61.9 Friday, Jan. 6 Cardiac morphology of Dermophis mexicanus through ontogeny HYKIN, SM; Univ. of California, Berkeley email@example.com
Caecilians are a little-known and understudied clade of limbless, often fossorial amphibians restricted to tropical regions around the globe. Heart development and function, especially regarding the extent of separation of oxygenated and deoxygenated blood within the heart, has been much speculated on but little explored. In the hope of shedding light on the dominant respiratory mode, development, and evolution of this clade, I examined histological sections of the heart in an ontogenetic series of the viviparous Dermophis mexicanus. At 8 mm TL, heart looping has commenced in a counterclockwise direction. By 10 mm TL, heart-tube looping is complete and interatrial septation commences via an extension of anterior myocardial cells through the atrial chamber. At 15 mm TL the valves of the atrioventricular funnel and conus begin to form from endocardial cushions. In the conus, what will become two sets of semilunar valves progress through a spiral configuration. At 20 mm TL the interatrial septum is complete at its right limit, but remains incomplete through the majority of the chamber. The valves of the atrioventricular funnel and the conus remain asymmetrical; however, the conus valves appear to be transitioning out of the spiral configuration. Finally, the newborn and adult forms are similar to one another, but dramatic changes have taken place between the 20 mm stage and parturition. From my observations, I conclude that heart-tube looping occurs approximately one month into the 11-month gestation period and differs from heart-tube looping seen in anurans. The interatrial septum commences development shortly after heart-tube looping completes. No evidence of vestigial interventricular septum development is seen, but it is noteworthy that some vestigial development of the spiral valve (seen in anurans) appears to be a component during early development of the semilunar valves in the caecilian.