56 



THE BOOK OF POULTRY. 



shell, causing the " double-yolked egg " so well 

 known to every poultry-keeper. 



Thus received into the oviduct, the yolk 

 becomes enveloped in a glairy fluid called the 

 white, or by chemists albumen. This is secreted 

 by the mucous membrane of the oviduct, and 

 added layer by layer as the egg passes on. The 

 uses of the white or albumen are manifold. It 

 is eminently nutritious, forming indeed the 

 chief nourishment of the chick during its growth 

 in the shell ; as it becomes absorbed by the 

 little animal, and forming as it does by far the 

 greater part of the egg when laid, it gives the fast- 

 growing little body the needed increase of room ; 

 it is a very bad conductor of heat, and hence 

 guards the hatching egg against the fatal chills 

 which would otherwise occur when the hen left 

 the nest ; and finally, it preserves the delicate 

 yolk and vital germ from concussion or other 

 violent injury. 



At a still farther point of the oviduct the 

 egg becomes invested with the skin or parch- 

 ment-like covering which is found inside the 

 shell. In reality this skin consists of two layers, 

 which can easily be separated ; and at the large 

 end of the egg they do separate entirely, form- 

 ing the air-chamber. At first this chamber is 

 small, but as the &gg gets stale it becomes larger 

 and larger, so that even in eggs stored it fills 

 at length a large portion of the space within the 

 shell, the egg itself drying up in proportion. In 

 eggs on the point of hatching it usually occupies 

 about one-fifth of the space. It has been proved 

 by experiments that the perforation of this air- 

 chamber, even by a needle-point, is an effectual 

 prevention of successful hatching. 



In the last portion of the oviduct the egg 

 becomes coated with that calcareous deposit 

 which forms the shell, after which it passes into 

 the cloaca and is ready for expulsion. In some 

 breeds colouring matter is added over the solid 

 ingredient, producing the deep-coloured eggs 

 of the Cochin, and in other birds the splashed 

 and spotted patterns so well known. In fowls 

 which lay coloured eggs similar splashes often 

 occur, and we have had Brahma hens which laid 

 eggs with a white ground, covered thickly over 

 by chocolate-coloured spots. We have had 

 others, again, lay eggs covered apparently with 

 a coat of whitewash, which on being rubbed off 

 with a rough cloth, revealed the usual buff-brown 

 tint beneath. All these things obviously depend 

 on some peculiar condition of the secreting 

 organs, as does the shape of the egg of each 

 bird when finally laid. 



Occasional departures from the ordinary 

 type of egg will now be understood. If the 

 latter portion of the oviduct be in an un- 



healthy condition, or if yolks be matured by 

 the ovary faster than shells can be formed by 

 that organ, "soft" or unshelled eggs will be 

 produced. If, on the contrary, the oviduct and 

 its glands be active, while the supply of yolks 

 is temporarily exhausted, the diminutive eggs, 

 which consist of only white and shell, and which 

 not infrequently terminate the laying of a long 

 batch, may be expected to occur. Disease ex- 

 tending to the middle portion of the passage 

 may result in eggs without even the membranous 

 skin ; and if the entire canal be in an unhealthy 

 condition, yolks alone may probably be dropped 

 without any addition whatever, even of white. 

 This last occurrence therefore denotes a serious 

 state of affairs, and should be met at once by 

 depletic medicines, or it will probably be fol- 

 lowed by the loss of the bird. 



Fig. 23. — Diagram of an Egg. 



B L, blastoderm. w v, white yolk. VY. yellow yolk. v, vitelline 

 membrane. F, layer of very fluid albumen round the vitelline membrane. 

 D, dense albumen enclosing the yolk with preceding envelopes. In this 

 envelope D are incorporated the ends of c h, the chalazse. w, body of 

 the albumen, c, somewhat denser layer of albumen, surrounded by a 

 fluid layer. M M^ outer and inner shell membrane, separated at A, air- 

 chamb-r. s, shell. 



Let us now consider the egg itself which is 

 a much more complicated organism than many 

 people are aware of There is much even in 



the shell S (Fig. 23) to excite our 

 Structure interest. It is composed chiefly of 

 theEffe prismatic particles, so arranged as 



to leave pores or interspaces between 

 them. As laid, the shell is of enormous strength, 

 so that it will resist great pressure between the 

 palms of the hands applied to the opposite ends ; 

 though it is not correct that, as we have seen 

 stated, " the strongest man cannot break it " in 

 this way. Still, for its thickness and te.xture, 

 its strength is phenomenal. As hatching pro- 

 ceeds, however, the carbonic acid and dioxide 

 formed by the breathing of the chick, dissolved 

 in fluid, gradually dissolve a portion of the 



