The enrolment having been completed in this manner, those of the tribunes on whom this duty falls collect the newly-enrolled soldiers, and picking out of the whole body a single man whom they think the most suitable make him take the oath that he will obey his officers and execute their orders as far as is in his power.
Then the others come forward and each in his turn takes his oath simply that he will do the same as the first man.
At the same time the consuls send their orders to the allied cities in <a class="linkToPlace" target="_blank" href="/place?placename=Italy&groupId=656&placeId=1199">Italy</a> which they wish to contribute troops, stating the numbers required and the day and place at which the men selected must present themselves.
The magistrates, choosing the men and administering the oath in the manner above described, send them off, appointing a commander and a paymaster.
The tribunes in <a class="linkToPlace" target="_blank" href="/place?placename=Rome&groupId=935&placeId=1669">Rome</a>, after administering the oath, fix for each legion a day and place at which the men are to present themselves without arms and then dismiss them.
When they come to the rendezvous, they choose the youngest and poorest to form the velites; the next to them are made hastati; those in the prime of life principes; and the oldest of all triarii,
these being the names among the Romans of the four classes in each legion distinct in age and equipment.
They divide them so that the senior men known as triarii number six hundred, the principes twelve hundred, the hastati twelve hundred, the rest, consisting of the youngest, being velites. If the legion consists of more than four thousand men, they divide accordingly, except as regards the triarii, the number of whom is always the same.
Walbank Commentary