ajax ไม่ทำงานคับ มันไม่ส่งค่า post ไปอีกหน้า รบกวนช่วยชี้แนะด้วยคับ

<script type="text/javascript">  
$(function(){  
    $("select#Fac_no").change(function(){  
		$.post('ajax.php',  $("#myform").serialize(), function(data) {
			$('#Dep_no').html(data);
			alert(data);
		});
    });  
});  
</script>   


<select id="Fac_no" name="Fac_no" onchange="CallAjax('Fac_no');">
   <option value="">สำนักวิชา</option>
   <? 
	$query1 = mysql_query("SELECT
	faculty.Fac_no,
	faculty.Fac_name
	FROM
	faculty
	");
	while ($Row1=mysql_fetch_array($query1)){
?>
         <option value="<?=$Row1[Fac_no]?>"><?=$Row1[Fac_name]?></option>
         <? } ?>
         </select>


<div id="data2"><select name="Dep_no" id="Dep_no"><option>สาขา</option></select></div>

<?
include_once("../../class/ConnectClass.php");
$ConnectClass = new ConnectClass(); 

	$sql = "SELECT
				depart.Dep_no,
				depart.Dep_name,
				depart.Fac_no
				FROM
				depart
			WHERE
			depart.Fac_no = '".$_POST['Fac_no']."'";
		$query = mysql_query($sql);
			while($Row=mysql_fetch_array($query)){
				echo "<option value=\"$Row[Dep_no]\">$Row[Dep_name]</option>";
			}


?>