Day 13 covers all of Chapter 8, The Assurance of Future Buddhahood of the Five Hundred Disciples.

Purna, the son of Maitrayani, is one of the 10 great voice hearers. But he is not just any Sravaka:

Thereupon the Buddha said to the bhiksus:

“Do you see this Purna, the son of Maitrayani, I always praise him, saying that he is the most excellent expounder of the Dharma. I also praise him for his various merits. He strenuously protects my teachings, and helps me propagate them. He shows the Way to the four kinds of devotees, teaches them, benefits them and causes them to rejoice. He explains my right teachings perfectly, and gives great benefits to those who are performing brahma practices’ with him. No one except the Tathagata excels him in eloquence.

Purna is not an ordinary Sravaka seeking to remove himself from the World of Endurance. He is a son of the Buddha, a secret Bodhisattav:

Bhiksus, listen to me attentively!
The Way practiced by my sons
Is beyond your comprehension
Because they learned how to employ expedients.

Knowing that people wish to hear
The teachings of the Lesser Vehicle,
And that they are afraid of having the great wisdom,
[My sons, that is,] the Bodhisattvas transform themselves
Into Sravakas or cause-knowers,
And teach the people with innumerable expedients.

Saying to the innumerable living beings, [for instance,]
“We are Sravakas.
We are far from the enlightenment of the Buddha,”
They save them, and cause them to attain [Sravakahood]
Even the lazy people who wish to hear the Lesser Vehicle
Will become Buddhas with this expedient
in the course of time.

My disciples are performing
The Bodhisattva practices secretly
Though they show themselves in the form of Sravakas.
They are purifying my world
Though they pretend to want little
And to shun birth-and-death.
In the presence of the people,
They pretend to have the three poisons and wrong views.
They save them with these expedients.
They change themselves into various forms.
If I speak of all their transformations,
The listeners will doubt me.

After assuring the 1,200 Arhats of their future Buddhahood, the 500 Arhats tell the PARABLE OF THE PRICELESS GEM. Telling the Buddha that he is like the friend who sewed a priceless gem in the garment, they said:

We are like the poor man.
In the long night you taught us
Out of your compassion towards us,
And caused us to aspire for unsurpassed [enlightenment].

Because we had no wisdom, we did not notice that.
The Nirvana we attained was only part [of your wisdom].
Satsifying ourselves with it,
We did not wish to attain anything more.

Now you have awakened us, saying:
“What you attained was not true extinction.
When you have the unsurpassed wisdom of the Buddha,
You will attain true extinction.”

Hearing from you that we are assured
Of becoming Buddhas one after another,
And that our worlds will be adorned,
We are joyful in body and mind.