Ernst Haeckel (1834–1919)

Romanes' 1892 copy of Ernst Haeckel's allegedly fraudulent embryo drawings (this version of the figure is often attributed incorrectly to Haeckel).^[6]

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. The series of twenty-four embryos from the early editions of Haeckel's Anthropogenie remain the most famous. The different species are arranged in columns, and the different stages in rows. Similarities can be seen along the first two rows; the appearance of specialized characters in each species can be seen in the columns and a diagonal interpretation leads one to Haeckel's idea of recapitulation.

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.^[7] In and around 1800, embryology fused with comparative anatomy as the primary foundation of morphology.^[8] 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.^[9] 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.^[10] 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.^[11] 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.^[12]

Haeckel argues that certain features in embryonic development are conserved and palingenetic, while others are caenogenetic. Caenogenesis represents "the blurring of ancestral resemblances in development", which are said to be the result of certain adaptations to embryonic life due to environmental changes.^[13] In his drawings, Haeckel cites the notochord, pharyngeal arches and clefts, pronephros and neural tube as palingenetic features. However, the yolk sac, extra-embryonic membranes, egg membranes and endocardial tube are considered caenogenetic features.^[14] The addition of terminal adult stages and the telescoping, or driving back, of such stages to descendant's embryonic stages are likewise representative of Haeckelian embryonic development. In addressing his embryo drawings to a general audience, Haeckel does not cite any sources, which gives his opponents the freedom to make assumptions regarding the originality of his work.^[15]

Karl Ernst von Baer (1792–1876)

Haeckel was not the only one to create a series of drawings representing embryonic development. 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. In relation to developmental timing, von Baer's scheme of development differs from Haeckel's scheme. Von Baer's scheme of development need not be tied to developmental stages defined by particular characters, where recapitulation involves heterochrony. Heterochrony represents a gradual alteration in the original phylogenetic sequence due to embryonic adaptation.^[16] As well, von Baer early noted that embryos of different species could not be easily distinguished from one another as in adults.

Von Baer's laws governing embryonic development are specific rejections of recapitulation.^[11] As a response to Haeckel's theory of recapitulation, von Baer enunciates his most notorious laws of development. Von Baer's laws state that general features of animals appear earlier in the embryo than special features, where less general features stem from the most general, each embryo of a species departs more and more from a predetermined passage through the stages of other animals, and there is never a complete morphological similarity between an embryo and a lower adult.^[17] Von Baer's embryo drawings^[18][19] display that individual development proceeds from general features of the developing embryo in early stages through differentiation into special features specific to the species, establishing that linear evolution could not occur.^[20] Embryological development, in von Baer's mind, is a process of differentiation, "a movement from the more homogeneous and universal to the more heterogeneous and individual."^[21]

Von Baer argues that embryos will resemble each other before attaining characteristics differentiating them as part of a specific family, genus or species, but embryos are not the same as the final forms of lower organisms.

Wilhelm His (1831–1904)

Wilhelm His was one of Haeckel's most authoritative and primary opponents advocating physiological embryology.^[22] His Anatomie menschlicher Embryonen (Anatomy of human embryos) employs a series of his most important drawings chronicling developing embryos from the end of the second week through the end of the second month of pregnancy. In 1878, His begins to engage in serious study of the anatomy of human embryos for his drawings. During the 19th century, embryologists often obtained early human embryos from abortions and miscarriages, postmortems of pregnant women and collections in anatomical museums.^[23] In order to construct his series of drawings, His collected specimens which he manipulated into a form that he could operate with.

In His' Normentafel, he displays specific individual embryos rather than ideal types.^[24] 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.^[25] 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.^[26] His' embryology is not explained in terms of ancestral history.

The debate between Haeckel and His ultimately becomes fueled by the description of an embryo that Wilhelm Krause propels directly into the ongoing feud between Haeckel and His. Haeckel speculates that the allantois is formed in a similar way in both humans and other mammals. His, on the other hand, accuses Haeckel of altering and playing with the facts. Although Haeckel is proven right about the allantois, the utilization of Krause's embryo as justification turns out to be problematic, for the embryo is that of a bird rather than a human. The underlying debate between Haeckel and His derives from differing viewpoints regarding the similarity or dissimilarity of vertebrate embryos. 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. His also counteracts Haeckel's sequence of drawings in the Anthropogenie with what he refers to as "exact" drawings, highlighting specific differences. Ultimately, His goes so far as to accuse Haeckel of "faking" his embryo illustrations to make the vertebrate embryos appear more similar than in reality. His also accuses Haeckel of creating early human embryos that he conjured in his imagination rather than obtained through empirical observation. His completes his denunciation of Haeckel by pronouncing that Haeckel had "'relinquished the right to count as an equal in the company of serious researchers.'"^[27]

Early opponents: Ludwig Rutimeyer, Theodor Bischoff and Rudolf Virchow

The first suggestion of fakery against Haeckel was made in late 1868 by Ludwig Rutimeyer in the Archiv für Anthropogenie.^[28] Rutimeyer was a professor of zoology and comparative anatomy at the University of Basel, who rejected natural selection as simply mechanistic and proposed an anti-materialist view of nature. Rutimeyer claimed that Haeckel "had taken to kinds of liberty with established truth".^[29] Rutimeyer claimed that Haeckel presented the same image three consecutive times as the embryo of the dog, the chicken, and the turtle.^[30]

Theodor Bischoff (1807–1882), was a strong opponent of Darwinism. As a pioneer in mammalian embryology, he was one of Haeckel's strongest critics. Although Bischoff's 1840 surveys depict how similar the early embryos of man are to other vertebrates, he later demanded that such hasty generalization was inconsistent with his recent findings regarding the dissimilarity between hamster embryos and those of rabbits and dogs. Nevertheless, Bischoff's main argument was in reference to Haeckel's drawings of human embryos, for Haeckel is later accused of miscopying the dog embryo from him.^[28] 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".^[31]

Contemporary criticism of Haeckel: Michael Richardson and Stephen Jay Gould

Michael Richardson and his colleagues in a July 1997 issue of Anatomy and Embryology,^[32] demonstrated that Haeckel falsified his drawings in order to exaggerate the similarity of the phylotypic stage. 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.^[33] On the other hand, one of those who criticized Haeckel's drawings, Michael Richardson, has argued that "Haeckel's much-criticized drawings are important as phylogenetic hypotheses, teaching aids, and evidence for evolution".^[34] But even Richardson admitted in Science Magazine in 1997 that his team's investigation of Haeckel's drawings were showing them to be "one of the most famous fakes in biology."^[35]

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 .^[36]