I want to preface this summary with a quick heads-up: this sutta has a few points that will strike modern readers as very strange, and perhaps problematic. Remember that these texts are around 2,500 years old. They come from a time and culture which was, in some respects, completely unlike our own. It is important that we avoid misinterpreting passages by viewing them through our own cultural lens, and we must also remind ourselves that passages which may seem bizarre to us would have been perfectly understandable to the intended audience.
I will do my best to explain these points when they come up - usually, I would leave this sort of background information in the endnotes, but for this sutta I feel it is important enough to leave in the main body of text.
The Buddha was traveling through the kingdom of Kosala with around 500 monks. They reached Icchānankala, a village full of Brahmins, where the Buddha and his monks stopped to stay in the jungle. In the nearby village of Ukkhattha, there were a couple of famous and well-respected Brahmins - the teacher Pokkharasāti, and his student Ambattha. They lived on property given by the Kosalan king, Pasenadi.
Ambattha was from a high-class family, and he was considered a fully-educated Brahmin. Pokkharasāti had already given him his approval by telling him, "What I know, you know." This meant he had memorized the Three Vedas1, and (most importantly for this story) he was trained in recognizing the 32 Marks of a Great Man.
What are these 32 Marks? By the time the Pali Canon was finalized, there seems to have been a belief that someone could be determined to be a "Great Man" (mahāpurusa) by inspecting their bodies for certain features and markings. A person born with these marks would go on to realize one of two achievements: they would either become a holy king, or they would renounce the world and become fully enlightened. I won't go into the specifics here - both for the sake of brevity, and because I don't think they're important.2
Pokkharasāti had heard the rumors of the Buddha's greatness, and found himself curious. He told his student Ambattha to go and investigate; if the rumours were true, the Buddha would bear the 32 Marks. Since Ambattha was trained in recognizing these signs, he could report back with his findings.
Ambattha left in a horse-drawn chariot, accompanied with a group of other young Brahmins. They rode as far as they could, and then walked the rest of the way through the jungle. They came across some monks, and as a testament to Ambattha's reputation, they actually recognized him. They pointed out the Buddha's dwelling, and instructed the Brahmins on the proper way to enter - they should approach quietly, without rushing; then, they should cough and knock on the door; the Buddha himself would rise and open the door for them.
After joining the Buddha, Ambattha's friends exchanged pleasantries with him and sat down respectfully. Ambattha, however, ignored the usual etiquette for such a meeting; he paced around while the Buddha was seated, and continued speaking after the others had finished with the usual small-talk.
The Buddha asked him outright, "Ambattha, would you behave this way with your Brahmin teachers?" Ambattha said, "No, I wouldn't - but you're no Brahmin teacher. This is the way I speak with lowly, bald-headed, fake ascetics. It's the way I speak with those who are dark, those who are born from the feet of Brahma."3
The Buddha simply responded by saying, "You've come to see me for some specific reason, but you're not focusing on that. Clearly, you haven't perfected your training." This enraged Ambattha, who began cursing at the Buddha. He began disparaging the Shakyas, the clan from which the Buddha came. He ranted and raved about how rude and poorly-behaved they were - ironic, given his own demeanor. He said that Shakyans were a lowly people, and that they don't revere Brahmins the way they should.
"Why do you have such ill-will towards the Shakyans?" the Buddha asked. Ambattha told the Buddha that once he had visited the Shakyans in their capital, Kapilavatthu, while running an errand for his teacher Pokkharasāti. Having entered their meeting hall, he was shocked at their behavior - the Shakyans did not come down from their high seats, instead laughing with one another, poking each other and having fun. They did not offer Ambattha a seat. Ambattha took this to be an insult to him personally.
The Buddha told him this was an insignificant thing, completely undeserving of the level of hatred Ambattha had fostered. He told the Brahmin, "Even a quail is allowed to behave how it wants in its own nest. You were in the home of the Shakyans, Ambattha - their behavior was fine."
Ambattha, then, begins ranting about the four social classes. In the Buddha's time, society was broadly divided into four classes or castes: Brahmins (the priestly class), the Khattiyas (warriors and aristocrats), the Vaishyas (merchants and farmers), and the Shudras (commoners, laborers, and artisans). These four classes existed in a hierarchy of superiority, rooted in ideas of spiritual purity.4 Traditionally, Brahmins were seen as the most pure of the classes, but by the Buddha's time, shifting socio-economic conditions had given rise to the Khattiya class being empowered enough to claim supremacy for themselves, at least in certain regions. As Walshe notes in his translation of this sutta, in Kosala (where this sutta takes place), and in the west of the Indian subcontinent generally, Brahmins enjoyed supremacy - however, in Sakya, where the Buddha was born, Khattiyas were at the top of the chain.
Ambattha, being a Kosalan, insisted that Brahmins were the highest of the classes, and that the other three - including the Sakyan Khattiyas - should be subservient to them, and revere them as superiors.
The Buddha asked Ambattha about his own clan heritage. He answered, "I am a Kanhāyana." The Buddha then revealed something to him, and his Brahmin friends: the Kanhāyanas descended from a slave-girl, who once belonged to a Sakyan master.
To explain, the Buddha first recounted the story of King Okkāka, the ancestor of the Sakya clan. When it came time for him to prepare one of the princes to take his throne, he chose the son of his beloved queen. He banished the other princes: Okkāmukha, Karandu, Hatthinīya, and Sīnipura. They settled in the Himalayas, beside a lotus pond, in a large grove of teak-trees (sāka). They created families with their own sisters, in an attempt to keep their bloodlines "pure."5
Eventually, the king's ministers told him about the lives his exiled sons had built for themselves. He swelled with pride, impressed at the way they were thriving despite having been banished. "They truly are Sakyans, as strong as the teak-trees where they live!" This, the Buddha explained, was the origin of the Sakyan clan and their name.
This set the stage for the origin story of Ambattha's clan, the Kanhāyanas. King Okkāka had a slave-girl named Disā. This slave gave birth to a "black boy," a Kanha. When it was born, it exclaimed, "Wash me, mother! Remove this filth which covers me! I will bring you great fortune!" However, the people were frightened by him... they proclaimed, "He is a newborn, yet he speaks! He is a Kanha, he is a goblin!" The Buddha explained that in the time of King Okkāka, people used the term "kanha" as an insult in much the same way as contemporary people used the term "goblin" (pisāca).6
I want to stop here and pull away from this story in order to address something: we must not read this passage in the light of our own cultural framework. The word "kanha" here is complicated. It can simply refer to the color black, or darkness in general - including that of the skin. However, we shouldn't understand this in any sort of racial context. As Christopher Titmuss notes in his book "The Explicit Buddha," Brahmins often had lighter skin because their social privelege allowed them to stay out of the sun. Members of the lower-class, on the other hand, often had to toil in the sun, leading to darker complexions. They also did not wander around outdoors, as the Buddha and other holy mendicants did. Therefore, the Brahmins took pride in their lighter complexions, and used the word "black" or "dark" as an insult to lower-class people. The Buddha telling a Brahmin he was descended from a "Kanha," a "black boy," would thus have been shocking - a pointed attack on the prejudice they held. Most importantly, as we shall see, the Buddha went on to subvert this idea when he told the rest of Kanha's story.
Finally, this word has a metaphorical dimension as well, in the sense of "darkness" as the opposite of "light." This is strongly implied by the supernatural elements of this story, such as the boy speaking immediately after being born, and being equated to a goblin. When we read the story of Kanha, we need to keep all of this context in mind.
At this point, Ambattha's friends spoke up. "Please, do not try to humiliate Ambattha this way! He is high-born, well-educated, and he's perfectly capable of demonstrating his nobility!" The Buddha retorted, "If this is true, let him speak for himself - otherwise, I should just have this discussion with all of you instead." They backed down.
"I have an important question for you," the Buddha said to Ambattha, "and it's one you won't like. You should answer it anyways. However, I must warn you - if you evade the question, your head will be split into seven pieces." (This will be explained soon!)
"Ambattha, have your Brahmin teachers told you this before, regarding the roots of your clan?" Ambattha remained silent. The Buddha asked once more, only to be met with silence again. The Buddha warned him once more: "Ambattha, do not be silent. If someone thrice ignores a genuine question asked by a Tathāgata, their head will be split into seven pieces."
A yakkha (nature spirit) named Vajirapani was in the air above Ambattha, holding a massive weapon called a "vajira." This weapon was surrounded with fire, glowing red, and Vajirapani was waiting eagerly for his chance to bring it down on Ambattha's head.
Here, again, I want to interrupt the story in order to provide some information which is too important to leave for the endnotes. This passage is striking, and many people may be shocked to see the Buddha talking about someone's head being blown apart by a weapon-wielding spirit.
As Walshe notes in his translation of this sutta, this idea of someone's head being split into seven pieces for ignoring a question seems to be a pre-Buddhist idea which found its way into the suttas. It appears twice; here, and again in the Cūlasaccakasutta (MN 35). In both suttas, the relevant passages are nearly identical. In both instances, the yakkha Vajirapani was said to be the spirit ready to enact the violence. This was not something the Buddha wanted to happen - he was simply warning the people with whom he was arguing that there was a spirit eager to attack them if they ignored his question. This was not something he was ordering or commanding. The Buddha himself was threatened similarly by a yakkha in the Ālavaka Sutta (SN 10.12 / Sn 1.10).
This raises two questions: 1. What is a yakkha? 2. Who is Vajirapani? In the suttas, yakkhas are a type of nature spirit. They are ruled by King Vessavana (also known as Kuvera, or Vaiśravana in Sanskrit). King Vessavana is one of the high-ranking devas known as the Four Great Heavenly Kings. The suttas depict various yakkhas with a wide range of personalities and demeanors. Some are benevolent and even volunteer themselves as guardians of the Buddha's teachings. Others are openly hostile and can be understood as demons.7
"Vajirapani" is a name which means "Vajira-Wielder." "Vajira" is the Pali form of the word "Vajra," which refers to a type of weapon from Indian mythology. Originally used by the Vedic god Indra, it came to be a symbol of ritual power in Buddhism. In fact, in later commentaries, Buddhaghosa understands Vajirapani as referring to Indra himself, but I find this strange - Indra is usually referred to as Sakka in the suttas, and he is understood as a great deva, much higher-ranking than a yakkha. Admittedly, I usually disregard the commentaries, and I certainly won't claim to be more knowledgable than the Venerable Buddhaghosa.
Those familiar with later Buddhist traditions may be familiar with Vajrapani, a bodhisattva and Dharma Guardian who bears the same name as this yakkha and also wields a vajra. Some sources claim that these figures should be understood as the same being - I'm not so sure, but won't make a hard claim one way or the other.
Ambattha saw Vajirapani above him, and was (understandbly) terrified. He crouched down beside the Buddha, and asked him to repeat his question one last time. The Buddha asked, "Have your teachers told you about the origins of your clan?" Ambattha admitted that he had, in fact, been taught the history of his clan. He already knew he was descended from the son of a Sakyan slave, despite using his supposed "high birth" to speak poorly of the Buddha and his people.
Ambattha's friends became wild. "Our very own Ambattha is a low-born descendant of a Sakyan slave! This is outrageous! The Buddha spoke the truth when he claimed that his ancestors were the masters of Ambattha's ancestors... we were wrong to doubt him!"
The Buddha, out of compassion for Ambattha, stepped in to stop the other Brahmins from belittling him. He had already achieved his first part of his goal - to show that Ambattha was not superior to the Sakyans. Now it was time to teach the real point of his lesson: no one is greater than or lesser than anyone else simply because of their heritage.
"Brahmins, you should not disrespect Ambattha for being a descendant of Kanha. You see, Kanha became a holy man. He traveled to the south country, and he learned the great mantras of the Brahmins. Eventually, he asked King Okkāka to marry one of his daughters, Maddarūpi. The King was outraged that someone like Kanha would be so bold as to ask for his daughter's hand. He grabbed his bow, knocked an arrow, and aimed it at Kanha, but he found himself unable to loose the arrow or relax the string."
The Buddha continued. "The king's ministers ran over to Kanha, begging him to spare their ruler. Kanha reassured them that he had no plans to harm the king, but warned them that the king should obey him if he wanted to keep his kingdom safe from harm: if the king shot his arrow downwards, his realm would be devastated by an earthquake; if he shot an arrow upwards, the gods would curse his kingdom with a drought for seven years. The only way to ensure safety was for the king to aim his readied arrow at the crown-prince. He assured everyone that no one would be harmed, not even the prince. The king obeyed, and just as Kanha said, nobody was harmed."
"Being filled with fear by Kanha's display, he agreed to let the low-born Kanha marry his royal daughter. Despite his low birth, Kanha had become a great sage, skilled in the Brahmin mantras. Ambattha has no reason to feel ashamed for being his descendant." With this conclusion to the story, the Buddha had inverted all of the ideas of anyone being superior to another by being of a higher birth. He countered Ambattha's claim of superiority over the Sakyans by revealing the Brahmin's ancestors as Sakyan slaves; however, he also countered the idea that Ambattha should be ashamed of his birth, because Kanha was able to become a holy man despite being considered a low-born.
The Buddha asked Ambattha to consider a few hypothetical scenarios. "Imagine if a young Khattiya man married a Brahmin woman, and they had a son together - Brahmins would offer their son a seat and give him water; the son would be given food at funeral-rites, rice-offerings, and when received as a guest in a Brahmin home; he would be taught the mantras; and finally, he would be allowed in the presence of Brahmin women without them being covered, unlike those considered to be low-born. Isn't this true?" Ambattha agreed.
The Buddha continued. "However, the Khattiyas would not annoint this son as a king; isn't that right, Ambattha? Why is that?" Ambattha answered, "Because the Khattiyas do not recognize someone born from a Brahmin woman as high-born."
The Buddha then presented the opposite scenario. "Imagine if a young Brahmin man married a Khattiya woman, and they had a son together - Brahmins would receive their son as a Brahmin high-born, but again, the Khattiyas would not recognize someone born from a Brahmin man as a high-born. He would not be annointed by the Khattiyas. Isn't this true?" Ambattha, once again, agreed. "This demonstrates that the Khattiyas are of a higher social standing than the Brahmins, Ambattha."
The Buddha presented yet another hypothetical scenario. "If a Brahmin had been punished by his peers, his head would be shaved, he would be punished with a bag of ashes8, and he would be exiled. He would no longer be treated with the honors we previously discussed. However, if a a Khattiya were likewise punished by his own peers, he would still be honored in all of those ways by the Brahmins. Isn't this true?" Ambattha agreed. "So even an exiled Khattiya is of a higher social standing than a Brahmin."
"However, there is a verse which was once spoken to me by the great god Brahmā Sanankumāra9:
'The Khattiyas are the highest among those who revere clans,
But one with knoweldge and conduct is the highest among both gods and men.'"
"Ambattha, the message of this verse is true, and I make the same claim myself." The Buddha, here, insisted that one's birth and social standing is not important—knowing what is true and living a virtuous life is infinitely better.
The Buddha's words must have moved Ambattha, because the Brahmin asked him to explain the ideas of knowledge and conduct. "Ambattha," he replied, "those established in knowledge and conduct are not slaves to ancestry, social status, or pride. By abandoning prideful nonsense such as this, one can achieve supreme knowledge and conduct." Ambattha asked the Buddha to say more.
At this point, the sutta presents the Sīlakkhandhavagga Pericope, a stock passage which appears in the 13 suttas of the Sīlakkhandhavagga. It begins with the Buddha discussing the appearance of a Tathāgata in the world, and the attainment of enlightenment. For the purposes of this sutta, this means the realization of "supreme knowledge and conduct." The passage then introduces a hypothetical person which hears these teachings and renounces his life as a householder to become a monk.
The next part of this passage is the list of ethical practices from DN 1). Our hypothetical monk perfects these ethical practices; next comes guarding the sense gates; being mindful & alert; being content; abandoning the 5 Hindrances; and the Four Jhānas. The Buddha explained that these practices make up "the supreme conduct taught by a Buddha."
The passage continues, and the Buddha explained that "supreme knowledge" consists of the various attainments which arise from the mind being immersed in samadhi, including insight into the nature of the body & consciousness; the various supernatural powers; the destruction of the defilements; and the cessation of continued existence. The Buddha concluded by saying, "there is no further development of knowledge or conduct beyond these things."
Next, the Buddha told Ambattha about four "drains," or paths leading to failure, which must be avoided by those who pursure supreme knowledge and conduct. Basically, these were four examples of ways someone attempting to become a religious sage could get in over their heads.
"First," the Buddha began, "there is the case where someone who has yet to achieve knowledge and conduct will boldly enter the wilderness, carrying his belongings in a cloth tied to a pole over his shoulder, believing he will survive by eating fruit which has fallen from the trees. Such a person will only find success in being a servant to someone who has achieved knowledge and conduct."
"Second, there is the case where such a person will enter the wilderness. Having been unable to live off of the fallen fruits, he brings a spade and a basket, believing he will survive by eating roots and tubers from the ground. Such a person will only find success in being a servant to someone who has achieved knowledge and conduct."
"Third, there is the case where such a person - unable to survive in the wilderness by any means - sets up a fire-hearth at the edge of some village or town. There, he tends the sacred fire. Such a person will only find success in being a servant to someone who has achieved knowledge and conduct."
"Finally, there is the case where such a person - unable to survive in the wilderness, and unable to get by tending the sacred fire on the outskirts of a village - will move to the central square of some town, or some crossroads, and build a fire-hearth. There, he waits for some holy man to come by, so that he may venerate them. Such a person will only find success in being a servant to someone who has achieved knowledge and conduct."
"Ambattha," said the Buddha, "tell me this: have you or your teacher achieved this supreme knowledge and conduct? Is it found in your tradition?" Ambattha replied, "No, Master Gautama. We are far from these achievements." The Buddha followed up by asking, "Ambattha, have you or your teacher even attempted the Four Drains?" Ambattha replied, "No, Master Gautama."
"So, Ambattha, not only has no one in your tradition achieved the supreme knowledge and conduct - they have not even gone down the four paths which lead to failure! Despite this, your teacher Pokkharasāti dares to speak down on those he considers to be lowly, dark, born from the feet of Brahma - those he calls fake ascetics. He has achieved so little, but disparages those who have achieved more. Don't you see he has led you astray?"
"Consider this, Ambattha. Your teacher Pokkharasāti lives off land and food provided to him by King Pasenadi. However, the king won't even meet him face-to-face! When they have an audience together, the king stays behind a curtain."10
"Consider this, too, Ambattha - you Brahmins claim to have the authority of the Vedic seers of the past. However, let's imagine a commoner were to stand where the king once stood, and spoke as if he had the king's authority. Does this make him equal to the king, or even one of the king's ministers?" The Brahmin answered, "No, Master Gautama."
"It is the same with the Brahmins. In the past, the Vedic seers authored the hymns of the Vedas.11 Their names were Atthaka, Vāmaka, Vāmadeva, Vessāmitta, Yamadaggi, Aṅgīrasa, Bhāradvāja, Vāsettha, Kassapa, and Bhagu. Today, you Brahmins study their verses and learn their mantras. However, you are not seers. You Brahmins may imagine yourselves as standing in their place, but you do not."
"Tell me, Ambattha - when you have studied these seers, did they live the way you and your master do today? Did they take luxurious baths and perfume themselves? Did they neatly groom their hair and beards? Did they adorn themselves with garlands and wreaths? Did they dress in white clothing? Did they indulge themselves, addicted to sensual pleasure? Did they eat only the whitest grains of rice, with a wide range of soups and sauces on the side? Did they enjoy the company of sexy women? Did they gallop around the town in fancy chariots? Did they keep themselves surrounded by armed thugs, holed up in fortresses?"
"No," Ambattha said, "they did not." "So, Ambattha, neither you nor your master are sages - nor are you close to becoming sages! Yet you have doubts about me. Why don't we resolve this? Ask me whatever you want."
The Buddha and Ambattha got up and started walking around. Ambattha used this opportunity to inspect his body for the 32 Marks. He was able to see all of them except two - a "sheathed penis"12, and an unusually large tongue.
The Buddha realized what was happening, and decided to remove Ambattha's doubts for good. Using his supernatural powers, he revealed his "sheathed penis,"13 then stuck his tongue out and moved it across his entire face in order to show how long it was.
...This passage is strange. Funny, even. I'd simply like to remind you about what I said earlier: most likely, these passages about the 32 Marks are later monastic insertions, added to give the Buddha a type of authority which would have been acknowledged by certain audiences in the time when this scripture was finalized. It's bizarre to us today, but to the intended audience, it was probably perfectly acceptable.
Finally, Ambattha was convinced - the Buddha was the real deal. He was truly a Great Man. He bid farewell to the Buddha and took his leave. He walked back to his chariot and rode back to see his teacher. Pokkharasāti had left Ukkaṭṭhā, and was waiting for his student in his park. Ambattha left his chariot and walked to his teacher, bowing and sitting to his side.
"Ambattha," said the teacher, "did you see Master Gautama?" "Yes, sir, I did." "Are the rumors true?" "Yes, sir, they are. He bears the 32 Marks."
Pokkharasāti told his student to recount the conversation he had with the Buddha. Ambattha told him everything, and the Brahmin teacher was horribly embarassed by his student's behavior. He was angry, and he blamed Ambattha's behavior for the things the Buddha said about them and the Brahmin tradition. He was so angry, in fact, that he began kicking and verbally abusing Ambattha. He then decided to go and visit the Buddha himself, but some of the other Brahmins told him it was too late to depart now - so, he retired for the night, determined to set out the next morning.
The Brahmin teacher had a bunch of food prepared to be given as an offering, and left with it by chariot with a number of torchbearers in his company. Like Ambattha before him, he rode as far as the horses could go, then embarked on foot. He met the Buddha, exchanged polite greetings, and sat down to his side. Pokkharasāti asked the Buddha to recount what had happened with Ambattha, and the Buddha described their discussion. When the Buddha had finished, the Brahmin said, "Master Gautama, my student is a fool. Please forgive him."
The Buddha responded warmly, simply saying, "May your student Ambattha be happy, Brahmin." Pokkharasāti began to inspect the Buddha's body, and the scenario played out just like the one before it - the Buddha recognized what was happening, and revealed the final two marks to the Brahmin. In this way, Pokkharasāti also became convinced of the Buddha's authenticity. "Master Gautama," the Brahmin asked, "might I offer a meal for you and your monks today?" The Buddha consented by remaining silent.14
The Buddha, his monks, and the Brahmin traveled to Pokkharasāti's home, and he served the Buddha some food while his Brahmin students served the Buddha's monks. When the Buddha had finished eating, Pokkharasāti took a seat beneath the Buddha and to his side (a sign of respect). The Buddha proceeded to teach him gradually and thoroughly, leading up to the Four Noble Truths. Pokkharasāti attained the Dhamma-eye, and attained insight into impermanence. He was so moved that he praised the Buddha, and declared himself (along with his children, wives, ministers, and counsellors) as lay followers. He tells the Buddha that he is welcome to visit whenever he wishes, and assures him that the Brahmin children will honor him and grant him hospitality.
This concludes the Ambattha Sutta.
The corresponding sutra from the Chinese Canon is Dīrghāgama 20. I do not have access to a translation of the sutra, so I cannot provide a comparison. I will try to update this section if I ever get the chance.
 The Vedas are a collection of Sanskrit hymns and liturgies composed by mysterious Indo-European peoples. Even during the Buddha's time, they were quite old, and the Brahmin religion was rooted in these texts. They contained hymns to many gods, verses of mythology, and instructions for elaborate fire rituals which only Brahmins were allowed to perform. The many traditions we would now call Hinduism emerged from this tradition.
 When it comes to the Early Buddhist texts, I tend to err on the side of conservatism and I interpret most things literally - the idea of the 32 Marks is an exception, and I feel completely comfortable saying that it is a later monastic insertion. Bhikku Sujato notes that DN 30, which lists the 32 Marks, has several qualities which betray its nature as a late text: the poetic meter of its verses is inconsistent with the earlier texts, and the prose sections have no counterpart in the Chinese Canon.
It is also important to note that, if the Buddha's body literally had these 32 qualities, he would have looked like a space alien. His would have had arms that stretched down to his knees, elongated fingers and toes, a massive torso, a tongue long enough to lick his entire face, and a tuft of white hair in the middle of his brow.
The suttas clearly demonstrate that the Buddha's appearance was not this striking. In DN 2, King Ajattasattu could not distinguish the Buddha from the many monks in his company - he had to ask his royal physician to point out the Buddha. In MN 140, the Buddha lodges with a monk named Pukkusāti and remains completely unrecognized until the Buddha revealed his identity. The commentary attempts to explain that the Buddha used his mental powers to mask his 32 Marks, so as to avoid being recognized by others, but there is nothing in the canonical sutta to indicate this.
Here, one may ask: where did the idea of the 32 Marks come from? Sujato notes that the suttas claim the idea comes from the Brahmin scriptures, but no evidence of any such passages exists today. He raises the possibility that the idea actually comes from Babylonian influence in India. For more information, read his blog post here.
 In the Vedas, there is a hymn called the "Hymn of Purusha." It is a relatively late composition found in the Rg Veda. It describes how the gods created the world by sacrificing and dismembering a giant cosmic person (Purusha). Different parts of the body were used to form parts of the cosmos and human society - even the Vedas themselves were created this way.
This text claims that the four classes of human society were created from four parts of the Purusha - the Brahmins, the highest class, come from the mouth. They serve as the "voice" of the universe, bearing the sole empowerment to recite the sacred hymns and perform rituals. The Kshatriyas were born from the arms, and they serve as rulers and warriors. The Vaishyas were born from the thighs, and they hold up the rest of society as merchants and farmers. Lastly came the Shudras, born from the feet of the Purusha. They were the lowest class of people still recognized as part of society: commoners, laborers, and people who made a living as artists or performers.
It seems that in the time of the Buddha, this idea had changed and the god Brahma replaced the cosmic Purusha in this myth. Otherwise, the general idea remained intact, and the Brahmins happily used it to explain to others why they were of a higher pedigree. Ambattha was referencing this hymn when saying that the Buddha was "born from the feet of Brahma," despite the fact that the Buddha was usually considered a Kshatriya and not a Shudra. Ambattha was clearly trying to be offensive.
 This broad, fourfold system is still relevant in India today, though over time it evolved to include finer levels of nuance and detail. In the Buddha's time, he spoke out against the follies of classism, and in modern times Buddhism has been used as a sort of revolutionary vehicle for empowerment of the lower classes.
 Inbreeding among royal families has always been a common part of various cultures in human history. It's an uncomfortable, but not unexpected detail in this story.
 This is a common occurence throughout the suttas when the Buddha told historical or mythological stories - he would explain to his audience that certain words, phrases, or ideas in the story correspond directly to contemporary examples from his own time. Someone more knowledgable than me could probably explain why this occurs in the suttas as much as it does.
 For some examples: In the Ātānātiya Sutta (DN 32), King Vessavana came to the Buddha in order to ask him why some of the yakkhas dislike the Buddha and his teachings. He provided the Buddha with a protective prayer to be recited by his followers, so that they could be protected from the hateful yakkhas which would attack them in the wilderness. In the Ālavaka Sutta (SN 10.12; also found in the Sutta Nipata, at Sn 1.10), the Buddha was staying where a yakkha named Ālavaka lived. Amusingly, this spirit went back and forth commanding the Buddha to enter, then leave, over and over. Eventually the Buddha refused, saying "Do what you will." Ālavaka then asked the Buddha a question, and gave him a warning very reminiscent to the one the Buddha gave to Ambattha - if the Buddha did not answer the question, the yakkha would either confound his thoughts, tear out his heart, or throw him across the ocean. Boldly, the Buddha told the yakkha that was not powerful enough to make good on his threats, but agreed to answer anyways. Ālavaka became a disciple of the Buddha.
 I have no idea what this means.
 This god is one of the high-ranking gods known as Mahā Brahmās. As Walshe notes in his translation of this sutta, this verse the Buddha quotes occurs several times in the suttas; sometimes, as in this story, it is attributed to Brahmā Sanankumāra. Elsewhere, it is attributed to the Buddha himself. The Sanankumāra Sutta (SN 6) tells the story of the time when the god visited the Buddha and spoke this verse. He is also mentioned in the Mahāsamaya Sutta and the Janavasabha Sutta.
 There are numerous examples in the suttas of King Pasenadi and the Buddha meeting face-to-face. I suppose the implication here is that the king does not really respect Pokkharasāti.
 Tradition held that the Vedas were revealed to the Vedic seers, the ancient Brahmins. The term "seer" is translated from "rshi," which literally means something like "those who heard." The idea was that the Vedas were not authored by any human, but they were divinely produced and merely recorded by the seers. Therefore, it's possible that the Buddha saying that the seers composed the Vedic hymns is a subtle jab at the Brahmin religion.
 I have seen this interpreted as either referring to the possession of a foreskin, or a penis which actually retracts inside of the body. Either way, the idea seems to be that the penis is concealed in some way.
 I have no idea what it means to use psychic powers to reveal your penis... it's probably not worth thinking about too much...
 This is a behavior we see throughout the suttas - I assume this was the most polite way for the Buddha to agree to things being offered to him.