Once the septum is in place, it is then extended, as you can see in this Bacillus subtilis, enlarging the nascent spore and ultimately separating it from the mother cell envelope. Note that the monoderm mother is producing a diderm spore, surrounded by two layers of its mother’s membrane. As this occurs, a copy of the genome is pumped through a specialized protein nanomachine. The increased turgor pressure from the highly-concentrated DNA helps the forespore round and expand. Usually, the mother cell membrane finishes closing off at the cell pole, but occasionally this occurs instead on the side of the mother cell (⇩).
When the process of engulfment is finished, the forespore’s cell wall is reinforced and expanded into a tough “cortex,” and an extra protein coat is added to the outside, completing the sturdy envelope that will protect the spore from harsh environments (⇩). Finally, when the spore is ready, the mother cell lyses, releasing the time capsule to its fate.
The process of forespore engulfment by the mother cell membrane culminates in closure of the membranes, creating an outer spore membrane and an inner spore membrane on either side of the sporulation septum. The final point of separation is usually at the center of the cell pole, but not always, as you can see in this Bacillus subtilis forespore being engulfed from the side of the mother cell. The layer to the right of the developing spore may be a precursor of the coat (⇨).
In this later stage of sporulation, you can see that the Bacillus subtilis forespore has now separated completely from the mother cell envelope. It has also moved away from the cell pole. The layer flanking the forespore may be the assembling spore coat.
