clicking to add content is adding it outside of the page's table

Clicking the "add" button in my nested form is adding the content
outside of the page's main table, how can I get it to add the html
into the table that the link is contained in?

I followed the Railscast

My view file:

<table style="width:100%" >
  <tbody class="fields">
      <% echantillon_form.fields_for :treatments do |builder| %>
         <%= render :partial => "treatment_fields", :locals => {:f =>
builder} %>
      <% end %>

      <%= link_to_add_fields "Add treatment",
echantillon_form, :treatments %>

      <%= echantillon_form.submit "Submit" %>
      <% end %>

and my partial:

<div class = "fields">

<tr><td><%= f.label(:treatmenttype, "type de traitement") %></td>
<td><%= f.text_field :treatmenttype %></td></tr>

<tr><td><%= f.label(:treatmenttype, "commentaires sur la preparation")
<td><%= f.text_field :preparation %></td></tr>

<tr><td><%= f.hidden_field :_destroy %> </td></tr>

<tr><td><%= link_to_remove_fields "remove", f %></td></tr>