DEDIFFERENTIATION IN PEROPHORA 645 
sensitive in the region of the two siphons. It is connected with 
the stolon by a narrow tube of less diameter than the stolon, 
separating above into two tubes; this is generally longer in 
proportion in older individuals. The stolon may grow in 
length and form buds at the proximal end or the distal end 
or both. 
Suitable food for Perophora has not yet been discovered ; 
but in spite of this stolon-zooid preparations may be kept alive 
in the laboratory for a considerable length of time. 
2. DEDIFFERENTIATION, 
(a) General.—Processes may occur in living matter 
whereby whole organisms or parts of them become visibly 
simpler. This occurs, for instance, in Clavellina when kept in 
unfavourable conditions, in Hydra when starved (Schultz, 1906), 
and in various other Coelenterates, in eneysting protozoa and 
in other protozoa in the ordinary course of the life-cycle, with- 
out encystment (Lund, 1917), in sponges (Maas, 1910 ; Miiller, 
1911), &e. Such a process is the reverse of differentiation, and 
is best called dedifferentiation. It has also been termed 
involution and reduction. ‘The latter word will here 
occasionally be used as a convenient synonym for the more 
accurate but clumsier term. 
In Clavellina the original observations of Driesch and the 
later work of Schultz was carried out on half-animals, 
the individuals being cut in two and the half containing 
the branchial sac (pharynx) used for the experiments. This 
portion proved capable of regenerating the whole organism. 
Sometimes it remained intact and produced a restitution-bud 
in which the missing organs were formed; at other times it 
dedifferentiated completely to form an opaque spheroid which 
later redifferentiated into a normal whole individual; or it 
might show a combination of the two processes. Here, when 
dedifferentation occurred, it was as the result of the shock 
of the operation and of the changes produced by it. 
