12 LABORATORY OUTLINES FOR EMBRYOLOGY 



5. The Hen's Egg 



a. A Raw Egg 



Crack slightly the broad end of a hen's egg. Carefully 

 pick off a small portion of the shell. Notice that beneath 

 the shell there is a white leathery tissue. This is called 

 the shell membrane. How many layers are there in the 

 shell membrane? If the egg is not perfectly fresh, there 

 will be a space at this end of the egg. This is called the 

 air-chamber. Where is the air-chamber with reference 

 to the shell and the membrane? Immediately beneath 

 the shell membrane is the colorless albumen, the white 

 of the egg. Pour the egg out into a saucer. Examine 

 a piece of the shell with the dissecting microscope. 

 Notice that it is filled with very small holes. What is 

 the function of these holes? Notice that the white is not 

 all of the same consistency. What is the location of the 

 more fluid part ? From the opposite sides of the colored 

 part of the egg, the yolk, extend two twisted chords 

 called the chalazcB (hail stones). Are they attached to 

 the yolk? Are they attached to the shell? 



The yolk is enclosed in the vitelline membrane. What 

 other name could be applied to the vitelline membrane? 

 If the egg has been fertilized, there will be at a point on 

 the surface of the yolk, lying immediately underneath the 

 vitelline membrane, a small white disc about 4 m.m. in 

 diameter. This is the blastoderm. If the egg has not 

 been fertilized, this spot will be much smaller. It 

 is the nucleus and cytoplasm of the cell. The blasto- 

 derm is composed of a layer of cells. The yolk, as in all 

 eggs, is for the nourishment of the developing embryo. 



