difficulties, and many other obstacles which we encounter when we
would make an extensive study of the fertilised mammal. The chicken
has, therefore, always been the chief object of study in this
connection. The excellent incubators we now have enable us to observe
it in any quantity and at any stage of development, and so follow the
whole course of its formation step by step.

By the end of the seventeenth century Malpighi had advanced as far as
it was possible to do with the imperfect microscope of his time in the
embryological study of the chick. Further progress was arrested until
the instrument and the technical methods should be improved. The
vertebrate embryos are so small and delicate in their earlier stages
that you cannot go very far into the study of them without a good
microscope and other technical aid. But this substantial improvement
of the microscope and the other apparatus did not take place until the
beginning of the nineteenth century.

Embryology made scarcely any advance in the first half of the
eighteenth century, when the systematic natural history of plants and
animals received so great an impulse through the publication of
Linne's famous Systema Naturae. Not until 1759 did the genius arise
who was to give it an entirely new character, Caspar Friedrich Wolff.
Until then embryology had been occupied almost exclusively in
unfortunate and misleading efforts to build up theories on the
imperfect empirical material then available.

The theory which then prevailed, and remained in favour throughout
nearly the whole of the eighteenth century, was commonly called at
that time "the evolution theory"; it is better to describe it as "the
preformation theory."* (* This theory is usually known as the