J. W. Jenkinson 
103 
been — in actual observation — included as large deviations on the opposite side of 
the curve*. 
In the case of the angle between the first furrow and either the sagittal plane 
or the plane of symmetry, this is unavoidable, since the two ends of the furrow 
ai-e, externally, alike ; but it would be possible — I am sorry to say I neglected to 
do this — to distinguish between deviations which are 180" apart and of opposite 
sign in the case of the plane of symmetry and the sagittal plane, since each of 
these is polarized, there being a larger extent of unpigmented yolk at one end 
of the plane of symmetry than at the other, and the sagittal plane being marked, 
at one end only, by the dorsal lip of the blastopore. 
Indirectly, it is true, the two ends of the first furrow might be distinguished 
from one another by the position of the furrow on the bilaterally symmetrical 
unpigmented yolk area ; but at the large deviations in question — about 90^ — this 
would hardly be practicable. 
With regard to the first point of difference between the frequency polygons 
and the normal curves, Professor Pearson suggested to me that the discrepancy 
might possibly be due 
(1) to a tendency of the planes not only to coincide, but to lie at 180° with 
one another, the two positions being indistinguishable in observation ; 
(2) to the existence of two kinds of eggs, one in which the planes practically 
always coincide, another in which they deviate one from another at random. 
The first supposition is untenable. 
As Schulze and Roux have pointed out, the dorsal lip always appears on one 
side of the egg, at one end of the plane of symmetry, namely on the side of the 
grey crescent, where the unpigmented area extends most nearly to the equator. 
With regard to the first furrow there is, externally, no difference between its ends ; 
the only internal difference is in the position in it of the male and female pronuclei, 
which lie a little away from, but on opposite sides of, the axis. One end of the 
plane of the first furrow might therefore be termed male, the other female. The 
male pronucleus must lie on that side of the egg on which the spermatozoon 
has entered, and this is always (Schulze and Roux) on the side opposite to the grey 
crescent. This plane could not, therefore, under any circumstances, deviate by both 
0° and as much as 180^' from either the plane of symmetry or the sagittal plane. 
With regard to the second proposed explanation. 
In Table X. will be found the parentage of the eggs used in the several 
experiments, with the date of each. 
* Professor Pearson obtained general formulae for fitting normal curves to the observations, by 
supposing the extremities of such normal curves beyond 90° cut off, reversed and added to the frequency 
on the opposite side ; but even so the observations failed to fit the normal curve modified in this 
manner. 
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