(Mishnah): If a man did not write a Kesuvah to his wife, it is an enactment of Beis Din that a Besulah collects 200 and a widow collects 100;

If he wrote that she should collect a field worth 100 in place of 200, or did not write that there is a lien on all his property to the Kesuvah, he is obligated (she has a lien and collects 200), for this is a Tenai (stipulation of) Beis Din.

(Gemara): Our Mishnah is like R. Meir, for R. Meir says that anyone who [gives a Kesuvah] less than 200 Zuz for a Besulah or 100 for a widow, it is Bi'as Zenus.

According to whom was it needed (to teach the Seifa, that if he did not write that there is a lien on all his property to the Kesuvah, he is obligated)? It is for R. Meir. Even though R. Meir says that a document without Achrayus of land cannot be collected [from Meshubadim], he agrees that here she collects.