Limb Buds and Webbed Toes: A Year of Embryos

Four Seasons at the Linden Park Nature Center

1Linden Park is a large city park with a pond, a sandy hillside and a small nature center. Every year the center runs a program called "Before They Hatch." Naturalist Odalys Ferreira-Quist leads it, and this year eighth grader Kwabena Lindgren signed up as a volunteer. No wild nests are disturbed. Instead, the center shows photos and models of embryos from university labs and matches them with the animals visitors can see in the park.

2In spring, mallard ducks nest on the pond's island, and their ducklings paddle out on webbed feet about four weeks later. Kwabena helped set up a display of duck and chicken embryo photos. A chicken hatches after about 21 days. Early on, both kinds of embryos grow small bumps on their sides called limb buds, which later become wings and legs. A few days later, the toes of both embryos are joined by thin webbing. In the duck, the webbing stays. In the chicken, the cells of the webbing die off and are cleared away by about day 10, leaving separate toes.

3In early summer, painted turtles climb the sandy hillside to dig nests and lay eggs. Turtle embryos grow front and hind limb buds too, and their webbed toes stay webbed, which helps adult turtles swim.

4In fall, the center adds photos of embryos from animals that do not live in the park. A human embryo grows limb buds at about four to five weeks, and its fingers and toes are webbed until about week eight. A bottlenose dolphin embryo grows front limb buds, which become flippers. It also grows hind limb buds, but these shrink away before birth, and adult dolphins have no hind legs. A python embryo grows no front limb buds, but it grows small hind limb buds that stop growing early. On the outside, an adult python shows just two tiny spurs near its tail.

5By winter, Kwabena had filled a table with his notes. Ms. Ferreira-Quist reminded visitors that biologists compare early stages like these because organisms that share a pattern of development often share a common ancestor, even when the adults look very different.