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 in 

 appearance through the superimposed surface-colour, or they may 

 not be visible at all. The Gallinse 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 Eggs). 



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 

 Monstrosities). 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 (c/. 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 



