Commander Hange,
For now, leave Reiner be.
If Bertholdt already knows of his survival or starts asking questions, then tell him the truth and leave it at that. If he does not know and he has not asked, I do not believe he will. I was clear to them what the consequences of failure would be; he will not ask for confirmation of what he already knows. The same goes for his father. The fewer allies he believes he has left in Marley, the more easily he will stay focused on maintaining the right path.
Reiner is not aware of Bertholdt's survival either. I am certain you understand why.
That is quite the repertoire of experiments you have carried out on your own. From one scientist to another, color me impressed.
Will you publish your findings in a scientific journal? Does Paradis have scientific journals? The Eldian Society would be stoked to see a rare Eldian publication rear its head, and you may prove to the world that Paradis possesses the prowess and intelligence expected of a civilized nation after the Marleyan empire destroyed our predecessors' body of work. You may contribute to a positive image and engender change. However, I do not know how far Paradis' scientific community has advanced.
What strikes me as notable is that your tests are not just aimed at your ultimate goal; you truly wish to gain an understanding of how the titans work, even if the information you obtain remains locally concentrated around its use for the titan shifter. Your work speaks of a certain curiosity—a desire to catalog the world as you observe it and paint a fuller picture for the sake of knowledge. You even scrutinized some of my data to confirm their validity and come to your own conclusions. This adherence to a peer-reviewed scientific method offers me faith that you are the right person to conduct this research.
I would be interested in reading your reports and offering my advice on how to prepare them for publishing.
Let me list some guiding principles for you to get started. I have full confidence that you will find your way towards a fitting method.
You will not find that, cerebrospinal fluid aside, any of a titan shifter's biological matter differs much from that of a regular Eldian's in structure and purpose. Most of Marley's efforts are focused on blood, musculoskeletal and organ samples, and cerebrospinal fluid. While you may extract others for the sake of documentation and verification, you will not find much there.
I regret to inform you that titan serum is one of Marley's most guarded secrets, especially to us Warriors. We could run off and transfer our titan to another Eldian; such knowledge is dangerous in our hands. For this reason, your unregulated research is pivotal to Eldian liberation.
What I do know is that the serum is composed of cerebrospinal fluid that is activated in some way to attune it to our Founder Ymir. This does not need to be from a titan shifter; any Eldian's cerebrospinal fluid will suffice. However, since you are less eager to let Eldians perish for this cause than Marley is, I suggest extracting from titan shifters only. Please use any samples you obtain to their fullest potential. Any transformation, addition, subtraction, and other chemical process you can think of, you should apply.
Its extraction is where pain becomes an issue.
A healthy adult has roughly 90 to 150 mL of cerebrospinal fluid in his body, of which 30 to 60 mL resides in the spine. With Bertholdt's size, I would estimate closer to the upper limits. In regular human beings, this supply is renewed three to four times a day.
In order to harvest this spinal fluid, you will need to carry out a lumbar puncture, also known as a spinal tap. My associates will provide you with the necessary tools for this. You will want to puncture his spine between his lumbar vertebrae, specifically between L3 and L4, or between L4 and L5. Go deep enough to enter his spinal column, but not so deep that you tap into his central nerves. This procedure is met with a great level of pain, especially if you hit any major nerve bundle or the needle scrapes against the vertebrae. In regular humans, this procedure has a mortality rate of above 95%. Fortunately, a titan shifter's healing will counteract this rate and he is not at risk of death if given proper care: nutrition, hydration, and sterilization and post-procedural disinfection of the area.
Cerebrospinal fluid is the one component which a titan shifter cannot easily regenerate. My untested hypothesis is that the spinal titan parasite suppresses the production of cerebrospinal fluid to have better control over its natural environment and lower risk of being attacked by its host's immune system, and as such, a titan shifter needs more time to regenerate lost cerebrospinal fluid. You will need patience. The volume you can extract at a time is best kept minimal. I recommend no more than 10 mL per week.
After the procedure, he may suffer from headaches, nausea, backaches, swelling, and general unwellness. He may also temporarily become paralyzed from the waist down. Do not worry about this: time will offer him a full recovery. On top of that, meningitis, brain herniation, and spinal damage may occur. After each procedure, you will need regular intervals where you guide him to heal potential unseen damage before it can cause issues. Until a week after the procedure, walk your fingers over his spine to guide his healing and request him to focus on the varying regions of the brain using touch to identify the locations he needs to heal twice a day. In case of paralysis, manually exercising his legs and abdomen three times a day for a half hour interval each will speed up recovery.
In order to acclimatize Bertholdt to these invasive extractions, I suggest getting him used to needle punctures. As preliminary work, you can harvest and store blood from varying sources. When he is used to this, harvest fluids from more uncomfortable regions, namely the abdominal cavity. You can take samples from his stomach, kidneys, liver, gallbladder, bladder, intestines, and even from his peritoneal cavity. Ensure your needle fully punctures the peritoneum during all procedures.
You can gradually expose him to increasing degrees of pain so that when the first spinal tap comes, the puncture itself will be secondary to the rest of the procedure. Like a frog in a pot of heating water that lets itself be boiled, he will be less likely to decline a test when the pain level is familiar to him.
It is your full discretion to decide how quickly you build this up. My advice is not to leave any less than two weeks between the first blood draw and the first spinal tap.
Should you require any further guidance or instructions on how you should approach this, I will be happy to assist you. In the meantime, I have attached a visual guide on how to carry out a spinal tap and an anatomical guide to extracting fluids from the abdominal organs, copied from a few anatomical books I found in our library. Forgive the crudeness; I am no artist.
Yours faithfully,
Warchief Zeke Yeager
25th of December, 851