From: parker@ehsn17.cen.uiuc.edu (Robert S. Parker)
Subject: Re: When is a fetus not a person?
Organization: University of Illinois at Urbana
Distribution: usa
Date: Mon, 25 Jan 1993 23:52:46 GMT

sfm@manduca.neurobio.arizona.edu (Stephen Matheson) writes:

>From article <1993Jan19.004526.25747@Princeton.EDU>,
>by datepper@phoenix.Princeton.EDU (David Aaron Tepper):

>> In article <1993Jan18.230432.20379@organpipe.uug.arizona.edu>,
>> sfm@manduca.neurobio.arizona.edu (Stephen Matheson) writes:

>>>From article <1993Jan17.085933.8172@Princeton.EDU>,
>>>by datepper@phoenix.Princeton.EDU (David Aaron Tepper):

>>>> I finally found the article I was looking for: _The_New_Republic_,
>>>> Dec. 7, 1992 issue. There's a story about the use of RU-486 as a
>>>> "morning-after" pill. It seems that the egg, up until 14 days or so
>>>> after conception, has nothing that can confer even numerical
>>>> identity on it. I'm butchering the article badly, of course, but
>>>> do read the article.

[discussion of humble admissions and punishments omitted]

>> Okay, the gist of the article: Let's say there is an egg that splits
>> into two, forming what will become two identical twins. The question
>> is, what happened to the original egg?

>> a) The egg is dead. If this were true, the mother should not only
>> be rejoicing that she has two healthy children, but mourning the
>> loss of the original egg. Nonsense.

>Uh-huh.
[Is that agreement?  I would *think* so...but he almost sounds sarcastic.]

>> b) One of the twins is the "real" original egg; the other twin is
>> only a copy. Yet both twins have an equal claim to be that original
>> egg. How can one differentiate which one is the original egg? Scrap
>> this one.

>Okay.

Besides, if that were a reasonable way of thinking of it then only one of
your cells would be the *real* you, and it would probably have died a long
time ago.  Too bad.   ;)

>> c) The egg is split up among the two new twins. While true as far as
>> this goes, this means that at the two-cell stage, half of each twin
>> is the original egg, and the other half is material gathered from
>> "outside" the original. Which leads to saying that neither is the
>> "real" egg.

Actually, material is not "gathered from outside" the egg until implantation,
which does not occur until after many many divisions (at which point twinning
is not possible).  I'm sure you knew that.  Each one is only *half* the "real"
egg.  They are able to develop to full size because they can absorb material
through the placenta.  (I don't think non-mammals have twins comming from the
same egg, which is because they only have the material in the egg itself.)
If you *meant* gathered from the placenta, well that doesn't happen until
implantation, and it accounts for essentially all of the mass of a newborn.
(otherwise, we'd all be the size of a pinpoint)


>> The article goes on to note that there is nothing in the genome that
>> codes for identical twinning -- instead, it is caused by environmental
>> factors, e.g. a temporary cutoff of oxygen. Since twinning can occur
>> up until about 14 days after conception,

>Really?!?  I can see a morula, or even a blastula, dividing into 2
>identical halves, but at 14 days I'm guessing that the embryo is
>awfully complex...  I'll look this up...

I would doubt twinning at 14 days.  If someone has a reference that it is
possible...ok.  I remember learning that cell differentiation is significant
after 8 divisions (256 cells), which I'm pretty sure happens before
implantation which is (I hope) sooner than 14 days.  After that point,
separation would not lead to two well-developed twins.  There may be some
other mechanism of twinning, but I don't know how it could occur.

>> until that point there is
>> no reason to believe that the egg even has "numerical identity". (To
>> tell the truth, I see where he is going, but it could still be argued
>> that as long as only one copy of the egg exists, it certainly does
>> have numerical identity. The logic here, now that I've gone over it,
>> is admittedly fuzzy.)

That piece of fuzz under your bed is also "unique".  I guess that would
make it "bad" to vaccuum it up and destroy it's structure...  Kinda silly,
huh?   ;)

>My problem with all this is that IMO the egg ceased to exist the
>moment it divided.  In developmental biology, we talk of symmetric
>and asymmetric cell division.  The cleavages of the egg and of the
>blastomeres are examples of symmetric division: one cell divides to
>yield 2 daughters that are roughly equivalent.  In this case, it
>becomes rather odd to consider the continued existence of the
>original cell.

I don't think that's a very good way of looking at it.  All of your cells
will either die or divide symetrically within the next week.  (maybe less)
Does that mean that you cease to exist a week from now?  Hopefully not.
A better way to look at it might be that the egg is "developing" (but the
same egg).  After all, a chicken egg does not become a different egg just
because the chicken develops from a single cell to a pre-born chick.

>                The repeated divisions of some neuroblasts are
>examples of asymmetric division: one cell divides to yield a
>daughter and another neuroblast identical to the first.  In this
>case, the neuroblast is considered to be repeatedly dividing
>asymmetrically to yield progeny; it is considered to "survive"
>the divisions.

But this doesn't happen early in the development where twinning is possible,
does it?  (in humans)

>I don't claim to understand the spiritual implications of twinning,
>given a definition of personhood or ensoulment beginning at
>conception.  But I'm also not sure how useful this "numerical
>identity" stuff can be.

>Steve Matheson

-Rob
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