What does Tertullian mean by the Son being a "portion of the Father's substance"?

What does Tertullian mean by the Son being a "portion of the Father's substance"? > "For the Father is the entire substance, **but the Son is a derivation > and portion of the whole**" ([*Against Praxeas*, Chapter 9](http://www.newadvent.org/fathers/0317.htm)) . Tetullian seemed to contradict the Scriptures: > For **the entire fullness of God's nature dwells bodily in Christ**, > > Colossians 2:9 (HCSB) If not, what does he mean by the Son being a derivation and portion of the whole substance?