158 Symmetry of Egg and Symmetry of Emhryo in the Frog 
I. When the eggs are close and their axes horizontal the First Furrow tends 
to lie parallel to the length of the slide. This is the position it ought to occupy 
when under the influence of pressure in that direction. It is however doubtful 
whether the pressure is in reality responsible for this very marked effect. 
II. When the eggs are close, but the axes vertical, the First Furrow still lies 
in the direction of pressure, but to a less extent. It would appear then that this 
plane tends to some extent — a slight extent, truly — to place itself at right angles 
to the plane of gravitation, and this supposition is borne out by the persistence of 
this tendency when the eggs are spaced and the pressure therefore removed, but 
the axes horizontal (III.), and by its disappearance when the eggs are spaced and 
the axes vertical (IV.). 
In fact the value of the standard deviation of the distribution about 90° 
steadily increases as first gravity only, then pressure only, and thirdly both ftxctors 
are eliminated. 
There seems, therefore, no escape from the conclusion that the First Furrow 
places itself in the direction of what I will still call the " pressure," but perpen- 
dicular to the gravitation plane. Gravity, therefore, even during the short interval 
that elapses between insemination and the turning over of the egg can apparently, 
though only to a slight extent, influence the position of the First Furrow. 
In permanently inverted eggs the First Furrow, as has been mentioned, tends 
mainly to lie in or at right angles to or at 45° to the gravitation plane. Here only 
the second of these positions can be detected. It would not perhaps be going too 
far to attribute the difference to the continued action of gravity in the other case. 
The direction of division depends on the position in the egg of the fertilization 
spindle, since the Furrow passes in the equator of the mitotic figure. This depends 
in turn on the direction of elongation of the spindle, or on that of the initial 
division of the centrosome, and it is not difficult to understand how the centro- 
somes might preferably divide in, or the spindle be brought into the meridian of 
streaming set up by gravity in the cytoplasm. Should this streaming be more 
marked and lasting the two pronuclei, i.e. the equator of the spindle, not its axis, 
might be dragged into this plane and then division would be in the plane instead 
of at right angles to it. 
In Table XIV. II. and Fig. 1 II. it will be seen that the frequency at 0" 
is high ; the same peculiarity is observable in IV. I cannot find any explanation 
for this except by supposing that on some of the slides the rows and not merely 
the eggs in each row may have been in contact, and hence that there may have 
been a little "pressure" across, in addition to that along the slide. 
It would seem that light exerts very little, if any, effect upon the position of the 
Furrow. In column I. B of Table XIV. are given the frequencies for the First 
Furrow when the eggs are kept in darkness. There is, as far as can be gathered 
from so small a number, the same tendency for the Furrow to lie in the direction of 
pressure as in the whole series (I. A) in which the majority were exposed to day- 
light falling upon them from one end of the slide. 
