J. W. Jenkinson 149 
The results of this investigation are presented in tabular form herewith 
(Table L). 
TABLE I. 
First Furrow 
and Sagittal 
Plane 
Plane of 
Symmetry and 
Sagittal Plane 
Plane of Symmetry and First Furrow 
All cases 
About 0° only 
About gO"" only 
o- 
p 
40-39° + -65 
•138 + -0:31 
29-75" + -63 
•372 ±-025 
48-33°+ 1-U 
•087 ± -032 
18-70° + -60 
23-29° ±-86 
In this experiment the eggs were placed, haphazard with regard to the direction 
of their axes, in rows parallel to the length of long glass slides, and the angles were 
measured between the various planes and lines ruled across the slides. Under these 
circumstances the eggs in each row become pressed against one another by their 
jelly-membranes, the direction of the pressure being parallel to the length of the 
slide ; and since the axis may make any direction with the vertical, it is possible 
that during the half-hour which elapses before the eggs turn over into their normal 
position with the axes vertical and the white (yolk) pole below, gravity may act 
upon, and cause, as in Pfliiger's permanently inverted eggs, a redistribution of the 
contents (the heavy yolk granules sinking to the lower side, the lighter cytoplasm 
and pigment rising to the upper side) and so impress upon the eggs a gravitation 
plane of bilateral symmetry (the plane, including the egg-axis and the vertical, 
on each side of which the yolk descends, the cytoplasm ascends in a corresponding 
manner). 
It is known of course that when the eggs are kept inverted this plane becomes 
the median plane of the embryo, that side on which the white pole is upturned 
being dorsal, and further that the First Furrow, while it may make any angle with 
this plane, tends usually to lie in, or at right angles to or at an angle of 45° to it. 
It is also well known that when eggs ai'e strongly compressed at right angles to- 
the axis the First Furrow lies in the direction of pressure, the median plane of the 
embryo at right angles to it (0. Hertwig). Thirdly, the direction of the incident 
light (day-light) may conceivably exert some influence upon the position of one or 
more of these planes (though Roux has indeed stated that this is not the case), and 
so upon the angles they make with one another. 
In order to discover therefore whether any of these external agencies do affect 
the direction of the planes and so of the angles between them, the eggs have been 
examined under four principal different conditions. 
I. The eggs were placed close together in longitudinal rows, with their axes 
horizontal, and the white pole facing the same side of the slide so that each egg 
had to turn through 90° to gain its normal position. The pressure therefore is 
parallel to, the gravitation symmetry plane at right angles to the length of the 
slide. 
