Editing Embryo drawing (section)

==Famous embryo illustrators==
===Ernst Haeckel (1834–1919)===
[[Image:Haeckel drawings.jpg|thumb|250px|Romanes' 1892 copy of [[Ernst Haeckel]]'s allegedly fraudulent embryo drawings (This version of the figure is often attributed incorrectly to Haeckel.)<ref>Richardson and Keuck, "Haeckel's ABC of evolution and development," p. 516</ref>]]

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 [[Karl Ernst von Baer#Baer's laws|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.<ref>Nyhart, ''Biology Takes Form'', pp. 132–133</ref> In and around 1800, embryology fused with comparative anatomy as the primary foundation of [[Morphology (biology)|morphology]].<ref>Hopwood, "Pictures of Evolution and Charges of Fraud", p. 264</ref> 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.<ref>Richardson and Keuck, "Haeckel's ABC of evolution and development," p. 497</ref> 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.<ref>Nyhart, ''Biology Takes Form'', p. 9</ref> Haeckel proposes that all classes of vertebrates pass through an evolutionarily conserved "[[Phylotype|phylotypic]]" stage of development, a period of reduced phenotypic diversity among higher embryos.<ref name="Keuck p. 506">Richardson and Keuck, "Haeckel's ABC of evolution and development," p. 506</ref> 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.<ref>Nyhart, ''Biology Takes Form'', p. 159</ref>

Haeckel argues that certain features in embryonic development are conserved and [[Palingenesis|palingenetic]], while others are [[Caenogenesis|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.<ref>Richardson and Keuck, "Haeckel's ABC of evolution and development," p. 499</ref> In his drawings, Haeckel cites the [[notochord]], [[Branchial arch|pharyngeal arches]] and clefts, [[pronephros]] and [[neural tube]] as palingenetic features.  However, the [[yolk sac]], extra-embryonic membranes, egg membranes and [[Endocardium|endocardial]] tube are considered caenogenetic features.<ref>Richardson and Keuck, "Haeckel's ABC of evolution and development," p. 500</ref> 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.<ref>Hopwood, "Pictures of Evolution and Charges of Fraud", p. 270</ref>

===Karl Ernst von Baer (1792–1876)===
Haeckel was not the only one to create a series of drawings representing embryonic development.  [[Karl Ernst von Baer|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.<ref>Richardson, Michael K. and Gerhard Keuck, "Haeckel's ABC of Evolution and Development," p. 506</ref>
As well, von Baer early noted that embryos of different species could not be easily distinguished from one another as in adults.

[[Karl Ernst von Baer#Baer's laws|Von Baer's laws]] governing embryonic development are specific rejections of recapitulation.<ref name="Keuck p. 506"/> 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.<ref>Gould, ''Ontogeny and Phylogeny'', p. 56</ref> Von Baer's embryo drawings<ref>{{cite web|url=http://www.sites.hps.cam.ac.uk/visibleembryos/s3_1.html|title=Histories of development|series=Making Visible Embryos}}</ref><ref>{{cite journal|author1=Erki Tammiksaar |author2=Sabine Brauckmann |volume=26|number=3/4|journal=History and Philosophy of the Life Sciences
|title=Karl Ernst von Baer's 'Über Entwickelungsgeschichte der Thiere II' and its Unpublished Drawings|year=2004|pages=291–308, 471–474|jstor=23333718|pmid=16302690}}</ref> 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.<ref>Richards, ''The Meaning of Evolution'', pp. 57–59</ref> Embryological development, in von Baer's mind, is a process of differentiation, "a movement from the more [[wikt:homogeneous|homogeneous]] and universal to the more [[heterogeneous]] and individual."<ref>Richards, ''The Meaning of Evolution'', pp. 59–60</ref>

Von Baer argues that embryos will resemble each other before attaining characteristics differentiating them as part of a specific [[family (biology)|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.<ref>Gould, ''Ontogeny and Phylogeny'', p. 189</ref> 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.<ref>Hopwood, "Producing Development", p. 38</ref> 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.<ref>Hopwood, "Producing Development", p. 36</ref> 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.<ref>Hopwood, "Producing Development", p. 50</ref> 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.<ref>Di Gregorio, ''From Here to Eternity'', p. 277</ref> 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 (anatomist)|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.'"<ref>Hopwood, "Producing Development", p. 61</ref>