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Haeckel's Drawings

Explore Evolution incorrectly asserts that Haeckel’s Biogenetic Law claims that the earliest stage of embryos are most similar. Haeckel's concept of caenogenesis fully acknowledged that there can be signficant differences between embryos including at the earliest stages of development. This misrepresentation of the Biogenetic Law allows Explore Evolution to set up erroneous claims that Haeckel committed fraud and that the distinctions between the early stages of development of different classes of vertebrates argue against common descent.

Following Darwin's lead, Haeckel tried to discover the evolutionary history of various animals by studying their embryos. He produced a set of influential drawings showing that the embryos of various classes of vertebrates were very similar during their earliest stages of development.
Explore Evolution, p. 66. Emphasis added.

This claim is false. Haeckel was aware of significant departures in the morphology of early embryos from what would be predicted by his Biogenetic Law. These differences in order, timing and pattern of embryogenesis were termed as caenogenesis by Haeckel. He also understood that a prominent reason for why embryos showed a signficant difference in their cleavage and gastrulation patterns was due to the amount of yolk in their eggs. As Michael Richardson (the developmental biologist whose work re-ignited the interest in Haeckel's embryos) and Gerhard Keuck observe:

The early divergence, which violates some of von Baer's laws, is due to differences in egg size and patterns of cleavage and gastrulation among species. Recent explanations of the conservation of midembryonic stages, despite variations in early development, include the idea that they are subject to strong stabilizing forces (e.g. selection, pleiotropy; see Wagner & Misof, 1993; Raff, 1996; Wagner, 1996). Haeckel was aware of these early differences, and they were included among his caenogenetic exceptions.
Richardson and Keuck, 2002. "Haeckel's ABC of Evolution and Development," Biol. Rev. 77 , p. 507

Why are Haeckel's embryos focused upon in this section? Most likely because this diagram was found in many high school biology textbooks, and that Michael Richardson's research on vertebrate embryos rekindled an old controversy about whether the early embryos in the diagram are accurate. However, as Richardson and colleagues note, this hardly undermines the strong support for common descent from embryology, despite the claims of Creationists and ID proponents.

Data from embryology are fully consistent with Darwinian evolution. Haeckel’s famous drawings are a Creationist cause célèbre (3). Early versions show young embryos looking virtually identical in different vertebrate species. On a fundamental level, Haeckel was correct: All vertebrates develop a similar body plan (consisting of notochord, body segments, pharyngeal pouches, and so forth). This shared developmental program reflects shared evolutionary history. It also fits with overwhelming recent evidence that development in different animals is controlled by common genetic mechanisms. (4)
Richardson et al. (1998) "Haeckel, Embryos and Evolution." Science, 280:983

Finally, given that Explore Evolution claims:

This book is one of the first textbooks ever to use the inquiry based approach to teach modern evolutionary theory. It does so by examining the current evidence and arguments for and against the key ideas of modern Darwinian theory.
Explore Evolution, preface

it is remarkable that Explore Evolution fails to include recent work in evo-devo in the Embryology: Case For section. We now know there is an evolutionarily conserved "genetic toolkit" - a set of genes responsible for constructing all animals, from sea anemones to fruit flies to humans (Carroll et al. 2005, Davidson 2005). The only mention of the genetic regulation of development refers to the outdated macromutation theory from the 1940's by Richard Goldschmidt. So while Explore Evolution generally acknowledges that modern biologists consider embryology to provide strong support for common descent, students are at a high risk of incorrectly concluding that this support is from the diagram of Haeckel's embryos.