ajax初学者的疑问…

Question

<script type="text/javascript">
        function Creat1() {
            alert("one");
            if (window.ActiveXObject) {
                xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
            }
            else if (window.XMLHttpRequest) {
                xmlhttp = new XMLHttpRequest(); //创建xmlhttprequest
            }        }
        function Creat2() {
            Creat1();
            //var txt = document.getElementById("Text1").value;
            //alert(txt);
            var url = "ajax_fwq.aspx";
            // var url1 = "ajax1.aspx";
            alert(url);
            xmlhttp.onreadystatechange = Callback1;
            xmlhttp.open("GET", url, true);
            xmlhttp.send(null);
        }
        function Callback1() {
            //alert('back');
            if (xmlhttp.readyState == 4) {
                if (xmlhttp.status == 200) {
                    //do things成功执行                    //alert(xmlhttp.responseText);
                    var ttt = xmlHttp.responseXML.getElementsByTagName("span").value ;
                    //var ttt = xmlhttp.responseText;
                    alert(ttt);
                                    }
            }        }
    </script>
<input id="Button1" type="button" value="button" onclick="Creat2()"/>
.net  C#:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="ajax_fwq.aspx.cs" Inherits="Ajax试炼_ajax_fwq" %>
<p>
    <input id="span" type="text" value="sssssssss"/></p>这是我的的js代码，我为什么获取不到var ttt = xmlHttp.responseXML.getElementsByTagName("span").value ;呢？是不是我哪里写错了，我初学者，用var ttt = xmlhttp.responseText;这个就能得到值，为什么XML不行？