Without it, would William Harvey discovered the blood flow? The cold-blooded animals like toads or frogs have a slow heart that is easier to analyze than mammals. It is by looking at them that Harvey made his first observations. In his work Motus Cordis, he assures that the blood is expulsed by the heart to the artery and goes by through the veins. “This is how I start to wonder if there was a circulatory move of the blood”. But, Harvey died before finding out what becomes to the blood between the arteries and the veins.

Once again, we have to thank the frog for solving the enigma of the blood circulation. Observing under a microscope the lung of a frog, Marcello Malpighi noticed very thin blood vessels - the capillaries - that link the small arteries to the small veins.

In 1661, he wrote :

“Things are much easier to see with the frogs (…). The microscopic observation revealed things even more prodigious (…). I clearly saw that the blood got divided and circulated in tortuous vessels”

It is by crossing the lungs that the veins’ blood goes back to the arteries. But, to get this result, Malpighi admitted, even glorified himself:

“I have almost scarified the entire race of frogs, something that never happened before, even during the furious battle between rats and frogs described by Homer”.

Theories of “generation” – that is of reproduction – had aroused passionate quarrels. One of them was about fecundation: what part plays the man seed? The frog, as a very common animal, and reproducing quickly, has the merit of transparence… It reproduces by external fecundation, the eggs laid by the female are straight after recovered by the male seed.

Jan Swammerdam, Biblia naturae, Leyde, 1737-1738 [manuscript from 1679]

In the XVII th century already, Jan Swammerdam was interested by the frog’s reproduction. With a small magnifying lens, he observed the cellular division of a fecundated frog’s egg. In 1768, Lazzaro Spallanzani dressed a male frog with a little panty made of leather, not without difficulty, because the animal tried to get rid of it!

“What assure the whole, is that I’ve putted some straps to these panties. I slipped it on the harms of the male frog, under his head, between his body and the female’s one”

The females mated with these males wearing panties freed their eggs, but these ones decayed and didn’t transform in tadpole. But, inside the panties, Spallanzani found drops of transparent liquor. He took a sample of virgin eggs in a frog’s ovary, knowing by experience that they can spontaneously develop. He damped them with the collected seed and noticed a few days after that the eggs developed as well as if they were naturally fecundated by the male.

“He just came to realize the first artificial insemination in a laboratory” wrote Jean Rostand. This series of experiences – with more than 200 frogs – allowed Spallanzani to ruin the hypothesis of fecundation at distance: the direct touch of the egg and the male seed is essential.

In the middle of the XVII th century, the Dutch naturalist Jan Swammerdam explored another field of life: the transmission of the nerve impulse. A frog can keep on swimming even if one’s take away its heart, while it comes to a standstill if one’s take away its brain. The blood circulation is not necessary to move (at least for the frog). In 1568 he demonstrated, in front of the Duke of Toscane, the contraction of the frog’s muscle, separated from the frog with its nerve, under the action of a simple compression of the nerve. While there’s no more links between the nerve and the spinal cord, the contraction can be repeated at will. Opposing to the most widespread hypothesis, Swammerdam concluded of his experience that the muscle contraction can’t only be explained by the action of a fluid that flowed inside of the nerve, from the spinal cord to the muscle.

Jan Swammerdam, Biblia naturae, Leyde, 1737-1738 [manuscript from 1679]

One of the experiences of Swammerdam on the muscle of a frog. “If one irritates the B nerve with scissors – or with any instrument [the muscle contracts] and move the two hands that handle the tendon closer”	In one more precise version of the experiment “put the muscle in a A glass tube and pierce the tendons with two fine needle BB […] from which the tip is fixed in a piece of cork. If one’s irritates the nerve C, one can see the muscle moving the needles closer in DD; while most of the muscle becomes considerably thicker […] and blocks the tube”.

The details and conclusions of Swammerdam’s experiments are not much diffused because, renouncing to science to focus to spirituality, he burned a part of his manuscripts. What’s left will be published half a century later, but his public demonstrations and his correspondence made his works known by European scientists.

The extreme sensitivity of the frog and the facility by which one can isolate its nerves had made it a preferred subject in the study of the nervous command. In the second part of the XVIII th century, the nerves of the frog’s tights are excited by pressure, by “irritation”, by chemical action of opium or curare, or more, by the direct use of electricity.

The new stimulant discovered by Galvani – a distant spark – caused a general astonishment and new wave of researches.

We won’t tell here the story of the adventures that took Galvani’s frogs to the Volta battery. But, we should note that Volta joined Galvani in his praise to the precious batrachians:

“I’ve chosen the frog among any other animals, because it is gifted by a very sustainable vitality and because it is very easy to prepare it”. (Letter to Cavallo, 1793)

The “galvanists” in favor of animal electricity and the “voltaists” in favor of the metallic electricity enrolled the animal in their controversy. The great traveler and naturalist Alexander von Humboldt transported in his luggage, “even riding a horse”, pliers, scalpels, and metallic patch to defend the thesis of animal electricity on local frog, always accessible in neighboring pond.

Today, animal manipulation is no more allowed in schools, and public demonstration had to be replaced by another media.

The frog’s career in electricity didn’t end in 1800. After the announcement of the existence of electromagnetic waves by Heinrich Hertz at the end of the 1880’s, many scientists and inventors looked for diverse detector of these mysterious waves. Thus, in 1912, Lefeuvre, professor in physiology at the University of Rennes, realized a “physiological detector”, capable to detect waves of telegraphic wireless emitted in Paris. Before being able to diffuse speech and music, the Hertzian waves transmitted Morse code telegraphic signals. The core of Lefeuvre’s apparatus is nothing else than a frog, which tight contracts at the signal reception! One dash: an extended contraction, on dot: a brief contraction.

Pr. Lefeuvre "physiological sensor"

J.A. Fleming, Principles of electric wave telegraphy and telephony, Longmans, 1916.

The popular wisdom in weather prevision uses many of sayings that refer to the frog:

“If the weather is nice, the frog croaks on the banks of its ponds”; in the vase, it digs, if the bad weather is coming”. “Frogs that croak at day, rain in the three days” “When frogs sing at night, in the morning, the sun shines"

In the 70’s, on the French radio Europe 1, the famous hoarse-voiced meteorologist Albert Simon awarded the quality of his prevision to his frog which, in its bowl, went down or up its ladder in accordance with the variation of the barometer.

A “privileged fragment of the living world”

Let’s go back to life science. The study of the parthenogenesis - that is the reproduction without male - by the biologist and popularizer Jean Rostand was also done with frogs. By inflicting an appropriate trauma on a non-fecundated egg, one can obtain the cellular division of this egg. Thousands of toads, frogs and tadpoles were submitted to these experiments:

“We can study all the problems on frogs. People think that frog is a small subject but it isn’t. All biology is in a frog. We can study cells, sperm, eggs, parthenogenesis, etc. Lastly, even the mutation and the heredity. We can study everything on a frog”, it constitutes a “privileged fragment of the living world”.

A new form of reproduction without male, the cloning, was realized for the first time in 1962 on a frog, before the ewe Dolly, by the introduction of the nucleus of an intestinal cell in a nucleus-free egg.

We can let George Canghilem, philosopher and historian of life science, conclude that:

“The frog was, so to speak, a good girl for physiologist; they used it a lot and it was very useful to them”.