בית שאור

A leavening container (this was an earthenware container divided into two compartments, one held salt or other ingredients, the other held leavening, a mixture of dough and active yeast such as sourdough)

sealed with a tightly fitted cover

and placed inside a tanur,

the leavening and the sheretz inside of it [the container]

and the partition [that separates the two compartments of the container] between them,

the tanur is tamei (as the sealed container does not prevent the spread of tumah to its exterior)

and the leavening is tahor (as the sealed container does prevent the spread of tumah to its interior).

And if there was an olive's volume of a cadaver [inside the container]

the tanur and the house [that the tanur was in] are tamei

and the leavening is tahor.

And if there is an opening of a handbreadth (some say this is in the partition, others say it's in the lid)

everything is tamei. (According to those who say the hole is in the partition, this is only in the case of a cadaver; in the case of a sheretz that is not the case, as the container is still complete for the purposes of protecting the leavening.  According to those who say the opening is in the lid, then the container is no longer sealed, and the tumah of a sheretz will also spread to the leavening.)

פרק ח משנה ז

השרץ

A sheretz

that was found in the eye of a tanur (the eye was a small opening in the wall of the vessel, some say it was near to the top to let out smoke, others that it was near the bottom to allow in air or fuel for the fire; all opinions agree that the eye would be closed at its inner edge while baking),

in the eye of a kirah,

[or] in the eye of a kupach,

if [the sheretz is found] from the inside edge [of the eye's opening] and outwards (i.e. it did not make it past the inner edge of the eye, through the walls of the oven, and into its interior space)

it [the oven] is tahor (although the eye is tamei),

and if it was in the air (most say this refers to the oven being outdoors and not under a canopy)

even [if] an olive's volume of a cadaver [was inside the eye]

[the oven] would be tahor;

If there was there

an opening of a handbreadth (aside from being such a large opening , it is assumed that this gives the eye a 'roof' of its own that has the area of at least a handbreadth square; either due to the thickness of the wall of the oven, or because the eye was extended out from the oven using a pipe)

Everything is tamei (the top of the eye acts as a ohel, or a canopy, and the opening of a handbreadth in the wall of the oven allows the tumah of the cadaver to spread to it).