Dr John Davy on the Eggs of Birds. 251 
so, the ratio referred to certainly would not be one of perfect 
exactness. Probably there is another specific element con- 
cerned—vital force ; thus, though the egg of the bantam is 
very much smaller than that of the barn-door fowl, yet the 
same time is required for its hatching. 
Whatever the degree of thickness of the shell, it is in- 
variably pervious to air, and chiefly, I believe, through 
minute apertures—foramina—in the crust. In the egg of 
the common fowl these are tolerably conspicuous. In every 
instance that I have put an egg under water deprived of air 
by the air-pump, on fresh exhaustion air has been seen to 
rise in currents from particular points, affording proof of 
the existence of such foramina. Granting their existence, 
they are certainly covered internally with membrane, 
through which the air must pass, it may be inferred, by 
pores of such minuteness as to escape detection under the 
highest power of the microscope. In a solitary instance, 
on removing an abortive egg which had been twenty-one 
days under a hen, on breaking the shell I found it’s inner 
membrane covered in part with mould (fucor mucedo) the 
spores of which must have entered, it may be presumed, 
through the foramina in the crust and the insensible pores 
just referred to. 
In accordance with the necessity for the aération of the 
embryo and foetus in process of development, the egg of 
every kind of bird has an air-cell formed by the separation 
of the two layers of the internal lining membrane, and this 
at the end which is generally largest, and first presents in 
the act of being laid. The contained air, in every instance 
that I have examined it, has been found to differ but little 
from atmospheric air ; and this whatever the age of the egg 
“British Birds and their Eggs.’ London, 1842.—The Martin (Hirundo 
urbica), from 12 to 18 days; Swift (H. apus), 16 to 17; Eagle-owl (Bubo 
maximus), 21; Goshawk (Astur palumbarius), 21; Sparrow-hawk (Accipiter 
fringillarius), 21 ; Stockdove (Columbo cenas) 17; Turtledove (C. turtur), 16 to 
17; Pheasant (Phasianus colchicus), 24 to 26; Cock-of-the-wood (Tetrao urogallus), 
28; Black Grouse (7. tetriz), 21; Partridge (7. perdix), 21; Swan (Cygnus olor), 
85 to 42; Wild Duck (Anas boschas), 29. The variation as to time, as in the 
well-known instance of the common fowl, depends probably on the tempera- 
ture of the atmosphere and the quality of the parent bird, whether a good or 
bad sitter. 
