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 
toa polyp again.’ 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 
egg. 
1Part II, p. 627. 
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