1G2 Sijinmetry of Egg and Symmetry of Embryo in the Frog 
ought to be produced by gravitation, as in I. A and B, does not appear. The 
number of instances (85) is perhaps too small. The effect of gravity is noticeable 
enough in III. A, but there is no excess in the frequency at —90° compared with 
that at + 90", only a slight rise between 0° and — 90°, which is not represented on 
the other side. I believe, though I am not certain, that the light came from + 90° 
in this case. Nevertheless we may conclude from the position of the Plane of Sym- 
metry under all these conditions of pressure, gravity, and light in varying combina- 
tion, that its direction in the egg may most certainly be affected by gravity and to 
some extent by light. It does not appear to be affected by pressure at all. It tends 
to lie in the gravitation symmetry plane, and the grey crescent appears on the side 
on which the white pole was upturned. It is clear, therefore, that the half-hour that 
passes before the egg assumes its normal position is time enough for gravity to 
impress a permanent effect upon the egg, and so affect the position of the Symmetry 
Plane. That gravity is not indispensable may, of course, be at once inferred from 
the appearance of the grey crescent in eggs which have never had to turn over at all. 
C. We have now to enquire whether and how far the egg symmetry becomes 
the Symmetry of the Embryo. 
The frequencies are given in Table XVI., the corresponding polygons in Fig. 3. 
It will be seen immediately that in some cases at least the direction of the 
Sagittal Plane follows very closely on that of the Plane of Symmetry. 
In column I. B and Fig. 3. I. B, are given the positions of this plane in eggs 
which were placed closely together, had their axes horizontal, and were kept in 
darkness until segmentation had begun, but thereafter exposed to day-light 
coming from -|- 90°. The Sagittal Plane exhibits a very marked tendency to lie 
in the gravitation plane, the dorsal lip being turned towards 0°, the side on 
which the yolk is upturned. Its direction follows, in fact, very closely upon that 
previously assumed (Table XV., I. B) by the Plane of Symmetry, the standard 
deviation for both having practically the same value, and, as we shall presently 
see, the correlation between the two is under these circumstances very high, 
though not complete. 
Turning to column I. A and Fig. 3. I. A we again find a close correspondence 
between the position of the Sagittal Plane and that previously occupied by the 
Plane of Symmetry (Table XV., I. A, and Fig. 2. I. A). The correspondence is, 
however, not perfect, for there is here as high, or rather a higher frequency about 
-I- 90° as about — 90°. This may be due to the light. 
III. Again, when the eggs are spaced, but the axes still horizontal, there is 
the same high frequency at 0° and low frequency at 180° (III. A and B) ; in both 
these cases there is also a high frequency at -)-90° and another at -90°. In III. B 
the direction of the light is known, from + 90, and, as I have said, I believe that it 
had the same direction in the A series. Should this be so, the high frequency 
at -I- 90° of the Sagittal Plane as compared with the lower frequency at the same 
point of the Plane of Symmetry (Fig. 2. III. A) must, I think, be attributed to 
