EMBRYOLOGY 197 
arrangement, as may be seen in hard-boiled eggs. Some of the 
layers of denser albumen, surrounding the fluid layer next the 
vitelline membrane, extend as twisted cords or “ chalazex” towards 
the two poles of the egg. They do not quite reach the outer layer 
of the white, although the cord next the pointed pole of the egg 
ultimately becomes somewhat superficially attached to the lining 
membrane of the eggshell. The chalaze serve to suspend the yolk 
by acting as elastic pads, and thus keeping it in position. The 
interior of each cord presents the appearance of a succession of 
opaque white knots, hence the name of chalazez or hailstones. 
When the egg has arrived at the narrow constriction of the 
oviduct (which seems to take place in the common Fowl in from four 
to six hours after its entrance into the infundibular upper end of 
the oviduct), the mucous membrane of the latter produces a denser 
layer of albumen mixed with several laminze of felted fibres, which 
approach the nature of connective tissue. This is the shell-mem- 
brane which gives the egg its final size and shape, and consists of an 
inner and an outer layer, both of which remain permanently in 
close apposition over the greater part of the egg, and adhere to the 
shell, but at the broad end they tend to separate, and develop an 
air-chamber between them. ‘This chamber does not exist in per- 
fectly fresh eggs, but makes its appearance and increases in size as 
the white of the egg loses in bulk from evaporation. 
From the narrow isthmus the egg passes into the uterine or 
shell-forming dilatation; here it remains from twelve to twenty 
hours. The whole shell is deposited as an accessory sheath by the 
thickish white excretions of the glandular walls of the uterus. This 
excretion forms an organic basis or matrix, impregnated with cal- 
careous matter, which coagulates and crystallizes partly in the shape 
of felted strands. The shell rests with so-called mammillary processes 
upon and partly in the shell-membrane ; the mammillz themselves 
are comparatively poor in inorganic matter. The interstices be- 
- tween them and the shell-membrane are continued through the 
calcareous layer of the hard shell as vertical canals. These canals 
are branched only in the Ostrich, and converge towards the bottom 
of the little pits on the surface of the egg; in the Rhea only two 
canals seem to open into each pit; in all other birds each pit leads 
only into one vertical canal. Besides this mammillary and the 
porous layer, the shell of most birds possesses a cuticular layer. 
This outermost layer is the most variable part of the shell; it 
is apparently structureless, either very poor in calcine salts, and 
in this case smooth and shiny, or considerably infiltrated with 
calcareous matter, and then exhibiting the well-known chalky and 
often rough appearance of the eggs of the Ani, Cormorants, Grebes, 
and Flamingos. Even when well developed, this cuticular layer is 
always extremely thin. In the Ostrich and in Rhea it is very 
