what is this part?

[dieseltrip]

1996 E300D- I was driving home last night and the "low oil" light came on. Also I noticed a ton of smoke coming out the back of the car. I pulled over at the next exit and popped the hood. Basically the engine was pissing oil straight up on the underside of the hood, then falling down all over everything including the headers (thus the smoking I saw). I Checked the oil via the dipstick- empty . I refilled it with 4 quarts and the turned the engine on and saw where the leak was coming from- a small crack on this plug looking thing on the passenger side of the engine head, near the front, just above where the water pump attaches. It's about 1.5 inches diameter and 2 inches long. I appears to be a plug inside a plug. The outer plug has a hex nut type collar and the inner plug is an large allen type. The rate of loss was about 1 quart per minute, so I had it towed home. What a mess... oil everywhere and all down the underside of the car and even in the wheels.

Anyone know what this part is? Can I just remove it and bring it to the deal for a replacement. Is anything going to spring out of the head if I remove it? Thanks in advance for your help.