Haeckel and his Embryos

A Note on Textbooks
by Ken Miller and Joe Levine

Page 223 of the Lion Book (BIOLOGY - The Living Science) and page 283 of the Elephant Book (BIOLOGY by Miller and Levine) each contain drawings of the early stages of embryonic development in several vertebrates. Although neither of these drawings are identical to his, they are based on the work of Ernst Haeckel (portrait at left), a 19th century German Biologist who was a pioneer in the study of embryonic development.

Haeckel noticed that vertebrate embryos pass through a series of similar stages in early development, and argued that there was a good reason for this. As an organism evolves, he reasoned, it does so by tacking on new stages to its process of embryonic development. Therefore, as an organism passes through embryonic development it actually re-traces every stage of its evolutionary ancestry. This idea became known as "Ontogeny recapitulates Phylogeny," which literally means "Development is a replay of Ancestry."

(At Left): For examples of the early stages of development in vertebrate embryos. Unlike Haeckel's drawings, these photographs are absolutely accurate and have been taken at similar stages of development.

Top: a Fish Embryo
Next: a Chick Embryo
Next: a Pig Embryo
Bottom: a Human Embryo

All of these photomicrographs were taken by the Swedish biophotographer Lennart Nillson, and can be viewed directly at the Odyssey of Life website (part of the NOVA science series on Public Broadcasting)

As you read this, you may wonder why evolution should be limited to changes tacked on at the end of the process of development. So did evolutionary biologists, and Haeckel's idea was quickly discarded. In fact, evolution can affect all phases of development, removing developmental steps as well as adding them, and therefore embryology is not a strict replay of ancestry. Nonetheless, many of the stages that embryos pass through can indeed be understood as remnants of their evolutionary past.

One example is the fact that the embryos of all placental mammals (including humans) form a yolk sac during their development. Why is this important? Because the eggs of these organisms do not have large amounts of stored yolk, and therefore their yolk sacs are empty! Nontheless, the persistence of a yolk sac stage makes perfect sense when one considers that these animals are descended from egg-laying reptiles in which the sac encloses a massive amount of yolk to support embryonic development

(Above) A Developing forelimb in the Human Embryo


This idea has been pushed back into the news recently by the news that Haeckel's drawings of embryonic similarities were not correct. British embryologist Michael Richardson and his colleages published an important paper in the August 1997 issue of Anatomy & Embryology showing that Haeckel had fudged his drawings to make the early stages of embryos appear more alike than they actually are! As it turns out, Haeckel's contemporaries had spotted the fraud during his lifetime, and got him to admit it. However, his drawings nonetheless became the source material for diagrams of comparative embryology in nearly every biology textbook, including ours!

More information on Embryonic Development:

 

So, what have we done?
Well, we fixed it!

In 1998 we rewrote page 283 of the 5th edition to better reflect the scientific evidence. Our books now contain accurate drawings of the embryos made from detailed photomicrographs:

 

Update written on 12/21/97 by Ken Miller

Kenneth R. Miller
Professor of Biology
Brown University
Providence, RI 02912

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