To what did the animals now have to show reverence?

In Ch.5. of "Animal Farm" all the animals were now asked to show reverence to the skull of old Major.

"The skull of old Major, now clean of flesh, had been disinterred from the orchard and set up on a stump at the foot of the flagstaff, beside the gun. After the hoisting of the flag, the animals were rquired to file past the skull in a reverent manner before entering the barn."