654 MANUAL OF PHYSIOLOGY. 



brane of the uterus becomes so hypertrophied and developed as 

 to retain the ovum when it reaches that organ. 



The human ovum is a cell consisting of a mass of protoplasm 

 inclosing a nucleus and a nucleolus, and surrounded by a cell- 

 wall. On its outer surface is an irregular layer of cells, the re- 

 mains of that part of the tunica granulosa which encircled the 

 ovum iu 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 vitellusoryolk, 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 



FIG. 248. 



Ovum. (Robin.) a. Zona pellucida or vitelline membrane, b. Yolk. 

 c. Germinal vesicle or nucleus, d. Germinal spot or nucleolus. e. Interval 

 left by the retraction of the vitellus from the zona pellucida. 



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

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

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

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

 white being arranged in thin layers, which separate the yellow 

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

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



