74 SEX AND HEREDITY 



bobs upwards. The air-space is of importance to the 

 young bird, for when, just before hatching, it begins 

 to struggle within the egg-shell, its beak penetrates the 

 air space, it takes its first breath of air, and so invigorated 

 the young chick is able to break the shell and step into 

 the world which lies outside. 



The egg-shell varies much in different birds. Where it 

 is freely exposed to view and to daylight it often shows 

 a beautiful protective colouring which renders it very 



FIG. 43. 



View of a Hen's egg, freshly laid, with part of the shell broken away so as to 

 expose the contents. 



The " yolk " or true egg is seen in the centre with the whitish protoplasmic 

 portion (blastoderm) uppermost, a.s, air space ; alb, albumen or white ; ch, 

 denser strand of albumen towards each end ; s.m, shell-membrane. (From 

 Graham Kerr's Embryology.) 



inconspicuous, as for example in the case of the Peewit 

 and other kinds of Plover. Its shape also varies : it is 

 often pointed at one end, moulded within the parental 

 body while still soft by the squeezing pressure of the 

 oviduct forcing it onwards. And Nature has exaggerated 

 this pointed shape in some of the eggs which are laid 

 on shelves of rock, such as those of the Razorbill or 

 Guillemot, so that when it gets a knock the egg merely 

 runs round without rolling for any distance. 



The young bird shows many interesting developments 

 while it is contained within the egg-shell. The yolk 



