The central zone has smoother structure compared with the surrounding granular cytoplasm. Is it not a big vacuole with (densely, dark structures) packed membranes? It looks like there is a membrane delineating the two zones.
Nicolas, I see what you mean. When I first looked at the 'central structure,' I didn't see the enclosing membrane. Usually, when I see things that look like the 'central structure', they turn out to be Golgi in other sections. Looks like further sections would have cleared things up!