164 Symmetry of Eyg and Symmetry of Embryo in the Frog 
II. When the eggs are close and the axes vertical the high frequency at 0° 
does not appear (II. A and B). The frequency at +90°, when the eggs are 
illuminated from + 90° (II. A), is less than that at — 90° but not so very much 
less as it is in the case of the Plane of Symmetry (Table XV., II. A and Fig. 2. 
II. A). In the case of the eggs illuminated from 0° (II. B) no relation can be 
traced between the directions of the Sagittal and Symmetry Planes. 
IV. Lastly, in those eggs which are freed from the influence of pressure and 
gravity, but exposed to light (coming from 90°) (IV.), there is a rough agreement 
in the position taken up by the two planes except that, as in the previous series, 
the frequency in the region of + 90° is increased. 
Taken as a whole this third set of observations appears to indicate that the 
dorsal lip of the blastopore shows some tendency to be formed on that side of the 
egg which is turned towards the light. There is, however, no evidence that its 
position is affected by the pressure. 
Speaking generally then we may say, as the result of the whole series of 
measurements, that 
(1) the position of the First Furrow depends upon the direction of pressure in 
a very marked degree ; its tendency is to coincide with that direction. In a less 
degree, it is dependent on the gravitation plane, since it to some extent avoids 
that plane, and places itself perpendicular to it. The First Furrow is not, however, 
influenced by the direction of incident light, 
(2) the Plane of Symmetry is independent of the direction of pressure, but is 
very markedly dependent on the gravitation plane, with which it exhibits a strong 
tendency to coincide. It is also apparently influenced by light, being placed in 
this plane with the grey crescent turned away from the source of light, 
(3) the Sagittal Plane is as independent of the pressure as is the Plane of 
Symmetry. It indeed follows the latter very closely, and would appear to be 
largely determined by it, though not completely, for while the dorsal lip exhibits 
nearly as great a tendency to appear on that side of the egg on which the white 
hemisphere was turned up as does the grey crescent, it also displays an inclination, 
unlike the grey crescent, for that side of the egg which is turned towards the light. 
More recently (this Spring, 1909) another series of experiments has been 
carried out with the view of ascertaining more exactly the effect of light upon the 
eggs when gravity and pressure were both removed. 
The eggs were all spaced and their axes were vertical. 
In one lot (A) they were exposed to light (from — 90°) throughout — from the 
beginning of fertilization till the appearance of the dorsal lip of the blastopore. 
In the second lot (B) they were kept in darkness till the beginning of segmen- 
tation. Some of these (B a) were then exposed to light coming from the same 
direction as before, while the remainder were kept permanently in darkness (B yS). 
The results of this experiment are tabulated in Tables XVII. — XXIII. 
