$before_widget and $after_widget not present?

Hi,

I am working on a new plugin and have problem echo the content of $before_widget and $after_widget. Content from these two variables are not present in the sourcecode. Why is this happening?

function widget() {
// Some code..
echo $before_widget;
echo $before_title.'<a href="'.get_category_link($category_id).'">'.$title.'</a>'.$after_title;
echo '<ul>';
wp_list_categories( array('parent' => $category_id, 'hide_empty' => 1, 'show_count' => 1, 'title_li' => null) );
echo '</ul>';
echo $after_widget;
}