Tech tree symbol question

[ret]

What does the symbol near the top right of some of the techs in the tech tree represent?

Its the circle w/ a line through it (no symbol)

Also how can you tell what techs from one era lead into which techs from other eras?

 

[RobKid]

Same answer for both questions. The techs with the crossed out circle are the ones you do not have to research to progress to the next age.

 

[Diety]

you don't need those to get to the next age

oops, i should read the other answers *before* posting