C# - Get Div Control In ContentPlaceHolder?

I've created very simple web site for the test purpouses. Only one master page and one content page.

My content page looks like this:

<%@ Page Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" Title="Untitled Page" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<div id="TestDiv1">bla bla</div>
<div id="TestDiv2">ble ble</div>
</asp:Content>

Now, basing on some condition I would like to show/hide a given div. So I am trying to reach one of those divs by Controls collection, like this:

public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
ContentPlaceHolder myContent = (ContentPlaceHolder)this.Master.FindControl("ContentPlaceHolder1");
myContent.FindControl("TestDiv1").Visible = false; //this is not working
}
}
}
}

But the above example is not working. None of the two div control exists in the myContent.Controls collection. If I place for example a TextBox on my content page, I can reach it through Controls.

So what should I do to be able to access the div control?