ses, from a morphological point of
view, little value. For this reason it seems necessary to introduce into
scientific morphology the concepts of morphological elements and
divisions" (ii., p. 84).

Von Baer exercised a very considerable influence upon the subsequent
trend of morphological theory. By his criticism of the Meckel-Serres
theory, he rid morphology for a time of an idea which was leading it
astray; by his substitution of the law that development is always from
the general to the special, he set morphologists looking for the
archetype in the embryo, not in the adult alone, and made them realise
that homologies could often best be sought in the earliest stages of
development; by formulating the germ-layer theory he supplied
morphologists with a new criterion of homology, based upon the special
relations of the parts (germ-layers) which are first differentiated in
all development. He made the study of development an essential part of
morphology.

[166] _De generatione Animalium_.

[167] _De formato foetu_, ? 1600; _De formatione
foetus_, 1604.

[168] _Exercitationes de generatione animalium_, 1651.

[169] _De formatione pulli in ovo_, 1673; _De ovo
incubato_, 1686.

[170] _De formatione pulli in ovo_, 1757-8; _Sur la
formation du coeur dans le poulet_, 1758.

[171] _Theoria generatioinis_, 1759; _De formatione
intestinorum_, 1768-9.

[172] _Beitraege zur Entwickelung des Huehnchens im Ei._
Wuerzburg, 1818. Also in Latin in shorter form, 1817.

[173] _Untersuchungen ue. die Entwickelungsgeschichte der
Fische_; Leipzig, 1835.

[174] Cuvier, in 1812, _Ann. Mus. d'Hist. Nat._, xix.; von
Baer in 1816, _Nova Acta Acad. Nat. Cur._ See
_Entwickelungsgeschichte der Thiere_, i., p. vii., f.n.

[175] Compare a parallel passage in Prevost et Dumas:--"At
the very first sight one will be struck with the
resemblance between the forms of the very early embryos
of these two classes, a resemblance so extraordinary
that one cannot refuse to admit the conclusions
resulting from it. The resemblance is so striking that
one can defy the most experienced observer to
distinguish in any way the embryos of dog or rabbit ...
from those of fowls or ducks of a corresponding
age."--_Ann. Sci. nat._, iii., p. 132, 1824.

[176] _De l'organisation des Animaux_, i., p. 140, 1822.

[177] "Ueber das aeussere und i