Organisms whose cells contain nuclear material without a membrane are called prokaryotes.

Here's why:

* Prokaryotic cells are simpler in structure than eukaryotic cells. They lack a true nucleus and other membrane-bound organelles. Instead, their genetic material (DNA) is located in a region called the nucleoid, which is not enclosed by a membrane.

* Eukaryotic cells have a well-defined nucleus enclosed by a nuclear membrane, which separates the DNA from the rest of the cytoplasm.

So, while prokaryotes have their genetic material (DNA), it's not contained within a separate membrane-bound compartment like a nucleus. This is a key difference between prokaryotes and eukaryotes.

Examples of prokaryotes include bacteria and archaea.