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Cornerstone of the Corps (兵団の礎 Heidan no Ishizue?) is the 1st chapter of the 12th volume and the 41st chapter overall of the Attack on Titan: Before the Fall manga, written by Ryō Suzukaze and illustrated by Satoshi Shiki.
As two Military Police soldiers approach the cottage, the timely arrivals of both Jorge Pikale and the trainees buy Kuklo enough time to escape. Kuklo and Jorge catch up with what the other has been up to since they last saw each other and plans are revealed for the next step of rebuilding the Survey Corps.
As the two Military Police members begin to approach the cottage, Kuklo resolves that he has no choice but to fight and starts to get dressed. Suddenly, Rosa, Kai and Ivo approach the cottage and nearly get shot. Kai tells them that they are only trainees and are not up to anything suspicious. The two MPs ask what they are doing here at this hour and inquire about the cottage. Rosa claims they found it abandoned and are using it as a way point for training. Not fully convinced, the MP's move towards the cottage once more. After the trainees continue to impede them, the two soldiers forcibly push Rosa aside; Kai retaliates and grabs one of them, only to be hit with the butt of a rifle. As Ivo tries to interfere, the other soldier aims his rifle at his head.
Just then, Jorge arrives with Hugo and Felix, demanding to know what is going on. As the two MP's put away their rifles, one of them claims the trainees are harboring remaining members of the Dissidence Movement and they interfered with their search. Laughing, Jorge brushes the claims aside and correctly guesses the two did not inform the trainees of their intentions. He then walks past the two MP's and opens the door with a loud crash. Jorge notes that the door was lighter than he expected and invites them to search the premises.
Finding nothing, the two MP's apologize to the trainees for the rough treatment and leave. Rosa then wonders where Kuklo disappeared to; she soon gets an answer when Jorge tells Kuklo to come out and the latter flies out of a nearby tree. Noticing the trainees' surprise, he briefly explains that Jorge loudly opening the door gave him enough time to use the device and escape. Jorge requests to speak with Kuklo before they can go any further.
Kuklo notifies Jorge about what happened to him in the Industrial City and asks about Sharle's whereabouts. He is relieved to hear that Sharle is alive and well, attending a ball in the city with Xavi. However, Jorge mentions that due to the rebellion, they have lost access to the Industrial City for the time being. Captain Dafner was removed from his position and now Vice Commander Bernhart is in command of the Industrial City. Changing the topic, Jorge decides to allow the other trainees inside to fulfill their curiosity about the device; Kuklo is hesitant to, but Jorge reveals that Carlo and Cardina are working on a new training regimen that will train members of the Survey Corps with using the device. As the trainees come flooding in, Kuklo smiles at the fact that the rebuilding of the Survey Corps can truly begin thanks to Jorge, Carlo and Cardina's efforts, as well as Xenophon and Sharle.