Before mandatory log-ins to post comments, there was a diagram (I believe by Amagin) that was the sole victim of a spambot. It has since been deleted. People kept bumping it for some reason and it would take up more than half of the comments page.
The paste core is the yukkuri's brain. Once they've reached their full size during development, it remains that size for the rest of the yukkuri's life. Thus, the ability to transplant an adult's core into a koyukkuri, which essentially puts an adult into a child's body.