HETEROMORPHOSIS 121 



I distinguish in Tubularia, according to the differences 

 in irritability, between the stems and the root. By the root 

 is understood that part of the Tubularian which is endowed 

 with a special contact-irritability (stereotropism), by virtue 

 of which it attaches itself to solid bodies and keeps the 

 animal in a fixed position. By the stem is understood that 

 part of the animal which bears the sexual elements and the 

 polyps, and which is endowed with the opposite irritability, 

 in consequence of which it grows away from the substratum 

 to which the animal is attached. This simple terminology, 

 which is based upon the irritability of the organs, will 

 suffice for our purposes. Of the entire animal only the 

 polyps can move spontaneously ; the stem is immovable. If 

 we cut a piece out of a stem, we must discriminate between 

 its oral and aboral ends, according to the orientation of the 

 piece in the original uninjured animal. The oral end is 

 that which was originally directed toward the polyps, the 

 aboral end, that which was directed toward the root. I 

 shall now describe the main experiments individually. 



1. I cut off the roots and polyps of a series of stems, 

 and put these mutilated stems with their aboral ends ver- 

 tically into the sand sufficiently deep to keep them in a ver- 

 tical position. At the free oral ends, which were surrounded 

 on all sides by sea- water, new polyps were formed in a short 

 time at the proper temperature and with favorable speci- 

 mens within two days. These corresponded in form with 

 the old polyps. No growth took place at the ends which 

 were buried in the sand, no matter how long the observations 

 were carried on (in some instances for several months). 



When I put stems with their oral ends in the sand, a 

 polyp was formed at the free, aboral pole. In favorable 

 cases this was formed in a few days. Neither a polyp nor a 

 root was formed at the oral end, which had been covered by 

 sand, no matter how long I continued my observations. 



