198 EMBRYOLOGY 
hard and brittle, like the glaze of pottery; in the common Fowl 
and Turkey it is parchment-like ; in Auks, and apparently in Gulls, 
it is absent. The cuticle is spread over the whole surface of the 
egg, extending unbroken over and into the pits or surface ends of 
the air-canals, which are therefore closed when such a cuticle is pre- 
sent. The latter, however, readily admits the passage of air when 
dry, but when wet or moist is impermeable to air. 
The colour of the shell is produced by pigment-corpuscles, which 
may be deposited in various levels of the shell. Sometimes the 
pigment is restricted to the cuticular membrane, or when the 
latter is absent it is deposited on. the surface of the porous or 
calcareous layer. In most eggs pigment exists also in the deeper 
strata of the calcareous layer; interrupted deposition produces 
the spots, those which are deepest being naturally modified m 
appearance through the superimposed surface-colour, or they may 
not be visible at all. ‘The Gallinzee seem to be the only birds in 
which the spots, when such occur at all, are restricted to one 
stratum, while the spots of other birds’ eggs are both deep and 
superficial. In many eggs, whether spotted or plain, the deepest 
strata of the porous layer of the shell are uniformly coloured. As 
a rule spots are more frequent and larger towards the blunt pole 
of the egg, and there exists a distinct resemblance between the 
eggs, even between those of successive clutches, laid by the same 
bird (see Eas). 
Abnormal eggs, occasionally of the most perplexing shape, are 
of common occurrence in domesticated birds where, especially in 
Fowls, the artificial overproduction of eggs tends to overstrain 
and to exhaust the oviduct. Want of calcareous food may explain 
the soft-shelled or “wind” eggs. Sometimes eggs with two yolks, 
but otherwise normal, are met with, and that twins have been hatched 
out of such an egg has been observed beyond doubt (see also 
MonstTROsITIES). Eggs which contain intestinal worms, blood clots, 
inorganic concretions, and similar strange enclosures are quite abnor- 
mal. Such substances, when once inside the oviduct, seem to 
stimulate its walls like an ovarian egg and receive the ordinary 
albuminous and calcareous supplementary coatings. 
When the eggshell is completed, the egg is protruded into the 
cloaca and out through the vent, by the violent contractions of the 
uterine and cloacal walls, head foremost, i.e. the blunt pole appears 
first (cf. p. 185, note), and not the pointed end, as some have stated. 
Controversies have often arisen ‘on this point. Mechanical 
reasons plainly indicate, not the impossibility, but the greater 
difficulty of an egg moving with its pointed end forwards. A 
wedge or a cone enclosed within or driven into an elastic substance 
slips out towards its broad basis, not in the direction of its apex.} 
1 Direct observations of hens when in the act of laying are rare and not free 
