The Development of the Chick 91 



attached to it, are the chalaz&, which seem to 

 be merely more condensed portions of the 

 albumen that are twisted, and have been said 

 to serve to hold the yolk in the centre of the 

 egg, though it is difficult to see how they can 

 serve any such purpose, as they are not at- 

 tached at their outer ends. 



The yolk is the essential part of the egg and 

 corresponds, as has been previously pointed 

 out, to the true ovum or egg of the frog, or 

 other animals. It is bright yellow in color, 

 spherical in shape, and about an inch in diam- 

 eter. It is surrounded and held in shape by 

 the thin, elastic vitelline membrane, and ex- 

 hibits on one side, normally the upper one, no 

 matter how the egg has been opened, a small, 

 whitish circle, the blastoderm or cicatricula 

 (Fig. 33, b /). The yolk substance is made up 

 of a great number of yolk granules, of which 

 two main kinds may be distinguished, the yel- 

 low and the white. If the yolk of a hard-boiled 

 egg be carefully cut, with a sharp knife, ver- 

 tically through the blastoderm, it will be 

 noticed that the white and the yellow yolk are 

 arranged in concentric layers, the yellow being 

 the more abundant ; and that there is a flask- 

 shaped mass of white granules in the centre of 



