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 possesses, 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.