JQuery :: Inserting The Nested Tags?

I want to insert a nested <div> into an existing span in a number of table cells, each of which has different content, so:

<td><span>Content</span></td>

would be 'converted' to

<td><span><span>Content</span></span></td>

I have tried:

$('tr td span').prepend("<div>");
$('tr td span').append("</div>");

However, this actually seems to produce

<td><span><div></div>Content</span></td>

I have also tried something along the lines of

$('tr td span').html("<div>" + $(this).text() + "</div>");

but have been having trouble using the this keyword to produce the required effect.