Kesubos, Chapter 4, Mishnah 7. This mishnah and those that follow it, through the end of the chapter, all rest on a single foundation: the kesubah is a condition established by beis din. Beis din is what created the obligation of the kesubah in the first place, and all of its various components were likewise instituted and fixed by beis din.
Two conclusions follow from this, and they are what the mishnayos before us come to teach:
He wrote no kesubah at all: even though he never obligated himself in a physical document, the obligation stands in full force.
He omitted some of the conditions: conditions instituted by beis din that were not written in the document are still fully binding.
"Lo kasav lah kesubah":
A man who did not write a kesubah for his wife - "besulah goveh masayim v'almanah maneh" - a woman who married as a virgin collects two hundred zuz, which is the basic sum of the kesubah, and a widow who marries a second time collects one hundred zuz. And the reason is: "mipnei shehu tenai beis din" - it is a condition that beis din placed upon marriage, and it applies whether or not the husband ever put his obligation in writing.
"Kasav lah sadeh shaveh maneh tachas masayim zuz":
As part of the kesubah, a lien is placed on all of the husband's property to guarantee its payment. The mishnah now discusses a man who did write a kesubah, but designated within it a particular field worth one hundred zuz in place of his true obligation to her - two hundred zuz, in the case of a woman who married as a virgin - "v'lo kasav lah" - and he did not write for her the clause he was supposed to write: "kol nechasim d'is li achra'in l'kesubasich" - all the property that I own is pledged to your kesubah (this clause, like all these formulas, is written in Aramaic).
Even so, "chayav" - he is liable, and all his property is pledged to this debt, "shehu tenai beis din". It was beis din that established that all of the husband's property is bound and pledged to the payment of the kesubah, and he cannot tell her that she has nothing but that one field he designated for her in the document.
In summary: this mishnah teaches that the kesubah and all its conditions are conditions of beis din, and therefore their force does not depend on being written: one who wrote no kesubah at all - a virgin still collects two hundred and a widow one hundred; and one who designated a field worth one hundred and did not write the lien on all his property - all his property is nevertheless pledged to her kesubah.