[If] one says to his sons, “I will slaughter the pesach for the first among you to reach Jerusalem,” [then] as soon as the first one has put his head and the greater part of his body inside, he has acquired his portion, and he acquires his brothers' [portions] for them.  We may always be registered for it as long as there is [at least] an olive's volume for each one.  They may be registered or withdraw from it until it is slaughtered; R' Shimon says: Until the blood is thrown for it.