Only a Gen's body produces selyn in any way perceptible or measurable by instruments. 

Ancients and animals do not register as producing selyn, though theoretically they produce and use selyn instant by instant in exactly equal amounts. 

Children of two Simes or two Gens or a Sime and a Gen do not produce selyn, but don't use much either.  The fetus draws selyn from the mother, with that draw spiking just before birth, supplying the child's body with all the selyn necessary until Establishment or changeover

Of course, in the early centuries of the mutation, many mysterious child-deaths were due to selyn deficiency -- a fact completely unknown and un-discoverable at that time.  By the time it could have been observed, it no longer happened as the defective genetic strains had died out.  Deaths at changeover from insufficient selyn are still observed in the Interstellar era, and no remedy has been found -- not for lack of trying. 

This mechanism of pre-natal selyn draw is one crucial reason the "artifical womb" will not work for Simes and Gens whereas it does work just fine for Ancients. When the child's body reaches puberty there is a one in three chance they will begins to produce Selyn, becoming Gen.

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