link between electricity and the nervous system by studying frogs. He created one of the first tools for measuring electric current out of a frog leg. In 1852, H. F. Stannius used a frog's heart in a procedure called a Stannius ligature to demonstrate the ventricle and atria beat independently of each other and at different rates. The African clawed frog or platanna (Xenopus laevis) was first widely used in laboratories in pregnancy tests in the first half of the 20th century. A sample of urine from a pregnant woman injected into a female frog induces it to lay eggs, a discovery made by English zoologist Lancelot Hogben. This is because a hormone, human chorionic gonadotropin, is present in substantial quantities in the urine of women during pregnancy. In 1952, Robert Briggs and Thomas J. King cloned a frog by somatic cell nuclear transfer. This same technique was later used to create Dolly the sheep, and their experiment was the first time a successful nuclear transplantation had been accomplished in higher animals. Frogs are used in cloning research and other branches of embryology. Although alternative pregnancy tests have been developed, biologists continue to use Xenopus as a model organism in developmental biology because their embryos are large and easy to manipulate, they are readily obtainable, and can easily be kept in the laboratory. Xenopus laevis is increasingly being displaced by its smaller relative, Xenopus tropicalis, which reaches its reproductive age in five months rather than the one to two years for X. laevis, thus facilitating faster studies across generations. Genomes of Xenopus laevis, X. tropicalis, Rana catesbeiana, Rhinella marina, and Nanorana parkeri have been sequenced and deposited in the NCBI Genome database. As pets Due to being inexpensive and relatively easy to care for, many species of frogs and toads have become popular as exotic pets. They are undemanding and require low maintenance. Both frogs and toads can be housed in paludariums, terrariums and aquariums. Pharmaceutical Because frog toxins are extraordinarily diverse, they have raised the interest of biochemists as a "natural pharmacy". The alkaloid epibatidine, a painkiller 200 times more potent than morphine, is made by some species of poison dart frogs. Other chemicals isolated from the skins of frogs may offer resistance to HIV infection. Dart poisons are under active investigation for their potential as therapeutic drugs. It has long been suspected that pre-Columbian Mesoamericans used a toxic