Fig. 5.

Hypothesis of sternum, plastron, and gastralia development. The turtle is depicted on the Top and the alligator at the Bottom of the schematic ventral side of an embryo. Anterior is to the Right. Chondrogenic sternal morphogenesis begins on lateral sides of the embryo extending from the posterior end of the forelimb posteriorly, based on chick and mouse studies (black dashed rectangle), but it does not form in turtles. This may be due to the osteogenic field of plastron bone condensations (circles) overlapping the sternal morphogenic area during the same developmental window. In alligators, it is not known when or where sternal morphogenesis begins. The gastralia begin to develop one to two developmental stages later than the plastron, and they develop in posterior-to-anterior sequence (black arrow); the lateral parts of the gastralial bones (orange) form first, and this is followed by the formation of the medial parts (blue). Gastralia lie posterior to the sternum. The osteogenic gastralia development is unlikely to coincide with chondrogenic sternal development. Diagram is not to scale. Circles: red, epiplastron; yellow, hyoplastron; magenta, transient mesoplastron; green, hypoplastron; blue, xiphiplastron; f, fontanelle.