The eggs of Ascaris lumbricoides can be found in human stool specimens in 2 main states: fertile or infertile. Fertile eggs each contain a visible embryo in an early stage of development. They are round to oval, measure 45 to 75 micrometers in greatest dimension and have a thick shell and a bumpy mammillated outer layer. In comparison, infertile eggs are oval to elliptical and measure up to 90 micrometers in greatest dimension. The shell is difficult to visualize in infertile eggs and no internal embryo is present. Like the fertile eggs, they have an outer mammillated layer. It is important to note, however, that both fertile and infertile eggs can lose their outer mammillated coat. When this happens, they are said to be decorticated. In the decorticated state, the fertile eggs can resemble those of hookworm, but are differentiated by their significantly thicker shell. The thick shell protects the developing embryo in the environment and allows the eggs to remain viable, even in harsh conditions. The images below show an A. lumbricoides egg in the process of hatching. The eggs had been stored in formalin in my lab's refrigerator for several months, but were clearly still viable, and would have been infectious if ingested!

.