egg divides, before the incubation has proceeded twelve hours, into
two different layers, an external serous layer and an internal mucous
layer; between the two there develops later a third layer, the
vascular (blood-vessel) layer.* (* The technical terms which are bound
to creep into this chapter will be fully understood later
on.--Translator.)

Karl Ernst von Baer, who had set afoot Pander's investigation, and had
shown the liveliest interest in it after Pander's departure from
Wurtzburg, began his own much more comprehensive research in 1819. He
published the mature result nine years afterwards in his famous work,
Animal Embryology: Observation and Reflection (not translated). This
classic work still remains a model of careful observation united to
profound philosophic speculation. The first part appeared in 1828, the
second in 1837. The book proved to be the foundation on which the
whole science of embryology has built down to our own day. It so far
surpassed its predecessors, and Pander in particular, that it has
become, after Wolff's work, the chief base of modern embryology.

Baer was one of the greatest scientists of the nineteenth century, and
exercised considerable influence on other branches of biology as well.
He built up the theory of germinal layers, as a whole and in detail,
so clearly and solidly that it has been the starting-point of
embryological research ever since. He taught that in all the
vertebrates first two and then four of these germinal layers are
formed; and that the earliest rudimentary organs of the body arise by
the conversion of these layers into tubes. He described the first
appearance of the vertebrate embryo, as it may be seen in the globular
yelk of the fertilised egg, as an oval disk which first divides into
two layers. From the upper or animal layer are developed all the