s
relation to transcendental anatomy was in some ways a close one, though
he was a trenchant critic of the extreme views of the school.[182] He took
from Oken the idea that a simple fundamental plan rules the organisation
of all Vertebrates; "That jaws and limbs are modifications of one
fundamental form is readily apparent, and, after Oken, the fact ought to
be accepted by the majority of those naturalists who do not refuse to
admit the existence of a general type from which the diversity of
structure is developed" (i., p. 192). He accepted the vertebral theory
of the skull in its main lines, and used his embryological knowledge to
support the idea that jaws correspond to limbs--the latter point as part
of the transcendental idea that the hind end of the body repeats the
organisation of the anterior part (i., p. 192). The particular form
which his theory of the relation of jaws to limbs took is shown in the
following passage:--"The maxillary bone has ... the significance of an
extremity and at the same time that of a rib or lower arch of a
vertebra, just as the pelvic bones unite in themselves the signification
of ribs and proximal members of the hinder extremity" (Meckel's
_Archiv_, p. 367, 1826).

He appreciated the morphological idea of the serial repetition of parts,
and gave it accurate formulation. The whole vertebrate body, he
considered, was composed of a longitudinal series of _morphological
elements_, each of which was made up a section from each of the
fundamental organs--a vertebra, a section of the nerve-cord, and so on
(_Entwickelungsgeschichte_, ii., p. 53). Groups of these morphological
elements formed _morphological divisions_, such as the vertebral
segments of the head with their highly developed neural arches, or the
segments of the neck with their undeveloped haemal arches. The
morphological elements are clearly shown only in the animal parts, but
there are indications in the embryo of a segmentation also of the
vegetative parts,--the gill-slits, for instance, and the vascular
arches. The vegetative parts, however, develop on the whole
unsymmetrically (_cf._ Bichat). These elements which von Baer
distinguishes are morphological units, as he himself points out,
contrasting them with organs which are not usually units in a
morphological sense. "We call organ," he writes, "each part that has by
reason of its form or its function a certain distinctiveness, but this
concept is very indefinite, and posses