rrow, 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
four plans in the process of formation. But his greatest scientific
achievement is perhaps the discovery that all animals originate in eggs,
and that all these eggs are at first identical in substance and structure.
The wonderful and untiring research condensed into this simple statement,
that all animals arise from eggs and that all those eggs are identical in
the beginning, may well excite our admiration. This egg consists of an
outer envelope, the vitelline membrane, containing a fluid more or less
dense, the yolk; within this is a second envelope, the so-called
germinative vesicle, containing a somewhat different and more transparent
fluid, and in the fluid of this second envelope float one or more
so-called germinative specks. At this stage of their growth all eggs are
microsopically small, yet each one has such tenacity of its individual
principle of life that no egg was ever known to swerve from the pattern of
the parent animal that gave it birth.

III.

From the time that Linnaeus showed us the necessity of a scientific system
as a framework for the arrangement of scientific facts in Natural History,
the