48
J. CLAVERT
FIG. 9. Photograph showing a blastodisc of an avian egg removed from the uterus
about 2 hr before laying. The eccentric pellucid area is quite apparent and the plane of
bilateral symmetry is determined.
has to give the egg a chosen tilt and presentation (the axis of the
cylinder and that of the egg coinciding). If the cylinder is horizontal and
rotates clockwise (viewed from the right), with the sharp end of the egg
also at the observer's right, the orientation of the embryo will follow
the rule of von Baer. If the blunt end is at the observer's right the
embryo will be directed the other way. In the first case, if the cylinder
axis is tilted down to the right, the cephalo-caudal axis of the embryo
shifts towards the sharp end. The greater the tilt, the further it shifts:
with a tilt of 60° it shifts through 90° to the right, and above this limit
there is no effect. We were unable to formulate a precise mathematical
relation since the egg structure is variable. As mentioned above, the
thickness of the inner layer of liquid albumen plays an important part
in maintaining egg equilibrium and in permitting compensating movements. In vitro we were able to verify that orientation is established by
the last rotation; even if this is done several hours before the critical
phase. We were able to show that rotations are effective as long as the
area pellucida has not begun to appear. The formation of the area
pellucida corresponds with some important transformation of the germ
which we are now trying to identify.
J. CLAVERT
FIG. 9. Photograph showing a blastodisc of an avian egg removed from the uterus
about 2 hr before laying. The eccentric pellucid area is quite apparent and the plane of
bilateral symmetry is determined.
has to give the egg a chosen tilt and presentation (the axis of the
cylinder and that of the egg coinciding). If the cylinder is horizontal and
rotates clockwise (viewed from the right), with the sharp end of the egg
also at the observer's right, the orientation of the embryo will follow
the rule of von Baer. If the blunt end is at the observer's right the
embryo will be directed the other way. In the first case, if the cylinder
axis is tilted down to the right, the cephalo-caudal axis of the embryo
shifts towards the sharp end. The greater the tilt, the further it shifts:
with a tilt of 60° it shifts through 90° to the right, and above this limit
there is no effect. We were unable to formulate a precise mathematical
relation since the egg structure is variable. As mentioned above, the
thickness of the inner layer of liquid albumen plays an important part
in maintaining egg equilibrium and in permitting compensating movements. In vitro we were able to verify that orientation is established by
the last rotation; even if this is done several hours before the critical
phase. We were able to show that rotations are effective as long as the
area pellucida has not begun to appear. The formation of the area
pellucida corresponds with some important transformation of the germ
which we are now trying to identify.