As was pointed out yesterday, if the רישא of the mishna is ברי וברי why is the din יחלוקו even according to סומכוס there should be a שבועה of מודה במקצת?
The שיטה quotes the ראב"ד who answers as follows. He says that the case of the Mishna is where the לוקח was תופס the cow or שפחה. Therefore the whole dispute is only on the ולד and there is no מודה במקצת. This very similar to saying הילך however, here it is לכ"ע as he actually was תופס already.