Reference specific fields in template

I am trying to create my own list template. I want to reference specific fields in my Participant Database. How can I, say, access the value of last_name in this script?

$this->show_search_sort_form();
<tbody>
<?php while ( $this->have_records() ) : $this->the_record(); // each record is one row ?>
<tr><td>
<?php while ( $this->have_fields() ) : $this->the_field(); // each field is one cell ?>
<?php echo $this->field->name() ?>=<?php $this->field->print_value() ?><br>
<?php endwhile; // each field ?>
</td></tr>
<?php endwhile; // each record ?>
</tbody>`