prepare() function doesn't work well in migrate module
Unfortunately you can't set the value directly like that, you need to reference the language and delta in the classic, hated method:

public function prepare($entity, stdClass $row) {
  $row-&gt;title = "This is title 1";
  $row-&gt;field_CT_body[LANGUAGE_NONE][0]['value'] = t("This is body");
}


Note: This function takes 2 params, the entity object being built and the source row. So, since we're altering the entity object, we gotta manipulate it as such.  (I'm not sure, but you might be able to wrap that object in a metadata wrapper in order to use the cleaner API)

Also, please note that the "value" key will depend on the field type in use.

Note: untested code but it should be fine! :)