166 Symmetry of Egg and Symmetry of Embryo in the Frog 
Although, therefore, the differences in distribution observed in the different 
experiments may, in the case of the first furrow and sagittal plane, and the plane 
of symmetry and the first furrow, be possibly applied, as Professor Pearson has 
suggested, to the explanation of the discrepancy between the observed polygon 
and the corresponding normal curve, such an explanation will hardly hold good in 
the case of the plane of symmetry and the sagittal plane. 
I may add here that my results do not seem to lend support to Morgan's 
statement that, when the first furrow lies in the plane of symmetry, the sagittal 
plane coincides with both. 
This position of the first furrow occurs in my experiments B, G, G, H, I and J ; 
but in G, H and / the distribution of the angle between the first furrow and 
the sagittal plane is markedly crowded about 0", in B, G and J it is random. 
Further, in the " scatter " diagram (Fig. 9) of the correlation between the plane 
of symmetry and the first furrow, the dots which signify coincidence of the two 
are of course those which lie pretty thickly ranged along the diagonal. On 
Morgan's view all these dots should lie in the centre of the table : it is plain 
that they do not*. 
In conclusion, I have to express my thanks to Mr E. H. J. Schuster for the 
generous loan of his calculator, and to Professor Pearson for the suggestions he 
has been good enough to make. 
* It should be pointed out however that the tendency of the sagittal plane to lie in the plane of 
symmetry does increase slightly as the angle between the first furrow and the plane of symmetry 
diminishes. 
Thus the value of the standard deviation for the angle between plane of symmetry and sagittal 
plane for all the cases (Table IV.) is 
<7 = 29-75°±-63 (;t = 509, il/=2-23°±-89). 
For the 397 cases where the first furrow is also known (Table II.) 
<T = 30-16°±-72 (n = 397, M=3-41°±l-02). 
But if those cases only are considered in which the angle between first furrow and plane of symmetry 
is not greater than 45° (as in Table III.), then 
(r = 28-41° ±-87 (m = 238, Jlf=5-10° ± 1 -24). 
By taking the two middle arrays only of Table III. — those cases in which the said angle is not 
greater than 15° — 
(7 = 27 -94° ±1-09 (n=148, Jlf =5-16°±l-55) ; 
while when the range of the difference between first furrow and plane of symmetry is restricted to 5° (by 
taking the diagonal strip of Fig. 9), 
(r = 27-46°±l-32 (rt = 98, 4-84° ±1-87). 
