On the dispute between R. Elazar ben Yaakov and our mishnah, with regard to who goes out to confess over the eglah arufah - which leadership? And the verses that get him to his position. Also, when the dead body is found in a pile of stones, among other cases, there wouldn't be an eglah erufah. The halakhic rationale here is derived from the parallel to Shikhcha, the forgotten gleanings from the field. Plus, the pronouncement that Jerusalem does not do eglah erufah, even if it's the nearest city, but otherwise, they literally measure the body to the various potentially relevant cities - with a dispute whether the key focus is the body or the head of the body, and what part of the body is the point of measurement?