By analyzing the examples from fairytales and epics , we can get two single stories and the inferior type of this theme and by studying the arrangement of the story from the order of the plot , we can get four mechanisms of development : multiple starting points ; reoccurrence ; heterozygosis and branch expansion , which explain the inherent principle for a continual development and super length of the story 从神话、史诗中搜集到的该类型的例证，分析可得该主题下的两类单体故事以及更次一级的型式类别，并在情节序列上考察目连救母故事的组构情况，总结得出“复合起点” 、 “反复再现” 、 “异型融合”和“旁支扩张”四种生长机制，以期从一个侧面说明该故事持续生长并构成超长篇规模的内在原理。
