building and saving an assembly dinamically

I need to build some classes dynamically and save the assembly
to disk,however when I examine a dynamically built class
using ildasm, and compare it with a statically built class
with the same member structure, I realized that the 
manifest of dynamically built ones seem to be cut off,
I mean that its manifest is much simpler, besides when
statically referencing the dynamically generated dll from
another program class members don't show. Here is a snippet
with 2 very simple and identical classes one built
statically and the other dynamically to compare their manifest.
I cant figure out what is wrong, please can someone help me?


      public class SimpleClass
      {
       public string Name;
       public int Age;

      }

    class Program
   {
      static void Main(string[] args)
      {
        AppDomain appDomain = AppDomain.CurrentDomain;
        AssemblyName aName = new AssemblyName();
            aName.Name = "SimpleReflection";
        AssemblyBuilder assemBuilder = appDomain.DefineDynamicAssembly(aName,
           AssemblyBuilderAccess.Save);
        ModuleBuilder modBuilder = assemBuilder.DefineDynamicModule(aName.Name,"SimpleDyn.dll");


        TypeBuilder tb = modBuilder.DefineType("SimpleDyn.SimpleClass1",
            TypeAttributes.Public | TypeAttributes.BeforeFieldInit, typeof(object));

        FieldBuilder field1 = tb.DefineField("Name", typeof(string), FieldAttributes.Public);
         FieldBuilder field2 = tb.DefineField("Age", typeof(int), FieldAttributes.Public);

         tb.CreateType();
         assemBuilder.Save("SimpleDyn.dll");


        }
    }