LABORATORY EXERCISES 369 



(1) Strength of the egg shell. Let each student hold a hard- 

 shelled egg between the clasped hands, the ends of the egg in the hollow 

 of the hand, and try to break it. 



Observe the great strength of the egg, due to the arrangement of 

 the particles of the shell in an arch similar to the stones or bricks in 

 the arch of a bridge. 



(2) The contents of an uncooked egg. (a) Break a fresh, uncooked 

 egg in a saucer by separating the shell in the middle. 



Observe the "germinal disc," which appears as a light-colored spot 

 usually to be found on the upper surface of the yolk. 



The germinal disc contains the life principle of the egg. Being 

 on the upper surface, it remains in close contact with the source of heat 

 during natural incubation. 



(b) Note the "chalaza," or the whitish cords of denser albumen 

 on the sides of the yolk toward either end of the egg. These cords of 

 denser albumen serve to keep the yolk properly suspended within the 

 albumen. Thus the chick which develops on the upper surface of the 

 yolk is protected from injury, if, through rough handling, it should 

 come in contact with the shell. 



(c) Note the transparent, watery appearance of the albumen 

 (white of the egg). 



The albumen supplies the food by which the chick grows within 

 the shell, in liquid form. 



(rf) Examine the shell and note the air-space usually found near 

 the large end. Observe the two tough membranes, best seen at the 

 air-space where the membranes separate. 



The air-space furnishes a readily available supply of fresh air to the 

 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. 



(e) 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 through the shell more 

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

 broken egg, the chick will be smothered. 



(/) Note the pigment of the shell, which gives to each egg its char- 

 acteristic color. 



Observe how the first eggs laid for a brood are more pronounced 

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

 laid, due to exhaustion of the supply. 



