The middle form of the verb, which has a reflexive force. It is when an act returns back in its effect on oneself: see Note e, Heb. 1.3. Paul was to act thus in Acts 22.16; not to baptise himself, that would be active; but 'get baptised.' They passed through the sea and so got baptised. There was no action of course of a baptiser here, hence the middle voice.
'Are lawful' is the verb, of which exousia, the word translated 'right' or 'liberty' in ch. 8.9, and 'power' in Matt. 10.1, is the noun. The word translated 'liberty' in v. 29 is different, and is opposed to 'bondage,' as in John 8.32,36; 2Cor. 3.17; Gal. 5.13.