Why does a simpel AJAX script not work with Masterpage?

I have tried to make a simpel ajax script in a aspx page, which use MaterPage. Nothing happens, when I click the button. Can I not use the script, when I use masterpage? what is the problem?

<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage/MasterPage.master" AutoEventWireup="true" CodeFile="TetraSMSSendStatus.aspx.cs" Inherits="Features_TetraSMSSendStatus" %><asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server"><script>
      function loadXMLDoc() {
        var xmlhttp;
        if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
          xmlhttp = new XMLHttpRequest();
        }
        else {// code for IE6, IE5
          xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        xmlhttp.onreadystatechange = function () {
          if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
            document.getElementById("myDiv").innerHTML = xmlhttp.responseText;
          }
        }
        xmlhttp.open("GET", "ajax_info.txt", true);
        xmlhttp.send();
      }</script></asp:Content><asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server"><div id="myDiv"><h2>Let AJAX change this text</h2></div><button type="button" onclick="loadXMLDoc()">Change Content</button></asp:Content>

The script code is from: http://www.w3schools.com/ajax/ajax_example.asp