Don Wiebe, age 12, of  Rolling Meadows, I11., for his question:

PLEASE EXPLAIN INCUBATOR HATCHING OF CHICKEN EGGS.

Most baby chickens purchased by farmers come from large hatcheries. The eggs are placed in incubators which provide the just7right temperature and humidity for the chick to develop properly. A constant temperature of from 99.5 degrees F. to 100 degrees F. is considered best, and a system of fans circulates warm air through the egg chamber and over the eggs.

Chicken eggs need about 21 days to develop. During this time special devices in the incubator turn the eggs several times each day, the relative humidity is kept at about 60 percent and the oxygen level is kept at 21 percent--the normal level of outside air. Up to 100,000 eggs can be held in large incubators, and some hatcheries can hatch up to a million eggs at one time.