THE EGG AND DEVELOPMENT OF THE YOUNG BIRD 93 



The "White 

 and Shell. 



gradually thinner, especially round its 

 greatest diameter or equator, whieli then 

 exhibits a pale zone or belt called the 

 stigma. Finalh% whether or not feciuida- 

 tion takes place, the sac ruptures at the 

 stigma, and the liberated yolk and germ, 

 surrounded by a very thin and delicate 

 membrane, is received by the I'unnel-shaped 

 opening of the oviduct or egg-passage, 

 whose ofh.ce it is to convey it to the outer 

 world, and on its way to clothe it with 

 the other structures needful for its develop- 

 ment and preservation. ... It will easily 

 be seen how izvo yolks may become detached 

 and enter the ovnduct at nearly the same 

 time ; in which case they are likely to 

 be enveloped in the same white and shell, 

 causing the ' double-yolked egg ' [so well 

 known to every breeder of cage-birds], 



" 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 nuicous 

 membrane of the oviduct, and added layer 

 by layer as the egg passes on. The uses 

 of the white or albvuncn are manifold. 

 It is eminently nutritious, forming, indeed, 

 the chief novirishment 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 parchment-like covering which is fovuid 

 inside the shell. In reality this skin con- 

 sists of two layers, which can easily be 

 separated ; and at the large end of the 

 egg they do separate entirely, forming 

 the air-chamber. At first this chamber is 

 small, but as the egg 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 

 Ijy 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." 



The majority of the shells of eggs con- 

 tain colouring pigment, or are coated with 

 it, hence the varying shades. 

 " All these things obviously 



Coloration 

 of Egg-shells. 



depend on some jieculiar 

 condition of the secreting organs, as does 

 the shape of the egg of each bird when 

 finally laid." At the time of laying it is 

 expelled from the uterus by violent muscu- 

 lar contractions, and passes with its narrow 

 end downwards along the remainder of 

 the oviduct to reach the exterior. 



" Occasional dejiartures from the 

 ordinary tj'pe of egg will now be under- 

 stood. II' the latter portion 



Variations t ii • i .. i 



, „ of the oviduct be m an lui- 



of Eggs. 



healthy condition, or if yolks 

 be matured by the ovary faster than shells 

 can be formed by that organ, ' soft ' or 

 luishclled 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." 



This, however, more particidarly applies 

 to fowls, though we have known of some 

 isolated instances of yolkless eggs being 

 laid by Canaries. 



" Disease, extending to the middle por- 

 tion of the passage, may result in eggs 

 without even the membranous skin ; and 

 if the entire canal be in an unhealthy con- 

 dition, yolks alone may probably be 

 dropped without any addition whatever. 



