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"Urodeles," a class of vertebrates that includes newts and salamanders, have the enviable ability to regenerate arms, legs, tails, heart muscle, jaws, spinal cords, and other organs. "Planaria," simple worms, can be sliced and diced in hundreds of pieces, with each piece giving rise to a completely new animal. However, while both "urodeles" and "planaria" have the capacity to regenerate, they use different means to accomplish this feat.

In effect, a "urodeles" specimen turns back the biological clock. First, the animal heals the wound at the site of the missing limb. Then various specialized cells at the site, such as bone, skin, and blood cells, lose their identity and revert to cells as unspecialized as those in the embryonic limb bud. This process is called dedifferentiation, and the resulting blastema, a mass of unspecialized cells, proliferates rapidly to form a limb bud. Ultimately, when the new limb takes shape, the cells take on the specialized roles they had previously cast off.

In contrast, "planaria" regenerate using cells called neoblasts. Scattered within the planarian body, these neoblasts remain in an unspecialized, stem-cell state, which enables them at need to differentiate into any cell type. Whenever "planari"a are cut, the neoblasts migrate to the site and form a blastema by themselves. It is interesting to note that this mechanism is similar to that following reproductive fission in these animals, and that species incapable of this form of asexual reproduction have poorly developed regenerative capacities.

1. The primary purpose of the passage is to:

(A) describe the roles of blastemas in regenerating "urodeles" and "planaria."
(B) describe how "urodeles" use the process of dedifferentiation to regenerate.
(C) contrast the mechanisms by which "urodeles" and "planaria" accomplish regeneration.
(D) show how methods of cellular regeneration have evolved in different animal species.
(E) explain the link between reproductive fission and regeneration in simple worms.  






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