W. Rowan, K. M. Parker and J. Bell 
U5 
The Length of egg L may be considered as the easiest cliaracter to determine 
and needs no further comment. 
The Breadth of egg B should be closely related to the Lateral Girth Gi, and 
in most cases the relationship Gij = 7rB is very closely satisfied. If we sum and 
take the means we have 
7r=Mean Lateral Girth/Mean Breadth. 
This gives in the present material : 
7r = 3-224. as against 3142, 
which marks an error of about 2'6%, rather larger than we might anticipate, and 
possibly due to the inclusion of a certain number of slightly damaged eggs, and 
the measurement of the eggs in the field and not in the laboratory. The relation 
between and i? is a useful test of accuracy and should be determined with a 
slide rule before the egg is finally replaced in the nest, or lost sight of. 
The Longitudinal Girth Gi is somewhat more difficult to measure, and a rough 
test of its accuracy not so easy to determine as in the case of Gi,. We have, how- 
ever, developed a formula for determining Gi in terms of B and L, and on testing 
it we find that as a rule the differences are below 1'5 mm. Such a formula may 
be useful as emphasising the need for remeasurements, when the observed and 
calculated girths have values much in excess of I'o mm. We are not prepared 
to say, however, that the coefficients in this formula can be extended beyond 
the case of the Common Tern. 
While the Length-Breadth Index is valuable as giving a measure of the 
ellipticity of the egg, it is not of much influence on the apparent oval shape, 
unless we suppose some theoretical geometrical construction for the egg. If we 
suppose the blunt end of the egg to be approximately spherical, the hemisphere 
ending with the maximum breadth, then the egg might be considered as divided 
into two portions, the upper or hemispherical with radius ^B and the lower with 
length from the base of the hemisphere (or 'equator') to the lower pole =L — \B. 
The ratio of these two segments of the length depends only on the index BjL. 
Thus it is conceivable that this index has actually as much association with 
ovality as with ellipticity, although without some geometric theory of egg-shape, 
we are not able to make any dogmatic assertion as to the value of BjL. It 
seems, however, a character of considerable interest as being free of absolute size 
and also some measure of shape. If I = BjL and 0 be the ratio of ^B to L — ^B, 
HIT 
i.e. 0 = 2 — j^jii ^"^^ i^i^y consider 0 a measure of the ovality, and we 
have correlated 0 for eggs of the same clutch as well as /. Of course, since 0 is 
a fuiiction of /, there will be relatively little difference in the results. 
Biometrika x 19 
