FruitlessFetusSign upYour embryo is now about a millimetre long, the size of an adult Gyrodactylus, a microscopic flatworm that clamps onto the skin and fins of freshwater fish with a ring of tiny hooks. It is one of the most common fish parasites in the world and a genuine menace in aquariums and fish farms.
Here is why it earns a place on this particular website. Gyrodactylus is born already pregnant. Each worm carries a fully formed embryo inside it, and that embryo already contains the beginnings of a third generation, nested like a set of Russian dolls. A single founder can seed a whole population in weeks without ever laying an egg.
If you have ever kept guppies, you have likely hosted this parasite without knowing it.
Meanwhile, inside you
Your embryo this week is firmly implanted and has split into layers that will become every organ you have. They are not much to look at yet, but they are laying down their own architectural plans.