AA'A TOMY OF BIRDS 



329 



the egg-shell, and decorate it with pigment. The rest of the tube 

 is vaginal, being merely the passage-way by which the perfected 

 ovum is discharged into the cloaca, to be expelled |>^'r unim. The 

 muscular walls of the oviduct consist of both circular and longi- 

 tudinal unstriped fibres, like those of intestine — the latter especially 

 in upper portions and at the infundibulum, the former more con- 

 spicuously below, where they form a sort of os tinccB at the bottom 

 of the calcific portion, and a kind of spUnckr ncjincc at the end of 

 the tube. A recognisable ditoru is developed in many birds. 



The deposition of the white and of the shell remains to ]je 

 noticed. The first deposit upon the yelk-ball consists of a layer of 

 dense and somewhat tenacious albumen, called the cludaziferous 

 membrane (Gr. ^^dAa^'a, 

 chahiza, a tubercle, and 

 Lat. few, I bear). As the 

 egg is urged along liy the 

 peristaltic action of the 

 tube, it acquires a rotation 

 a])Out the axis of the 

 tube ] the successive layers 

 of soft albumen it re- 

 ceives are deposited some- 

 what spirally ; and the 

 chalaziferous membrane 

 is drawn out into threads 

 at opposite poles of the 

 egg. These threads, which 

 become twisted in opposite 

 directions during the ro- 

 tation of the egg, are 

 called chalazce ; they are 

 the " strings," rather un- 

 pleasantly evident in a soft-boiled egg, which serve the im- 

 portant office of mooring and steadying the yelk in the sea 

 of white by adhesions eventually contracted with the membrane 

 which immediately lines the shell. They are also entrusted 

 with the duty of ballasting, or keeping the yelk right side up. For 

 there is a " right side " to the yelk-ball, being that on which floats 

 the cicatricle, or " tread." This side is also the lightest, the white 

 yelk being less dense than the yellow ; and the chalazre are attached 

 a little below the central axis. The result is, that if a fresh egg be 

 slowly rotated on its long axis, the tread will rise by turning of the 

 yelk-ball in the opposite direction, till, held by the twisting of the 

 chalazce, it can go no farther ; when, the rotation being continued, 

 the tread is carried under and up again on the other side, resuming 



Fig. 110.— Hen's egg, nat. size, in section ; from Owen, 

 after A. Thompson. "A, cicatricle or "tread," with its 

 nucleus, of white germ-yellc, floating on surface of pale 

 thin nutritive yelk, leading to central yelk-cavity, x ; a, 

 the yellow yelk-ball, deposited in the successive layers, 

 forming a set of lialones, and enveloped in the chalaziferous 

 membrane which is spun out at opposite poles into the 

 twisted strings, chalaza, c, c; h, h', successive invest- 

 ments of softer white albumen ; d, membrana putaminis, 

 the " soft shell " or egg-pod, between layers of wliich at 

 the great end of the egg is the air space, /; c, the shell. 



