NATURAL HISTORY. 



DIAGRAMMATIC SECTION OF A FOAVL 8 EGG. 



dbl) Blastoderm ; 



Shell Membrane; () ExtWnaf Layer of ditto ; (.e) Shell. 



that is to say, the outer pores of the shell are closed under the influence of moisture. This may be seen 

 by removing the outer layers, when air or water Avill pass in quite easily. These canals are said to be 

 branched in the Ratite birds, and to be simple in the Carinatse. The shell is lined by the shell-mem- 

 brane, which, again, is made up of two layers. 

 At the broad end of the egg these two layers 

 are separated from one another, and so give rise 

 to that air-chamber which is found in stale eggs, 

 and increases in size as the egg grows older and 

 the yolk evaporates. 



The shell-membrane is in direct contact with 

 the white of the egg (albumen). This, in its fresh 

 state, consists of fluid albumen, arranged in 

 layers, which are separated from one another by 

 networks of fibres, in the meshes of which, 

 however, fluid albumen is also to be found. 

 There are. further, two special sets of fibrous 

 cords in the white of the egg; these extend 

 somewhat along the long axis of the egg, though 

 they do not reach to the shell-membrane. From 

 their bead-like character they are known as 

 chalazce (hailstones), but their more common English name is that of the " tread." 



The " white " is separated from the yolk by the so-called vitelline (or yolk) membrane ; the greater 

 part of this yolk is known as the yellow yolk, and is made up of minute albuminous granules, but its 

 outermost part is formed of a thin layer of a somewhat different substance, which goes by the name of 

 the white yolk. The spheres of this latter are still smaller than those of the yellow yolk, and they 

 are also found to form layers at various levels in it. At one point the white yolk becomes 

 a good deal thicker, and forms, as it were, a pad for a small white disc, which, in ordinary circum- 

 stances, is always found uppermost when an egg is opened. This disc is formed of an encircling 

 white rim, and within it there is a rounded transparent region, the centre of which is more opaque. 



This region is known as the blastoderm, and is that part of the egg from which the chick, with its 

 organs and complicated vessels, muscles and bones is soon to be developed. In the laid egg, this 

 blastoderm consists of two layers of cells, as do at a certain stage the eggs of all but the very simplest 

 of animals. The dissection of a laying fowl will probably reveal the presence of eggs at an earlier 

 stage, and from their study the following history has been made out : the ellipse-shaped egg r 

 when about to leave the ovary, is a yellow body enclosed in a fine membrane, and possessing at one 

 pole a small (germinal) disc ; this disc contains a smaller germinal vesicle, and a still smaller germinal 

 spot ; when this body is ripe, it escapes from its enclosing capsule, and the germinal vesicle disappears. 

 As the egg passes down the oviduct the albumen becomes deposited around it, and part of it is con- 

 verted into the shell-membrane. The egg now becomes subjected to a thick, white fluid, which is 

 gradually converted into the shell. 



While these additions to the substance of the egg are going on, the germinal disc undergoes 

 the remarkable process known as segmentation, in which it becomes divided into two, four, eight, 

 sixteen, thirty-two (and so on) masses, which arrange themselves in two distinct layers, the presence 

 of which has been already noted in the laid ess. 



v OO 



This is not the place in which it is possible to follow out the various future changes undergone, 

 but the condition of the young birds on escaping from the egg is widely different in some of the 

 larger groups of birds. Some young birds, on their exclusion from the egg, are able to shift for them- 

 selves, and are covered with down ; while others are born naked and helpless, and require food 

 from their parents for some time after they are hatched. Of the first section, an ordinary chicken 

 is a familiar example, while a young Thrush or a Sparrow illustrates the second. There are, however, 

 manifest exceptions to this rule, as in the Herons, for instance, where the young are densely clothed 

 with feathery down, but are helpless for a long time after they are hatched. 



Finally, it must be stated that all birds possess an oil-gland (known as the uropygial), situated 



