DEVELOPMENT OF THE EMBRYO 439 



end of the ovum, where the embryo is to develop, is called the animal 

 pole. It is nearly free from yolk, and appears at the time of laying as 

 a circular whitish area known as the blastoderm (Fig. 251, b), and 

 measures from three to four millimeters in diameter. As the animal 

 pole is not as dense as the surrounding material, it is always found on 

 top of the yolk, no matter which way the entire egg be turned, provided, 

 of course, that the yolk is free to rotate. 



The more central portion of the animal pole is rather translucent 

 or pellucid and therefore is called the area pellucida (Fig. 251, C). This 

 central portion is surrounded by a whitish or opaque region called the 

 area opaca. The yolk itself is called deutoplasm, and is divided into two 

 types of material, white and yellow yolk. The white yolk occupies the 

 region just below the blastoderm, and is rather shaped like a flask, as 

 shown in the figure. It extends to the center of the yolk. It will be 

 noticed that the yolk is thus arranged in various concentric layers. A 

 layer of thick yellow yolk alternates with a thinner stratum of white 

 yolk. The two types of yolk differ in physical characteristics and in 

 chemical composition. 



The vitellus or true egg-cell alone is formed in the ovary. Such 

 structures as develop within the ovary proper are called primary. Struc- 

 tures, such as chorionic membranes (found in most of the higher forms), 

 are known as secondary structures, while those particular regions which 

 are formed by accessory reproductive organs, such as the white of the 

 egg and the shell, are said to be tertiary structures. The white of the 

 egg is composed of albuminous matter which is chemically quite com- 

 plex. It will be remembered that the protoplasm in all living cells is 

 largely albuminous. 



Toward each end of the newly-laid egg, one finds a dense, opaque 

 twisted cord extending through the white of the egg from opposite sides 

 of the yolk toward the apices of the shell. These twisted cords are called 

 chalazae (Fig. 251, C). They are continuous with a very thin, dense layer 

 of albumen surrounding the yolk. This thin layer is called the chalazif- 

 erous layer. It is generally assumed that the chalazae assist in holding 

 the yolk in position, though this has been disputed by several biologists, 

 primarly because the ends nearest the shell are not attached. Immedi- 

 ately outside the chalaziferous layer there is another thick, dense layer 

 of albumen, and superficial to this is a still thicker layer of a more fluid 

 albumen. The hard-boiled egg in which the albumen has coagulated 

 lends itself well for the observation of these various layers. Usually, in 

 observing such hard-boiled eggs the albumen is seen to be arranged in 

 spiral sheets. 



The ovoid shell which surrounds the entire egg is quite resistant to 

 gradually applied pressure, but easily broken if the blow be sharp. The 

 shell in turn is covered superficially by a thin cuticle perforated by many 

 pores. The main substance of the shell is made up of loosely arranged 



