Chrysostom: He privately insinuates Himself to be the Lord; for it is He Himself who is Lord of the harvest. For if He sent the Apostles to reap what they had not sown, it is manifest that He sent them not to reap the things of others, but what He had sown by the Prophets. But since the twelve Apostles are the labourers, He said, "Pray ye the Lord of the harvest, that he would send labourers into his harvest;" and notwithstanding He added none to their number, but rather He multiplied those twelve many times, not by increasing their numbers, but by giving them more abundant grace.
Remigius: Or, He then increased their number when He chose the seventy and two, and then when many preachers were made what time the Holy Spirit descended upon the believers.
Chrysostom: He shews us that it is a great gift that one should have the power of rightly preaching, in that He tells them that they ought to pray for it. Also we are here reminded of the words of John concerning the threshing-floor, and the fan, the chaff, and the wheat.
Hilary: Figuratively; When salvation was given to the Gentiles, then all cities and towns were enlightened by the power and entrance of Christ, and escaped every former sickness and infirmity. The Lord pities the people troubled with the violence of the unclean Spirit, and sick under the burden of the Law, and having no shepherd at hand to bestow on them the guardianship of the Holy Spirit. But of that gift there was a most abundant fruit, whose plenty far exceeded the multitude of those that drank thereof; how many soever take of it, yet an inexhaustible supply remains; and because it is profitable that there should be many to minister it, He bids us ask the Lord of the harvest, that God would provide a supply of reapers for the ministration of that gift of the Holy Spirit which was made ready; for by prayer this gift is poured out upon us from God.