'On neither of these two ways those small creatures (flies, worms, &c.) are continually returning of whom it may be said, Live and die. Theirs is a third place. 'Therefore that world never becomes full (cf. V, 3, 2). 'Hence let a man take care to himself! And thus it is said in the following Sloka:—

In plain words: Tiny creatures like flies and worms follow neither path; they just keep being born and dying over and over in a third place. That is why the other world never fills up. So, the teacher says, a person should take care of their own soul.