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Jan 14 2013

What I taught today: a little old-school history of embryology

This is an abbreviated summary of my class lecture in developmental biology today. This was the first day of class, so part of the hour was spent on introducing ourselves and going over the syllabus, but then I gave a lightning fast overview of the history of developmental biology.

Classical embryology began with Aristotle, whose work was surprisingly good: he approached the problem of development with relatively few preconceptions and fairly accurately summarized what was going on in the development of the chick. Most of this old school embryology is descriptive and was really a narrow subset of anatomy, but there were a few major conceptual issues that concerned the old investigators, in particular the question of preformation (the plan of the embryo is laid out in the egg) vs. epigenesis (the plan of the embryo emerges progressively). Aristotle, by the way, was on the right side of this debate, favoring epigenesis.

In the 19th century, development was seen as a progressive process that paralleled the hierarchical organization of nature — that is, developmental biology, what there was of it, was coupled to the great ladder of being. This is not an evolutionary idea, but reflects the view that there was a coherent pattern of greater and lesser development that was part of a coherent divine plan for life on earth. The German ‘Natural Philosophers’ pursued this line of reasoning, often to degrees that now look ridiculous in hindsight. In contrast, there were developmental biologists like Karl Ernst von Baer who wanted nothing to do with a cosmic teleology but instead preferred to emphasize observation and data, and simple minimal hypotheses.

In the late 19th century, developmental biology split into two directions. One was a dead end; Ernst Haeckel basically lifted the explanatory framework of the natural philosophers, replaced divinity with evolution, and tried to present development as a parallel process to evolution. Von Baer had already demolished this approach, and despite a few decades of popularity Haeckelian recapitulation died as a credible framework for studying evolution in the early years of the 20th century. The other direction developmental biology took was Wilhelm Roux’s Entwicklungsmechanik, or experimental embryology. This was an approach that largely eschewed larger theoretical frameworks, and focused almost exclusively on observation and experimental manipulation of embryos. It was a successful discipline, but also divorced mainstream developmental biology from the evolutionary biology that was increasingly influential.

As examples of Entwicklungsmechanik, I discussed Roux’s own experiments in which he killed one cell in a two-cell embryo and saw partial embryos result, an observation that fit with a preformationist model, but more specifically a mosaic pattern of development, in which patterns of development were encoded into the cytoplasm or cortex of the egg. Those experiments were seriously flawed, however, because the dead cell was left attached to the embryo, and could have deleteriously affected development. The experiments of Hans Driesch were cleaner; he dissociated embryos at the four cell stage, cultured each blastomere independently, and discovered that each isolated cell developed fully into a complete, miniature larva.

Driesch, unfortunately, interpreted these results to imply that there was an entelechy, or guiding intelligence outside the embryo, and that the only conceivable explanation was the existence of purpose behind embryology. This was also a dead end; the modern explanation for the phenomena is that they regulated, that is, that cells determine their fate by interacting with one another, rather than some kind of cosmic plan. And that’s really going to be a major focus of this course: how do cells communicate with one another, how are genes regulated to set up coherent and consistent patterns of gene expression that produce the organized cell types we find in an adult multicellular plant or animal?

That set up the next lecture. Entwicklungsmechanik, while representing a solid and productive research program, quickly reached its limits, because what we really needed to examine were those patterns of gene expression rather than trying to infer them from observations of morphology. The big breakthrough was the melding of developmental biology and molecular biology — most of the modern developmental biology literature focuses on examining interactions between genes. So on Wednesday we’ll get another fast overview of the molecular genetics research program, and a bit of evo-devo.

Slide thumbnails (PDF)

19 comments

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  1. 1
    David Marjanović

    An interesting evo-devo paper just came out in Nature (online early). So did an amazing paper on the vertebral columns of Ichthyostega, Acanthostega and Pederpes

  2. 2
    David Marjanović

    *headdesk* Forgot the main reason for making that comment in the first place:

    Entwicklungsmechanik, or experimental embryology

    Literally “development mechanics”, probably alluding to the hands-on manipulation of embryos, perhaps also to a theory-free or -reduced approach.

  3. 3
    PZ Myers

    Yep. Very reductionist, very focused on careful observation and experimental manipulation with a minimum of theoretical overloading.

  4. 4
    Karen Locke

    Thank you for the summary. Keep it up, and bio-ignorant people like me might learn more!

  5. 5
    ChasCPeterson

    Surely this “history” is incomplete. It mentions vortices nowhere.

  6. 6
    PZ Myers

    Or toroids!

    That’s for the advanced class.

  7. 7
    Alexandra (née Audley)

    … how do cells communicate with one another, how are genes regulated to set up coherent and consistent patterns of gene expression that produce the organized cell types we find in an adult multicellular plant or animal?

    Oooo, I’m tingling with anticipation!

    (And gracias for the .pdf warning! ♥)

  8. 8
    poxyhowzes

    And by a tiny amount, the total amount of ignorance in the pharyngulasphere was reduced today. Thank you, PZ. I shall look forward to future lessons, even if the playback is set to “fast forward.”

    And, having omitted to pay my tuition, I shall refrain from asking what happened between the time of Aristotle and the time people learned to speak German.

    pH

  9. 9
    rubberband

    So, does this mean I’m gonna get to follow the whole semester here? If yes, COOL! Great lectures without the tuition.

  10. 10
    No One

    PZ Myers

    Yep. Very reductionist, very focused on careful observation and experimental manipulation with a minimum of theoretical overloading.

    Is that like creationists favorite science “observational science”?

  11. 11
    Nerd of Redhead, Dances OM Trolls

    Is that like creationists favorite science “observational science”?

    My impression was like evolutionary psychologists who actually look at the genome rather than presupposing “adaptations” without evidence…

  12. 12
    makeinu

    Poor Ernst Haeckel. If only he’d stuck to his delightful drawings of natural forms. That was his real, and probably only, talent.

  13. 13
    Azkyroth Drinked the Grammar Too :)

    Cue Pitwits crowing that PZ is a hypocrite for talking about chicks…

  14. 14
    broboxley OT

    David (or anyone else) Entwicklungs is development. What is the roots of that? Reason I ask is that it blew my very rudimentary German up.
    PZ thanks for the lecture always look forward to learning new stuff

  15. 15
    gmoney

    PZ thanks for this. I wish I had taken developmental biology as a part of my degree. The only exposure I had to developmental biology was in Penn State’s Bio 102 with Dr. Richard Cyr, 20 years ago. Hoping you’ll continue to post about what you are teaching, so I can be a bit of a free rider.

  16. 16
    David Marjanović

    Entwicklungs is development. What is the roots of that?

    Entwicklung is development, yes; the -s- connects the parts of the word. Literally, Ent-wickl-ung historically means un-wrapp-ing, exactly as de-velop-ment does, because that’s how it was once thought to work.

  17. 17
    broboxley OT

    thank you David

  18. 18
    wbenson

    P.Z. Myers says: “Karl Ernst von Baer who wanted nothing to do with a cosmic teleology but instead preferred to emphasize observation and data, and simple minimal hypotheses.”

    What Karl von Baer actually said: “If . . . the general result which has just been expressed [i.e., “[t]he history of the development of the individual is the history of its increasing individuality in all respects”] be well based and true, then there is one fundamental thought which runs through all forms and grades of animal development, and regulates all their peculiar relations. It is the same thought which collected the masses scattered through space into spheres, and united them into systems of suns; it is that which called forth into living forms the dust weathered from the surface of the metallic planet. But this thought is nothing less than Life itself, and the words and syllables in which it is expressed are the multitudinous forms of the Living.” [Pp. 237-8, in Henfrey & Huxley (eds.) 1853, ‘Scientific Memoirs, selected from . . . etc., etc.,’ translation by Thomas H. Huxley, downloaded from archive-org].

    Sounds pretty much like cosmic teleological woo to me. Ernst Haeckel proposed that shared developmental sequences resulted from common descent rather than Baer’s “thought” that collected matter “into systems of suns.”

    Cheers!

  19. 19
    lpetrich

    As to “learning to speak German”, German is a descendant of earlier languages, just as English, Greek, and every other present-day or historical one is. In fact, back in Aristotle’s day, English and German didn’t exist — they were still proto-Germanic, spoken in northern Germany and southern Denmark.

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