efinitive whole exercises
guidance. This guidance is shown most clearly in the regulatory
processes of the germ, whereby the large individual variations commonly
presented by the early embryo are compensated for or neutralised in the
course of further development. Baer in this shows himself a vitalist.

It is, however, the third and subsequent Scholia which must here
particularly occupy our attention, for it is in these that von Baer
comes to grips with morphological problems. Already in the second
Scholion he had definitely enunciated the law which runs as a theme
throughout the volume, the observational and the theoretical part alike,
the law that development is essentially a process of differentiation by
which the germ becomes ever more and more individualised. "The essential
result of development," he writes, "when we consider it as a whole, is
the increasing independence (_Selbstaendigkeit_) of the developing
animal" (p. 148). In the third Scholion he elaborates this thought and
shows that differentiation takes place in triple wise. The three
processes of differentiation are "primary differentiation" or
layer-formation, "histological differentiation" within the layers, and
the "morphological differentiation" of primitive organs.

The first of these differentiations in time is the formation of the
germ-layers, which takes place by a splitting or separation of the
blastoderm into a series of superimposed lamellae. Baer's account of the
process in the chick is as follows:--

"First of all, the germ separates out into heterogeneous layers, which
with advancing development acquire ever greater individuality, but even
on their first appearance show rudiments of the structures which will
characterise them later. Thus in the germ of the bird, so soon as it
acquires consistency at the beginning of incubation, we can distinguish
an upper smooth continuous surface and a lower more granular surface.
The blastoderm separates thereupon into two distinct layers, of which
the lower develops into the plastic body-parts of the embryo, the upper
into the animal parts; the lower shows clearly a further division into
two closely connected subsidiary layers--the mucous layer and the
vessel-layer; the original upper layer also shows a division into two,
which form respectively the skin and the parts which I have called the
true ventral and dorsal plates--parts which contain in an
undifferentiated state the skeletal and muscular systems