Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!boulder!eddy
From: eddy@boulder.Colorado.EDU (Sean Eddy)
Newsgroups: sci.bio
Subject: Re: How do twins work?
Message-ID: <8276@boulder.Colorado.EDU>
Date: 21 Apr 89 18:09:14 GMT
References: <8243@boulder.Colorado.EDU> <3791@silver.bacs.indiana.edu>
Sender: news@boulder.Colorado.EDU
Reply-To: eddy@boulder.Colorado.EDU (Sean Eddy)
Organization: University of Colorado, Boulder
Lines: 21

<3791@silver.bacs.indiana.edu>chiaravi@silver.UUCP (Lucius Chiaraviglio) writes:
>	Not likely, unless an aberrantly large polar body had been formed.  I
>have seen scanning electron micrographs of eggs with polar bodies attached,
>and the polar bodies have < 1/64 of the volume of the egg.  The smallest
>fraction of an egg (formed by mitotic division of the zygote followed by
>separation of the resulting cells) that I have heard of being able to form a
>viable embryo in mammals is 1/16 (and that only in some mammals), so a polar
>body of the size shown in these electron micrographs would die, possibly after
>a few divisions.

Your argument is not a valid one. Separating cells at two-cell,
four-cell (or 16-cell) stages of development and trying to get
the separated cells to develop into a viable embryo 
addresses the question of when crucial developmental information 
in the embryo gets segregated into different cells.
Such an experiment does not address the question of how much
egg cytoplasm is required to get a viable embryo.

- Sean Eddy
- Molecular/Cellular/Developmental Biology; U. of Colorado at Boulder
- eddy@boulder.colorado.EDU		!{hao,nbires}!boulder!eddy