While sea urchins, molluscs, and nematodes, and more recently, Drosophila, have each become an important vehicle for the study of different aspects of development, the Amphibia and chordates in general have been especially important as the vehicle for the study of inductive regulative mechanisms. The early development of all chordates is marked by two revolutions in the control of early pattern formation: dorsalization at the blastula stage and gastrulation—primary induction caused by the “organizer” —which was studied in great detail in Amphibia by Spemann and his coworkers and continues to be the subject of intense scrutiny. The early phases of development in Amphibia exemplify rather well some of the problems in discovering the causal processes in development, whether in the egg, at fertilization, in the blastula, or in gastrulation itself. The short discussion of early development in Amphibia that follows is meant to exemplify rather than catalogue these questions. The oocyte in amphibians is radially symmetrical. A first axis of symmetry is established between a more heavily pigmented animal hemisphere and a less pigmented vegetal hemisphere. Before fertilization the egg is covered with a transparent vitelline membrane. When fertilization occurs, the vitelline membrane lifts from the surface of the egg and the egg is then free to rotate inside it so that the animal hemisphere lies uppermost and the vegetal hemisphere is lowermost. This rotation is apparently a response to gravity, which means that the vegetal hemisphere is heavier, almost certainly a result of the concentration of more and larger yolk granules in the vegetal hemisphere. Therefore, if the egg rotates to a new orientation with the yolk down and the animal hemisphere up, it must be the case that at this point the yolk and other egg contents are not free to be redistributed within the egg but are secured in place. The animal vegetal axis now marks the future anteroposterior axis of the embryo.
Divergence of gene regulatory network linkages during specification of ectoderm and mesoderm in early development of sea urchins
