Secondary coverage of the yolk by the body wall in the direct developing frog, Eleutherodactylus coqui: an unusual process for amphibian embryos.

Eleutherodactylus coqui develops directly from a large 3.5-mm egg to a froglet, without an intervening tadpole stage. We have examined the development of the body wall, a structure whose behavior has been altered in this derived development. In an event that is unusual for amphibian embryos, the yolk mass is secondarily surrounded by the body wall, which originates near the embryo's trunk. The epidermis of the body wall is marked by melanophores, and the rectus abdominis, which will form the ventral musculature, is near its leading edge. As the body wall expands, the epidermis, melanophores, and rectus abdominis all move from the dorsal side to close over the yolk at the ventral midline. The original ectoderm over the yolk undergoes apoptosis, as it is replaced by body wall epidermis. Intact muscles are not required for ventral closure of the body wall, despite their normal presence near the advancing edge. Comparative examination of embryos of Xenopus laevis and Rana pipiens suggests that ventral closure does not occur in species with tadpoles. The expansion of dorsal tissues over the yolk, as illustrated by E. coqui, may have been important in the origin of amniote embryos.