skeletal axis of the vertebrate. In the lowest of the vertebrates, the
amphioxus, the internal skeleton consists only of this cord throughout
life. But even in the case of man and all the higher vertebrates it is
round this cord that the spinal column and the brain are afterwards
formed.

However, important as these and many other discoveries of Baer's were
in vertebrate embryology, his researches were even more influential,
from the circumstance that he was the first to employ the comparative
method in studying the development of the animal frame. Baer occupied
himself chiefly with the embryology of vertebrates (especially the
birds and fishes). But he by no means confined his attention to these,
gradually taking the various groups of the invertebrates into his
sphere of study. As the general result of his comparative
embryological research, Baer distinguished four different modes of
development and four corresponding groups in the animal world. These
chief groups or types are: 1, the vertebrata; 2, the articulata; 3,
the mollusca; and 4, all the lower groups which were then wrongly
comprehended under the general name of the radiata. Georges Cuvier had
been the first to formulate this distinction, in 1812. He showed that
these groups present specific differences in their whole internal
structure, and the connection and disposal of their systems of organs;
and that, on the other hand, all the animals of the same type--say,
the vertebrates--essentially agreed in their inner structure, in spite
of the greatest superficial differences. But Baer proved that these
four groups are also quite differently developed from the ovum; and
that the series of embryonic forms is the same throughout for animals
of the same type, but different in the case of other animals. Up to
that time the chief aim in the classification of the animal kingdom
was to arrange all the animals from lowest to highest, from the