D-L configurations: synthesis

I was looking at this picture. I don't understand what is going on.

I understand that we start with isoserine. Then, two different processes are shown.

But, when we go from glyceric acid to glyceraldehyde, why does the arrow point right?

Attached Images

Screen Shot 2014-07-13 at 12.02.56 PM.png (66.1 KB)