234 OUTLINES OF CHORDATE DEVELOPMENT 



Surrounding the vitellus or egg cell, are several nutritive and 

 protective egg membranes, all of the tertiary class, i.e., formed 

 by the accessory reproductive organs, the vitellus alone having 

 been formed within the ovary; there are no secondary or chori- 

 onic membranes. Immediately surrounding the vitellus is the 

 " white' 7 or albumen. The chemical composition of this is 

 quite complex, various albumens forming the predominating 

 constituents. Two denser, opaque, twisted cords, the chalazce, 

 extend through the albumen from opposite sides of the vitellus, 

 toward the apices of the shell. These are continuous with a 

 very thin, dense layer of albumen surrounding the vitellus, the 

 chalaziferous layer. Outside of this the albumen forms a rather 

 thick dense layer, and superficially there is a still thicker layer of 

 more fluid albumen. In the hard-boiled egg the albumen can 

 often be seen to be laid around the vitellus in spiral sheets. 



The entire ovum is enclosed in a definite ovoid shell, very 

 resistant to pressure applied gradually, though easily broken 

 by a sharp blow. The shell is covered superficially by a thin 

 structureless cuticle perforated by numerous pores. The 

 chief substance of the shell is composed of loosely arranged 

 particles of the carbonates and phosphates of calcium and 

 magnesium, in an organic matrix. The inner surface of the shell 

 is formed by a thinner but denser layer of inorganic salts. 

 The dried shell is very porous and affords an easy pathway for 

 the passage of gases and water vapor. 



Lining the shell is the tough shell membrane. This is a 

 double sheet of fibrous connective tissue; at the blunter end of 

 the shell its two layers are separated by an air space, which is 

 often of considerable size in eggs that have been laid for some 

 time. 



2. The Reproductive Organs of the Fowl 



A better understanding of the structure of the egg can be had 

 from the study of its formation, but first we must review the 

 main facts regarding the re productive system of the fowl. These 

 organs are asymmetrically developed, those of the right side 



