ere first published in Burdach's
treatise on physiology. Continuing his investigations alone von Baer
extended them to the evolution of organisms generally, and after a sojourn
at Berlin he was invited by his old teacher Burdach, who had become
professor of anatomy at Koenigsberg, to join him as prosector and chief of
the new zoological museum (1817). Von Baer's great discovery of the human
ovum is the subject of his _Epistola de Ovo Mammalium et Hominis Genesi_
(Leipzig, 1827), and in the following year he published the first part of
his _History of the Evolution of Animals_ (_Ueber die
Entwickelungsgeschichte der Thiere_), the second part following in 1837. In
this work he demonstrated first, that the Graafian follicles in the ovary
are not the actual eggs, but that they contain the spherical vesicle, which
is the true ovum, a body about the one hundred and twentieth of an inch in
diameter, wherein lie the properties transmitting the physical and mental
characteristics of the parent or grandparent, or even of more remote
ancestors. He next showed that in all vertebrates the primary stage of
cleavage of the fertilized egg is followed by modification into leaf-like
germ layers--skin, muscular, vascular and mucous--whence arise the several
organs of the body by differentiation. He further discovered the
gelatinous, cylindrical cord, known as the _chorda dorsalis_, which passes
along the body of the embryo of vertebrates, in the lower types of which it
is limited to the entire inner skeleton, while in the higher the backbone
and skull are developed round it. His "law of corresponding stages" in the
development of vertebrate embryos was exemplified in the fact recorded by
him about certain specimens preserved in spirit which he had omitted to
label. "I am quite unable to say to what class they belong. They may be
lizards, or small birds, or very young mammalia, so complete is the
similarity in the mode of formation of the head and trunk in these animals.
The extremities are still absent, but even if they had existed in the
earliest stage of the development we should learn nothing, because all
arise from the same fundamental form." Again, in his _History of Evolution_
he suggests, "Are not all animals in the beginning of their development
essentially alike, and is there not a primary form common to all?" (i. p.
223). Notwithstanding this, the "telic" idea, with the archetypal theory
which it involved, possessed von Baer to the