_Articulates_,--animals in which all parts are arranged in a succession of
articulated joints along a longitudinal axis. Cuvier has expressed this
jointed structure in the name _Articulates_; whereas Baer, in his name of
_Longitudinal_, referred only to the arrangement of joints in longitudinal
succession, in a continuous string, as it were, one after another. For the
_Doubly Symmetrical_ type his name is the better of the two; for Cuvier's
name of _Vertebrates_ alludes only to the backbone,--while Baer, who is an
embryologist, signifies in his their mode of growth also. He knew what
Cuvier did not know, that in its first formation the germ of the
Vertebrate divides in two folds: one turning up above the backbone, to
inclose all the sensitive Organs,--the spinal marrow, the organs of sense,
all those organs by which life is expressed; the other turning down below
the backbone, and inclosing all those organs by which life is
maintained,--the organs of digestion, of respiration, of circulation, of
reproduction, etc. So there is in this type not only an equal division of
parts on either side, but also a division above and below, making thus a
double symmetry in the plan, expressed by Baer in the name he gave it.
Baer was perfectly original in his conception of these four types, for his
paper was published in the very same year with that of Cuvier. But even in
Germany, his native land, his ideas were not fully appreciated: strange
that it should be so,--for, had his countrymen recognized his genius, they
might have claimed him as the compeer of the great French naturalist.

Baer also founded the science of Embryology, under the guidance of his
teacher, Dollinger. His researches in this direction showed him that
animals were not only built on four plans, but that they grew according to
four modes of development. The Vertebrate arises from the egg differently
from the Articulate,--the Articulate differently from the Mollusk,--the
Mollusk differently from the Radiate. Cuvier only showed us the four plans
as they exist in the adult; Baer went a step farther, and showed us the