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.

His ’ depiction of embryological development strongly differs from Haeckel ’ s depiction, for His argues that the phylogenetic explanation of ontogenetic events is unnecessary.

As Erland Nordenskiöld argues, charges of fraud against Haeckel are unnecessary.

Furthermore, Haeckel even proposed theoretical life-forms to accommodate certain stages in embryogenesis.

Haeckel ’ s opponents found his methods problematic because such simplification eliminates certain structures that differentiate between higher and lower vertebrates.

In his drawings, Haeckel cites the notochord, pharyngeal arches and clefts, pronephros and neural tube as palingenetic features.

Throughout Haeckel ’ s time, criticism of his embryo drawings was often due in part to his critics ' belief in his representations of embryological development as “ crude schemata .” In this way, Haeckel specifically selected relevant features to portray in his drawings.

The features uniting the Lissamphibia were first noted by Ernst Haeckel.

In modern biology ( e. g. Haeckel and Fritz Müller ), palingenesis has been used for the exact reproduction of ancestral features by inheritance, as opposed to kenogenesis, in which the inherited characteristics are modified by environment.

Haeckel introduced the concept of " heterochrony ", which is the change in timing of embryonic development over the course of evolution.

Haeckel ’ s illustrations show vertebrate embryos at different stages of development, which exhibit embryonic resemblance as support for evolution, recapitulation as evidence of the Biogenetic Law, and phenotypic divergence as evidence of von Baer ’ s laws.

Haeckel ’ s embryo drawings are primarily intended to express his idiosyncratic theory of embryonic development, the Biogenetic Law, which in turn assumes ( but is not crucial to ) the evolutionary concept of common descent.

The term, ‘ recapitulation ,’ has come to embody Haeckel ’ s Biogenetic Law, for embryonic development is a recapitulation of evolution.

Haeckel was not the only one to create a series of drawings representing embryonic development.

In response to Haeckel ’ s evolutionary claim that all vertebrates are essentially identical in the first month of embryonic life as proof of common descent, His responds by insisting that a more skilled observer would recognize even sooner that early embryos can be distinguished.

Haeckel encountered numerous oppositions to his artistic depictions of embryonic development during the late nineteenth and early twentieth centuries.

Haeckel ’ s opponents believe that he de-emphasizes the differences between early embryonic stages in order to make the similarities between embryos of different species more pronounced.

Haeckel promoted and popularized Charles Darwin's work in Germany and developed the controversial recapitulation theory (" ontogeny recapitulates phylogeny ") claiming that an individual organism's biological development, or ontogeny, parallels and summarizes its species ' evolutionary development, or phylogeny.

Haeckel advanced a version of the earlier " recapitulation theory ", previously set out by Étienne Serres in the 1820s and supported by followers of Geoffroy including Robert Edmond Grant, which proposed a link between ontogeny ( development of form ) and phylogeny ( evolutionary descent ), summed up by Haeckel in the phrase " ontogeny recapitulates phylogeny ".

Haeckel ’ s ‘ Biogenetic Law ’ portrays the parallel relationship between an embryo ’ s development and phylogenetic history.

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.

Karl E. von Baer and Haeckel both struggled to model one of the most complex problems facing embryologists at the time: the arrangement of general and special characters during development in different species of animals.

As a response to Haeckel ’ s theory of recapitulation, von Baer enunciates his most notorious laws of development.

Haeckel then provided a means of pursuing this aim with his biogenetic law, in which he proposed to compare an individual's various stages of development with its ancestral line.

For Haeckel, language specifically represented the concept that all phenomena of human development relate to the laws of biology.

" Haeckel ’ s ABC of evolution and development.

Ernst Haeckel ( 1866 ), in his endeavour to produce a synthesis of Darwin's theory with Lamarckism and Naturphilosophie, proposed that " ontogeny recapitulates phylogeny ," that is, the development of the embryo of every species ( ontogeny ) fully repeats the evolutionary development of that species ( phylogeny ), in Geoffroy's linear model rather than Darwin's idea of branching evolution.

Haeckel, who admired Darwin's work, defined ecology in reference to the economy of nature, which has led some to question whether ecology and the economy of nature are synonymous.

On the other hand, Michael K. Richardson, Professor of Evolutionary Developmental Zoology, Leiden University, while recognizing that some criticisms of the drawings are legitimate ( indeed, it was he and his co-workers who began the modern criticisms in 1998 ), has supported the drawings as teaching aids, and has said that " on a fundamental level, Haeckel was correct "

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.

Although Rutimeyer did not denounce Haeckel ’ s embryo drawings as fraud, he argued that such drawings are manipulations of public and scientific thought.

In a March 2000 issue of Natural History, Stephen Jay Gould argued that Haeckel “ exaggerated the similarities by idealizations and omissions .” As well, Gould argued that Haeckel ’ s drawings are simply inaccurate and falsified.

Some version of Haeckel ’ s drawings can be found in many modern biology textbooks in discussions of the history of embryology, with clarification that these are no longer considered valid.

In fact, ontogeny evolves – the phylogenetic history of a species cannot be read directly from its ontogeny, as Haeckel thought would be possible, but characters from ontogeny can be ( and have been ) used as data for phylogenetic analyses ; the more closely related two species are, the more apomorphies their embryos share.

In the case of echinoderms, he argued that the bilateral larvae must have been introduced after the establishment of the existing classes, and he challenged Haeckel ’ s view that these larvae are evidence that echinoderms evolved from bilateral ancestors.

Baruch Spinoza, Gottfried Leibniz, Gustav Theodor Fechner, Friedrich Paulsen, Ernst Haeckel, Charles Strong, and partially William James are considered panpsychists.

One issue they highlighted was Wells ' accusation that Haeckel forged images of embryos that are allegedly still in biology books.

Ernst Haeckel claimed that Negroes have stronger and more freely movable toes than any other race which is evidence that Negroes are connected to apes because when apes stop climbing in trees they hold on to the trees with their toes, Haeckel compared Negroes to “ four-handed ” apes.

Ernst Haeckel was particularly ardent, aiming to synthesise Darwin's ideas with those of Lamarck and Goethe while still reflecting the spirit of Naturphilosophie.

Ernst Haeckel expanded on the ideas of Gegenbaur while advocating the concepts of Charles Darwin.

In contrast, Anthropoidea by Mivart dates to 1864, while Simiiformes by Haeckel dates to 1866, leading to counterclaims of priority.

The presence of gill-like slits in the neck of the developing human embryo famously led Ernst Haeckel to postulate that " ontogeny recapitulates phylogeny "; this hypothesis, while false, contains elements of truth, as explored by Stephen Jay Gould in Ontogeny and Phylogeny.