Gill slit

[Pete]

I'm confused about something. Its probably a mere problem of semantics but I figured I'd ask to make sure.

 

The following site defines Gill slit (http://en.wikipedia.org/wiki/Gill_slit) as follows

Gill slits are gills with individual openings rather than an outer cover.

So gill slits are a type of gill. But at the bottom it says

In the 19th century, gill slits of vertebrate embryos were erroneously thought to be actual gills, and thus evidence for the recapitulation theory.

Those two statements taken together are confusing to me.

 

gill slits of vertebrate embryos -Are they are or are they not gill slits?

 

Is this an issue of semantics?

Edited January 24, 2009 by Pmb

[Daecon]

Maybe it means the form of gills, but not the function?

 

As in, the embryo couldn't use them to "breathe" with, it has to rely on the mother's blood for oxygen?

[Pete]

Maybe it means the form of gills, but not the function?

 

As in, the embryo couldn't use them to "breathe" with, it has to rely on the mother's blood for oxygen?

Doesn't that imply that comment? Gill slits on fish embyros are not actual gills? What use is such a comment? Its like saying fetal legs are not actual legs because the fetus can't use them to walk.

 

If that's all there is to it then this was a dumb question. Lol!

Edited January 24, 2009 by Pmb

[Daecon]

Well, I don't really know for sure. Maybe the gill slits on the embryos aren't connected to the circulatory system, so they don't actually work despite being 'real' gills.

 

Perhaps the genes for gill creation are still encoded in the DNA, and expressed during embryonic pre-development, but are suppressed or overwritten by lung genes being dominant when the lung genes start to become expressed in later embryonic development. Or I could be talking rubbish. *shrug*

 

Anyway... aren't fish vertebrates? They have spines, don't they?

[Pete]

Note - Pmb = Pete

Well, I don't really know for sure. Maybe the gill slits on the embryos aren't connected to the circulatory system, so they don't actually work despite being 'real' gills.

 

Perhaps the genes for gill creation are still encoded in the DNA, and expressed during embryonic pre-development, but are suppressed or overwritten by lung genes being dominant when the lung genes start to become expressed in later embryonic development. Or I could be talking rubbish. *shrug*

 

Anyway... aren't fish vertebrates? They have spines, don't they?

 

I think that the problem lies in the fact that I came to think about this because a theist claimed that human embryos don't have gills. So the real problem is that I started discussing science with a theist (even though I am one).

[Mokele]

Basically, what human and all vertebrate embryos have are properly called Pharyngeal Pouches. In fish, the pouches continue to develop, and open to the outside, forming slits, while gills form behind them (and are actually part of a separate developmental pathway). In mammals and other non-gilled vertebrates, the pouches recede, and the area becomes the neck (some traces of the originally patterning is still evidence in nerve positions, etc).

 

There is one exception - the first gill slit. In jawless fish (lampreys and hagfish), it becomes a true gill slit. However, in jawed fishes, the bones on either side have become part of the jaw apparatus, and the first gill slit is reduced to a tiny hole. This tiny hole is visible behind the eye of modern sharks and rays, called a spiracle, and has no associated gills (though it can be used as a passage for water, allowing the animals to breathe while on the bottom). It actually persists in all vertebrates, though often covered by skin, and in everything that has ears, it's been re-routed to the inner ear. You know how your ears can "pop"? That's the tube (called the Eustachian tube in humans) between your mouth area and your ear (technically, the opening is in the nasopharynx, the throat behind the nose but above the mouth).

 

Mokele

[Pete]

Basically, what human and all vertebrate embryos have are properly called Pharyngeal Pouches.

Thank you. I didn't mention that since I looked that term up before I started this thread and came across sources which implied that it was just a synonym for gill slits. E.g. see

http://en.wikipedia.org/wiki/Recapitulation_theory

For example, Haeckel believed that the human embryo with gill slits (pharyngeal arches) in the neck not only signified a fishlike ancestor, but represented an adult "fishlike" developmental stage.

This page seems to indicate that gill slit is a synonym of pharyngeal arche. I also learned about this in Biology, by Sylvia S. Mader, page 607

The diagram illustrates that the embryonic chordates have a notochord and a dorsal hollow nerve cord. Another characteristic of all vertebrate embryos is the presence of gill pouches or slits (fig. 27.9). Only the lower vertebrates (fishes and amphibian larvae) use the gill slits as functioning structures. The fact that higher forms go through this embryonic stage indicates that the higher forms are related to lower forms. The phrase “ontogeny (development) recapitulates (repeats) phylogeny (evolutionary history)” was coined some years ago as a dramatic way to suggest that animals share the same embryonic stages.

and from B]What Evolution Is, by Ernst Mayr (http://home.honolulu.hawaii.edu/~pine/book1qts/embryo-compare.html)

...why should the embryos of birds and mammals develop gill slits, like fish embryos?

All this seems to imply that Pharyngeal Pouches is just a formal/scientific name for gill slits. These are simply more reasons why I asked Is this an issue of semantics?

[iNow]

Note - Pmb = Pete

OT, but are you saying that you have two accounts, and that PMB is the same person as Pete?

 

 

EDIT: Nevermind. I see this has already been cleared up elsewhere in the forum. They are the same guy, but this is not an issue. Please carry on. Sorry for the distraction.

Edited January 24, 2009 by iNow

[Mokele]

All this seems to imply that Pharyngeal Pouches is just a formal/scientific name for gill slits. These are simply more reasons why I asked Is this an issue of semantics?

 

They're not the same thing, and the examples above are either sloppy writing, or simplifications for a lay audience.

 

In all vertebrate embryos, you get pharyngeal pouches, but they're just pouches. A slice through the embryo would show you a series of thickenings joined by thin sheets of tissue, but without any actual "slit" or other communicating opening to the outside. In gill-less animals, the pouches never become true openings - the thick-thin-thick pattern is gradually overgrown by surrounding cells. Only in animals with actual gills do the thin portions become thinner and eventually open to the outside.

 

So basically, the terms are not synonymous - a "gill slit" can only apply when there are actual openings, which doesn't happen unless the animals actually has gills.

 

Now, as for recapitulation theory, part of it was because embryology wasn't very advanced yet, so it may not have been obvious to early embryologists that there were no actual openings. But some of it was simply wishful thinking and a refuted hypothesis. Organisms *do* share some developmental basis, and there are developmental relicts of our ancestry(such as in human kidney development), but it's much more complex than the simple 'recapitulation' that was initially favored.

 

Mokele

[Pete]

They're not the same thing, and the examples above are either sloppy writing, or simplifications for a lay audience.

Thanks. That answers my question. Much appreciated.

Now, as for recapitulation theory, ...

I only saw the term "gill slits" from that page on which recapitulation theory was menioned. And I only came across that page when I did a search on that term. I looked through a book by Richard Dawkins and found recapitulation theory in it. He referred to recapitulation theory as unfashionable.

 

The only assumption I made was what I learned in that biology text, i.e. that in some species those things become gills which in other species they become other things. From what I read, in humans the first gill pouch forms the middle ear and the Eustachian tube, the second becomes the tonsils, the third and fourth become the thymus and parathyroid and the fitfh pouch disappears.

---

Merged post follows:

Consecutive posts merged

They're not the same thing, and the examples above are either sloppy writing, or simplifications for a lay audience.

Why would Mayr refer to pharyngeal pouches as gills?

 

Also, Mayr implies that there is an structure in fetal embryos which become tails in monkeys and disappears in humans. Is there a name for that structure?

Edited January 24, 2009 by Pete
Consecutive posts merged.

[Mokele]

Why would Mayr refer to pharyngeal pouches as gills?

 

Because he's an ornithologist, not an embryologist. The "gill slits" story is pretty prevalent, and I only know it to be false after having some embryology eduction.

 

Because biology is so phenomenally diverse, once you get far beyond a scientist's relatively small area of expertise, they're often no better than a well-read layman - I'm a world expert in snake locomotion, but on molecular biology, I'm pretty awful, and there are likely plenty of undergrads and laymen who know more about molecular biology than I do.

 

Mokele

[Pete]

Because he's an ornithologist, ...

 

Its my understanding that he started out as an ornithologist and ended up as an evolutionary biologist. The Harvard University Press refers to Mayr as a ..giant among evolutionary biologists

http://www.hno.harvard.edu/gazette/daily/2005/02/04-mayr.html

Widely considered the world's most eminent evolutionary biologist and even one of the 100 greatest scientists of all time, ...

Pretty impressive credentials!!

 

Nature refers to him as The evolutionary biologist... They write (http://www.nature.com/news/2005/050131/full/news050131-19.html)

The evolutionary biologist Ernst Mayr died on 3 February at the age of 100, after a short illness. A hugely prolific writer and researcher, he was instrumental in developing modern ideas in evolutionary theory.

FYI - See also

http://www.achievement.org/autodoc/page/may1bio-1

http://www.pbs.org/wgbh/evolution/library/06/2/l_062_01.html

...I'm a world expert in snake locomotion..

I suffer from ophidiophobia so I'm not responsible for my actions if your "little friends" get near me.

The "gill slits" story is pretty prevalent, and I only know it to be false after having some embryology eduction.

Perhaps its a matter of interpretation and semantics. E.g. if the embryo is at a stage where the structures that become the eyes are present then it may be legitimate to refer to them as the eyes even though they aren't fully formed. Perhaps when Mayr refers to the pharyngeal pouches as gills that he's doing something analogous.

Edited January 24, 2009 by Pete

[Mokele]

Perhaps when Mayr refers to the pharyngeal pouches as gills that he's doing something analogous.

 

Probably. In technical literature, such a glossing-over of differences would be impossible, but in popular literature, it's practically a necessity (otherwise it winds up as unreadable as the technical literature). It may even have been insisted upon by the editor or publisher of his book.

 

Mokele

[Pete]

Probably. In technical literature, such a glossing-over of differences would be impossible, but in popular literature, it's practically a necessity (otherwise it winds up as unreadable as the technical literature). It may even have been insisted upon by the editor or publisher of his book.

 

Mokele

I read that book and recall that chapter. If that was his intent then it was a very serious mistake. I.e. the purpose of that portion of the book was to argue that early embryonic stages have features in common across certain species. If he didn't really means gills then it would appear as if he was intentionally trying to mislead the reader. I don't know how else to interpret it otherwise.

[Mokele]

The pharyngeal pouches are indeed common across vertebrates. The difference is that only in gilled animals do they perforate and become gill slits.

 

I should also note that this is precisely why books are considered inferior to scientific articles - books aren't peer-reviewed, so mistakes can slip through.

 

Mokele

[Pete]

The pharyngeal pouches are indeed common across vertebrates. The difference is that only in gilled animals do they perforate and become gill slits.

Why is that anyway? Do you think' date=' perhaps, that ist an evolution thing like I've read about?

 

Eyes become eyes, heads become heads, but pharyngeal pouches become various things. From what little I know of evolution it seems like its related to the idea that, loosely speaking, animals evolved from fish. So perhaps those structures originally all became gills. And perhaps that's what Mayr was arguing.

I should also note that this is precisely why books are considered inferior to scientific articles - books aren't peer-reviewed, so mistakes can slip through.

What kind of mistake could he have made? Do you believe Mayr made an mistake in his reasoning or his or was the mistake in his knowledge of embrology? I don't mean to be difficult but Mayr was one of the greatest scientists of his time and a very proliphic writer. His books for the layman aren't exactly easy reading so its not as if he dumbed them down. I just find it a hard to accept that Mayr made a bogus arguement on what must have been to him an obvious falsehood. For this reason I'd prefer to find out why he wrote what he did rather than to dismiss it as a mistake.

 

I know of no example of a physics book where the author used such a horribly weak arguement.

Edited January 25, 2009 by Pete

[Mokele]

He was clearly mistaken in using "gill slits", probably due to lack of knowledge of embryology.

 

The underlying point about similarities in embryology is true, he was just wrong about the stage.

[Pete]

Okay. I think I pretty much have what I came to ask.

 

Here is the gist of what I got from all this:

 

Mayr used the term gill to refer to the structure that will become the gill if the embryo is a fish. These same structure become other things in other species.

 

Analogy - The term eye could be used to refer to the structure that will become the eye.

Edited January 25, 2009 by Pete

[Mokele]

Pretty much, yeah.