Crack.
A soft, clean sound. The bone marrow biopsy needle bored a small opening into the lumbar vertebra.
"Hmph."
Bone marrow punctures normally required anesthetic, but as with every procedure before this one, Sasuke had chosen not to use any, wanting to sense every change in his body as clearly as possible.
Even after steeling himself mentally, the piercing pain of the needle breaching the marrow made his body shudder involuntarily, and a muffled grunt escaped him.
The Shadow Clone at the instrument watched the original for a moment, confirmed he was alright, and spoke. "Bone puncture complete. Initiating injection of the First Hokage's cells. Remain alert."
Sasuke gave a small nod.
The clone pressed a button. The drilling stopped and the needle shifted to injection mode. The pale green luminescent solution of the First Hokage's cells began flowing slowly through the tube and into the marrow cavity.
The moment the cells entered, Sasuke's brow furrowed sharply. Cold sweat broke across his forehead.
"Vital signs showing irregularities. Heart rate and blood pressure are abnormal."
The clone observed the readings on the instrument display, logged them in the experiment notes, then stepped closer to the original and asked in a low voice.
"What does it feel like?"
The original couldn't spare hands to record data during the procedure, so that responsibility fell to the clone.
"It feels like something is burning inside my body. Like ants tearing into flesh, a mix of pain and itching. And a strange coolness. No..."
Sasuke carefully tracked the sensations moving through him, was quiet for a moment, then continued. "Better described as refreshing. Like standing in a field or a forest with a clean breeze moving across your face."
"Noted."
The clone kept writing as the original spoke, then asked. "Any detectable increase in dojutsu power or physical strength?"
Sasuke shook his head. "Nothing yet. But I can feel the cells in my body meeting the First Hokage's cells, tangling with them, pushing against them. As though each side wants to consume the other."
The cellular vitality in the First Hokage's cells was extraordinarily strong. They carried an intense invasive quality. In an ordinary person, injecting them into the body would cause the First Hokage's cells to immediately begin devouring the person's own cells.
But that wasn't the positive outcome it might sound like. The result was a violent rejection reaction between the First Hokage's cells and the host's existing tissue, causing severe deterioration of the body's vital signs, and ultimately death, the host's body transforming into something resembling a tree.
Sasuke's situation was quite different. He possessed the Eternal Mangekyou, and while his current strength didn't yet match Hashirama Senju at his peak, both of them operated at the same fundamental tier of Yin and Yang chakra.
Even if the First Hokage's cells were individually stronger than Sasuke's, the sheer quantity of Sasuke's own cells more than made up for it.
What was essentially a life-or-death gamble for an ordinary person carried relatively little risk for Sasuke.
"Now I'm going to try guiding the First Hokage's cells to fuse with the bone marrow cells."
As he spoke, the Eternal Mangekyou in his eyes began to turn slowly. He directed the Yin chakra of his Sharingan through his meridians, pushing it carefully toward the lumbar spine.
As his Yin chakra arrived at the site of the injection, it met the Yang chakra within the First Hokage's cells and reached a subtle equilibrium. The two began slowly merging.
Time passed. Sasuke maintained careful control over the fusion while the clone moved continuously between the instruments surrounding the procedure table, monitoring everything.
"Heart rate and blood pressure returning to normal."
The clone swept its eyes across the display and entered all the readings into the documentation. "Is it working?"
Hearing the question, Sasuke slowly opened his eyes. "More or less."
The discomfort inside his body had faded entirely. What remained was a feeling of extraordinary ease, a warm, gentle comfort saturating him from within, something deeply pleasant that resisted easy description.
If he had to put it into words, it felt like having the finest practitioner in the shinobi world give him a proper sauna treatment.
"The First Hokage's cells have fully disappeared within my body. They've entered an initial fusion with my existing bone marrow cells, transforming into bone marrow stem cells that belong specifically to me."
"Understood." The clone continued recording as it listened. "Any notable changes in dojutsu power or physical vitality?"
Sasuke shook his head. "No significant changes. A marginal increase in both, nothing more."
"The new cells produced by the fusion of the First Hokage's cells with my own, what I'll call Sasuke cells, currently represent a negligible fraction of the roughly one hundred thirty trillion cells in my body. There's been no qualitative shift in my strength. For these cells to gradually differentiate throughout my entire body and replace the others is going to be a long process."
The clone nodded, unsurprised.
Madara Uchiha had bitten off a piece of Hashirama Senju's flesh, cultivated it, and transplanted it onto himself, growing something like a secondary heart from it.
Strictly speaking, his Rinnegan had awakened from the fusion between his own Yin chakra and the Yang chakra within that piece of flesh. But beyond that piece, every other organ, tissue, and cell in his body remained Uchiha Madara's own.
It had never truly been a fusion between the two.
What Sasuke was doing, injecting the First Hokage's cells into the bone marrow cavity, allowing them to fuse, and then letting the resulting embryonic stem cells self-replicate and differentiate outward from the inside, transforming every organ, bone, nerve, and cell into something new, was fusion in a genuine sense.
The downside of this method was simply that it took considerably more time.
The newly generated Sasuke cells currently numbered only in the tens of thousands. The average human body contained somewhere between forty and sixty trillion cells. In this world, the human body was more exceptional still, holding one hundred thirty trillion. Several times the figure from his previous life.
Don't ask why. The answer was chakra.
Replacing one hundred thirty trillion cells starting from a base of tens of thousands of new Sasuke cells was by definition a slow process.
To accelerate it, Sasuke would need to continue injecting the First Hokage's cells repeatedly over a long period, alongside ongoing guidance through his Yin chakra.
... Though I doubt I'll need every cell in my body to convert before I'm eligible to try awakening the Rinnegan.
Sasuke closed his eyes, exhaled slowly, and summarized his thoughts.
The piece of Hashirama's flesh that Madara had grown on his chest had probably accounted for at most five or six trillion of his one hundred thirty trillion cells.
If that small amount of Yang chakra from those cells was enough for Madara to awaken the Rinnegan, then the cells Sasuke was generating from his fusion with the First Hokage's cells, while somewhat reduced in vitality compared to the original, should give him a qualifying chance once they reached twenty or thirty trillion.
That was the theory, at least. But Sasuke was also aware that awakening the Rinnegan wasn't as simple as hitting a number.
It was worth remembering that Madara had transplanted that piece of Hashirama's flesh in his prime, at a point where his own strength and mastery over the Sharingan far surpassed anything Sasuke could currently match.
Opening the Fourth Form Susanoo without eyes. Facing all nine Tailed Beasts alone alongside a young Gaara and Kurama-empowered Naruto.
Sasuke currently could manage the Fourth Form of Susanoo at most.
Facing nine Tailed Beasts with both eyes wide open, he would have one thing to say: goodbye.
Even with all of Madara's overwhelming strength, he hadn't awakened the Rinnegan immediately. He had spent decades, holding on until he was nearly at death's door, before it finally appeared.
Clearly, awakening the Rinnegan was not something that simply happened on schedule.
Sasuke even suspected that beyond the reunion of Yin and Yang power, there might be other hidden conditions. Something like experiencing the proximity of death, truly brushing against it, before the requirements were fully met.
After all, the Rinnegan was the eye of creation, said to hold the power to build worlds, and equally the eye of destruction, said to hold the power to unmake them.
It represented both life and death. Perhaps Yin and Yang Release alone were not sufficient.
But these were questions for future experiments. His current goal was to accelerate the generation of more Sasuke cells.
"Should we proceed with the injection of Naruto's cells?"
The clone asked while Sasuke was still thinking.
The question broke through his thoughts. Sasuke looked up and shook his head.
"Not yet. I haven't fully assessed the new cells yet. I'll wait until I have a clearer picture before moving forward."
He closed his fist, sensing the subtle shifts in his body, and said in a measured voice. "The Sasuke cells are still too few to produce any meaningful change in my strength. I'll wait until they've differentiated to the point where I can clearly sense the change in my body, then decide on the next step."
The clone nodded without comment, logged all the data into the documentation, walked over to disconnect the various tubes and attachments from the original, and then dissolved.
Sasuke dressed himself again, found a chair to lean back in, and closed his eyes to rest.
He was tired.
___
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