166 
EGGS OF BIRDS. 
of nature. The calcareous covering- of an egg- is concreted and formed 
in a most expeditious manner ; a few hours only seems necessary for 
this work. Only one vitellus separates from the ovarium at a time, 
(except as we shall hereafter mention,) till the exclusion of which no 
other succeeds. But as this is a daily production, with few exceptions, 
there is no more time allowed for perfecting the albumen and shell 
than twenty-four hours. 
As the course of nature is sometimes obstructed by remote causes, 
there are few general rules without some exceptions ; but an individual 
deviation by no means perverts the law of nature, but is simply an 
individual defect. We shall here instance imperfect eggs sometimes 
produced, such as want the vitellus^ and* others containing two yolks ; 
and although there are probably very few instances of such productions 
from birds in their wild state, yet it is no uncommon thing in domestic 
fowls. The egg of a hedge-sparrow and of the common tern, are the 
only instances, in a state of nature, we ever remember finding yolk- 
less, and were of course not half the usual size. We are also told of 
eggs with double shells ; and we have frequently seen eggs without 
any calcareous covering, but wrapped in a soft pliable skin like vellum. 
To account for these extraordinary productions, we must conceive a 
defect exists at the time in some part of the animal body ; and as the 
vitellus and albumen derive their origin from different parts, it is na- 
tural to conclude, in those preternatural eggs destitute of yolk, the cause 
proceeds from some defect in the ovarium. May it not be occasioned 
by an unequal stimulus in the parts necessary to perfect the egg, and 
that the growth of the vitellus is not in proportion to the albumen 9 
Thus while the vesicles appointed for collecting, preparing, and uniting 
the organic particles of the albumen in the uterus have all their effec- 
tive powers, those of the ovarium are weak ; of course the one goes 
on with the operations appointed by nature, while the other is stopped 
in its progress for want of a sufficient quantity of organic matter to 
bring it to perfection at the same time. Hence the reason of the com- 
mon fowl sometimes producing three or four imperfect eggs following, 
by which the vitellus becomes sufficiently large, and a regular succession 
of perfect eggs are produced. On the contrary, when, from the same 
causes, the growth of the vitellus in the ovarium is too luxuriant, two 
yolks pass the oviduct together, which being surrounded with the usual 
‘ The Centininum Ovum, of naturalists, vulgarly called a cock’s egg. This name 
has been given it from a supposition that it was the hundredth egg, or the last the bird 
could lay. 
