"Submit" Comment Button Disappeared

For some reason I am not able to explain, the “Submit” button of the WordPress comment section (disregarding Facebook’s and Google’s) has disappeared completely. I’ve checked the code gazzilion times, I have removed widgets and placed them again, deactivated plugins, etc. I don’t know what is happening. Can somebody help me?

This is my blog: http://pmrb.net/blog

A particular post: http://pmrb.net/blog/2010/07/18/material-educativo-de-frege-revisado-v-0-2/