302 FORMATION OF THE EGG AND CHICKEN 
yolk be examined there will be found, just beneath the vitelline 
membrane, a small circular whitish body, about one-eighth of an 
inch in diameter, which will always float uppermost, no matter in 
what position the egg is laid. In the fertilized egg this disk has 
three well-defined areas,—an outer white rim, within which is a 
clear zone, and in the centre of this zone a somewhat elongated 
structure which is the embryo itself. These three parts constitute 
what is known as the blastoderm of the hen’s egg, the part whose 
function is reproduction; all the rest of the egg is directly utilized 
for nutrition and protection. F 
The passage of the egg down the oviduct, from this time on, 
is marked by the following processes: 
By the muscular contraction of the walls of the oviduct, the 
yolk is propelled to the cloaca. The passage of the yolk stimulates 
the walls of the duct to secrete successive layers of albumin. The 
first layer is thick, and has thickened portions which are fastened 
to the opposite poles of the yolk, to hold it in place. These thick- 
ened portions are called chalaze; they are heavy twisted threads 
of albumin. After the thick layer of albumin two layers are added, 
each of which is thinner and more watery than the preceding one. 
When the egg reaches the lower or contracted part of the oviduct, 
two very tenacious but thin layers of albumin are added, to form 
the inner and outer shell membranes. The air cell, which is always 
perceptible at the larger end of the fully developed egg, is not 
present at first, but is formed by the evaporation of the fluids of 
the albumin and the entrance of atmospheric air to replace these 
fluids after the egg is laid. Just before the egg is discharged into 
the cloaca, it passes through a section of the oviduct where the 
shell is added, in the form of a coating of thin liquid secretion 
containing lime, which hardens quickly. 
The perfect egg with its hardened shell is retained in the cloaca 
a short time before being laid. The power to hold the egg in the 
cloaca is under the control of the bird, so that, if she is frightened 
from her nest or unable to reach it, she can retain the egg for some 
time, the exact period varying with individuals. , 
The parts of the completed egg are the shell, shell membranes, 
albumin, and yolk (Fig. 146). The use of each is discussed in the 
following paragraphs. 
The shell is the outer envelope of the egg, and its function is 
primarily that of protection. Soft-shelled eggs are those that have 
either no shell or else a very thin one. This is due to a deficiency 
