2 BULLETIN 471, U. S. DEPARTMENT OF AGRICULTURE. 
The eggs of wild birds, once commonly used, are still eaten, but 
in a limited way. Plovers' eggs are prized by epicures and the eggs 
of certain kinds of sea birds have been used to some extent. 1 
Other eggs besides those of birds are also eaten. Turtles' eggs are 
generally highly prized and very commonly eaten where the} 7 can be 
obtained. The eggs of the terrapin are served with the flesh in some 
of the dishes prepared from it. Fish eggs, especially those of stur- 
geon, are eaten in large quantities preserved with salt, under the 
name of caviar. Shad roe is another illustration of the use of fish 
eggs. 
Notwithstanding these minor instances, the term "eggs," when 
used in connection with food topics, refers to the eggs of birds, 
usually domestic poultry, and more particularly hens' eggs, and, 
unless qualified in some way, is used in the last sense in this bulletin. 
Although man has been guided by instinct and experience in the 
selection of eggs for food and of birds for domestication as egg pro- 
ducers, he has nevertheless chosen kinds having distinct physiological 
characteristics. In this respect birds may be divided into two groups : 
(1) Those in which the young are hatched full-fledged and ready in 
a great measure to care for themselves, and (2) those in which the 
young are hatched unfledged and remain entirely dependent upon the 
parents for some time. Hens and guinea fowls are familiar examples 
of the first group ; robins and sparrows of the second. The eggs of 
the two classes differ materially in composition, as might be expected 
from the role they play in the development of the young bird, more 
nutritive material being needed in the first case than in the second, 
since growth is continued in the egg until the bird reaches a more 
advanced stage of development. It is interesting to note that the first 
includes the birds whose eggs are economically important. 
The appearance of an egg — the shell with its lining of membrane 
inclosing the white and the yolk — is too familiar to need any discus- 
sion. Each fertile egg contains a germ, or embryo, and is at the 
same time a storehouse of material for the development and growth 
of the young individual until it has reached such a stage that life is 
possible outside the narrow limits of the shell. The embryo is in close 
relationship with the yolk, which furnishes the nutritive material for 
its early development, the white being used later. Naturally, yolk 
and white differ in composition as much as they do in appearance, 
their food value for man corresponding to the part they play in the 
growth of the chick. 
1 The danger of exterminating these desirable birds by gathering their eggs for food 
lias been discussed in D. S. Dept. Agr. Yearbook, 1899, p. 270. 
