Chicken eggs are rated by their quality as AA, A, B, and Inedible. I guess you nearly have eaten "Inedible" chicken eggs but not embryo egg or balut.
An embryo egg(balut) is done in method:
Method #1
1. Prepare a big wooden box; line this with heated rice hull at the bottom about 6-8 inches thick.
2. Arrange the eggs in bags of about 20-50 pieces each, put them on the heated hull and cover again with the same thickness of heated hull.
3. Cover the box very well so as to prevent quick loss of heat.
4. Every morning inspect the eggs to see if the rice hull needs reheating. The hull must be about 38°C.
5. On the third day, examine the eggs against a bright light to see if the germ plasm has developed. This is the part of the egg that becomes the chick if the egg is fertile. If no germ plasm develops remove these eggs. These are sold as penoy. Put back into the heated hull those eggs with germ plasm.
6. On the 13th day examine again the egg against the light. Remove the eggs without germ pasm. At this stage, those with developing germ plasm are the “balut sa puti”.
7. On the 17th day the chicks will be growing little feathers. These may now be cooked.