ARTIFICIAL PARTHENOGENESIS IN ANNELIDS 691 



We generally consider development as a process which 

 can only occur in one direction, or, in other words, is irre- 

 versible. But this is certainly not generally the case. I 

 showed in a recent paper that the morphogenetic processes 

 in Hydroids are reversible. If the polyp of a Campanularia 

 is brought in contact with a solid body, it is transformed into 

 undifferentiated material and later into a stolon. If the 

 same organ is brought in contact with sea-water, it gives rise 

 to a polyp again. 1 The same may be done with Margelis 

 and other Hydroids. In Antennularia a change in the 

 orientation of a branch with polyps will bring about the 

 transformation of this material into a stolon. Between the 

 two phases the material must pass through an undifferentiated 

 stage where it is neither polyp nor stolon. It will be the 

 task to determine how far in the animal kingdom the develop- 

 mental processes are found to be reversible. It is obvious 

 that in a form with a reversible development death will not 

 necessarily follow a certain stage of development (corre- 

 sponding to senility in man). 



It is not impossible that "natural" death is comparable 

 to the situation which is present in the mature egg after it 

 leaves the ovary. Nature has shown us the way by which 

 at this critical point death can be avoided in the case of the 



i Part II, p. 627. 



