譬えば竹の節を一つ破ぬれば余の節亦破るるが如し - The joints in a piece of bamboo: if one joint is ruptured, then all the joints will split.

本文
　
釈尊・塵点劫の間・修行して 仏にならんと はげみしは 何事ぞ 孝養の事なり、然るに 六道四生の 一切衆生は皆 父母なり 孝養おへざりしかば 仏に ならせ給はず、今 法華経と申すは 一切衆生を仏になす 秘術まします御経なり、所謂 地獄の一人・餓鬼の一人・乃至 九界の一人を 仏になせば 一切衆生・皆 仏になるべき ことはり顕る、譬えば竹の節を一つ破ぬれば 余の節 亦 破るるが如し
　

法蓮抄（父子成仏抄）（１４５９頁）　
建治元年４月

通解（意訳）
　
釈尊が塵点劫もの間、修行して仏になろうと励まれたのは何のためか。一切衆生、すべての生命への孝養のためである。さて、その一切衆生の過去の人間は皆、辿っていけば我が祖先であり、父母である。釈尊も、孝養を終えないうちは、仏になることはできなかったのである。今、法華経というのは、一切衆生を仏にする秘術がある御経である。生命の状態をあらわした、いわゆる十界論の地獄界の一人・餓鬼界の一人・ないし九界の中の一人、この「一人の人」を仏にすることで、「全ての人々が皆、同じように仏になることができる」という道理が顕れたのである。
譬えば、竹の節を縦に一つ割れば、他の節もそれにしたがって、一瞬で縦に割れるようなものである。このように、地獄界にいたとしても餓鬼界にいたとしても、南無妙法蓮華経と唱えた瞬間に仏界に到達し、仏性を開いて、自己の無限の力、可能性を涌現させることができるのである。
　
　
English Translations

" Letter to Hōren "
　
 What inspired Shakyamuni to devote himself to religious practice over kalpas equal in number to dust particles in an effort to attain Buddhahood? It was nothing other than the ideal of filial devotion. All the living beings of the six paths and the four forms of birth are our fathers and mothers. Therefore, as long as Shakyamuni was unable to treat them all with filial devotion, he refrained from becoming a Buddha.
 The Lotus Sutra offers a secret means for leading all living beings to Buddhahood. It leads one person in the realm of hell, one person in the realm of hungry spirits, and thus one person in each of the nine realms of existence to Buddhahood, and thereby the way is opened for all living beings to attain Buddhahood. The situation is like the joints in a piece of bamboo: if one joint is ruptured, then all the joints will split.
 

"The Writings of Nichiren Daishonin", Vol.1, P 512
[ --- ] is liberal translation

Interpretation (liberal translation)
　
 What inspired Shakyamuni to devote himself to religious practice over kalpas equal in number to dust particles in an effort to attain Buddhahood?
 It was nothing other than the ideal of filial devotion. When everyone is retracing, people in the all the living beings past are my ancestor and are my parents. Therefore, as long as Shakyamuni was unable to treat them all with filial devotion, he refrained from becoming a Buddha.
 The Lotus Sutra offers a secret means for leading all living beings to Buddhahood. By the so-called Ten world theory which showed the state of the life, It leads one person in the hell world, one person in the hungry world , and thus one person in each of the nine world of existence to Buddhahood, and thereby the way is opened for all living beings to attain Buddhahood.
 This one person attain to the Buddahood, so the law by which all people can reach everyone in Buddhahood in the same way was publicized.
  For example,  A plant as a bamboo and this bamboo have many joints. The joint, that one is lengthwise, when breaking it, a remaining joint also breaks naturally with the division. The same way, Even if a person is in the hell world, The moment of he chant with the myoho-renge-kyo, he's life attain to the Buddhahood, an open the Buddha nature, and he can publicize the infinite power and possibility for orginally from the his life. 

参考文献：「御書をひもとく」「英語で学ぶ御書」　第三文明社
Bibliography : "The Writings of Nichiren Daishonin"
"The Record of The Orally Transmitted Teachings"