Seventeen verses are to be recited to correspond to the 'seventeenfold' Praj[=a]pati.

But 'some say' twenty-one verses; and he may recite twenty-one, for if 'the three worlds' are added to the above seventeen one gets twenty, and the sun (_ya esa tapati_) makes the twenty-first! As to the number of worlds, it is said (_ib_.

I.2.4.

11, 20-21) that there are three worlds, and possibly a fourth.
Soma is now the moon, but as being one half of Vritra, the evil demon.
The other half became the belly of creatures (_ib_.

I.6.3.