302 FORMATION OF THE EGG AND CHICKEN 



yolk be examined there will be found, just beneath the vitelline 

 meml^rane, a small circular whitish body, about one-eighth of an 

 inch in diameter, which will always float uppermost, no matter in 

 what position the egg is laid. In the fertilized egg this disk has 

 three well-defined areas, — an outer white rim, within which is a 

 clear zone, and in the centre of this zone a somewhat elongated 

 structure which is the embryo itself. These three parts constitute 

 what is known as the blastoderm of the hen's egg, the part whose 

 function is reproduction; all the rest of the egg is directly utilized 

 for nutrition and protection. 



The passage of the egg down the oviduct, from this time on, 

 is marked by the following processes: 



By the muscular contraction of the walls of the oviduct, the 

 yolk is propelled to the cloaca. The passage of the yolk stimulates 

 the walls of the duct to secrete successive layers of albumin. The 

 first layer is thick, and has thickened portions which are fastened 

 to the opposite poles of the yolk, to hold it in place. These thick- 

 ened portions are called chalazse; they are heavy twisted threads 

 of albumin. After the thick layer of albumin two layers are added, 

 each of which is thinner and more watery than the preceding one. 

 When the egg reaches the lower or contracted part of the oviduct, 

 two very tenacious but thin layers of albumin are added, to form 

 the inner and outer shell membranes. The air cell, which is always 

 perceptible at the larger end of the fully developed egg, is not 

 present at first, but is formed by the evaporation of the fluids of 

 the albumin and the entrance of atmospheric air to replace these 

 fluids after the egg is laid. Just before the egg is discharged into 

 the cloaca, it passes through a section of the oviduct where the 

 shell is added, in the form of a coating of thin liquid secretion 

 containing lime, which hardens quickly. 



The perfect egg with its hardened shell is retained in the cloaca 

 a short time before being laid. The power to hold the egg in the 

 cloaca is under the control of the bird, so that, if she is frightened 

 from her nest or unable to reach it, she can retain the egg for some 

 time, the exact period varying with individuals. 



The parts of the completed egg are the shell, shell membranes, 

 albumin, and yolk (Fig. 146). The use of each is discussed in the 

 following paragraphs. 



The shell is the outer envelope of the egg, and its function is 

 primarily that of protection. Soft-shelled eggs are those that have 

 either no shell or else a very thin one. This is due to a deficiency 



