The most awesome to date was a box of squid, fixed and embedded in plastic over ten years ago. These squid provided the data for the study which proved horizonal transmission of the symbiotic bacteria in the accessory nidamental gland.
I know, right? Isn't that amazing?
Um. Okay, let me explain. Female squid have a complicated set of reproductive organs to make jelly capsules for their eggs. Bacteria live inside one of these organs (the AN gland), and from there they colonize the jelly capsule when it is formed. But where did the bacteria in the AN gland come from to begin with? They could have colonized the embryonic female squid while she was developing in the jelly capsule her mother made (vertical transmission from parent to child) or they could have been hanging around in the ocean, waiting to join up with her later in life (horizontal transmission from the environment). By embedding a bunch of squid of all different ages in plastic, slicing them very thinly, and looking for bacteria through an electron microscope, these scientists figured out that female squid don't even have AN glands until they are 87 days old. (Female humans, by contrast, are born with all of their reproductive organs in place.) So there is basically no way the bacteria could be transmitted vertically via the jelly capsule, because there's nowhere for them to live in an embryonic female squid. As further evidence, the researchers found that when the AN gland first develops in that 87-day-old squid, it has holes that open right out to the ocean--allowing easy colonization by horizontal transmission!
So that was a pretty cool study, published back in 1998, and ever since then the box of squid has been sitting around in the lab, patiently waiting for anyone who might want to check their data, or do a follow-up study, or look at something completely different that involves electron microscopy on a developmental series of squid.
And now Dave is retiring, so the box of squid has moved to my lab.
That is so freaking cool!