580 STRUCTURE OF THE BIRD'S EGG, - 
own substance is scarcely distinguishable. Not only thousands 
but tens of thousands of eggs are often produced by a ane 
individual, their aggregate forming what is known as 
“hard roe ;” whilst the “soft roe” or “milt” is the corre- 
sponding mass of sperm-cells produced by the male. 
755. After the ovum of the Bird has quitted the ovarium, 
and is passing through the oviduct towards its outlet, it 
receives layer after layer of albumen poured out in a viscid 
condition from the lining membrane of the oviduct, forming 
the “white” of the egg (fig. 315, g) ; and this is inclosed in 
Fig. 315.—Section or Fowt’s Eae : 
4a, cicatricula; 6, yolk-bag; c, membrane lining shell; d, attachment of chalaze; 
e, chalaze; f, air-space; g, albumen, p 
a double membrane composed of a network of fibres, which 
is formed by the consolidation of a plastic exudation (§ 391), 
poured out after the albuminous exudation has been com- 
pleted. The outer layer of this membrane is consolidated by 
the deposit of calcareous particles in the interspaces of its 
fibrous matting, so as to form the “shell” of the egg; an 
arrangement that gives the necessary protection, without cut- 
ting-off the contents of the shell from that communication 
with the atmosphere which is requisite for the development. 
of the embryo. The inner layer, which forms a lining to the 
shell, separates into two lamine at the large end of the egg; 
and, inclosed between these, there is a bubble of air (f), which 
serves to give the young bird, just before it is hatched, the 
