nt of a
Vertebrate accordingly one would not expect the vertebrae to appear
before the embryo had passed through many Invertebrate stages. But
experience shows the direct contrary, for in the chick the rudiments of
the vertebral axis appear sooner than any other part.

The theory of parallelism or recapitulation then is not borne out by the
facts, and clearly cannot be the law which we are seeking. But what then
is the true relation between the variety of development and the variety
of adult structure? Before answering this question we must review the
varied forms of adult organisation and consider in what relations they
stand to one another. In particular we must enquire whether they belong
to one type or to many. One point is here cardinal--we must distinguish
between the _type_ of organisation and the _grade_ of differentiation.
By "type" von Baer means the structural plan of the organism. "I call
the _type_ the spatial relationship of the organic elements and organs"
(p. 208). Each type of organisation characterises one of the big groups
of animals; the lesser groups represent "grade" modifications of the
type. "The product of the degree of differentiation and the type gives
the several great groups of animals which are called classes" (p. 208).
_Ausbildung_ (differentiation) takes place in one or other of several
directions, in adaptation, for instance, to life in the water or to life
in the air.

There are, von Baer considers, four main types--(1) the peripheral or
radiate type, (2) the longitudinal type, (3) the massive or molluscan
type, (4) the vertebrate type. The radiate type is shown by discoid
infusoria, by medusae, by starfish and their allies. The longitudinal
type characterises such genera as _Vibrio_, _Filaria_, _Gordius_, and
all the annulate animals. Mollusca, rotifers, polyzoa, and such
infusoria as are not included in types (1) and (2) belong to the massive
type, in which the body and its parts form rounded masses. The
longitudinal type is predominantly "animal," the massive type
predominantly "plastic" (vegetative). The vertebrate type has both the
"animal" and the "plastic" organs highly developed. In the symmetrical
arrangement of the animal parts it resembles the longitudinal type; its
plastic parts with their asymmetrical arrangement and rounded shape
belong to the massive type.

These types of von Baer inevitably recall the "Embranchements" of
Cuvier, with which they more or less coincide.