Full Idea
In Frege's system 'concept' and 'extension of a concept' are primitive notions; whereas 'zero' and 'successor' are defined.
Gist of Idea
For Frege 'concept' and 'extension' are primitive, but 'zero' and 'successor' are defined
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Charles Chihara - A Structural Account of Mathematics 7.5
Book Reference
Chihara,Charles: 'A Structural Account of Mathematics' [OUP 2004], p.204
A Reaction
This is in contrast to the earlier Peano Postulates for arithmetic, which treat 'zero' and 'successor' as primitive. Interesting, given that Frege is famous for being a platonist.