380 Animal Husbandry 
(a) Observe the air-space and the two membranes, which are 
separated with difficulty. Note that the outer membrane is the 
thicker and tougher. 
(b) Cut the egg lengthwise through the middle. Observe the 
lighter-colored, flask-shaped center of the yolk, and the darker 
yolk arranged around it in concentric layers. Note the “ ger- 
minal vesicle,” or ‘‘germinal disk” at the upper part of the light 
yolk. Observe that the yolk is at one side and not in the center 
of the white of the egg. Note also that the germinal disk is on 
the upper side of the yolk. This is because the yolk is lighter in 
weight than the albumen and hence floats. The germinal disk 
on the surface of the white yolk is lighter than the dark yolk. 
Snyder gives the chemical composition of the dry substance of 
the inside of the egg as: — 
PRorEIN Far 
Albumen sia of the ee gee ee ae Slag: ee’, 2BBE92Z 53 
Yolk . . Be os chy ye 4B. ee: det: cee: LOG?! 64.43 
It will be seen that there is a large amount of fat in the yolk 
and almost no fat in the albumen. Fat is lighter than albumen, 
hence rises to the surface. This may be observed in practice 
by holding a fresh egg in front of an egg-tester and noting the 
tendency of the yolk to float upward. 
This tendency of the yolk to float to the surface makes it neces- 
sary to turn eggs frequently when they are kept for hatching, 
otherwise the yolk will rise until the germinal disk comes in con- 
tact with the shell membrane. It will then become dry by evapo- 
ration and adhere to the membrane. If the egg is then turned 
the germ will be killed. 
4. Make a drawing of the longitudinal section of the egg, show- 
ing: (a) The shell and its pores. (6) The two shell membranes 
turned back from the shell. (c) The air-space. (d) The three 
layers of albumen. (e) The vitelline membrane surrounding the 
- yolk. (f) The vitellus contained within the vitelline membrane. 
