Abraham Trembley and the Hydra

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Abraham Trembley and the Hydra

Overview

The hydra is a small organism, often less than an inch (2.5 cm) long, that became the focus of much attention and debate during the eighteenth century. Abraham Trembley (1710-1784), along with a number of others, used the hydra to investigate basic issues concerning development, regeneration after damage, and the differences between plants and animals. Because of this and related work, by the end of the century the extents of both the plant and animal kingdoms were more clearly defined and how development occurs was more fully understood.

Background

Trembley, who was born in Geneva, Switzerland, was working as a tutor in Holland when he first encountered a green hydra called Chlorohydra viridissima in a sample of pond water. It was clinging to a plant, and at first he thought it was itself a plant because of its green color. But when he saw that its finger-like projections or tentacles actually moved, a characteristic of animal, not plant, life, he became uncertain as to how to classify this creature. Some few plants do move quickly when stimulated, as is the case with the Venus flytrap, which rapidly closes its leaf-shaped "trap" when a fly lands on it, but these quick movements are always triggered by some stimulus, they are never spontaneous as the movements of the hydra were.

To try to determine whether the hydra was a plant or an animal, Trembley decided to cut the organism in two. He reasoned that if it were an animal, this operation would kill it, but if it were a plant, each of the two parts was likely to survive. He split the hydra so that one part had all the tentacles. Over the next several days, Trembley continued to observe the hydra segments and found that each regenerated to the point where it looked like the original hydra. In another experiment, he removed the tentacles from several hydras, each of which he then put in a separate tank where they bore young by asexual budding with the buds separating from the parent, a separation that would not have occurred with plant buds. These experiments and the fact that the hydra had spontaneous movements made Trembley uneasy about classifying it as a plant.

To settle the question of whether the hydra was a plant or an animal, Trembley sent a letter describing his observations as well as some specimens to René Antoine Ferchault de Réaumur (1683-1757) at the Royal Academy of Sciences in Paris in 1741. Réaumur assured Trembley that the hydra was indeed an animal and that a related brown species, Hydra vulgaris, had been discovered by the great Dutch microscopist Anton van Leeuwenhoek (1632-1723) in 1702. After having Trembley's long letter read at two meetings of the Academy, Réaumur himself confirmed Trembley's results with his own experiments, as did two researchers in England. This work was exciting to those interested in zoology because this was the first case of an animal that could be multiplied by cutting it into pieces, though it was commonly known that many plant species could be propagated from cuttings. Here was a creature that seemed to be on the border between the plant and animal kingdoms.

Trembley continued to experiment with several species of hydra. He even managed to turn specimens of Hydra vulgaris inside out, and found that this reversed form could survive and feed without returning to its original form. He also performed the first permanent graft of animal tissues when he successfully fused two hydras by placing one inside the other: he pushed it in tail first through the mouth of the second individual.

His studies on the hydra were not Trembley's only contributions to the life sciences. He also was the first to identify the process of cell division by examining specimens of the onecelled diatom Synedra. This was cell division in the strict sense of the term, that is, a cell with one nucleus dividing to form two cells, each with a nucleus.

Impact

Trembley's research was important for a number of reasons. First, it focused attention on the hydra and other members of the phylum Cnidaria, formerly called Coelenterata, which includes jellyfish, corals, and sea anemones. Like the hydra, other cnidarian species fascinated students of nature because they seemed to have characteristics of both plants and animals. The branch-like structure of corals and the fact that most cnidarians spend part of their lifecycle attached to some solid surface made them seem plantlike. The sedentary form of the organism is called the polyp and the mobile form is the medusa, with jellyfish being the cnidarians with the most developed medusae. Extensive observations and experimentation, often following Trembley's lead, made it obvious that though they sometimes superficially resemble plants, these organisms were in fact animals since they could respond rapidly to stimuli, digest food, and reproduce in ways similar to other animals.

During the eighteenth century the idea of the great chain of being was still widely accepted. This held that all organisms could be organized into a chain from the simplest to the most complex—one-celled organisms would be at one end of the chain and humans at the other. It was further held that there were no breaks in the chain, that where there seemed to be a great difference between one link or organism type and the next, the assumption was that this was simply because missing links had yet to be discovered. This is where the term "missing link" arose; much later it was used in referring to an animal with a mixture of ape and human characteristics that was thought to have existed before the evolution of modern humans.

But the concept of a great chain of being was developed long before the idea of evolution became widely accepted, and in the seventeenth and eighteenth centuries missing links were sought not as support for evolution, but because it was thought that God had created a perfect chain of organisms, with all possible types. If this were the case, the apparent rather significant differences between plants and animals posed a problem, and some observers of nature saw the hydra and other cnidarians as links between the two. The debate over the proper way to classify cnidarians continued for some time until the evidence for their animal nature became overwhelming. Other research also made it clear that there were obviously major differences between plants and animals and no half-way organisms bridging the gap between the two kingdoms; so the idea of a great chain of being slowly lost support.

Trembley's studies were significant because they focused attention on the basic question of what makes plants different from animals. While this was usually an easy enough question to answer in the case of large plants and animals, at and near the microscopic level the distinctions were not so obvious. Also, at this time many botanists were seeking to understand plants in terms of animal anatomy and physiology. For example, they saw a similarity between the circulation of blood in animals and the movement of sap in plants, between the fertilization of eggs by sperm in animals and the fusion of eggs and pollen cells in plants. While the exploration of some of these similarities was useful, in many cases the assumption of similarity went too far. The careful observations of investigators like Trembley showed that surface similarities, such as green color, might hide deeper differences.

Another area that had long fascinated biologists is regeneration, the regrowth of structures that have been removed from an organism; it is interesting to see regrowth of a kind that is not possible in humans and mammals in general. While a hydra seems to be able to easily regrow a tentacle, we can't regrow our own limbs. The fact that a portion of the hydra could give rise to an entire organism was also appealing to researchers because it provided support for the idea of epigenesis, the concept that during development tissues are organized and changed radically in form as organs arise from shapeless masses of cells. Epigenesis was opposed by those who accepted preformation, the idea that in the plant seed or beginning animal embryo was a tiny copy of the adult of that species, so that all that happens during development is that the copy enlarges and its organs become functional.

Preformation seems a rather odd idea today, especially because it implies that within each tiny adult form is a still smaller copy representing the next generation. But preformation was widely accepted during the seventeenth and eighteeenth centuries because many found it even more difficult to conceive of how an organism as complex as a human being could arise from a shapeless mass of cells. Experiments such as Trembley's, however, indicated that cells could indeed organize and reorganize themselves into different formations, that a whole hydra could develop from just a portion of one. If this were the case, then it became more likely that clumps of cells within an embryo could also organize themselves into more complex structures, as the theory of epigenesis suggested.

Finally, Trembley's work served as a model for the experimental study of organisms and for the use of the microscope in experimentation. The microscope had been developed in the seventeenth century and had opened up a whole new world of organisms to investigation. Before this time, no one had suspected that pond water, saliva, and milk were teaming with life. By the eighteenth century, looking through a microscope and being amazed by the great enlargement of everything from a hair to a snowflake became a popular form of entertainment, and the microscope became less a tool of science than of amusement. Investigators like Trembley showed the tremendous power of this instrument to also further understanding of living things. Some of the hydras he worked with were only a quarter of an inch long; it would have been impossible to manipulate these creatures as he did—cutting them into pieces, forcing one inside the other, and observing the results of these operations—without the aid of magnifying lenses. His research and that of his contemporaries foreshadowed the experiments on embryos that were done in the nineteenth century and that led to the flowering of the fields of experimental zoology and embryology.

MAURA C. FLANNERY