That's not
quite right - the master page doesn't call the content pages to display them. Instead, the content page defines its master page, and the content to display inside the content placeholders.
ASP.NET Master Pages - MSDN[
^]
Beginner's Tutorial on Master Pages in ASP.NET[
^]
<%@ Page Language="C#" MasterPageFile="~/Site.master" Title="Content Page"%>
<asp:Content runat="server" ContentPlaceholderID="head">
(Header content goes here...)
</asp:Content>
<asp:Content runat="server" ContentPlaceholderID="MainContent">
(Main content goes here...)
</asp:Content>