Select/Unselect ALL checkbox not working

No checkbox is being shown for the Select/Unselect ALL checkbox at the header template. Also when I run, all the checkbox is already supposed to be checked !

Here is the aspx codes
 

<asp:GridView ID="grdEventType" runat="server" AllowPaging="True" AllowSorting="True" AutoGenerateColumns="False" BackColor="White" BorderColor="#999999" BorderStyle="Solid" BorderWidth="1px" CellPadding="3" DataKeyNames="EventType_ID" datasourceid="sdsEventType" ForeColor="Black" GridLines="Vertical" Width="50%" OnSelectedIndexChanged="grdEventType_SelectedIndexChanged">
            <AlternatingRowStyle BackColor="#CCCCCC" />
            <Columns>
                                       
                <asp:TemplateField HeaderText="Select">
        <HeaderTemplate>
        <input id="chkAll" onclick="javascript:SelectAllCheckboxes(this);"
            runat="server" type="checkbox" />
    </HeaderTemplate>
    <ItemTemplate>
       <asp:CheckBox ID="chkSelect" runat="server" />
    </ItemTemplate>
    <HeaderTemplate>
    </HeaderTemplate>
   </asp:TemplateField>

                <asp:CommandField ShowDeleteButton="True" ShowEditButton="True"/>
                <asp:BoundField DataField="EventType_ID" HeaderText="EventType_ID" ReadOnly="True" SortExpression="EventType_ID" />
                <asp:BoundField DataField="EventTypeName" HeaderText="EventTypeName" SortExpression="EventTypeName" />
            </Columns>
            <FooterStyle BackColor="#CCCCCC" />
            <HeaderStyle BackColor="Black" Font-Bold="True" ForeColor="White" />
            <PagerStyle BackColor="#999999" ForeColor="Black" HorizontalAlign="Center" />
            <SelectedRowStyle BackColor="#000099" Font-Bold="True" ForeColor="White" />
            <SortedAscendingCellStyle BackColor="#F1F1F1" />
            <SortedAscendingHeaderStyle BackColor="#808080" />
            <SortedDescendingCellStyle BackColor="#CAC9C9" />
            <SortedDescendingHeaderStyle BackColor="#383838" />
        </asp:GridView>
        <asp:SqlDataSource ID="sdsEventType" runat="server" ConnectionString="<%$ ConnectionStrings:EventContext %>" DeleteCommand="DELETE FROM [EventType] WHERE [EventType_ID] = @EventType_ID" InsertCommand="INSERT INTO [EventType] ([EventType_ID], [EventTypeName]) VALUES (@EventType_ID, @EventTypeName)" SelectCommand="SELECT * FROM [EventType]" UpdateCommand="UPDATE [EventType] SET [EventTypeName] = @EventTypeName WHERE [EventType_ID] = @EventType_ID">
            <DeleteParameters>
                <asp:Parameter Name="EventType_ID" Type="Int32" />
            </DeleteParameters>
            <InsertParameters>
                <asp:Parameter Name="EventType_ID" Type="Int32" />
                <asp:Parameter Name="EventTypeName" Type="String" />
            </InsertParameters>
            <UpdateParameters>
                <asp:Parameter Name="EventTypeName" Type="String" />
                <asp:Parameter Name="EventType_ID" Type="Int32" />
            </UpdateParameters>
        </asp:SqlDataSource>
        <script lang="javascript">

 function SelectAllCheckboxes(spanChk){

   // Added as ASPX uses SPAN for checkbox
   var oItem = spanChk.children;
   var theBox= (spanChk.type=="checkbox") ? 
        spanChk : spanChk.children.item[0];
   xState=theBox.checked;
   elm=theBox.form.elements;

   for(i=0;i<elm.length;i++)
     if(elm[i].type=="checkbox" && 
              elm[i].id!=theBox.id)
     {
       //elm[i].click();
       if(elm[i].checked!=xState)
         elm[i].click();
       //elm[i].checked=xState;
     }
 }
</script>

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The code behind

 protected void grdEventType_SelectedIndexChanged(object sender, EventArgs e)
    {
        // StringBuilder object
        StringBuilder str = new StringBuilder();

        // Select the checkboxes from the GridView control
        for (int i = 0; i < grdEventType.Rows.Count; i++)
        {
            GridViewRow row = grdEventType.Rows[i];
            bool isChecked = ((CheckBox)row.FindControl("chkSelect")).Checked;

            if (isChecked)
            {
                // Column 2 is the name column
                str.Append(grdEventType.Rows[i].Cells[2].Text);
            }
        }

        // prints out the result
        Response.Write(str.ToString());
    }

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