EGGS OF BIRDS. 255 



poration after it is laid. The air accumulates between layers 

 of the ^ membrana putaminis' at the great end of the egg, fig. 130, 

 A,/; and in increased quantity as the other contents become 

 condensed into the tissues of the chick, when it is averred to 

 contain rather more oxygen than in ordinary atmospheric air. 

 Such is the complex structure of the egg of a bird prior to its 

 becoming subject to the influence of incubation. 



It diflfers from the egg of the cold-blooded, non-incubating 

 Ovipara, in the presence of the chalazas and of the air-chamber, 

 in the firmer and more complex structure of the shell, and in the 

 greater proportion of albumen : in all which differences may be 

 discerned a prospective adaptation to the business of hatching. 



The cicatricula, or germ, is on the uppermost part of the 

 floating yolk, the thinner part of which, occupying the nuclear 

 tract, fig. 130, B, c, makes that half of it the lightest. Pressure 

 of the upfloated germ against the shell-wall is moderated by the 

 weight of the denser albumen forming the chalazse, ib. a, c ; and 

 their usual attachments, a little below the axis of the yolk, help 

 also to make the cicatricular half the lightest and uppermost. 

 Under ordinary circumstances rotation of the egg takes place on 

 its long axis, and, if a fresh egg be so turned round, ^ the cica- 

 tricula will keep its position upwards for one turn or a little 

 more, and then, by the twisting of the chalaz^, the yolk is carried 

 completely round, and balances itself again with the cicatricula 

 uppermost in its new position.'^ The main function of the 

 chalazge is to keep the yolk more steady in the albumen, and to 

 moderate the effects of any \iolent movement or rotation of the 

 egg. The domed form of the hard shell enables it to bear the 

 superincumbent weight of the brooding mother. How these 

 modifications of the oviparous egg in anticipatory relation to the 

 needs and conditions of incubation can be brought about by 

 ^ selective ' or other operations of an unintelligent nature is not 

 conceivable by me. 



Birds differ in the number of eggs which they lay at one 

 breeding season, in the relative size, in the shape, colour, sur- 

 face, and thickness of the shell of the laid eggs. The Frigate 

 Bird, Albatross, Penguin, Fulmar, Petrel, Awks, and some other 

 sea-birds that brood on bare rocks, severally lay and hatch but 

 one egg at a season : the Skua Gulls (Lestris) have two eggs ; 

 the Common Gulls (^Larus) three eggs ; the Lamellirostres and 

 most GallincB hatch many eggs at a brood. 



The Cuckoo has the smallest egg in proportion to its size, the 

 ' cccvin. p. 65. 



