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[Ascaris lumbricoides]

Fertilized and unfertilized Ascaris lumbricoides eggs are passed in stool of the infected host.  Fertilized eggs are are rounded and have a thick shell with an external mammillated layer that is often stained brown by bile.  In some cases, the outer layer is absent (known as decorticated eggs).  Fertile eggs range from 45 to 75 µm in length.  Unfertilized eggs are elongated and larger than fertile eggs (up to 90 µm in length).  Their shell is thinner and their mammillated layer is more variable, either with large protuberances or practically none.  Unfertile eggs contain mainly a mass of refractile granules.

Three fertilized eggs (one decorticated, on the right) of A. lumbricoides.