Although we now know that convergent extension occurs, it has been difficult to study it in vivo. Hardin (1989) used the species Eucidaris tribuloides (a pencil urchin from the Caribbean) to study this process. The embryos of this species are incredibly transparent, and their secondary mesenchyme never attach to the animal pole. Since their SMCs can't be pulling upward to any appreciable extent, this provides a "natural experiment" for studying active convergent extension. In this case, convergent extension could be visualized directly.