J. W. Jbnkinson 
161 
A question that of course will obviously occur is, to what are the deviations 
from coincidence due ? They may be the result of internal or external factors, and 
of the latter heat and light and gravity at once suggest themselves as possible. 
Many years ago Pflliger showed that, by preventing the jelly from absorbing water, 
the egg of the frog could be prevented from rotating inside it and compelled to 
remain in any arbitrarily selected position. The first and second furrows were, 
however, shown to be vertical, the third horizontal as in the normal egg. The 
median plane of the embryo was determined by the plane which included the 
original, now tilted, egg-axis and the present vertical axis, a plane afterwards 
termed by Born, who examined the internal structure of eggs so placed in 
" Zwangslage," the " streaming meridian," since there occurred equally on each 
side of it an upward streaming of cytoplasm and pigment, a downward sinking of 
the heavy yolk granules. The first furrow, according to Pfluger, in such inverted 
eggs may make any angle with this plane; according to Born, it is generally either 
in or at right angles to it, and Roux corroborates this. 
It is evident that under the influence of gravity a very marked bilateral 
arrangement is conferred upon the constituents of the egg and that this symmetry 
impresses itself on segmentation and embryonic development, and it does not 
seem impossible that, before the fertilized egg, which is laid with its axis in any 
position, is able to rotate inside its jelly membranes, a slight bilateral symmetry 
may be conferred upon it under the influence of gravity, and that this may interfere 
with the other bilaterality produced by the entrance of the sperm. 
I attempted to measure the angle between the original position of the egg 
(before rotation), the plane of symmetry and the first furrow ; but the measure- 
ments are, 1 am afraid, too few and too inconclusive. I give here, however 
(Fig. 11), a curve of the angles made by the first furrow with the streaming 
40 
30 
S 20 
s 
w 
10 
0 10 20 30 40 50 60 70 80 90 
Fig. 11. The First Furrow and the ' Gravitation Symmetry Plane.' 
meridian (gravitation symmetry plane) of a number of eggs kept in " Zwangslage." 
If the measurements are not too few (215) to be trusted, the curve brings out the 
very interesting point that the first furrow tends to lie either in, or at right angles 
Biometrika v 21 
