THE ANATOMY OF BIRDS. — OOLOGY. 



227 



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 CQnceiitric 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^. 



its formation. A bird's egg is therefore meroblastic (Gr. fj-epos, meros, a part, and ^XaariKos), 

 and we must carefully discriminate between the great mass of yellow food-yelk, as it may be 

 called, and a small quantity of "white yelk," the true germ-yellc, 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 ^•itelline mem- 

 brane of the Dynamamceba or not; i. e., whether the food-yelk 

 lias accumulated inside or outside the original zona pellucida). 

 All this enormous accumulation, effecting what is called a meto- 

 vnm 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 iu 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 beiug usually only this one) becomes 

 liighly developed, 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 infundibidum, or fuimel-like mouth, corresponding to the fimbriated 

 extremity of the mammalian fallopian tube, and constituting a morsiis 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 sai<l to be from ^Jn to y}g of an inch in diameter. Taking it at jj,,, 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 ^pyornis, is said to have a content of 

 about a gross of hen's eggs — 144. Supposing the yelk of the yEpi/omis egg to bear the usual f)roportion 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 yEpyornis 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. 



