THE ANATOMY OF BIRDS. — OOLOGY. 
221 
its formation. A bird's egg is therefore meroblastic (Gr. fiepos, meros, a part, and ^Xaa-TiKOi) , 
and we must carefully discriminate between the great mass of yellow food-7jelJc, as it may bo 
called, and a small quantity of white yelk," the true germ-yelk, which alone is transformed into 
the body of the chick. The latter forms the cicatricle, vulgarly called the ''tread"; that small 
disc, visible in most birds' eggs to the naked eye, which appears 
upon the surface of the great yellow ball, floating in a pale thin 
yelk which penetrates the denser and yellower food-yelk by a 
cord of its own substance leading to a central cavity, the false 
yelk-cavity, around which the food-yelk is deposited in a series 
of concentric layers like a set of .onion-skins The whole mass 
is surrounded by a delicate structureless yelk-skin, called the 
vitelline membrane (whether this be the original vitelline mem- 
brane of the Dynamamceba or not ; i. e., whether the food-yelk 
has" accumulated inside or outside the original zona pellucida). 
All this enormous accumulation, effecting what is called a meto- 
vum or after-egg, to distinguish it from the protovum, or primitive 
state of the egg, goes on in the ovary, and in the ovisac of each 
ovum ; with the ripening of the ovum, the ovisacs become dis- 
tended to a corresponding size, and the whole ovary acquires 
the familiar bunch-of-grapes appearance. With such maturation 
of the fruit, the connection with the rest of the ovary lengthens 
into a stalk, or pedicel, by which the ripe ovum hangs to its 
stock, like any fruit upon its stem, ready to burst its skin and fall into the open mouth of the 
oviduct. Such rupture of the graafian follicle (ovisac), in its now distended state known as 
the capsule or calyx, occurs along a line where the numerous blood-vessels which ramify 
upon its surface appear to be wanting, called the stigma : this is rent ; the ovum slips out of 
its calyx, like the substance of a grape pinched out of its skin, and falls into the oviduct. 
After this discharge, the empty calyx collapses, shrivels, and ultimately disappears by ab- 
sorption. (See expl. of fig. 108). 
The ovum thus acquires the full size of its yelk in the ovary, — becoming, as in the case of 
the hen, a yellow sphere an inch in diameter.^ Notwithstanding its enormous distension with 
food-yelk, it is still morphologically a simple cell, affording the maximum dimension of any 
known protozoan or single-celled animal. Entering the oviduct, the germ-yelk part of the 
whole mass is fertilized by spermatozoa, unless this process has before occurred in the ovary, 
and in its passage through that tube the yelk -ball becomes invested successively with the 
mass of transparent albumen known as the " white" of the egg, and finally by the chalk shell 
— both secreted by the mucous membrane lining the oviduct. 
During its functional activity, the left oviduct (there being usually only this one) becomes 
highly dcA'^eloped, both as to its muscular walls, which by their contractility embrace the ovum 
closely and squeeze it along, and as to its mucous secretory surface. It is supported by perito- 
neal folds forming a mesometry, like the mesentery of the intestines ; its whole structure and 
office are quite like those of a length of intestine. The upper end of the singularly serpentine 
oviduct is dilated into an infundibulum, or funnel-like mouth, corresponding to the fimbriated 
extremity of the mammalian fallopian tube, ?Lud constituting a morsus diaboli, or devil's grip," 
^ How great this is can only be appreciated by comparison. The human egg, on escaping from the graafian 
follicle, is said to be from to of an inch in diameter. Taking it at 555, there would be 40,000 in a square inch, 
and in a cubic inch 8,000,000. The largest bird's egg known, that of the ^jnjornis, is said to have a content of 
about a gross of hen's eggs — 144. Supposing the yelk of the ^pyornis egg to bear the usual proportion to the 
other contents of the shell, and allowing for the difference in bulk between a sphere and a cube of equal diameters, 
there would still be somewhere about a billion human eggs in one jEpyornis egg-yelk, — roundly, a mass of them 
equal to that of the germs of more than one-half of the present population of the globe. 
Fig. 109. — Meroblastic ovum 
(yelk) of domestic fowl, nat. size, 
in section; after Haeckel. a, the 
thin yelk-skin, enclosing the yel- 
low food-yelk, which is deposited 
in concentric layers, c, d; b, the 
cicatricle or tread with its nu- 
cleus, whence passes a cord of 
white yelk (here represented in 
black) to the central cavity, d^. 
