210 NEW ZEALAND NATURE-STUDY BOOK 
Shape.—The hen’s egg is ovoid in shape—one end being 
distinctly more pointed than the other. The advantages 
of this shape have already been dwelt upon, so need not 
again be referred to. In colour the egg is either white 
or brown; its size, however, varies greatly. 
The Shell is hard, brittle and smooth, and is composed 
of organic matter and lime salts—principally carbonate of 
lime. If a little hydrochloric acid be poured on the shell, 
the effervescence marking the presence of a carbonate can 
readily be seen. Running from the outer to the inner 
surface are a number of minute openings, 7.¢., the shell, 
is porous. It is through these holes that the air passes 
which supplies the living germ with oxygen without 
which it could not develop into a young bird, and it is 
through them, moreover, that the impure air is got rid of. 
It is necessary that these pores be kept open ; the presence 
of too much moisture would tend to injure the growing 
chick by closing up the pores. Eggs laid in damp situa- 
tions, therefore, have a waterproof covering which admits 
of the passage of air. 
If eggs are to hatch they must be cooled several times 
aday. Heat causes substances to expand and cold makes 
them contract. As the egg becomes warm from the heat 
of the sitting-bird, the air inside expands and is forced 
out through the pores. When the egg cools, contraction 
takes place, thereby lessening the pressure within, with the 
result that the surrounding air is forced inside—in this 
way a constant supply of fresh air is secured. It will 
be noticed than a sitting hen leaves the nest from time to 
time. ‘This she does no doubt chiefly with the object of 
obtaining food, but her absence is necessary for the 
reasons stated above. 
A small piece of shell is easily broken, but the shape 
of a complete egg so distributes the pressure, especia 
