198
THE ESSEX NATURALIST.
under tail coverts, and a patch of white feathers on the middle
of the breast, forming an irregular horse-shoe shaped mark.
Ref.: Bulletin, Brit. Ornith. Club, vol. 35, pp. 45-6.
Part 4.—Abnormal Colouration in the Eggs of Essex Birds.
It has been shown that seven well-marked substances have
been found in the colouring matter of eggs and that all their
tints are due to the admixture of these in varying proportion.
A brown-red colour is the most important as it occurs in the shells
of a very large number of eggs, its entire absence being excep-
tional, and it is of a very permanent character. Two are of a
very fine blue with certain differences between them. The
fourth is a bright yellow, which when mixed with one of the
previous two gives a beautiful green colour. The fifth is a red-
dish-yellow pigment. The sixth has not been determined and the
seventh is described as a brick-red pigment. These form the
colouring matter and the next step is to consider how it is
applied to the shell of the egg. It is assumed that the pigment is
deposited by the walls of the oviduct and in the following manner.
The deposition occurs in two parts of the oviduct, during the
formation of the earlier layers of the shell, when a slight staining
results, and at a later stage when the egg is nearly complete. In
most cases there is no colouration until the egg has passed some
way down the duct. The ground colour is apparently first
deposited, followed by the application of the irregular markings
peculiar to the egg. If these are formed when the egg is at rest
a well defined spot results. If the deposition of the pigment
occurs while the egg is in motion, smears and blotches are pro-
duced, and streaks and lines are attributed to rotation. This
rough outline embodies the main facts but for amplification of the
subject Newton and Pycraft may be consulted. Before we pro-
ceed it must be pointed out that the colours of eggs are never
due to structure. So far as I am aware not even a plausible
explanation has been formed of the cause of abnormal colouration
of eggs.
Heterochrosis in eggs cannot be considered entirely under the
same heads as plumage. Of the four terms we know of only one
which has been used by oologists, namely erythrism, and this is
generally understood to mean an egg which exhibits abnormally
red colouration of some degree or other. Xanthism and melanism