In His' "Normentafel", he displays specific individual embryos rather than ideal types. His does not produce norms from aborted specimens, but rather visualizes the embryos in order to make them comparable and specifically subjects his embryo specimens to criticism and comparison with other cases. Ultimately, His' critical work in embryonic development comes with his production of a series of embryo drawings of increasing length and degree of development. His' depiction of embryological development strongly differs from Haeckel's depiction, for His argues that the phylogenetic explanation of ontogenetic events is unnecessary. His argues that all ontogenetic events are the "mechanical" result of differential cell growth. His' embryology is not explained in terms of ancestral history.