TWINNING IN EARTHWORMS 29 



structures of a single animal. While this process of 

 mesoblast elongation is taking place a longitudinal con- 

 striction occurs, beginning at about the middle and 

 running both forward and backward until it surrounds 

 the embryo. This furrow cuts more or less deeply 

 through the embryo, dividing the right half from the 

 left. In the majority of individuals the two half-bodies 

 remain connected in the middle region by only a few 

 large ectodermal cells. Each half-embryo, when thus 

 isolated, develops into an entire embryo. Here we have 

 a true case of duplicate twins derived from the right and 

 left halves of a single embryo. When the slightly 

 joined twin worms become active they complete their 

 physical separation by means of a series of rotations 

 which twist and finally break off the uniting cellular 

 cord. 



Double monsters result when the connection is too 

 thick to admit of twisting apart. Sometimes the 

 united region is of considerable extent, but the union 

 involves only the external epithelium and not at all the 

 internal structures. Sometimes the two components of 

 these double monsters are of very unequal size, one of 

 them being the equivalent of an autosite and the other 

 of a parasite, if we may use the terminology employed 

 for similar situations in vertebrate conjoined twins. 

 One interesting feature of Kleinenberg's work is that, 

 in the species studied by him, twinning appears to be 

 almost as regular and specific a process as it is in the 

 armadillos. In all of his numerous cases only a very 

 few instances occurred in which but a single worm 

 emerged from an egg capsule, and even these single 

 worms were probably survivors of twins, for rudiments 



