658 RuRi\L School Leaflet. 



embryo chick. The two membranes prevent the too rapid evaporation of 

 moisture through the pores of the shell, but allow oxygen to enter the egg 

 and carbon dioxid to pass out. 



(c) By placing a section of the shell under the lens, indentations or 

 pores in the shell may be observed. 



These thinner parts permit the gases to pass more readily through the 

 shell. If the pores of the shell are closed by oil, varnish, dirt or broken 

 egg, the pores will be closed and the chick smothered. 



(f) Note the pigment of the shell, which gives to each egg its 

 characteristic color. 



Observe in nature how the first eggs laid for a brood are more pro- 

 nounced in color, and how the color pigment decreases with each egg that 

 is laid, due to exhaustion of the supply. 



3. The content of a boiled egg. — Crack carefully, on the large end, 

 the shell of a hard-boiled egg; remove the shell carefully piece by piece 

 to avoid tearing the shell membrane. 



(a) Observe the air-space and the two membranes, which are sepa- 

 rated with difficulty. Note that the outer membrane is the thicker and 

 tougher. 



(b) Cut the egg lenthwise through the middle. Observe the 

 lighter-colored flask-shaped center of the yolk and the darker yolk ar- 

 ranged around it in concentric layers. Note the "germinal vesicle" or 

 "germinal disc" at the upper part of the light yolk. Observe that the 

 yolk is at one side and not in the center of the white of the egg. Note 

 also that the germinal disc is on the upper side of the yolk. This is 

 because the yolk is lighter in weight than the albumen and hence floats. 

 The germinal disc on the surface of the white yolk is lighter than the 

 dark yolk. 



The chemical composition of the dry substance of the inside of the 

 egg is (Snyder: Poultry Book, page 188) : 



Protein Fat 



White (albumen, white of the egg) 88.92 .53 



Yolk 20.62 64.43 



It will be seen that there is a large amount of fat in the yolk and almost 

 no fat in the albumen. Fat is lighter than albumen, hence rises to the 

 surface. This may be observed in practice by holding a fresh egg in front 

 of an egg-tester and noting the tendency of the yolk to float upward. 



This tendency of the yolk to float to the surface makes it necessary 

 frequently to turn eggs which are kept for hatching, otherwise the yolk 

 will rise until the germinal disc comes in contact witli the shell membrane, 

 which becomes dry by evaporation and allows the vitelline membrane to 

 adhere and thus become ruptured, killing the germ when tlie egg is moved. 



4. Rcviezv. — Make a drawing, longitudinal section (the outline of an 

 egg i]A times natural size showing directly from the egg itself) : 



