GET /embed.php?getEmbedPath=1 does not see header of X-G2-EMBED-PATH

I am trying to embed gallery 2.3 in wordpress. During:

the client expects to see a header of type:

but as you can guess, these never appear for the client.

I am running php 5.2.6, Gallery 2.3, and Apache 2.2.11 -- all quite recent. I am not using mod-rewrite beyond the Gallery2 plugin.

Any thoughts on how to discover the reason for the lost header?

Thanks,

Chris.