An Egg Full of Thorns is an extra chapter of Kodocha. In the tankōbon edition, it's included in the sixth book as chapter "30.5". It covers some details of Akito's childhood.
Synopsis[]
Key moments that shape the Hayama family into the way it is at the beginning of the series.
Plot[]
It's explained that when someones born, their heart is as smooth as an egg. Cut to Akito, age 3 - Natsumi who's 6 is being mean to him, including throwing a spoon at him and pushing him over. Natsumi lies, saying she had nothing to do with Akito falling, and their housekeeper Ms. Noguchi believes Natsumi. This happens often enough for Akito to feel discouraged from telling her about Natsumi's bullying. Akito goes outside and feeds some ants, before stepping on them. This causes thorns of spite, anger, sorrow, and pain to start growing on his heart.
Their dad works late, and Natsumi monopolizes the little family time they do have together. Akito and his dad's only bonding time is their nightly bath, but Akito again feels discouraged to tell him about Natsumi's treatment of him in fear it'd make his dad sad.
Finally one day as Natsumi is bullying Akito (pushing him off the couch) he talks back, and this makes Natsumi blow up about how their mom left to have him and never came back, and that she hates him - and their dad must also. Akito never knew where their mom was, and the insistence that their dad hates him too hurt him most. After that, Akito tells his dad he wants to take baths by himself. His dad worries for a moment before deciding it's nothing.
This continues, with Akito closing himself off over the belief his family hates him, their father being busy with work - and struggling with the grief of losing his wife, and Natsumi taking out her hurt on Akito. Koharu looks over them, sad at how things are going.
Akito is now 5, and Mr. Hayama is given tickets for a dinosaur exhibit, and the whole family goes (an unusual thing, since typically the only thing they did together was visit Koharu's grave). Natsumi gets two dinosaur toys, so Akito picks out too as well, and is happy to have them - holding them tightly to his chest.
On the way home, to try and make Akito smile, Mr. Hayama points to some gas tanks and says they're dinosaur eggs, which delights Akito. When he tells this to Tsuyoshi, he laughs and says they're gas tanks, and Akito feels lied to.
It then cuts to when Akito is 11, with Sana dragging Akito who has a fever to his father in Chapter 5, where Mr. Hayama carries him home. It's explained that a heart can grow so many thorns, that it once again becomes smooth and round, and that because of that everything will be okay.
In current day, Akito talks about the confusion he had as a kid regarding the dinosaur egg comment, and his dad doesn't recall it. Akito's three dinosaur toys are shown - the two he got as a kid, alongside the one Sana gave him.