cess of layer-formation
is due. Pander, however, had distinguished only three germ-layers, an
upper "serous" layer, a lower "mucous" layer and a middle
"vessel-layer." He it was who introduced the terms "Keimhaut"
(blastoderm) and "Keimblatt" (germ-layer).

[Illustration: FIG. 7.--Ideal Transverse Section of a Vertebrate Embryo.
(After von Baer.)]

The honour of being the founder of the germ-layer theory is sometimes
attributed to C. F. Wolff, notably by Koelliker and O. Hertwig. Wolff, it
is true, in his memoir _De formatione intestinorum_ (1768-9) showed that
the alimentary canal was first formed as a flat plate which folded round
to form a tube, and in a somewhat vaguely worded passage he hinted that
a similar mode of origin might be found to hold good for the other
organ-systems. But it seems clear that Wolff had no definite conception
of the process of layer-formation as the first and necessary step in all
differentiation. This, at any rate, was von Baer's opinion, who assigns
to Pander the glory of the discovery of the germ-layers. "You," he
writes, "through your clearer recognition of the splitting of the
germ--a process which remained dark to Wolff--have shed a light upon all
forms of development" (p. xxi.).

We have now seen, following von Baer's exposition, how development is
essentially a process of differentiation, a progress from the general to
the special, from the homogeneous to the heterogeneous; we have analysed
the process into its three subordinate processes--primary, histological
and morphological differentiation. So far we have considered development
in general and the laws which govern it; we have now to consider the
varieties of development which the animal kingdom offers in such
profusion, in order to discover what relations exist between them. This
is the problem set in the fifth Scholion. Baer at once brings us face to
face with the solution of the problem attempted in the Meckel-Serres
law. It is a generally received opinion, he writes, that the higher
animals repeat in their development the adult stages of the lower, and
this is held to be the essential law governing the relation of the
variety of development to the variety of adult form. This opinion arose
when there was little real knowledge of embryology; it threw light
indeed upon certain cases of monstrous development, but it was pushed
altogether too far. It complicated itself with a belief in a historical
evolution;--"People gradual