‘And Moses and Aaron took these men who are mentioned by name, and they assembled all the congregation together on the first day of the second month, and they declared their pedigrees (their ‘begetting') after their clans, by their fathers' houses, according to the number of the names, from twenty years old and upward, by their heads.'
These chieftains, these ‘men who are mentioned by name' because of their prominence, then returned to their tribes and gathered them on the day in question and each of the male members of the tribe ‘declared their begetting', that is, their claim to connection with the tribe, clan group by clan group, in their tribes, according to the number of names, for all those of twenty years old and upwards. In other words each tribe mobilised its fighting men, identifying them with the tribe. By now those of the mixed multitude (Exodus 12:38) who had determined to become Yahwists (Exodus 12:48) would have been incorporated into the tribes by adoption, and have become ‘children of Israel', and members of a chosen tribe, ‘descended' from the tribal patriarch.
“After their clans, by their fathers” houses, according to the number (assessment) of the names.' Note the division into clans, and then into wider families (fathers' houses), and then into smaller groupings (the ‘assessment of the names').