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early embryonic lineage consensus

Leon Avery leon at EATWORMS.SWMED.EDU
Mon Oct 28 07:22:22 EST 1996

The early embryonic lineage is invariant, and the P cell is always the
most posterior daughter, relative to the eggshell.  There is, however,
a reversal of division orientation in divisions 3 and 4 relative to 1
and 2.  This is because, as the first two divisions proceed, the P
cell gets pushed around the eggshell to a slightly dorsal position, so
that during the last two divisions posterior is the opposite direction
relative to the cell's axis as it was in the first two.  (I'm not sure
if this is making sense -- it's hard to describe without a picture.)

The reversal is most clearly seen when the eggshell is removed.  Then
all the P divisions are along the same axis, and the P granules go the
opposite direction in divisions 3 and 4 from 1 and 2.

Maybe this will clarify it:

   Without eggshell:

    AB  EMS  P4 D C

   With eggshell:
    /       C D \
   |AB         P4|

Well, maybe not.  Of course, in reality it doesn't look this simple
because AB and EMS have divided several times by the time D is

Leon Avery                                       (214) 648-2420 (office)
Department of Molecular Biology and Oncology              -2768 (lab)
University of Texas Southwestern Medical Center           -8856 (fax)
5323 Harry Hines Blvd                            leon at eatworms.swmed.edu
Dallas, TX  75235-9038                  http://eatworms.swmed.edu/~leon/

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