SEC. IV ANATOMY OF BIRDS 327 



form the embryo ; it is what the embryo feeds on during its forma- 

 tion. A bird's egg is therefore merohlastic (Gr. ^epos, meros, a part, 

 and ^Xaa-TiKos), 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-yelk, which alone is trans- 

 formed into the body of the chick. The latter forms the dcatricle, 

 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 viteUine membrane (whether this 

 be the original vitelline membrane of the 

 Bynamamoeha or not ; i.e. whether the 

 food-yelk has accumulated inside or out- 

 side the original zona pellucida). All this 

 enormous accumulation, effecting what is 

 called a metovum or after -egg to dis- 

 tinguish it from the jjrofoDMOT, or primi- 

 tive 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 distended to a corresponding 

 size, and the whole ovary acquires the (yeS^k Tmi^Sw. si^™ 

 familiar bunch - of - grapes appearance, section ; after HaBckei. a,thethm 



iTT-.i 1 , ,• !• ii 5 -i ii yelk-skm, enclosing the yellow food- 



Wlth such maturation 01 the iruit, the yelk, wUch is deposited m concen- 



connection with the rest of the ovary ItXfth"' fti \udcSf *wh^?e 

 lengthens into a stalk, or pedicel, by r%\-|enVd in waSo'to /if ceS 

 which the ripe ovum hangs to its cavity,*, 

 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 absorption. (See explanation 

 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- 



^ How great this is can only be appreciated by comparison. The human egg, on 



