ANSWER:
BLOOD SPOTS:
Blood spots occur when blood or a bit of tissue is released along with a yolk.  Each developing yolk in a hen's ovary is enclosed in a sack containing blood vessels that supply yolk building substances. When the yolk is mature, it is normally released from the only area of the yolk sac, called the "stigma" or "suture line", that is free of blood vessels.  Occasionally, the yolk sac ruptures at some other point, causing blood vessels to break and blood to appear on the yolk or in the white. As an egg ages, the blood spot becomes paler, so a bright blood spot is a sign that the egg is fresh.

Blood spots occur in less than one percent of all eggs laid.  They may appear in a pullet's first few eggs, but are more likely to occur as hens get older, indicating that it's time to cull.  Blood spots may be triggered by too little vitamin A in a hen's diet, or they may be hereditary - if you hatch replacement pullets from a hen that characteristically lays spotted eggs, your new flock will likely do the same.

MEAT SPOTS
Meat spots are even less common than blood spots. They appear as brown, reddish brown, tan, gray or white spots in an egg, usually on or near the yolk. Such a spot may have started out as a blood spot that changed color due to chemical reaction, or it may be a bit of reproductive tissue.  Since meat spots look unappetizing, cull a hen whose eggs characteristically contain them.

Excerpt from "Storey's Guide to Raising Chickens", by Gail Damerow ISBN: 1-5801-7325-X. Used with Permission.  This book is highly recommended and it is available from our Poultry Book Store at a 30% discount off the list price.