THE ANATOMY OF BIRDS. — OOLOGY. 
223 
putamen, a peel, rind), or egg-pod"; it is the final euvelope of such a " soft- shelled egg" 
as a hen drops when deprived of the lime required to enable her to secrete a hard shell. In 
the uterine dilatation of the oviduct a thick white fluid charged with earthy matter is exuded ; 
this condenses upon the egg-pod and forms the shell. The composition of this earth is chiefly 
carbonate of lime (common chalk), with some carbonate of magnesia, and phosphates of both 
of these bases — thus like that of bone as to ingredients, but in very difierent proportions. The 
shell does not simply overlie the pod in a distinct sheet, but is intimately coherent, the micro- 
scopic crystals or other particles of the earthy matter being deposited in the matted fibrous 
texture of the pod. The connection is most intimate in fresh eggs ; after a while, layers of the 
pod separate at the butt of the egg, forming the large air-space which every one has noticed in 
that situation. The shell being very porous, readily admits air. The air space enlarges during 
incubation, and the pod becomes more and more distinct from the shell, which latter also 
increases in porosity and fragility towards ^'full term." The rough or smooth appearance of an 
egg-shell, the pores which may be visible to the naked eye, and other physical characters, are 
due to the impression made upon it by the lining membrane of the ''uterus." The superficial 
deposit of chalk is so heavy, in some cases, as those of cormorants, etc., that it may be scraped 
oft' without interfering with the texturally firm shell-substance underlying. All the coloration 
of egg-shells, which frequently makes them pretty objects, is simply the deposit of pigment 
granules in or upon the shell. Such deposit may be perfectly uniform, as it is in the bluish- 
green egg of a robin, for instance, but it is oftener spotty — either upon a white or a whole- 
colored ground. The browns and neutral tints are the usual colors, particularly a bright 
reddish-brown; the same, lying in instead of upon the shell, gives the grays, "lilacs," and 
"lavenders" so well known. In ptarmigan, the pigment is so heavily deposited that the 
egg comes out pasty on the surface ; a sign of " fresh paint! " one must not disregard if he 
would not spoil the decoration. 
Oviposition. — The energy and rapidity with which the processes involved in the manu- 
facture of so complex a product as a bird's egg is now seen to be are extraordinary. A domestic 
fowl may lay an egg every day for an indefinite period. It is difficult to say how quickly an 
egg may ripen in the ovary; for, during the activity of that organ, several or many are to be 
found in all stages of immaturity, and the date of the initial impulse cannot well be determined. 
As there is probably but one egg at a time in the oviduct, the whole process of finishing oft" the 
yelk-ball with its chalaziform, soft albuminous, putaminous, and calcareous envelopes may go 
on in twenty-four hours, most of which time is consumed in the shell-formation. The number 
of eggs matured by the human female is or should be thirteen annually ; this is no large number 
for many of the gallinaceous and anatine birds to deposit in about as many days. But a 
probable average number is five or six. Defeat of the procreative instinct from any accident is 
commonly a stimulation to renewed endeavors to reproduce ; and very many birds rear two or 
three broods annually, though one clutch of eggs is the rule. Many, such as auks, petrels, and 
penguins, lay a single egg. Two eggs is the rule in humming-birds and pigeons. Three is 
normal to gulls and terns, though these often have but two. Four is the rule among the 
small waders of the limicoline groups. Some of the small Oscines lay over the average, 
having eight or ten ; among these, the European sparrow, Passer domesticus, is probably the 
most prolific. The parasitic cuckoos are said to lay the relatively smallest eggs ; that of the 
Apertyx is said to be the largest, weighing one fourth as much as the bird. The usual 
shape of an egg has given us the common names oval, ovate, and ovoidal, for the well-known 
figure. Some, as those of owls, woodpeckers, kingfishers, and others, more or less nearly 
approach a spherical shape. Eggs of grebes, herons, Totipalmate birds and various others 
are rather elliptical, or equal-ended, and narrow in proportion to their length. Eggs of the 
limicoline group are generally pyriform, — very broad at one end and narrow at the other. But 
