THE ANATOMY OF BUBJS&.^QOLOGY. 221 



its formation. A bird's egg is therefore merehlasUe (Gt. iiepos, meroS:,, a part, and ffkaariKos), 

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

 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 dcafy-ich, 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 Dynamameeba or not; i. e., whether the food-yelk 

 has accumulated inside or outside the original sona peUueida). 



AU this enormous accumulation, effecting what is called a meto- _ ,„„ ,, ^, ^ 



' " Pig. 109. — Meroblastic ovum 



vum or after-egg, to distinguish it from the protovum, or primitive (yelk) of domestic fowl, nat. size, 



state of the egg, goes on in the ovaiv, and in the ovisac of each *" section; after Haeckel. o, the 



f 7 . „ , 1 *''™ yelk-8kin, enclosing the yel- 



ovum ; with the ripening of the ovum, the ovisacs become dis- low food-yelk, which is deposited 



tended to a corresponding size, and the whole ovary acquires in concentric layers, c, <«; 6, the 



^, J. .,. , 1 Vc TTT- 1 -1 . cioatricle or tread with its nu- 



the lamiiiar buncn-oi-grapes appearance. With such maturation cleus, whence passes a cord of 



of the fruit, the connection with the rest of the ovary lengthens ^^'^ ^^'"^ '^^^^^ represented in 

 .. ,,, T-ii ■ , ,. black) to the central cavity, d'. 



mto a stalk, or peadcel, 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 ovaxy, 

 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 

 -rrr both secretcd by the mucous membrane lining the oviduct. 



During its functional activity, the left, oviduct (there being usually only this one) becomes 

 highly 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 mesomet/ry, 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 dilgited into an imfwnMbuhim, or funnel-like mouth, corresponding to the fimbriated 

 extremity of the mammalian fallopian tube, and constituting a morsus diaboli, or '' devil's grip," 



1 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 t^^ of an inch in diameter. Taking it-at ^^, 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 ^pyomis, is said to have a content of 

 ^out a gross, of hen's eggs — 144; Supposing the yelk of the jEpyprnis egg to bear the usual proportion to the 

 other contents of the shell, and aJlowing 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 ^.pyorois 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. 



