652 MANUAL OF PHYSIOLOGY. 



ovum in the Graafian follicle. The cell wall of the ovum is called 

 the vitelline membrane or zona pellucida, and the mass of granular 

 protoplasm it encircles, the vitellus or yelk, and in this is a nucleus 

 the germinal vesicle, which contains a nucleolus the germinal 

 spot. 



Beneath the outer covering of calcareous material of the hen's 

 egg there is a white membrane, which incloses a transparent albu- 

 minous substance known as the white of egg. Inside this is a 

 yellow fluid mass, the yelk, which is surrounded by a delicate 

 membrane, the vitelline membrane. The yelk is made up of two 

 varieties of material of different shades of color, the white and 



FIG. 248. 



Ovum. (Robin.) . Z>na pellucida or vitelline membrane. 6. Yelk. c. Germinal 

 vesicle or nucleus, d. Germinal spot or nucleolus. e. Interval left by the retraction of 

 the vitellus from the zona pellucida. 



the yellow yelk. Of these the yellow forms the greater part, the 

 white being arranged in thin layers, which separate the yellow 

 yelk into strata. In the centre of the yelk it .forms a flask- 

 shaped mass, with its neck turned to the upper surface, upon 

 which a portion of the yelk called the eicatrieula rests. This 

 cicatricula, which lies between the vitelline membrane and the 

 white yelk, is the active growing part of the egg, and out of it 

 are developed the chick and the embryonic membranes. 



Extending through the albumin from the vitelline membrane 

 to the ends of the egg are two twisted membranous cords the 

 chalazce, which fix and protect the delicate yelk from shocks, but 



