162 Symmetry of Egg and Symmetry of Embryo in the Frog 
to, or at 45" to the plane of symmetry, as though equally strong attractions were 
exerted by the two "predilection" planes, to use Roux's expression, upon the 
nuclear spindles. 
I have also made a few experiments (447 eggs) on the influence of heat and 
light upon the direction of the sagittal plane. The eggs were placed, as before, 
on slides ruled with parallel lines, in a damp chamber lined and covered with 
black cloth. They were then exposed continuously to the light and heat of an 
incandescent burner placed 15 inches away. As the curve (Fig. 12) shows, there 
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Fig. 12. Angle between direction of Light and the Sagittal Plane. 
seems to be a slight tendency for the sagittal plane to be diverted into either the 
direction of the incidence of light or a direction at right angles to it. The obser- 
vations are so few, and the tendency so slight, that I cannot lay especial stress 
upon the result, and publish it with all reserve. Further experiments with heat 
only, or light only, and light of various colours will perhaps make a more positive 
conclusion possible. 
The curves of Fig-s. 1, 5 and 8 are not and cannot be reduced to normal 
curves. I have drawn the appropriate normal curves on the top of these polygons 
and there is a complete absence of fit. The apex of the polygon in each case 
projects a good way above the apex of the probability curve, while the ends of 
the latter lie outside the ends of the polygon. 
The latter is very probably due to the fact that the limitation of the range 
of variability to 90° on each side is an artificial one. It is extremely likely that 
deviations slightly greater than 90° occur in small numbers, but that these have 
