Somc Phcnonu'iia of ItcgciifiMl ioii in Liiniiüilrilus and related Fornis. 433 



St'pta woro also beinu; formed in tlioü' iionnal position in those indi- 

 vidiials in wliich the intestinc was present and in those froni which 

 it ]iad beeil reinoved the inetaniorphosed ectoderm was entering the 

 coelom just as it does wlien tlie worm's body is in its normal position. 

 These results show that the presence of the neoblasts on the ventral 

 side of the worni is not controUed l)y the force of gravity. It is pos- 

 sible that their position depends, to a certain extent, upon tJie nerve 

 since in eases, not eonnected witli this experiment, in which during tlie 

 removal of the intestine the nerve had been cut so that it did not reacli 

 the end of the body, the neoblasts went no farther than the end of the 

 nerve. The experiment also indicates that the region in which the 

 ectoderm cells become enlarged is not dependent upon the position of 

 the worm's body. 



However, even if tJiere be a predetermined area within which the 

 ectoderm may be nietajnorphosed the exciting cause of the change is 

 still not evident. Among the possible Stimuli which may lead to the 

 metamorphosis of the ectoderm must be considered the nerve cord but 

 there is little to support the assumption that it is the cause of this 

 change. If it were, one would expect the enlargement to be centered 

 about the point at which the ventral nerve cord is in contact with 

 the ectoderm. This is not the case either in the normal regeneration 

 or in tlie abnormal conditio ns. The change under the latter circum- 

 stances is most marked dorsal to the nerve and opposite it the ecto- 

 derm is usually not enlarged and never but slightly so. Furthermore 

 in one instance the nerve touched the ectoderm in the normal manner 

 and even after three days there was no enlargement of the ectodorra. 

 No neoblasts were about the ectoderm, in fact there was only one neo- 

 blast to be seen and this was along the nerve some distance away (Fig. 2). 

 Neither does the severing of the body and the subsequent meeting of 

 the lacerated surfaces appear to act as the immediate Stimulus which 

 causes the ectoderm to enlarge. This is indicated by the instance 

 just cited with reference to the nerve in addition to which similar chan- 

 ges should occur at the anterior end wheii it is severed if the wound 

 alone is the cause of the transformation. Growth of course takes place 

 at the anterior end and cells from the ectoderm enter the coelom but 

 they undergo no such metamorphosis. Since these changes have 

 been observed repeatedly in the absence of the intestine a Stimulus from 

 this source is entirely out of the question. And when the intestine is in 

 contact with the ectoderm the same is true of the intestine asof the nerve, 

 namely that the transformation does not occur in its immediate vicinity. 



28* 



