 Even though this really looked like a spectacular confirmation of the semi-conservative scheme, Meselson and Stahl (1958) were quite cautious in stating their conclusions. Avoiding theoretical interpretation, all the experiment showed was that base nitrogen distributes evenly during replication. While this is inconsistent with the dispersive mechanism, it did not rule out the conservative mechanism. For it was possible that the material of intermediate density that the ultracentrifuge’s optical devices picked up did not consist of hybrid heavy/light DNA molecules at all, but of some kind of complex of heavy and light DNA. For example, Meselson and Stahl speculated that end-to-end covalent associations of old and newly synthesized DNA that was produced by the conservative mechanism would also have intermediate density and therefore produce the same band in their experiment. For this reason, could not completely rule out an interpretation of the results in terms of the conservative mechanism.