DEDIFFERENTIATION IN PEROPHORA 677 
differentiated organs of the subordinate region, and that 
resorption follows upon this. 
Numerous other cases of tissues, regions, and whole organisms 
being unable to maintain themselves as such in changed 
circumstances are known. Of these may be mentioned the 
degeneration of muscle-fibres when the nerves supplying them 
are cut. Here the ‘ normal environment ’ apparently includes 
constant nervous stimulation, and in the absence of this 
the elaborate structure of voluntary muscles cannot be main- 
tained in equilibrium. Similar dedifferentiation of muscle- 
fibres takes place in the stump of an amphibian limb which has 
been cut off preparatory to regeneration (Towle, 1901). 
In the interesting studies of Child on differential inhibition 
during development we do not get the total disappearance of 
one part of the system, but merely a change in the proportions 
of the various parts. The simplest example studied was the 
effect of dilute poisons upon the develcpment of the marine 
Polychaet worm Chaetopterus (Child, 1917). 
He found that during the earliest stages of development 
the apical region of the egg and blastula is the most susceptible 
to various poisons, in certain concentrations a regular death- 
gradient being obtained from the animal to the vegetative pole. 
By the time the early trochophore larvae has been produced, 
however, a new development occurs ; the posterior (previously 
vegetative) region suddenly becomes highly susceptible, its 
metabolic rate being raised apparently in preparation for the 
active growth-processes that are about to occur in this region ; 
for the formation of the permanent growth-zone, from which 
all the body-segments of the adult worm will be produced, 
takes place here. 
The death-gradient will now advance from the two ends of 
the larvae to meet in the middle region, which, with its lower 
metabolic activity, survives the effects of the poison longer 
than the rest. In the later larva the anterior region is differen- 
tiated as a head with ciliated band and apical tuft; and 
posteriorly there is a well-defined growing-region, with a small 
posterior prolongation. 
