work by Meckel (1815 and
1817), Tiedemann (_Anatomie u. Bildungsgeschichte des Gehirns_, 1816),
by Oken (_loc. cit., supra_, p. 90), and some others.

Pander, with whom apparently Doellinger and d'Alton collaborated, was the
first to publish his results;[172] von Baer, who through absence from
Wuerzburg had for a time dropped his embryological studies, started to
work in 1819, after the publication of Pander's treatise, and produced
in 1828 the first volume of his master-work, _Ueber
Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion_
(Koenigsberg, 1828). The second volume followed in 1837, but dates really
from 1834, and was published in an incomplete form. This second volume
is intended as an introduction to embryology for the use of doctors and
science students. In it von Baer describes in full detail the
development of many vertebrate types--chick, tortoise, snake, lizard,
frog, fish, several mammals and man, basing his remarks largely upon his
personal observations, but taking account also of all contemporary work.
A separate account of the development of a fish (_Cyprinus blicca_)
appeared in 1835.[173]

We shall concentrate attention on the first volume. This volume contains
the first full and adequate account of the development of the chick,
followed by a masterly discussion of the laws of development in general.

When we consider that von Baer worked chiefly with a simple microscope
and dissecting needles, the minuteness and accuracy of his observations
are astonishing. He described the main facts respecting the development
of all the principal organs, and if, through lack of the proper means of
observation, he erred in detail, he made up for it by his masterly
understanding and profound analysis of the essential nature of
development. His account of the development of the chick is a model of
what a scientific memoir ought to be; the series of "Scholia" which
follow contain the deductions he made from the data, and, in so far as
they are direct generalisations from experience, they are valid for all
time.

The first Scholion is directed against the theory of preformation, and
succeeds in refuting it on the ground of simple observation. The theme
of the second Scholion is that the essential nature (_die Wesenheit_) of
the animal determines its differentiation, that no stage of development
is solely determined by the antecedent stage, but that throughout all
stages the _Wesenheit_ or idea of the d