A number of additional measures were taken to insure that Janissaries' loyalty did not become divided. They were not allowed to have a job beyond their military duty. Initially Janissaries were required to be celibate. Later, they were allowed to marry, but their sons could themselves join the Janissaries, so that nepotism could not influence the balance of power within the corp. As time passed, the Janissaries gained power - both through their effectiveness as a practically unbeatable military force and by rioting for better pay and conditions. They gained the right to have their sons join the corp. Janissaries also started getting into trades and focusing on their businesses, not their military duties. Eventually, they gathered so much power that they were a law unto themselves. If they did not like what the Sultan was doing, they would revolt. The Janissaries were also resistant to change and did not attempt to adapt to modern military methods. By the 1800's they had become an expensive, political liability to the Sultan. In 1826, Sultan Mahmud 2 finally had to start a new European-style army to maintain the empire's military power. When the Janissaries revolted due to their replacement, their barracks were destroyed and those Janissaries who were not killed were banished.
Oh, and the term janissary, also spelled janizary, - it comes from the Turkish yan cheri. It means new army, as opposed to the previous Ottoman army which consisted of volunteers who were more interested in potential loot and glory than meeting any particular strategic goals.
More Janissary pictures at Art.com