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(Waddington 1956 ). The long-noted von Baer’s rule of thumb suggested a relation-
ship between the axis of the egg and that of the embryonic axis (von Baer 1828 ).
This axis is most often (~70 %) situated perpendicular to the long axis of the egg,
with the left side of the embryo oriented towards the blunt end. Eggs rotate clock-
wise about the long axis as they travel through the oviduct (~0.2 rpm) and continue
rotating in the uterus, where they acquire the shell membranes and shell. As the egg
rotates, the blastoderm is maintained at an angle of ~45°, balancing the inertia of the
rotation with the tendency of the buoyant blastoderm to float on the dense yolk
(Clavert 1962 ). In this arrangement, the anterior of the embryo forms at the lower
end. Most eggs enter the uterus and are laid sharp-end first and thus end up follow-
ing von Baer’s rule. The minority of cases where the blunt end enters first are thus
truly exceptions that prove the rule, since it is only the embryo’s orientation with
respect to the external egg shape that is changed; the posterior of the embryo still
forms at the upper end of the blastodisc.
Both in utero and in vitro experiments have defined a critical period for axis
establishment in the uterus (Vintemberger and Clavert 1959 ; Clavert 1961 ; reviewed
in Clavert 1962 ). Egg orientation was manipulated at different times prior to egg-
laying and the orientation of the embryonic axis was altered if the presentation of the
egg was changed at least six hours prior to laying. Additionally, eggs could be
removed and incubated in vitro in a rotating cylinder and the orientation of the
embryo was determined by the relative direction of rotation. Similar to the experi-
ments in frogs, the effects of earlier rotations could be overridden by later manipula-
tions up to the critical period of axis formation. This period correlates with the time
yolkalbumennucleus of Pandersubgerminal
ooplasm
direction of egg rotationpost.ant.post.ant.end view top viewFig. 6.5 Model for establishment of asymmetry in bird eggs. Left, sectional view of a uterine
chicken egg viewed from the sharp end. The direction of rotation is indicated; because of this rota-
tion, the lighter blastoderm cytoplasm is maintained off angle as it continually floats to the highest
point. The blastoderm is exposed to the subgerminal cytoplasm, which is hypothesized to contain
axis determinants (blue). At this stage, the blastoderm is several thousand cells and has not formed
the area pellucida epiblast. Right, top view of 2–3 day embryo showing anterior-to-posterior axial
polarity. This embryo would conform to von Baer’s rule, with head oriented away with the blunt
end positioned left. ant. anterior, post. posterior
D.W. Houston