Chicken – why is there no cavity in the whole chicken I just bought

If you're getting your eggs from a supermarket, they won't hatch. This is the case with eggs you get for eating from almost any source.

Hens lay eggs even if they haven't been, umm, mating with a male. Any egg laid in those circumstances will never hatch because it's unfertilized, and that's the standard practice for any commercial egg operation.

Now if you have a free ranging hen and a rooster too, and they're not kept apart, then yeah, you could get a fertilized egg. And that egg needs to be kept properly warm to hatch.

The shops are likely going out of their way not to get sued by sticking to safety guidelines rigidly. The packet instructions usually result in a dry and overcooked bird. The various other times and temperatures are all attempts to get a safely cooked, attractively browned bird without overcooking.

However, roasting meat based on oven temperature and the weight of the joint is the wrong way to go about things, because ovens are rarely calibrated correctly (the temperature is often wildly at variance to that set on the dial) and the weight of the meat is rarely nicely rounded.

If you really want to cook any meat (and many other foods) properly, you'll invest in a digital probe thermometer. Then you can simply cook the meat until it reaches a safe internal temperature and no more. They aren't expensive, you can get them from Amazon, and once you've got one you won't know how you did without it.