What were the Apis Bulls?

"The Apis Bull", wrote Herodotus, "is the calf of a cow which is never able after to have another. The Egyptians believe that a flash of lightning strikes the cow from heaven, and thus causes her to conceive the Apis. It has distinctive marks. It is black, with a white diamond on its forehead, the image of an eagle on its back, two white hairs on its tail and a scarab-beetle mark under its tongue".
The Apis bulls (along with the Mnevis bull and the Buchis bull) were considered to be the earthly incarnation of a god - but unlike other animals who could only intercede with deities on behalf of humans, the Apis Bull was believed to be the very god himself living amongst the human population. Apis was the manifestation of Osiris, Mnevis of Re-Atum and Buchis of Re, all three were chosen for their special markings and were indeed treated as gods on earth - when the bulls died they were then mourned, mummified and then buried with the same glory that occurred with the death of a pharaoh.
Although the Apis Bull associated with Osiris, the Apis Bull was worshipped in Egypt from a much earlier date than that of Osiris - Osiris became universally accepted as the god of the dead at the end of the Old kingdom, whereas there is mention of the Apis Bull as early as the First Dynasty and could even go back into Predynastic times. However, the Apis Bull was worshipped because he was Osiris and is found with the name Apis-Osiris - according to Plutarch "Apis was a fair and beautiful image of the soul of Osiris'.
All oxen which were 'clean' (entirely free from black spots, or even a single black hair) were chosen as a sacrifice to the Apis Bull.

Where did it Live?
It was in Memphis that the Apis bulls were kept and where the worship of them was particuarly keen. Psammaticus erected a grand court with columns '12 cubits' in height in which the Apis was kept when he was paraded in public. Attached to this court were two stables ('delubra' or 'Thalami') - and in front of these stables was a vestibule where the Apis was introduced to strangers, but also it was a place in which the mother of the Apis was fed.

The Festival of the Apis Bull

The festival in honour of Apis lasted seven days, during which time a large amount of people would gather at Memphis. The priests would led the sacred bull in a solemn procession through the crowds of people who welcomed the bull as he past - the children who smelt his breath were thought to be gifted with the power of predicting future events.

Apis as an Orcale

As the Egyptians considered the Apis Bull to be the living image of Osiris they consulted him as an oracle and took his actions to mean good or bad omens. If the bull took what food was offered him, after the asking of a question, then the omen was good - if the food was refused then it was considered a sinister omen.

Death of the Apis

When the Apis died, specially chosen priests went in search for another bull with the signs noted above. As soon as a bull was found he was taken to 'the city of the Nile' for 40 days before he was moved to Memphis - during this time only women were allowed to attend him. When the 40 days were completed the bull was transferred to Memphis using a boat with a specially constructed golden cabin.
Pliny and Ammianus state that the priests themselves drowned the Apis Bull had reached 25 years of age (the time prescribed to the life of Apis Bulls in the sacred books), beyond which the bull was forbidden to live. The body was then embalmed and a grand funeral procession took place at Memphis - the sarcophagus was placed on a sledge, followed by the priests dressed in leopard skins and uttering cries to their god.
When the Apis died of natural causes his funeral rites were much more ostentatious - so much so that the cost of the funeral placed a heavy burden on the Pharaoh in power at the time.
Whatever the cause of death, upon the death of an Apis Bull the people went into mourning, as if Osiris himself died, until his successor the Apis was found - then rejoicing commenced with as much joy as there was grief when the original Apis died.