Abraham Trembley was a pioneer in experimental morphology. He is primarily remembered for his experiments on the hydra, or freshwater polyp (Chlorohydra viridissima), tiny creatures about a quarter of an inch long. Trembley discovered that the polyp could produce separate and complete new animals after it had been cut in half. His experiments were described in Memoir on the Natural History of a Species of Fresh Water, Horn-shaped Polyps (1744). Trembley also made important observations of the reproduction of algae and protozoans.
Abraham Trembley was born into a prominent family in Geneva. His most famous discoveries, however, were made while he served as a private tutor to the children of wealthy families. He later accompanied the young Duke of Richmond on his grand tour of Europe (1752-1756). In recognition of Trembley's services, the duke granted him a pension. Trembley married in 1757 and settled near Geneva. The rest of his life was devoted to philosophical and religious studies.
Experiments on the hydra had a profound impact on eighteenth-century thinking about the nature of reproduction, development and differentiation, and regeneration. The tiny polyps had been observed by the great seventeenth-century microscopist Anton van Leeuwenhoek (1632-1723). Freshwater polyps are typically found among the roots of lily pads and other aquatic plants. Noting that the polyp reproduced by budding, Leeuwenhoek concluded that they were plants. After careful and patient observations, Trembley discovered that the hydra actually captured food in its tentacles. Moreover, the creatures had an interior stomach where the food was digested. Unlike typical plants, the hydra reacted to touch and used a foot-like part to move. Because the hydra could move and feed itself, Trembley and other eighteenth-century biologists thought that the polyp should be classified as an animal, although it seemed to represent the very bottom of the scale of animal forms. The polyp seemed to be at the border between the plant world and the animal world. According to modern taxonomists, Hydra is a genus of invertebrate freshwater animals of the class Hydrozoa (phylum Cnidaria). The genus includes about 30 different species, all of which feed on other small invertebrates, such as crustaceans.
Reasoning that plants and animals typically exhibited significant differences in their ability to regenerate parts, Trembley cut a polyp in half. He was amazed to see that each piece survived and produced a complete new specimen. To determine whether the new individuals needed specific parts of the original polyp, Trembley cut hydras lengthwise, crosswise, and even into several pieces. Much to his surprise, each piece of the hydra appeared to be capable of producing a new individual. This characteristic made the hydra more like a plant than an animal. The new hydras could also be cut up and the pieces would produce a third generation of hydras. In another series of experiments, Trembley succeeded in turning a hydra inside out, as if it were a glove, by inserting a bristle into the gut. These inside-out creatures survived and eventually appeared to become normal hydra. In other remarkable hydra experiments, Trembley made permanent grafts and observed cell division, long before the establishment of modern cell theory.
After Trembley published his experiments, other naturalists attempted to extend his observations to other species. Réné Antoine Ferchault de Réaumur (1683-1757) and Charles Bonnet (1720-1793) found that similar results could be obtained with freshwater worms. Naturalists were certain that freshwater worms were animals; therefore, the case for considering the hydra an animal was strengthened. If the polyp was indeed an animal, Trembley's discoveries led inexorably to a major philosophical dilemma. Other animals, such as salamanders and crabs, were able to regenerate missing parts. In such cases, the original individual survived and healed; the lost parts neither survived nor gave rise to new individuals. Thus, it could be said that the "soul" or "organizing principle" resided in the maimed individual. The ability of each piece of the polyp to produce a new individual challenged the belief in an organizing principle or soul. Of course, eighteenth-century naturalists could not solve this problem by invoking the modern idea that complex animals are composed of cells that contain the same genetic material in each of their nuclei. Some natural philosophers, such as Julien de La Mettrie (1709-1751) and Denis Diderot (1713-1784), concluded that Trembley's experiments supported a materialist and atheistic view of life. In other words, Trembley's polyp proved that there was no soul; "life" was distributed throughout the body.
The ability of the hydra to form new individuals when cut into pieces also had a profound impact on the debate between preformationists and epigenesists about the basis of generation. If pieces of an animal like the hydra could form two new individuals when cut in half, the existence of a preformed germ of the new individual in either the egg or the sperm seemed to be precluded.
LOIS N. MAGNER