An important histological discovery dating from this time is that of the finer structure of muscle, made by Stensen (or Steno) in 1664. He described the structure of muscle-fibres, resolving them into their constituent fibrils. To the microscope we owe not only histology but the comparative anatomy of the lower animals. Throughout the 17th and 18th centuries the discovery of structure in the lower animals went on continuously, as may be read in any history of Zoology.[22] We content ourselves here with mentioning only some representative names. In the 17th century Leeuenhoek, applying the microscope almost at random, discovered fact after fact, his most famous, discovery being that of the "spermatic animalcules." Swammerdam studied the metamorphoses of insects and made wonderfully minute dissections of all sorts of animals, snails and insects particularly. He described also the development of the frog. It is curious to see what a grip his conception of metamorphosis had upon him when he homologises the stages of the frog's development with the Egg, the Worm, and the Nymph of insects (_Book of Nature_, p. 104, Eng. trans., 1785). He even speaks of the human embryo as being at a certain stage a Man-Vermicle. In the 18th century, Reaumur and Bonnet continued the minute study of insects, laying more stress, however, on their habits and physiology than upon their anatomy. Lyonnet made a most laborious investigation of the anatomy of the willow-caterpillar (1762). John Hunter (1728-93) dissected all kinds of animals, from holothurians to whales. His interest was, however, that of the physiologist, and he was not specially interested in problems of form. It is interesting to note a formulation in somewhat confused language of the recapitulation theory. The passage occurs in his description of the drawings he made to illustrate the development of the chick. It is quoted in full by Owen (J. Hunter, _Observations on certain Parts of the Animal OEconomy_, with Notes by Richard Owen. London, 1837. Preface, p. xxvi). We give here the last and clearest sentence--"If we were to take a series of animals from the more imperfect to the perfect, we should probably find an imperfect animal corresponding with some stage of the most perfect." The tendency of the time was not towards morphology, but rather to general natural history and to systematics, the latter under the powerful influence of Linnaeus (1707-1778). The former tendency is well represented by Reaumur (1683-1757) with his observations on insects, the digestion of birds, the regeneration of the crayfish's legs, and a hundred other matters. To this tendency belong also Trembley's famous experiments on Hydra (1744), and Roesel von Rosenhof's _Insektenbelustigungen_ (1746-1761).