The image of a copulating frog dressed in tight-fitting pants sounds quite silly, but it was done in the name of serious research. In fact, more than one biologist dressed up their frogs to solve the mysteries of fertilization.
During the 18th century, there was some question as to how fertilization worked and whether semen (and sperm in particular) was necessary for fertilization. In 1677, Antonie van Leeuwenhoek spotted what he termed animalcules (little animals) in semen, but it was unclear what role they and ova played in forming an embryo.
The first pants-on-frog experiment was performed by René Antoine Ferchault de Réaumur, who wanted to figure out what substance, if any, frogs emitted during copulation. Réaumur kept meticulous notes on the various designs of these pants: He tried animal bladder, but waxed taffeta proved a better material. The leg holes were initially too big; the frogs could remove the pants by pushing their legs up through the holes—although this was solved by custom tailoring the pants. Apparently fit is important even when dealing with clothing for frogs. The frog pants even had shoulder straps to keep them in place.
Réaumur's focus was on the materials resulting from copulation, and he didn't keep notes on whether the eggs that resulted from these copulations developed. He was more interested in what the male frogs were leaving behind in their pants, and he wasn't finding much.
But Réaumur's experiments weren't in vain; a later physiologist, the priest and physiologist Lazzaro Spallanzani would put frog pants to better use. Spallanzani treated the pants as a prophylactic; he had frogs copulate while the males were wearing the pants and then watched the subsequent eggs to see if they developed. When they didn't Spallanzani took it as evidence that both semen (although not necessarily sperm) and ova were essential to biogenesis.
This would hardly be Spallanzani's last foray into researching the process of fertilization. He would go on to perform a successful artificial insemination of a dog, and his research would help debunk the theory of preformationism, which suggested that living beings develop from miniature versions of themselves.