Haeckel's embryo drawings are primarily intended to express his theory of embryonic development, the Biogenetic Law, which in turn assumes (but is not crucial to) the evolutionary concept of common descent. His postulation of embryonic development coincides with his understanding of evolution as a developmental process. In and around 1800, embryology fused with comparative anatomy as the primary foundation of morphology. Ernst Haeckel, along with Karl von Baer and Wilhelm His, are primarily influential in forming the preliminary foundations of 'phylogenetic embryology' based on principles of evolution. Haeckel's 'Biogenetic Law' portrays the parallel relationship between an embryo's development and phylogenetic history. The term, 'recapitulation,’ has come to embody Haeckel's Biogenetic Law, for embryonic development is a recapitulation of evolution. Haeckel proposes that all classes of vertebrates pass through an evolutionarily conserved “phylotypic” stage of development, a period of reduced phenotypic diversity among higher embryos. Only in later development do particular differences appear. Haeckel portrays a concrete demonstration of his Biogenetic Law through his "Gastrea" theory, in which he argues that the early cup-shaped gastrula stage of development is a universal feature of multi-celled animals. An ancestral form existed, known as the gastrea, which was a common ancestor to the corresponding gastrula.