the middle of what?

[unpakwon]

Please make the following in red clear for me.

On our planet, the earliest single-cell life forms were called prokaryotes. No more than a sack of cytoplasm with their DNA free floating in the middle, these cells came into existence roughly three and a half billion years ago.

In the above, does "the middle" refer to the middle of the sack of cytoplasm? Is it saying "prokaryotes were just a sack of cytoplasm that have their DNA free floating in the sack"?

Thank you.

[SoothingDave]

Yes.