nthusiasm of youth I applied myself to Anatomy, and
although I was interested in particular problems, yet I dared to pry
into them in the higher animals. But since these matters enveloped in
peculiar mystery still lie in obscurity, they require the comparison
of simpler conditions, and so the investigation of insects[20] at once
attracted me; finally, since this also has its own difficulties I
applied my mind to the study of plants, intending after prolonged
occupation with this domain, to retrace my steps by way of the
vegetable kingdom, and get back to my former studies. But perhaps not
even this will be sufficient; since the simpler world of minerals and
the elements should have been taken first. In this case, however, the
undertaking becomes enormous and far beyond my powers."[21] There is
something fine in this life of broad outlines, devoted whole-heartedly
to an idea, to a plan of research, which required a lifetime to carry
out.

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-